NOAA Technical Memorandum NMFS-SEFSC-490 

ANALYSIS OF SEA TURTLE BYCATCH IN THE COMMERCIAL 

SHRIMP FISHERIES OF SOUTHEAST U.S. WATERS AND THE 

GULF OF MEXICO 

Sheryan Epperly, Larisa Avens, Lance Garrison, Terry Henwood, Wayne Hoggard, 

John Mitchell, James Nance, John Poffenberger, Chris Sasso, 

Elizabeth Scott-Denton, Cynthia Yeung 

 

November 2002 

U. S. Department of Commerce 

National Oceanic and Atmospheric Administration 

National Marine Fisheries Service 

Southeast Fisheries Science Center 

75 Virginia Beach Drive 

Miami, FL 33149
NOAA Technical Memorandum NMFS-SEFSC-490 

ANAYLSIS OF SEA TURTLE BYCATCH IN THE COMMERCIAL 

Sheryan Epperly 

Lance Garrison 

John Poffenberger 

Elizabeth Scott-Denton 

U. S. DEPARTMENT OF COMMERCE 

NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION 

NATIONAL MARINE ISHERIES SERVICE 

Willi eries

November 2002 

npublished reports are used for documentation and timely communication of preliminary 

results, interim reports, or special-purpose information, and have not received complete 

formal review, editorial control, or detailed editing. 

SHRIMP FISHERIES OF SOUTHEAST U.S. WATERS AND THE 

GULF OF MEXICO

Larisa Avens 

Terry Henwood 

Wayne Hoggard 

John Mitchell 

James Nance 

Chris Sasso 

Cynthia Yeung 

Donald L. Evans, Secretary

Conrad C. Lautenbacher, Jr., Administrator 

F

am T. Hogarth, Assistant Administrator for Fish

U
NOTICE 

________________________________________________________________________ 

 

The National Marine Fisheries Service (NMFS) does not approve, recommend or endorse 

any proprietary product or material mentioned in this publication. No reference shall be 

made to NMFS, or to this publication furnished by NMFS, in any advertising or sales 

promotion which would indicate or imply that NMFS approves, recommends or endorses 

any proprietary product or proprietary material herein or which has as its purpose any 

intent to cause directly or indirectly the advertised product to be used or purchased 

because of NMFS publication. 

________________________________________________________________________ 

Correct citation of this report is: 

Epperly, S., L. Avens, L. Garrison, T. Henwood, W. Hoggard, J. Mitchell, J. Nance, J. 

Poffenberger, C. Sasso, E. Scott-Denton, and C. Yeung. 2002. Analysis of sea turtle 

bycatch in the commercial shrimp fisheries of southeast U.S. waters and the Gulf of 

Mexico. U.S. Department of Commerce, NOAA Technical Memorandum NMFSSEFSC-490, 88 pp. 

Copies of this report can be obtained from: 

National Marine Fisheries Service 

Southeast Fisheries Science Center 

Protected Species and Biodiversity Branch 

75 Virginia Beach Drive 

Miami, FL 33149 

or 

National Technical Information Service 

5285 Port Royal Road 

Springfield, VA 22161 

(503) 605-6000 

(800) 553-6847 (rush orders) 

http://www.ntis.gov/numbers.htm

National Marine Fisheries Service Southeast Fisheries Science Center, Division of Protected 

Resources and Biodiversity Contribution PRD-01/02-14 

ii 
TABLE OF CONTENTS 

hrimp Fishery Effort ......................................................................................1

....9

.12

......12

........................................................................23

Adjusted Catch Rates ..............................................................................44 

stimated Number of Interactions With Shrimp Trawls ..........................................49 

stimated Lethal Interactions in Shrimp Trawls ...................................................52 

TED Effectiveness .................................................................................52 

w Time and Water Temperature ..............................56 

Tow Times ..........................................................................................59 

Estima

teractions In Other Gears and the Bait Shrimp Fishery ........................................69 

Otter Trawls Used for Bait Shrimp ...............................................................69 

Roller Frame Trawls ...............................................................................71 

.......................................................72

Skimmer Trawls ....................................................................................72 

.....................................................................74

......................................................................76

............................................................77

..................................................................................78

iscussion ...................................................................................................80

esearch Recommendations .............................................................................82

.....................................................................................84

............................................................................................84

S

Gulf Effort – Hours Fished .........................................................................2

Atlantic Effort – Trips Offloaded ..................................................................2

Evaluation of Trip as the Appropriate Unit of Effort in the Atlantic .....................

Catch Rates of Sea Turtles in Shrimp Trawls .......................................................

 Foundation Data ..............................................................................

Aerial Survey Data .........

E

E

Mortality as a Function of To

 Estimated Mortalities ..............................................................................62

Estimated Mortalities in Trawls Equipped With TEDs With Larger Escape Openings ...64 

ted Number of Individuals Interacting With Shrimp Trawls ............................67 

In

 Beam Trawls .................................

Butterfly Nets ..................

Channel Nets ..................

Cast Nets .................................

 Other Gear .........

D

R

Acknowledgements ...

Literature Cited

iii 
ANALYSIS OF SEA T HE COMMERCIAL 

SHRIMP FISHERIES OF SOUTHEAST U.S. WATERS AND THE GULF 

OF MEXICO 

alculation of catch rates of sea turtles in shrimp trawls is necessary to evaluate the impact on 

teract

umber

crease in the size of the escape openings in turtle excluder devices (TEDs). The new 

at cat

number

arger turtles are less likely to escape through the openings) and the duration of tow times; this 

URTLE BYCATCH IN T

The coastal shrimp trawl fisheries have long been the focus of conservation actions to 

reduce turtle bycatch and mortality in the Gulf of Mexico and the U.S. Atlantic (NRC, 1990). 

C

sea turtle populations. In this paper we analyze sea turtle bycatch to provide an estimate of the 

current number of interactions with otter trawl gear as well as an estimate of the number of fatal 

in ions in Southeast U.S. waters and the Gulf of Mexico. We also provide an estimate of the 

n of individuals likely to die in the future with the new regulations that will require an 

in

regulations will allow many more turtles to escape. Other gears also are discussed. 

Our approach was to estimate the catch rates of sea turtles, by species, by geographic 

subregion, by depth stratum, and season, and to apply these catch rates to the reported effort of 

the commercial fleet during 2001 to estimate the total number of interactions. It must be noted 

th ch rate data exist only for otter trawls. Also, we estimate the number of interactions, not 

individual animals, as it is likely that animals interact with the fishery more than one time. The 

of fatal interactions is a function of the effectiveness of TEDs on various sizes of turtles 

(l

is a measure of number of turtles since an individual can only die once. 

terva

Finally, we note that there are numerous sources of variability and bias in this analysis. 

Where possible we attempt to quantify the magnitude and direction of these. Confidence 

in ls about the estimates are given where error can be quantified, but given the multitude of

assumptions and sources of variability that are unquantified, they may give a false impression of 

our confidence in the estimates. They imply bounds, when really those bounds are unknown. 

rimp

e number of interactions, the number of interactions resulting in mortalities under current 

ED regulations are enacted. We then discuss the potential for interactions in other gears and in 

commendations. 

A variety of gears are used to catch shrimp commercially in the Gulf of Mexico and the 

Atlantic (Table 1, Table 3). Recreational effort and catch estimates do not exist. Commercial 

catch generally is reported by fishing zones, which can be summarized into 35 statistical zones 

(Figure 1, Figure 2). In the Gulf of Mexico these zones are divided into 5 fm intervals and all 

This report is organized by sections. We first provide estimates of fishing effort by 

sh trawls. Next we provide estimates of sea turtle CPUE in shrimp trawls and adjust those 

estimates with aerial survey data. Following those sections, we provide the results: estimates of 

th

regulations, and the number of interactions which we expect will result in interactions once new 

T

the bait shrimp fisheries. Lastly, we provide the reader a summary discussion and make research 

re

Shrimp Fishery Effort 

1 
data ls

(1

45 fm are included in the > 45 fm locati (Poffenberger, 1991). Fathoms are 

e units of depth used for the analyses h lf data are reported already based on 

ese units.

In

n

re 2) 

orth

collected are reported in these zone/depth locations for a total of 11 possible location cel

inshore and 10 offshore) within a given zone; all fishery data collected in depths greater than 

on cell for the zone

th erein since the Gu

th

We divided the shelf into an inshore depth stratum (inside COLREG lines: bays and 

sounds), a nearshore depth stratum (10 fm) and an offshore depth stratum > 10 fm (Figure 1).

the Atlantic virtually all the shrimping effort occurs within 10 fm and thus there is no offshore 

stratum. Furthermore we divided the data temporally, into the “warm” or summer season when 

the shrimp fishery is most active (March-November) and the “cold” or winter season when the 

shrimp fishery is minimally active, mostly in offshore waters (see Figure 6-2, p. 88 in NRC, 

1990). Statistical zones were combined to form two geographic subregions in the Gulf: easter

Gulf of Mexico (zones 1-12) and western Gulf of Mexico (zones 13-21) (Figure 1, Figu

and three subregions in the Atlantic: South (zones 24-30), Central (zones 31-33) and N

(North Carolina). 

Gulf Effort – Hours Fished

In the Gulf of Mexico shrimp fishery statistics database, gear is divided into two 

categories: shrimp trawls and other shrimp trawls, including otter and mongoose trawls. In the 

latter category are butterfly nets and skimmer trawls, gears used exclusively in inshore waters. 

Bait shrimp catch and effort is not reported, but other gears, such as wing nets and cast nets may 

t

tlantic Effort – Trips Offloaded

be used to catch shrimp for bait. The estimation of shrimp fishery effort in the Gulf is dependen

upon data summarized by location cells (Nance 1992). Estimates of monthly shrimp trawl 

fishing effort for each location cell requires two elements: (1) total pounds of shrimp caught by 

gear, and (2) average catch per unit of effort (CPUE; pounds per hour fished) for that gear. Total

pounds caught is acquired from commercial seafood dealers located along the Gulf coast; CPUE 

is obtained from interviews with captains from shrimp vessels at the termination of their trip. 

Monthly effort (hours fished) for a location cell is estimated by dividing the monthly shrimp

landings from a type of gear used in a location cell by the average CPUE for that gear during the

same time and location cell combination. Otter trawl fishing effort (hours fished) in the Gulf of 

Mexico in 2001 is given in Table 2. This represents effort primarily for brown shrimp 

(Farfantepenaeus aztecus), pink shrimp (F. duorarum), and white shrimp (Litopenaeus 

setiferus), and to a lesser extent for rock shrimp (Sicyonia spp.), Trachypenaeus shrimp 

(Trachypenaeus spp.), seabobs (Xiphopenaeus kroyeri), and royal red shrimp (Pleoticus 

robustus). 

A

For fishing that occurred in the southeast U.S. Atlantic (i.e., off the coast from North 

Carolina south through the middle Florida Keys), fishery statistics are collected by the fishery 

2 
agency in each of the respective states and provided to the NMFS. The states of North Carolina

Georgia2

, and Florida3 have a trip ticket program that was operational during 2001. For these 

programs, dealers are required, by state law, to report all purchases of fish and shellfish landed 

(off-loaded) at ports in the respective state. In addition to information on the species purchased,

dealers are requ

1

, 

ired to provide information on the type of fishing gear and the location of the 

fishing trips. The duration of a fishing trip can be determined by the start date and unloading 

(landin

er

very

are reported as north or south of Cape Hatteras, within or outside state 

territorial waters. Inshore water body codes are more specific, but still some, such as the 

Intracoastal Waterway are problematic. For most water body codes we could assign the data to a 

those waters that bridged zones, we attributed the effort to the 

zone associated with the city/county of landing. This was especially problematic for the ocean 

catches s.

ta

e Gulf

on

g) date from the state’s trip ticket data. For South Carolina, which did not have a 

comprehensive trip ticket program active during 2001, data for individual trips were provided by 

the majority of the dealers voluntarily and submitted to the state

4

. Information on the type of 

shrimp that were caught, the type of gear used for the trip, and the location of the fishing trip was

provided. For South Carolina, however, duration of the fishing trip is not reported. 

Based on locations fished, we assigned the reported fishing effort to statistical zones and

depth strata (inshore vs ocean in the Atlantic), and based on date, assigned it to a season, summ

or winter (Table 4). Although location fished is reported in North Carolina, some codes are

broad: ocean catches

particular statistical zone. For

since ocean fishermen may be quite mobile, even fishing in waters off other state

Infrequently the place of landing was missing and then we attributed effort associated with that 

trip to zone 34. Bait shrimp statistics are reported separately in Florida and Georgia and are 

addressed later. 

Sources of Error 

NMFS and state port agents in the Gulf of Mexico collect shrimp statistics from two 

sources, seafood dealers and fisherman. Data on the amount and value of the shrimp from

individual trips that are unloaded or landed at the dealers are collected from dealer records. Da

that includes information on fishing effort and location for an individual trip is collected by 

interviewing the captain. Because the fishing trip is the basic sampling unit, the fundamental 

principle of the data collection procedures is to collect both the landing and interview data on a 

trip-by-trip basis. However, because the reported number of fishing trips that occur in th

shrimp fishery are in the several hundred thousand range, not every trip has information

 

1

 North Carolina Division of Marine Fisheries. Unpublished Data. Lees Sabo, NCDMF, Morehead City, NN. 

ersonal Communication (E-mail) October 31, 2002 to Sheryan Epperly, National Marine Fisheries Service, Miami,

L.

 Unpublished Data. Julie Califf, GADNR, Brunswick, GA. Personal

o John Poffenberger, National Marine Fisheries Service, Miami, FL. 

orida Fish and Wildlife Conservation Commission. Unpublished Data. Guy Davenport, National Marine 

Fisheries

Fisheries Service, Beaufort, NC. Personal Communication (E-mail) October 18, 2002 to Sheryan Epperly, National

Marine Fisheries Service, Miami, FL. 

P

F

2

Georgia Department of Natural Resources.

Communication (E-mail) October 17, 2002 t 3

Fl

 Service, Miami, FL. Personal Communication (E-mail) November 4, 2002 to Sheryan Epperly, National

Marine Fisheries Service, Miami, FL. 4

South Carolina Department of Natural Resources. Unpublished Data. Linda Hardy Bernstein, National Marine

3 
fishing effort and location from an interview. The port agent must assign a catch location for the 

landings for each trip, and uses information obtained from the dealer, other interviews, or 

historical knowledge of the fleet’s activity to perform this assignment activity. Thus, some erro

in assignment of locations of the catch can occur from the judgment of the port agents, or even 

during the interview process after a 60-day trip. However, these potential errors were conside

random in nature and the directional bias was considered negligible. 

The Gulf and South Atlantic Fisheries Foundation, Inc. received funding to address the 

question of possible location error and directional bias in the shr

r

red

ook, ELB)

that cou d accurately measure the magnitude and spatial patterns of fishing effort with a trip. 

This un

nd

.

e

h

dustry should be implemented using ELB technology and the port agent 

network to obtain more precise and accurate estimates of shrimp trawling effort with minimal 

ers.

ber, 

rtles

t

working inshore likely have less fishing power – they generally do not pull as many nets and 

often the nets are smaller. Equating their fishing power with that of the offshore fleets will result 

in an overestimation of turtle catch for the inshore vessels. 

The NMFS data collection program for shrimp statistics includes only the commercial 

sector landing table shrimp (shrimp for human consumption); live bait shrimp statistics are 

available only for Florida’s east coast and Georgia. The statistics do not include shrimp 

harvested by recreational fishermen, nor does it include catches by small, part-time commercial 

fishermen that sell their catches along the roadsides (Poffenberger, 1991). The “recreational” 

effort may be substantial, but because they generally are restricted to using a small amount of 

imp effort data. A proposal to

tackle this research question was successful submitted by LGL. The first year of the effort 

involved development of a simple, reliable, and low-cost GPS unit (Electronic Logb

l

it was successfully developed and has been shown to provide a very good measure of 

effort, with minimal inconvenience and cost (Gallaway, et. al., in press). 

This ELB unit has been successfully used in subsequent years in a small pilot study 

program to provided comparisons of actual areas fished, pounds landed from these areas, a

catch rates in these areas as measured using ELBs to the corresponding estimates made by port 

agents (135 trips of evaluation). The results from the pilot study program show that some

directional bias does occur, and that CPUE is often underestimated (Gallaway, et. al., in review)

If the results from this study are representative of the fleet as a whole, the shrimp effort in th

mid-shelf area could be overestimated, whereas the nearshore and deepwater effort could be 

underestimated. The results from the study suggest that a cooperative program involving bot

the NMFS and in

impact on the fish

Effort presented thus far is without consideration of fishing power relative to the 

vulnerability of sea turtles to capture. The catch of sea turtles may be influenced by the num

size, and characteristics of nets being fished as well as the speed of the vessel. Because data are 

not available to characterize the fleet, or to evaluate these factors on the catchability of sea tu

we must assume that each vessel can be characterized by the averages and assume that one uni

of effort has the same fishing power throughout the fishery in the Gulf or in the Atlantic, inshore

and in the ocean. In general, vessels working inshore waters are small compared to those in the 

ocean (an exception is the large vessels working inshore sounds of N.C.). Smaller vessels 

4 
gear in inshore waters, turtle mortality likely is not high. Failure to include this sector’s effort 

will result in an underestimation of the number of interactions with sea turtles.

The calculation of effort data in the Gulf is dependent on the assumption that the 

interviews accurately portray the catch and effort of the entire fleet for a particular location cell. 

During peak shrimp production about 70-80% of the landings have an average CPUE associated 

ith them. Infrequently location cells have landings reported but no interviews; usually the 

CPUE f

o.

w

is estimated from a month-specific statistical model based on the independent variables o

year and geographic location. This model was developed based on historical data and r2

=0.50 

for most monthly models. (Nance, 1992). 

Table 1. Estimated effort in 2001 (hours fished), reported by gear type, for the Gulf of Mexic

Bait shrimp trips are not included. 

 Gear 

Zone Shrimp Trawl Other Description of Other 

1 36787 0 

2 252021 0 

3 44014 0 

4 30537 0 

2 24320 0 

6 55090 0 

7 79948 76 skimmer trawls 

8 59995 0 

9 7616 0 

10 135119 2964 skimmer trawls 

11 458494 3772 skimmer trawls 

12 

225661 90119 

skimmer trawls and 

butterfly nets 

13 

851217 132194 

skimmer trawls and 

butterfly nets 

14 

645386 467629 

skimmer trawls and 

butterfly nets 

15 

468928 7746 

skimmer trawls and 

butterfly nets 

16 

475874 15745 butterfly nets 

skimmer trawls and 

17 

704769 2171 

skimmer trawls and 

butterfly nets 

18 482935 0 

19 1175512 0 

20 242435 0 

21 190378 0 

5 
Table 2. Shrimp trawl fishing effort (hours fished) estimated in the Gulf of Mexico in 2001 by

subregion and season. Bait shrimp trips are not included. 

Subregion 

Season / 

Depth stratum 

Western Gulf

(zones 13-21) 

Eastern Gulf 

(zones 1-12) 

Summer (Mar-Nov)

Inshore 734951 429285 

Nearshore (0-10 fm) 1933570 261442 

Offshore (> 10 fm) 1762230 462184 

Winter (Dec-Feb) 

Inshore 17851 18668 

Nearshore (0-10 fm) 342913 39870 

Offshore (> 10 fm) 445917 198151 

Table 3. Fish effort in 200 ), reported by

shrimp trips are not included, except in the Carolinas where they cannot be distinguished from

trips made for table shrimp. 

Gears 

ing 1 (trips gear type, in Southeast U.S. waters. Bait 

wl 

Butte

Net l 

Channel

p Seine Gear

540 0 0

0 0 0

0 0 0

0 0 3

0 0 22

0 0 19

0 0 12

0 0 1

0 0 11

4 0 31

6 7 1

Zone

Shrimp 

Tra

Skimmer 

Trawl 

rfly

Beam/

Roller

Traw Net 

Pot / 

Tra

Hand 

s

5 Other

25 0 0 281 0 0 0 

26 33 0 0 0 0 0 

27 38 0 0 0 0 0 

28 496 0 0 0 0 0 

29 223 0 0 0 0 0 

30 2302 0 0 0 0 0 

31 2969 0 0 0 0 3 10 

32 5950 0 0 0 0 0 

33 2067 0 0 157 6 9 0

34 6122 1760 0 0 1610 1 0 

35 3167 11 0 0 4 0 

 

5

 Hand gears include dip nets and cast nets. 

6 
Table 4. Shrimp trawl fishing effort (trips) in the Atlantic in 2001 by subregion and season. 

Bait shrimp trips are not included, except for in the Carolinas where they cannot be distinguished 

om trips made for table shrimp. 

e

fr

Subr gion

stratum (zone

th Cen ral No

er (Mare 4

r (Dec-Feb) 

e

6

Season /

Depth

Sou

s 24-30)

t

(zones 31-33)

rth

(zones ≥34) 

Summ Nov)

Inshor 54 411 7312 

Ocean 1906 9038 1902 

Winte

Inshor 101 36 39 

Ocean 31 1401 36 

7 
Figure 1. Statistical reporting zones and depth strata in the Gulf of Mexico. 

The nearshore stratum is 0-10 fm and the offshore stratum is > 10 fm. The inshore 

stratum, shown in white, is inside the COLREG lines. 

Figure 2. Statistical reporting zones for the Southeast U.S.

8 
Evaluation of Trip as the Appropriate Unit of Effort in the Atlantic

e probability of a turtle capture 

uring that trip also increases. Fishing trips are of variable durations and this factor is not 

accounted for in the effort data reported above for the Atlantic. Catch rates of turtles can be 

expressed as catch per unit effort, where that effort can be trip, day, hour, or other standard units 

(see section Catch Rates in Shrimp Trawls, below). The most comprehensive recent study of sea 

turtle interaction rates was conducted by the Gulf and South Atlantic Fisheries Foundation. If 

sampling by the Foundation to obtain the catch rates accurately mirrors the fleet in trip duration 

then there is no reason to convert reported trips to any other unit. If the duration of the sampled 

trips varies significantly from that of the fleet, it may then be necessary to use days or even to 

convert to hours fished. Doing so, though, introduces additional error. 

The Florida Trip Ticket system includes a field to record time (duration) of a trip, either 

in hours or days. About half of the trips reported duration in days (calendar days) while the 

remainder reported in hours. No effort was reported for 8 (0.25%) trips. Hours reported ranged 

from 1 to 48 per trip. A frequency distribution indicated a strong peak at 6-9 hours and most 

were less than 12 hours. We assumed that all trips reporting less than 12 hours fished were 1 day 

and that trips reporting > 12 hours were multiple days and estimated the duration by assuming 

that 12 hours represented a day. We assigned those trips reporting no effort the mean number of 

days fished by other vessels landing in Florida and fishing in the same season x subregion x 

depth stratum. Trips landing in Florida during the summer averaged 3.7 d (n=2482, SE=0.10, 

ode=1, range=1-65) and those landing during the winter averaged 3.6 d (n=745, SE=0.16, 

ode=1, range=1-30). Trips longer than a few days in duration likely are by vessels capable of 

individ ssels which allow them to 

stay at sea for extended periods of time and to hold shrimp until they are more marketable and 

prices increase. 

Days fished usually are recorded in the Georgia trip ticket data; however, 549 (17.7%) 

reported no effort. We assigned those trips reporting no effort the mean number of days fished 

by other vessels landing in Georgia and fishing in the same season x subregion x depth stratum. 

Trips landing in Georgia during the summer averaged 4.3 d (n=2122, SE=0.06, mode=3, 

range=1-36) and those landing during the winter averaged 4.1 d (n=435, SE=0.13, mode=3, 

range=1-30). 

The duration of trips offloading in South Carolina has not been recorded in recent years, 

but historically such data has been collected6

, most recently for 1989-1990 from the South 

Atlantic Detailed Shrimp Program (Anon., 2002). Many dealers reported weekly the number of 

trips offloaded (since this was not mandatory, some dealers reported only monthly and port 

agents had to impute the weekly data). Trips landing in South Carolina during the summer 

averaged 2.5 d (n=17847, SE=0.02, mode=1, range=1-78) and those landing during the winter 

averaged 2.3 d (n=1978, SE=0.03, mode=1, range=1-15). 

 

As the duration of a trip increases, it is expected that th

d

m

m

ually and quickly freezing the shrimp; some have resupply ve

6

 South Carolina Department of Natural Resources. Unpublished data. Linda Hardy ernstein, National Marine 

Fisheries Service, Beaufort, NC. Personal communication (E-mail) October 18, 2002 to Sheryan Epperly, National

Marine Fisheries Service, Miami, FL. 

B

9 
North Carolina trip tickets do not record the duration of a trip, but that information can be 

ferred by comparing the trip start date and the date of offloading, reported for all but 26 trips 

(0.25% se

ding

in

Source

is

s 

e

e ticket, that field was assigned by the port agent by comparing that ticket with 

the last date of unloading along with the amount of the catch and the agent’s fishery knowledge. 

This w

 

in

). Trips starting and landing on the same date were assigned a trip duration of 1 d. Tho

trips reporting no effort were assigned the mean number of days fished by other vessels lan

in North Carolina and fishing in the same season x subregion x depth stratum. Trips landing

North Carolina during the summer averaged 1.6 d (n=10352, SE=0.01, mode=1, range=1-18) and

those landing during the winter averaged 1.7 d (n=128, SE=0.14, mode=1, range=1-11). 

The average duration of Atlantic trips in our sampling data for sea turtle CPUE (see 

section on Catch Rates in Shrimp Trawls, below) is significantly greater than the duration of trips

by the fleet, as shown above. Thus days fished (not number of trips) were determined to be the

most appropriate measure of effort in the Atlantic (Table 5). 

s of Error 

The sources of error are many. Most important is the accuracy of the trip ticket data. We

are making assumptions concerning the duration of trips when data are missing or when the 

computed duration was 0 d, and about hourly data reported in Florida. We cannot evaluate the 

accuracy of these assumptions. Secondly, we assume that the proportion of time fishing is the 

same for all trips; some vessels may have more transit time, especially the freezer boats which 

are capable of ranging widely. Thus, we may be overestimating actual effort. For South 

Carolina we assume that data collected for 1989-1990 are representative of fishing today; there

no evidence to indicate that the fleet is fishing any differently then now.7

 Not all S.C. dealer

reported on trip tickets in 1989-1990. The program was voluntary (only monthly summaries wer

mandatory). Thus trip duration (calendar days) was imputed using reported catch divided by 

average catch/day from the trip tickets that month. Also, if the dealer did not fill out “calendar 

days fished’ on th

ould be a maximum estimate of the calendar days fished for the trip.8

 

7

 Nan Jenkins, South Carolina Department of Natural Resources, Charleston, SC. Personal communication (E-mail)

October 23, 2002 to Sheryan Epperly, National Marine Fisheries Service, Miami, FL. 8

 Nan Jenkins, S.C. Department of Natural Resources, Charleston, SC. Personal communication (E-mail) October 

16, 2002 to Sheryan Epperly, National Marine Fisheries Service, Miami, FL. 

10 
Table 5. Shrimp trawl effort (days fished) estimated in the Atlantic in 2001, by subregion and 

season. Bait shrimp trips are not included, except for in the Carolinas where they cannot be 

distinguished from trips for table shrimp. 

Subregion 

Season / 

Depth stratum 

South 

(zones 24-30)

Central 

(zones 31-33)

North 

(zones ≥34) 

Summer (Mar-Nov) 

Inshore 626 430 12352

Ocean 8331 26947 2418 

Winter (Dec-Feb) 

Inshore 116 36 44 

Ocean 2578 4164 84 

11 
Catch Rates Of Sea Turtles In Shrimp Trawls 

 

Observer data sets that quantify sea turtle catch per unit effort (CPUE) have very limited 

spatial and temporal coverage. The most comprehensive recent study on the interaction rates of 

sea turtles and shrim as conduct 97-199 ulf an Atlantic

Fisheries Founda , 19 r a ere much 

smaller in geog cope or used a net othe than a shrimp trawl. The

Foundation study wa to the wester lf of Mexico and the coas ntic between

northern Florida and South Carolina (see Foundation Data below). One option is to apply these 

rates to the entir Atlantic, respectively, for all times of the year. However, 

both the overall density and species composition of sea turtles are known to vary significantly 

across longitudinal and latitudinal scales. For example, sea turtle density is significantly higher 

in the eastern Gulf of Mexico and along the Flor a coast in comparison to the western Gulf of 

Mexico along Louisiana and Texas (McDaniel et al. 2000). Strandings, too, differ in number 

and species composition between the eastern and western Gulf of Mexico (Table 6). Catch rates 

for a region with relatively low turtle abundance (e.g., Western Gulf of Mexico) would be 

expected to be less than those in an area with significantly higher turtle abundance (e.g., Eastern 

Gulf of Mexico). The preferred option is to account for the relative density of turtles across 

geographic areas and adjust catch rates accordingly. Extrapolating CPUE information from

localized observations requires information on the relative density of turtles across geographic 

regions. For this, we used aerial survey data collected in the Gulf of Mexico and U.S. Atlantic. 

Foundation Data

p trawls w ed in 19 8 by the G d South

tion (1998; Jamir 99). All other ecent studies av ilable either w

raphic and temporal s r

s limited n Gu tal Atla

e Gulf of Mexico or

id

While aboard actively fishing commercial shrimp trawlers, the Foundation monitored the 

catch of sea turtles in nets not equipped with TEDs. From May 1997-May 1998 641 tows were 

observed in southeastern U.S. waters and 1,133 tows were observed in the waters of the Gulf of 

Mexico. In the Atlantic 274 turtles were captured and 269

 were captured in the Gulf. Tows in 

shallow waters (<15 fm) were restricted to 55 minutes during April through October and to 75 

minutes from November through March to minimize the mortality of sea turtles. Tow times in 

waters deeper than 15 fm were not restricted. Details on the study can be found in Gulf and 

South Atlantic Fisheries Foundation (1998) and Jamir (1999).

The distribution of the Foundation’s sampling effort was not proportional to the fleet’s 

effort in a given stratum (Figure 3, Figure 4) and therefore the samples cannot be considered to 

be random samples. Thus, samples were stratified so that catch rates for each stratum could be 

calculated. The stratification used is the same that was used to stratify effort data and is 

described above. 

 

9

 One tow in the Gulf of Mexico resulted in the capture of a Kemp’s ridley, but there was no effort (tow duration) 

recorded for that tow. Thus, our analysis is based on the capture of 25, not 26 turtles, in the Gulf. 

12 
Table 6. Stranding , the period of

me that the Foundation study was conducted. Note the difference in species composition 

tern Gulf and Western Gulf during each time period. Species codes are 

Cc=log

s of sea turtles in the Gulf of Mexico, May 1997-May 1998

ti

between the Eas

gerhead, Cm=green, Dc=leatherback, Ei=hawksbill, Lk=Kemp’s ridley, and 

Un=unidentified. 

Species Species

Cc Cc

Cm Cm

Dc Dc

Ei Ei

Lk Lk

Un Un

Total Total

Species Species

Cc Cc

Cm Cm

Dc Dc

Ei Ei

Lk Lk

Western Gulf

May 1997-May 1998 Species Proportions By Season - W. Gulf (zones 13-21) vs. E. Gulf (zones 1-12, 24, 25)

true strandings only - no H, no T, no PH, no IC

Winter (Dec - Feb)

Eastern Gulf

3

14

1

39

Number

16

5

0

Percent

41

13

0

8

36

3

Number Percent

12 20

35 59

3 5

2 3

5 9

2 3

59

Spring (Mar - May)

Western Gulf Eastern Gulf

Number Percent Number Percent

105 32 125 55

22 7 26 12

12 4 2 1

7 2 4 2

169 52 60 26

Un Un

Total Total

Cc Cc

Cm Cm

Species Species

Cc Cc

Cm Cm

Dc Dc

Ei Ei

Lk Lk

Un Un

Total 628 Total 469

22 4 29 6

304 48 94 20

17 3 10 2

12 2 7 2

45 7 80 17

228 36 249 53

All Seasons Combined

Western Gulf Eastern Gulf

Number Percent Number Percent

11 3 10 4

326 227

Summer (Jun - Aug)

Western Gulf Eastern Gulf

r Percent

46 37 38 53

9 7 14 19

Species Species

Cc Cc

Cm Cm

Dc Dc

Ei Ei

Lk Lk

Un Un

Total Total

Species Species

Number Percent Number Percent

61 44 74 67

9 7 5 4

0 0 2 2

3 2 2 2

59 43 15 13

6 4 13 12

138 111

Fall (Sep - Nov)

Western Gulf Eastern Gulf

Number Percent Numbe

Dc Dc

Ei Ei

Lk Lk

Un Un

Total Total

0 0 0 0

4 3 2 3

62 50 14 19

4 3 4 6

125 72

13 
The original catch and effort data collected in the Foundation study were re-analyzed

here. Catch and effort data were stratified by (1) OCEAN AREA - (i) western Gulf of M

(west of the 89oW), (ii) the Atlantic, (2) SPECIES

exico

, (3) SEASON - (i) summer = Mar

November, (ii) win

chter = December-February. The data were further stratified in the western Gulf 

by (4) DEPTH STRATA - (i) Nearshore: 0-10 fm, (ii) Offshore > 10 fm (there was no sampling 

effort in inshore waters), and in the Atlantic by (5) SUBREGION- (i) South: < 31o

N (statistical

zones 29-30), (ii) Central: ≥ 31o

N (zones 31-33) (Atlantic zones are the integer value of latitude

in decimal degrees). We attempted to align units of sampling effort with the units of total fishery 

effort collected by NMFS programs in the respective areas. A sampling unit in the western Gulf 

is a unique tow, and effort is measured in hours, the same units as reported for the fishery. Trips 

are the units used to report Atlantic fishing effort, but the trips that the Foundation sampled in the 

Atlantic generally were longer duration (mean=10.4 d, SE=1.83, mode=5 d, n=12 trips10) than 

reported for the fishery (see Evaluation of Trip as the Appropriate Unit of Effort in the Atlantic, 

above). Thus, trip is not a measure that is comparable in effort in the fishery and the Foundation 

study. Therefore, for this analysis a sampling unit in the Atlantic is a fishing day, which usually 

consists of more than one tow (Table 7). Catch and effort in the Atlantic are standardized by 

tows as in the Gulf data (Table 7C) to facilitate comparison between the two regions. Table 7

gives a summary of sampling effort.

Sampling effort in the Western Gulf totaled 5018.2 hours and in the Atlantic totaled 596.5 hours 

in 128 days of fishing (Table 7). No sampling occurred in the eastern Gulf or in the North 

subregion of the Atlantic. There was no sampling in winter in the Atlantic, and the only winter 

effort in the western Gulf occurred in the offshore stratum in 1997. The depth distribution of 

Foundation effort is shown in Figure 5. In the Atlantic, the effort was restricted to 0-10 fm. - 

less than 0.2% of the effort (1/641 tows) was in depths > 10 fm. Less than 0.5% of the effort 

(5/1133 tows) in the western Gulf was in depths greater than 40 fm. 

Stratified catch and CPUE statistics are tabulated in Table 8. Loggerheads (Caretta 

caretta) (western Gulf - 8, Atlantic - 201) and Kemp’s ridleys (Lepidochelys kempii)(western 

Gulf - 15, Atlantic - 67) were the most common species in the catch. Greens (Chelonia mydas) 

were rarely caught (western Gulf - 2, Atlantic - 5), and only one leatherback (Dermochelys 

coriacea) was caught in total (western Gulf). Atlantic catches were much higher than western 

Gulf catches, although Atlantic effort in hours was an order of magnitude lower than effort in the 

western Gulf. Observed mean CPUE = total catch/total effort (see effort in Table 7) in each 

stratum. CPUE was much lower in winter than summer. 

Table 9 shows the point and precision estimates of the CPUE by species and stratum. 

The number of observations or the sample size (n) is in tows in the western Gulf (A) and in days 

(B) or tows (C) in the Atlantic. The observed mean CPUE and standard deviation (std) are given 

for each stratum that had effort. Strata with no effort have no estimates. The 95% confidence 

limits (ucl = upper, lcl = lower) are computed by the non-parametric bootstrap percentile (PCTL) 

method. Bootstrap normal and parametric normal confidence limits also are computed for 

comparison (SAS Institute, 2000; Lunneborg, 2000). Bootstrapping was based on 1000 

 

10 5 of the 12 trips had effort in more than one subregion. Thus, it is not possible to report trip statistics by

subregion. 

14 
replicates of n observations within a stratum. The three methods gave very similar confidence 

interva s

pt that

rdinary estimator of mean CPUE should be relatively 

nbiased, and any gain with the delta-distribution method is expected to be minor, and in that 

is

on from which samples are 

rawn must be strictly defined and inferences applied on that particular population. Asymptotic 

in simp the

c

ably

It is a fundamental assumption of this analysis that the observed CPUE is representative 

of norm ot

g

e of 

8.0 hr/day (SE=0.38, n=1442). Sampling by observers of NMFS and the Foundation, 1997-2002 

(see Tow Times, below) yielded fishing time during the summer that averaged 6.5 hr/day 

ls, indicating that the bootstrapped distribution is roughly normal. The major advantage

of the bootstrap PCTL over the other two methods are that the limits do not extend into the 

negative range, and that no assumptions are made concerning sampling distribution, exce

sampling is random and representative. The coefficient of variation (cv) and variance (var) of 

the original observed dataset (sample size = n) are calculated for each stratum. 

Sources of Error 

Bycatch surveys typically contain a large proportion of zero observations, resulting in 

highly skewed sample distributions. The ordinary sample mean CPUE statistic may 

underestimate the true population mean, and to a greater extent, the ordinary sample variance 

may underestimate the true variability of the mean statistic. A widely practiced method is to 

separate zero and non-zero values (delta method) and fit a distribution model (often lognormal) 

solely to the non-zero values (Pennington, 1983). This method assumes a lognormal sample 

distribution for non-zero values, and is not robust to small departures from model assumptions 

(Myers and Pepin, 1990). With small sample sizes, such departures often cannot be detected, 

therefore obscuring the magnitude and direction of bias. As there are no extremely large 

observations in the bycatch data, the o

u

latter case if and only if the lognormal distribution is a good fit to non-zero catch rates. 

The confidence interval of the estimator of the mean is generated by the bootstrap 

percentile method, which does not rely on any assumptions regarding the underlying sample 

distribution. Its main virtue in this case is that the interval cannot extend beyond the possible 

range of values of the statistic. Simple random sampling and finite variance is assumed, but it

no remedy for inadequate sample size. For this analysis, the populati

d

le random samples, the bootstrap percentile method should have the same accuracy as

traditional normal approximation. Bias would be amplified in the same direction as the statisti

for which the interval was generated (i.e. negative in this case). The bootstrap percentile and the

bootstrap normal and parametric normal estimates of the confidence interval compare reason

well. The upper limit of the bootstrap percentile interval is slightly higher than the normal 

intervals, and that may compensate somewhat for the possible underestimation of the population 

mean by the ordinary sample mean. 

al fishery behavior and sampling locations. If the observed component of the fleet is n

representative then the analysis is biased. We are concerned about this assumption, especially as 

applied to the Atlantic where virtually all tows by the Foundation were limited to minimize turtle 

mortality. Time fishing (sum of all tows) averaged 4.8 hr/day (SE=0.25, n=71) in the Central 

subregion and 4.4 hr/day (SE=0.17, n=57) in the South subregion. It appears that the fleet may 

fish more hours during a day than sampled by the Foundation. Thus, we are underestimatin

catch per day. Boats landing in Georgia during summer 2001 reported average fishing tim

15 
(SE=0.31, n=142) in the Central subregion and 6.6 hr/day (SE=0.60, n=6) in the South 

subregion. 

Table 7. Effort in Foundation data (1997-1998) by YEAR, SEASON - (i) summer = MarchNovember, (ii) winter = December-February, in A) the western Gulf of Mexico ( o west of 89 W),

here a unique tow as a sampling unit and effort is measured in hours, B) the Atlantic, with a 

mpling unit, and C) in the Atlantic, with a tow as the sampling unit, for 

omparison with Gulf effort. Effort is further stratified by DEPTH STRATA in the western Gulf

(zones

ward of

n

w

fishing day as a sa

c

15-21) - (i) Nearshore: 0-10 fm, (ii) Offshore: >10 fm, and by SUBREGION in the 

Atlantic - (i) South: zones 29-30, (ii) Central: zones 31-32. There was no effort north

zone 32, and no effort in winter. The total number of samples (n), the mean, standard deviatio

(std), and the minimum (min) and maximum (max) effort per sample are tabulated. 

A. WESTERN GULF 

 hrs/tow 

YEAR SEASON

DEPTH 

STRATA 

total 

tows n 

total 

hrs mean std min max 

 

1997 summer Nearshore 321 291.7 0.91 0.3 0.2 5.6 

1997 summer Offshore 542 3089.3 5.7 3.7 0.2 14.5 

1997 winter Offshore 93 822.4 8.84 3.56 1.3 15.3

1998 summer Nearshore 106 101.4 0.96 0.15 0.2 1.2 

1998 summer Offshore 71 713.4 10.05 3.46 1.9 14.7

ALL 1133 5018.2 4.4291 4.1648 15.3 0.2 

 

B. ATLANTIC 

 hrs/day 

YEAR SEASON SUBREGION 

total 

days n 

total 

hrs mean std min max 

 

1997 summer Central 66 330.1 5 2.11 0.9 10.8 

1997 summer South 30 148.6 4.95 1.45 0.8 7.2 

1998 summer Central 5 13 2.6 0.67 1.8 3.6 

1998 summer South 27 104.8 3.88 0.83 0.9 4.8

ALL 128 596.5 4.6602 1.817 10.8 0.8 

 

 

C. ATLANTIC 

 hrs/tow 

YEAR SEASON SUBREGION 

total 

tows n total hrs mean std min max 

 

1997 summer Central 374 330.1 0.88 0.16 0.2 1.2 

1997 summer South 153 148.6 0.97 0.13 0.8 1.2 

1998 summer Central 15 13 0.87 0.13 0.4 0.9 

1998 summer South 99 104.8 1.06 0.16 0.6 1.2 

ALL 641 596.5 4.6602 1.817 10.8 0.8 

 

16 
Table 8. Catch statistics from the Foundation data in A) the western Gulf of Mexico, w

sampling un

here a

it is a tow, B) the Atlantic, with a fishing day as a sampling unit, and C) the Atlantic, 

ith a tow as a sampling unit for comparison with the Gulf data. Catch and effort data are 

ratified by SPECIES, YEAR, SEASON, DEPTH STRATA (Gulf)/SUBREGION (Atlantic). 

w

st

There was no winter effort in the Atlantic, where almost all effort was in <=10 fm (see Figure

5). 

A. WESTERN GULF 

 CPUE (catch/hr) CATCH (catch/tow) 

YEAR SEASON DEPTH

STRATA mean std min max total mean std min max

GREEN 

1997 summer Nearshore 0.0035 0.062 0 1.1111 1 0.0031 0.0558 0 1 

1997 summer Offshore 0 0 0 0 0 0 0 0 0 

1997 winter Offshore 0.0008 0.0077 0 0.0741 1 0.0108 0.1037 0 1 

1998 summer Nearshore 0 0 0 0 0 0 0 0 0 

1998 summer Offshore 0 0 0 0 0 0 0 0 0 

r Offs re 0 0 0 0 0 0 0 

1997 win

ho 0 0

ter Offshore 0 0 0 0 0 

1998 sum

0 0 0 0

mer Nearshore 0 0 0 0 0 

1998 sum

0 0 0 0

mer Offshore 0 0 0 0 0 0 0 0 0 

AD 

mer Nearshore 0.0138 0 4 0 0.1 1 

1997 sum

.123 0 1.1111 4 .0125 111 0 

mer Offs 7 .208 . 1 

1997 winte

hore 0.000 0.0113 0 0 3 2 0.0037 0 0607 0

r Offs 4 0.0131 0 0.126 0.0 0.103 0 1 

1998 summer

hore 0.001 6 1 108 7

0 

mer Offshore 0 0 0 0 0 0 0 0 0 

LEATHERBACK 

1997 summer Nearshore 0 0 0 0 0 0 0 0 0 

1997 summe

LOGGERHE

1997 sum

Nearshore 0.0079 0.0809 0 0.8333 1 0.0094 0.0971 0 1 

1998 summer Offshore 0 0 0 0 0 0 0 0 0 

RIDLEY 

1997 summer Nearshore 0.0329 0.218 2.5 9 0.028 0.1833 0 2 

1997 summer Offshore 0.0003 0.008 0 0.1852 1 0.0018 0.043 0 1 

1997 winter Offshore 0 0 0 0 0 0 0 0 0 

1998 summer Nearshore 0.0498 0.2261 0 1.1111 5 0.0472 0.213 0 1 

1998 sum

17 
B. ATLANTIC 

 CPUE (catch/day) CATCH 

YEAR SEASON SUBREGION mean std min max total 

GREEN 

1997 summer Central 0 0 0 0 0 

South 0.0333 0.1826 0 1 1 

r Central 2 1 

1998 summe uth 0.1111 0.3203 0 1 3 

1997 summer

1998 summe 0. 0.4472 0 1

r So

LEATHERBACK

1997 summer Central 0.01 0.1231 0 1 1 

h 0

ral 0 0

th 0

52

1997 summer Sout 0 0 0 0 

1998 summer Cent 0 0 0

1998 summer Sou 0 0 0 0 

LOGGERHEAD 

1997 summer Central 48 418 98

mer South 1.8 2.929 0 14

al 2 472 1

th 78 938 48

1.48 1.8 0 7

1997 sum 54 

1998 summer Centr 0. 0.4 0 1

1998 summer Sou 1.77 2.6 0 11 

RIDLEY 

1997 summer Central 64 877 9

th 33 732 16

mmer Central 0 0 0 0 0 

h 7 42

0.13 0.3 0 2

1997 summer Sou 0.53 0.9 0 4

1998 su

1998 summer Sout 1.5556 3.456 0 15 

YEAR SEASON SUBREG ON mean std min max total an std ax

GREEN 

1997 summer Central 0 0 0 0

h 8 1 0.0

ral 41 869 1. 1 1 0.066 0.258

98 summer S h 0.02 0.1436 0 0.8333 3 0.0303 1723 0 1 

0 0 0 0 0

1997 summer Sout 0.0073 0.089 0 1.1111 065 0.0808 0 1

1998 summer Cent 0.07 0.2 0 111 7 2 0 1

19 out 53 0.

 

C. ATLANTIC 

 CPUE (catch/hr) CATCH (catch/tow) 

I me min M

LEATHERBACK 

1997 summer Central 0.003 0.0575 0 1.1111 1 0.0027 0.0517 0 1 

1997 summer South 0 0 0 0 0 0 0 0 0 

1998 summer Central 0 0 0 0 0 0 0 0 0 

1998 summer South 0 0 0 0 0 0 0 0 0 

LOGGERHEAD 

1997 summer Central 0.3109 0.7133 0 5.5556 98 0.262 0.6001 0 5 

1997 summer South 0.3758 0.9138 0 5.5556 54 0.3529 0.8388 0 5 

1998 summer Central 0.0741 0.2869 0 1.1111 1 0.0667 0.2582 0 1 

1998 summer South 0.4338 0.8705 0 5.8333 48 0.4848 1.0138 0 7 

RIDLEY 

1997 summer Central 0.0241 0.1559 0 1.25 9 0.0241 0.1535 0 1 

1997 summer South 0.1144 0.3817 0 2.2222 16 0.1046 0.3472 0 2 

1998 summer Central 0 0 0 0 0 0 0 0 0 

1998 summer South 0.3732 0.8595 0 4.1667 42 0.4242 1.0111 0 5 

 

18 
Table 9. Catch per unit effort (CPUE) statistics from the Foundation data in A) the western Gulf 

of Mexico, where a sampling unit is a tow, B) the Atlantic, with a fishing day as a sampling unit, 

and C) the Atlantic, with a tow as a sampling unit for comparison with the Gulf data. The 

statisti are rther s tifi b SPE , STRATA (Gulf)/SUBREGION 

(Atl . a UE, standard deviation (std), and coefficient of variation (cv) 

of t n are given for each stratum that had effort. Strata with no effort are not listed. The 

95% i i l u er, lc low r) are computed by the non-parametric bootstrap 

perc ) m . a nd ra e al confidence limits are also 

com o a bas o 10 replicates of n observations within 

a stratum. 

A. WESTERN GULF 

 CPUE (catch/hr) 

SON DEPTH ST n an lcl cl method 

cs fu tra ed y CIES, SEASON DEPTH

antic) The observed me

he mea

conf mits (uc = pp l = e

entile (PCTL ethod B p norm pa m tric norm

puted for comparison. otstr pping was ed n

SEA RATA me

n CP

dence l

ootstr al a

B 00

std cv ucl 

GREEN 

summer Nearshore 427 0.0026 0.0538 1.0014 0.0077 -0.0025 Normal 

0.0078 -0.0026 Bootstrap Normal

0.0078 0 Bootstrap PCTL

 

summer Ofshore 613 0 - - - - - 

 

winter Offshore 93 0.0008 0.0077 0.9981 0.0024 -0.0008 Normal 

0.0023 -0.0007 Bootstrap Normal

0.0024 0 Bootstrap PCTL

 

LEATHERBACK 

summer Nearshore 427 0 - - - - 

 

.0231 0. 017 Boot ormal

0.024 0.

summer 3 0.0006 0.0106 0.7135 0.0014 -0.0002 Normal 

0.001 .0002 Bootstrap Normal

0.0015 0 Bootstrap PCTL

 0.004 -0.0013

- 

summer Offshore 613 0 - - - - - 

winter Offshore 93 0 - - - - - 

 

LOGGERHEAD 

summer Nearshore 427 0.0124 0.1143 0.4461 0.0232 0.0015 Normal 

0 0 strap N

 7 0026 Bootstrap PCTL

 

Offshore 61

4 -0

 

winter Offshore 93 0.0014 0.0131 0.9703 0.0041 -0.0013 Normal 

Bootstrap Normal

0 Bootstrap PCTL

0.0582 0.016 Bootstrap Normal

0.059 0.

0.0009 -0.0003 Bootstrap Normal

0.0009 0 Bootstrap PCTL

0 - - - - 

0.0041

RIDLEY 

summer Nearshore 427 0.0371 0.2199 0.2868 0.058 0.0162 Normal 

 8 0181 Bootstrap PCTL

summer Offshore 613 0.0003 0.0075 1.0097 0.0009 -0.0003 Normal 

winter Offshore 93 - 

 

 

19 
B. ATLANTIC 

 CPUE (catch/day) 

SEASON SUBREGION n mean std cv ucl lcl cl method 

GREEN 

summer South 57 0.0702 0.2577 0.4862 0.1386 0.0018 Normal 

 0.1364 0.004 Bootstrap Normal 

 0.1404 0.0175 Bootstrap PCTL

 

summer Central 71 0.0141 0.1187 0.9991 0.0422 -0.014 Normal 

 0.0407 -0.0126 Bootstrap Normal 

 0.0423 0 Bootstrap PCTL

LOGGERHEAD 

57 895 2.795 0.206 11 79

2.5263 1.0965 Boots

 

Central 71 1.3944 1.8085 0.1539 1.8224 0.9663 Normal 

.7996 0.9892 Bootstrap Normal

.7887 B otstrap PCTL

South 57 1.0175 2.5106 0.3268 1.6837 0.3514 Normal

14 37 Bo or

LEATHERBACK 

summer Central 71 0.0141 0.1187 0.9991 0.0422 -0.014 Normal 

 0.0421 -0.0139 Bootstrap Normal 

 0.0423 0 Bootstrap PCTL

 

summer South 1.7 9 2.53 1.04 Normal 

 2.4985 1.0804 Bootstrap Normal 

trap PCTL

summer

 1

 1 1 o

RIDLEY 

summer

 1.68 0.35 otstrap N mal 

1.7719 0.4561 Bootstrap PCTL

summer Central 71 0.1268 0.3753 0.3 0 .0 Normal 

28 07 Bo or

0.2254 0.0563 Bootstrap PCTL

 

SEASON ZO n uc cl

25 026 .0285 0 Normal 

92 Boots PCTL

38 843 .0085 -

513 .2156 0 379

 0.21 0.04 otstrap N mal 

 

C. ATLANTIC 

 CPUE (catch/hr) 

NE (oN) mean std cv l lcl method 

GREEN 

summer South 2 0.0143 0.1141 0.5 0 .0002

 0.0282 0.0004 Bootstrap Normal 

 0.02 0.0033 trap 

 

summer Central 9 0.0029 0.0563 0.9 0 0.0028 Normal 

 0.0084 -0.0027 Bootstrap Normal 

 0.0086 0 Bootstrap PCTL

38 843 .0085 -0.0028 Normal 

0.01 

0. 0.

LEATHERBACK 

summer Central 9 0.0029 0.0563 0.9 0

 0.0085 -0.0028 Bootstrap Normal 

0 Bootstrap PCTL

LOGGERHEAD 

summer South 252 0.3986 0.8958 0.1416 0.5097 0.2875 Normal 

 0.5099 0.2873 Bootstrap Normal 

 0.5105 0.2902 Bootstrap PCTL

 

summer Central 389 0.3018 0.703 0.1181 0.3718 0.2317 Normal 

 0.3699 0.2337 Bootstrap Normal 

 0.3703 0.2378 Bootstrap PCTL

RIDLEY 

summer South 252 0.216 0.6267 0.1828 0.2938 0.1383 Normal 

 0.2939 0.1382 Bootstrap Normal 

3015 1438 Bootstrap PCTL

 

summer Central 389 0.0232 0.1529 0.3342 0.0384 0.008 Normal 

 0.038 0.0084 Bootstrap Normal 

 0.0393 0.01 Bootstrap PCTL

 

20 
Figure 3. Fleet Effort (24 hr. days fished), 1997-1998 and Foundation Sampling 

Effort (hours fished) in the Gulf of Mexico. 

REPORTED SUMMER EFFORT FOR THE WESTERN

GULF

141383.95

114684.09

84706.22

INSHORE

NEARSHORE

OFFSHORE

REPORTED WINTER EFFORT FOR THE WESTERN

GULF

1282.86

17304.82

29635.81

INSHORE

NEARSHORE

OFFSHORE

FOUNDATION SUMMER EFFORT FOR THE

WESTERN GULF

393.1

3802.7

INSHORE

NEARSHORE

OFFSHORE

FOUNDATION WINTER EFFORT FOR THE

WESTERN GULF

822.4

INSHORE

NEARSHORE

OFFSHORE

FOUNDATION SUMMER EFFORT FOR THE

EASTERN GULF

No Foundation Effort

FOUNDATION WINTER EFFORT FOR THE

EASTERN GULF

No Foundation Effort

REPORTED SUMMEREFFORT FOR THE EASTERN

GULF

30500.77

24365.80

55027.53

INSHORE

NEARSHORE

OFFSHORE

REPORTED WINTER EFFORT FOR THE EASTERN

GULF

1259.00188

5840.43

18915.282

INSHORE

NEARSHORE

OFFSHORE

REPORTED SUMMER EFFORT FOR THE WESTERN

GULF

141383.95

114684.09

84706.22

INSHORE

NEARSHORE

OFFSHORE

REPORTED WINTER EFFORT FOR THE WESTERN

GULF

1282.86

17304.82

29635.81

INSHORE

NEARSHORE

OFFSHORE

FOUNDATION SUMMER EFFORT FOR THE

WESTERN GULF

393.1

3802.7

INSHORE

NEARSHORE

OFFSHORE

FOUNDATION WINTER EFFORT FOR THE

WESTERN GULF

822.4

INSHORE

NEARSHORE

OFFSHORE

FOUNDATION SUMMER EFFORT FOR THE

EASTERN GULF

No Foundation Effort

FOUNDATION WINTER EFFORT FOR THE

EASTERN GULF

No Foundation Effort

REPORTED SUMMEREFFORT FOR THE EASTERN

GULF

30500.77

24365.80

55027.53

INSHORE

NEARSHORE

OFFSHORE

REPORTED WINTER EFFORT FOR THE EASTERN

GULF

1259.00188

5840.43

18915.282

INSHORE

NEARSHORE

OFFSHORE

21
Figure 4. Fleet effort (trips) in 1997-1998 and Foundation sampling effort (days) in the Atlantic.

Zones 1-3 correspond to South, Central, and North subregions, respectively. 

 

REPORTED SUMMER EFFORT FOR THE ATLANTIC

10594

42560

23903 ZONE 1

ZONE 2

ZONE 3

REPORTED WINTER EFFORT FOR THE ATLANTIC

3168

5989

259

ZONE 1

ZONE 2

ZONE 3

FOUNDATION SUMMER EFFORT FOR THE

ATLANTIC

57

71

ZONE 1

ZONE 2

ZONE 3

FOUNDATION WINTER EFFORT FOR THE

ATLANTIC

No Foundation Effort

REPORTED SUMMER EFFORT FOR THE ATLANTIC

10594

42560

23903 ZONE 1

ZONE 2

ZONE 3

REPORTED WINTER EFFORT FOR THE ATLANTIC

3168

5989

259

ZONE 1

ZONE 2

ZONE 3

FOUNDATION SUMMER EFFORT FOR THE

ATLANTIC

57

71

ZONE 1

ZONE 2

ZONE 3

FOUNDATION WINTER EFFORT FOR THE

ATLANTIC

No Foundation Effort

22
23 

rt (1/641 tows) in

the Atlantic was in depths greater than 10 fm. 

Figure 5. Depth distribution of Foundation effort. Less than 0.5% of the effort (5/1133 tows) in 

the western Gulf was in depths greater than 40 fm. Less than 0.2% of the effo

Depth Distribution of Effort

20

30

40

50

60

70

80

90

percent

Western Gulf

Atlantic

0

10

0-5

>

5-10

>10-15

>15-20

>20-2

5

>2

5-30

>

30-3

5

>

3

5-40

>40-45

>45-50

>

50-5

5

>

5

5-60

>60-65

>65-70

fathoms

Aerial Survey Data

Results of aerial surveys conducted in the Gulf of Mexico and U.S. Atlantic were used to 

calculate relative abundance indices for sea turtles using line transect methodology. Derived 

relative abundance estimates will be used to extrapolate CPUE data from localized observer 

programs to other regions in the Gulf of Mexico and coastal Atlantic. The presented estimates 

do not represent absolute abundance but rather minimum population sizes. Sea turtles are easily 

missed during aerial surveys because they are relatively small, may have a similar coloration to 

the water in turbid areas, and spend a significant amount of time underwater. 

Aerial survey spatial and temporal coverage 

Extensive aerial surveys of the Gulf of Mexico were undertaken by the Southeast 

Fisheries Science Center (SEFSC) during fall (September – November) between 1992-1994 and 

1996. The surveys were stratified into inshore (0-10 fathoms) and offshore (10 –100 fathoms) 

areas. The western Gulf of Mexico, including the coasts of Texas and Louisiana, were covered 
in 1992 and 1996, the central Gulf from eastern Louisiana to the Florida panhandle during 1993, 

and the Gulf coast of Florida to Key West during 1994 (Figure 6). These surveys included effo

in bays and estuaries, but this effort was exclude

rt

d from the current analysis. 

An aerial survey of the coastal US Atlantic was undertaken from July-August 2002. The 

rvey included the coastline from Sandy Hook, New Jersey (40.5 oN latitude) to Ft. Pierce, FL 

(27.2 oN). The survey effort was allocated into 0-20m (10.9 fathoms) and 0-40m (21.9 fathoms) 

depth strata (Figure 7). 

Aerial survey methodology 

Both Gulf of Mexico and Atlantic surveys employed a DeHavilland Twin Otter aircraft 

flying at a ground speed of 204 km/hr and an altitude of 229 m. Survey effort was generally 

restricted to periods of calm sea state (Beaufort Scale 0-3) and good visibility conditions. Two 

visual observers were stationed in large bubble windows in the forward portion of the aircraft 

and looked outward and down to spot both marine mammals and sea turtles. Because the plane 

is a high-wing aircraft equipped with bubble windows, it was possible for the observers to 

visualize the trackline directly beneath the airplane. A third person was stationed at a recorder 

position and recorded all turtle and marine mammal sightings on a lap top computer. The data 

entry program also recorded aircraft position at 30 second intervals, and changes in 

environmental conditions (e.g., glare, sea state, weather) during survey operations. The species 

and number of turtles (group size) for each sighting were recorded while the observers were “on 

effort”. 

An angle measurement, θ, to each sighting was taken either by using an inclinometer or 

designating the sighting into 10o angle “bins” based upon markings on the bubble windows. For 

sightings where only an interval measurement was available, angle measures were “smeared” by 

adding a random value between –5 and 5 to the mid-point of the angle increment (e.g., 15 o, 

uckland et al.1993). The perpendicular distance to the sighting (PSD) in meters was 

alculated as PSD = tan(θ) * Altitude. During the analysis stage, sightings from the first two 

ngle bins (0-10o and 10-20o) were combined because it is often difficult to tell which bin the 

the extremely oblique angle. The resulting distance intervals were 0-83 m,

3-132 m, 132-192 m, 192-272 m, 272 – 396 m, 396-629 m, and > 629 m. 

imal at any 

istance away from the transect line is equal. Assuming that all animals or groups of animals 

either side of the line are observed, then the

ensity of animals in the area is: 

su

B

c

a

sighting fell in due to

8

Line transect analysis 

 

The standard theory for line-transect sampling is well developed and has been routinely

applied in a variety of wildlife population assessments in both terrestrial and marine habitats 

(Buckland et al. 1993). Given a random distribution of a particular survey line relative to the 

distribution of a population of interest, then the probability of observing an an

d

within a particular distance (W = strip width) on

d

(1) LW

n D

2 = , 

24 
where n is the number of groups observed and L is the length of the transect line. 

However, line transect theory as described in Buckland et al. (1993) recognizes that the 

nction, g(x). The sighting function can take any integrable form, 

owever in practice it is generally constrained to be monotonically decreasing vs. the distance 

e probability of sighting an animal within a strip is then the area 

nder this function divided by the total strip width:

probability of observing an animal or group generally declines with increasing distance away 

from the trackline. The distance sampling approach therefore examines the distribution of 

sighting frequency as a function of distance away from the trackline and corrects the density 

estimate for the sighting fu

h

away from the trackline, x. Th

u

(2) W

g x dx

P

w

a

∫ = 0 ( )

. 

 To calculate the probability of sighting an animal at any distance away from the trackline, the 

sighting function is rescaled to the probability distribution function (pdf), f(x) as: 

(3) ∫

w

g x dx

0 ( ) . 

The assumption is made that the sighting probability on the trackline is unity (g(0) = 1) allow

one to solve for the pdf at x = 0 as: 

(4) = w f 1 (0) , 

= g x f x ( ) ( )

ing

∫ g x dx

0

( )

and the quantity μ, or the effective strip width is: 

(5) ∫ =

w

g x dx

0

μ ( ) , 

lternatively expressed as μ = Pa * w. The density estimate given in eqn. 1 is therefore modified 

to: 

a

25 
(6) D = . Lμ

n

2

If the objects being observed occur in clusters, or groups, then equation 6 reflects the 

density of these groups and is simply modified to: 

(7) Lμ

nE s D

2

( ) = , 

where E(s) is the expected or average number of animals occurring in each group. The total 

abundance of animals in a given region of area A is then N = D * A. 

Variance in the abundance e te is calculated following the delta method outlined by 

Seber (1982) for combining uncorrelated variances. Thus, the variance in the density estimate is 

iven as:

stima

g

(8) 

 

 = ⋅ + + −2 var( ) var( (0)) var( ( )) var( ) n f E s D D 

  2 2 2

n f (0) E(s)

where var(x) indicates the variance of the respective quantities. The variance of mean group size 

[var(E(s))] is calculated using the standard expression for variance and the variance of the 

inverse sighting function [f(0)] is calculated based upon the maximum likelihood fitting 

procedure used to derive the sighting function. The quantity var(n) is the variance of the 

expected number of animals observed during the survey. The sampling unit for the current 

 all transect lengths, var(n) is estimated as: 

(9) 

survey is considered a single transect line. For each of k defined effort units where li is the 

length of each unit and L is the sum of

var( ) ( 1)

2

1

− 













 = ∑ − =

k

L

n

l

n

n L l

k

i

i

i

i

, 

 

where ni is the number of groups seen on transect i and n is the total number of groups observed 

uring the survey. This variance estimator assumes both independence of encounter rates 

r rates (n/l) are normally distributed. Severe 

iolations of these assumptions due to spatial contagion may result in inaccurate variance 

stimation. To account for these factors, variance may also be calculated through nonparametric bootstrap resampling of transects (Buckland et al. 1993). 

A fundamental assumption in h is that the probability of sighting animals on 

the trackline, g(0), is 1. This is required to accomplish the formulation of eqn. 4 and the solution 

kline, and therefore g(0) is < 1. The failure of this assumption introduces a direct, negative 

ias in the density estimate. This source of bias is termed “visibility bias”. 

d

between transect lines and that the mean encounte

v

e

this approac

for the effective strip width. In practice, it is likely that some animals will be missed on the 

trac

b

26 
Visibility bias can be separated into two somewhat independent components, availability 

bias and perception bias (Marsh & Sinclair 1989). Availability bias occurs when the animals can 

not be observed within the searched area. For example, if bird nests are obscured by vegetation 

r marine mammals are underwater. This type of bias is often accounted for by separate models 

f anim ior

als (Barlow, 1999). Perception bias results

om animals that were available to be seen, but were missed by the observers. Primary factors 

that influence perception bias include weather conditions, observer fatigue, and observer 

experience (Laake et al., 1997). Perception bias can often be reduced with adequate training of 

observers, frequent rotation to avoid fatigue, and limiting survey effort to periods where viewing 

onditions are favorable.

ied geographically into two subregions: the eastern (≤ 89 oE longitude ) and western (> 

9 oE longitude) Gulf, and into two depth strata: inshore (0-10 fathoms) and offshore (10-40 

fathoms, Figure 8) depth strata (Table 10). 

A total of 637 sightings of sea turtles were made in the Gulf of Mexico in waters < 40 

thoms in depth, and as expected there were strong geographic differences in sighting rates and 

h had

dley

ern

es were more 

roadly distributed and were observed primarily in the offshore strata (Figure 10B). Finally, 

many sightings could not be accurately identified to species and were described as un-identified 

hardshells (Figure 10C). The majority of these sightings occurred in southern Florida where 

green, hawksbill, ridley, and loggerheads were common. 

o

o al availability to the observer. For example, detailed models of observer search behav

and animal dive-surface intervals for marine mamm

fr

c

Survey results – Gulf of Mexico 

In the Gulf of Mexico, survey effort was post-stratified into four geographic and depth 

strata to reflect expected changes in turtle density and the distribution of shrimping effort. Effort 

was stratif

8

fa

species composition. In general, sighting rates were much higher in the eastern Gulf and inshore 

strata than in the western Gulf. Loggerhead turtles were sighted throughout the Gulf, thoug

a very low occurrence in the offshore strata in the Western Gulf (Figure 9A). Kemp’s ri

turtles were sited primarily in the inshore strata and most commonly occurred in the eastern Gulf

(Figure 9B). Green turtles occurred further offshore and were primarily sighted in the south

portion of the Florida Gulf coast (Figure 9C). Hawksbill turtles (Eretmochelys imbricata)

likewise occurred primarily in southwest Florida (Figure 10A). Leatherback turtl

b

ss all

. 

rom the 

characteristic of these

odels is that they assume a monotonic decline in sighting probability as the distance from the 

tracklin

f-fit. The "best"

Minimum abundance of sea turtles in each stratum were calculated employing line 

transect distance methods described above. A common sighting function was derived acro

species and strata to provide sufficient sample size for a robust estimate of the sighting function

The detection function was determined by evaluating the goodness-of-fit between several 

alternative models that satisfy the shape criterion for the relationship between distance f

trackline and sighting probability (Buckland et al. 1993). A primary

m

e increases. Sightings were grouped into the interval categories described above that are 

associated with the angle intervals used during the survey, and data were right-truncated at 629 

m. The histogram of sighting frequencies with respect to distance from the trackline was 

examined to determine the sighting function that best fit the data. Both half-normal and hazard 

rate models with polynomial expansion terms were evaluated for goodness-o

27 
detection function was selected based on the minimum Akaike Information Criterion (AIC, 

Buckland et al. 1993). 

ta as

g probability declines over this interval, the resulting 

abundance estimate is negatively biased due to the poor fit of the sighting curve. The sighting 

ective strip half-width (μ) of 324.9 m (CV = 4.3%). 

The sighting frequency indicates a decline in sighting probability from the trackline out 

to approximately 150 m PSD (Figure 11). This is a relatively common problem encountered in 

aerial surveys for turtles and results in an unknown degree of bias in derived abundance 

estimates. The sighting function providing the best fit to these data was the half-normal curve 

with no adjustment terms (Figure 11); however, this function did not adequately fit the da

indicated by a significant goodness of fit chi-square (χ

2

 = 11.46, df = 3, p < 0.01). The fitted 

sighting function projects a uniform sighting probability equal to 1 back from 130 m to the 

trackline. Because the actual sightin

function resulted in a calculated eff

rn

bution were not as strong. Loggerheads had 

pproximately equal densities between the inshore and offshore strata, though they were more 

he

homs as very little Atlantic shrimping 

ffort occurred outside this depth range (Figure 12, Table 12). 

urvey

er to

Minimum density estimates for each species and unidentified hardshell turtles are 

presented in Table 11. Note that these are minimum density estimates and should only be 

interpreted as relative abundance indices between strata. For most species, minimum turtle 

density is approximately 10x higher in the eastern Gulf of Mexico in comparison to the Weste

Gulf (Table 11). The patterns in depth distri

a

abundant in shallower water in the Western Gulf. Kemp’s ridley turtles had higher density in t

inshore Eastern Gulf subregion, while green turtles were generally more abundant in the offshore

stratum (Table 11). Hawksbill turtles were present only in the eastern Gulf and were

approximately evenly distributed between the inshore and offshore strata (Table 11). 

Leatherbacks were broadly distributed at low abundance throughout the Gulf with no clear 

spatial pattern in density (Table 11).

Survey results – coastal Atlantic 

Survey effort was stratified into three latitudinal strata again determined by the 

distribution of shrimping effort and available observer data. These included southern (28o - <31o

N latitude), central (31o - <34o N latitude), and northern (34o - <36o N latitude) strata. All 

analyses were limited to the depth strata between 0-10 fat

e

There were a total of 169 on effort sightings in depths <10 fathoms across the three 

latitudinal strata. Only loggerhead, leatherback, and un-identified hardshell turtles were 

observed during the survey. Loggerhead turtles were sighted consistently throughout the s

range in both deep and shallow water (Figure 13A). Leatherback turtles occurred only in the 

central stratum in water <10 fathoms, and were more common in deeper water (Figure 13B). 

There were 12 sightings that could not be identified to species, and these occurred most 

commonly in the central and southern strata (Figure 13C).

The sighting function for the Atlantic survey was developed in an equivalent mann

that of the Gulf of Mexico surveys. As in the Gulf surveys, the sighting function exhibited a 

significant drop-off near the trackline with peak sighting rates at approximately 150 m PSD 

28 
(Figure 14). The best fitting sighting function was a hazard rate function with no adjustmen

terms; however, the mod

t

el did not adequately fit the data (χ

2

 = 1834, df = 3, p < 0.001). The 

tted function results in an effective strip width (μ) of 340.0 m (CV = 4.92%). 

ces of

y

d

re

the

urtle

ensities showed a similar geographic pattern to that of loggerheads, with highest densities in the 

ct the

.

reflect

ce within a stratum. Thus, for a given species, i,: 

(10) 

fi

Minimum density estimates for identified turtles and unidentified hardshells are presented

in Table 13. As with the Gulf of Mexico surveys, these values should be considered indi

relative abundance as opposed to estimates of absolute numbers. The fact that no Kemp’s ridle

or green turtles were identified during the survey is problematic, as both species are known to 

occur along the Atlantic coast. It is highly likely that these relatively small, cryptically colore

species were missed during the survey. Loggerhead turtles were broadly distributed and we

abundant in all three strata. However, they were more abundant in the southern portion of

survey area in the Central and South subregions (Table 13). Leatherback turtles occurred in 

relatively low density and were most abundant in the Central subregion. Un-identified t

d

southern portion of the range (Table 13). 

Apportioning unidentified turtles: 

The species composition of turtles classified as unidentified hardshells should refle

regional species composition. However, this may be biased against difficult to identify species

For example, identification of loggerheads is fairly clear, but it is more difficult to distinguish 

between Kemp’s ridley and green turtles. Thus, un-identified turtles would include a higher 

relative proportion of the more difficult to identify species. Nonetheless, to accurately

regional relative abundance, it is necessary to apportion the hardshell turtles to species

categories. A proportion of the hardshell density is allocated to each species, excluding

leatherbacks, as their relative abundan

= s

i

i

D P

∑

j

Dj

1

where Dx is the density of a given species for all s hardshell species. For the Gulf of Mexico, the

hardshell species include loggerhead, green, Kemp’s ridley, and 

=

, 

hawksbill turtles. The corrected 

density of species i including unidentified hardshells is then: 

(11) i Di PiDhardshell correctedD = + , 

where Dhardshell is the density of unidentified turtles in the stratum. 

When apportioning unidentified turtles, it is also necessary to combine variances to arrive 

at an ap

e

it is

propriate estimator of variance for the corrected densities. Assuming that all densities 

values are uncorrelated, then the combined CV for two or more estimates is equal to the squar

root of the sum of squared CVs for each estimate, equivalent to equation 8 above. However,

29 
necessary to calculate and combine variances at each step in the calculation including the CV o

the denominator in equation 10, the CV of Pi, the CV of PiDhardshell, and the CV of correctedD

(equation 11). In estimating the CV for the total of all estimates (the denominat

f

i

or in equation 

10), we accounted for the fact that each estimate shares a common value for f(0) using 

tively small and easily missed by observers, and 

ecause they spend a considerable amount of time beneath the surface where they are not 

e decline in sighting rates near the trackline and poor fit of 

e sighting function introduces potential negative bias. 

f

xample, 

, for

ound

the perceived differences in animal density across geographic regions. The degree of this type of 

bias is likely to be relatively minor as the same aircraft and personnel were used and surveys 

were only conducted in good weather conditions. 

A potentially more serious source of relative bias in density estimates exists due to 

varying levels of sightability for the different species. It is notable in the Atlantic that no 

 to the water may limit the ability 

f visual observers to both see and identify these species during surveys, but the use of trained 

observers may increase the sightings of these sp

somewhat larger and their coloration causes them to stand out clearly against the greenish 

ackground of nearshore waters. The Gulf of Mexico surveys appear to have been more efficient 

t observing and identifying these species. It remains unclear as to whether this is due to 

ese species in the Gulf than 

e Atlantic. 

appropriate equations given in Buckland et al. (1993, pg. 100-101). Since it is likely that there is 

positive correlation between each value, the variance estimates are positively biased for the 

corrected densities. Corrected density estimates for each species and strata are shown in Table 

14. 

Sources of Error 

The density estimates presented here suffer from a number of potential biases. Most 

notably, the presented estimates are negatively biased by the assumption that all animals 

occurring on the trackline are seen by the observers. The level of visibility bias is likely to be 

severe for sea turtles because they are rela

b

available to the survey. In addition, th

th

However, for the purposes of this analysis the negative biases in the absolute density 

estimates are less important. These analyses are being used to assess the relative distribution o

sea turtles between regions, and we implicitly assume that the sighting probabilities, and 

associated biases, are equal across regions. This assumption may be violated due to potential 

variation in the sighting conditions within regions. In the Gulf of Mexico surveys, for e

the eastern and western strata were surveyed in different years. Differing water turbidity

example, between years could potentially influence sighting probabilities and would conf

Kemp’s ridley or green turtles were observed during the surveys; however, these species are 

known to occur in the central and southern strata and are frequently captured in shrimp trawls. 

The relatively small size of these turtles and similar coloration

o

ecies. In contrast, loggerhead turtles are 

b

a

differences in observer personnel or higher relative abundance of th

th

The variance estimates for all density measures were relatively large, particularly after 

combining estimates with unidentified hardshells. The high degree of variation in this data 

results from the relatively low abundances of sea turtles and more likely the relatively low 

30 
probability of actually seeing turtles. There are several assumptions made during the analysis 

that result in bias in the estimates of variance. First, during model fitting a relatively limited 

range of potential models was explored; forcing the choice of a monotonically declining functio

artificially reduces the degree of uncertainty in the estimate of effective strip width. Fit

more complex model to these data would result in a higher level of uncertainty because of the 

increase in parameters with a relatively small number of model degrees of freedom. Second,

apportioning hardshell turtles across the other species groups in the Gulf, we explicitly assume

there

n

ting a

in

is no correlation between identified and unidentified turtle densities. If the terms are in fact

orrelated, this would result in a decrease in variance of their combined density estimate. Thus, 

e “corrected density” estimates (Table 14) are likely positively biased. 

inally, variable sighting conditions due to environmental factors and weather conditions both 

c

the variances in th

F

during and between surveys will contribute additional unquantified variance to the resulting 

estimates. 

 turtle 

e

ever, given the quantified differences 

relative density, particularly in the Gulf of Mexico, these surveys provide useful information 

with w

The results of the Gulf of Mexico and Atlantic aerial surveys indicate differences in

density and species composition as a function of geographic region and water depth. Thes

estimates suffer from several sources of negative bias and the resulting variances suffer from

potential sources of both positive and negative bias. How

in

hich to infer relative catch rates in shrimp trawl fisheries across regions. 

Table 10. Summary of survey effort by strata during Gulf of Mexico aerial surveys. 

Stratum Boundary Stratum Area 

(km2

) 

Total Effort 

(km)

Total On-Effort 

Turtle Sightings

Western Gulf 

Inshore 0-10 fathoms 31,189 4,652 42 

Offshore 10-40 fathoms 72,262 5,915 15 

Eastern Gulf 

Inshore 0-10 fathoms 46,907 5,976 392 

Offshore 10-40 fathoms 105,158 3,693 188 

31 
Table 11. Minimum density estimates (N/sq km) for sea turtles by species in the Gulf of Mexic

Standard errors for each quantity are shown in parentheses. %CV=Percent coefficient of 

variation (100 * Standard Error / Estimate). 

Subregion Number

of Groups 

Group 

Density 

Average 

Group 

Size

Turtle 

Density 

Density 

%CV 95% Confidence Interva

o.

l

Loggerhead 

W. Gulf 

0-10 fm. 28 0.0093

(0.003) 

0.991 

(0.0245) 0.0092 33.92 0.0048 – 0.0176

W. Gulf 

10-40 fm. 7 0.0019

(0.002) 

1.143 

(0.143) 0.0021 108.47 0.0004 – 0.0118

E. Gulf 

0-10 fm. 

0.0371 1.069 144 (0.004) (0.002) 0.0397 11.84 0.0314 – 0.0500

E. Gulf 

10-40

0.0288 1.145 

 fm. 69 (0.004) (0.069) 0.0329 15.73 0.0242 – 0.0448

Kemp’s ridley 

W. Gulf 

0-10 fm. 1 0.0003(0.0004)

1 

(N/A) 0.0003 134.14 0.0001 – 0.0024

W. Gulf 

10-40 fm. 0 0 0 0 

E. Gulf 

0-10 fm. 20 0.0052

(0.002) 

1.150 

(0.109) 0.0059 32.94 0.0031 – 0.0111

E. Gulf 

10-40 fm. 1 0.0004(0.0002)

2 

(N/A) 0.0008 40.89 0.0004 – 0.0018

Green 

W

0 1 3

4) ( 000 9 . Gulf 

-10 fm. 

0.000

(0.000

2 0. N/A) 6 197.28 0.0001 – 0.007

(0) 0.

10 fm. (0.0006) 0.200) 50.3 0006

E. Gulf 

10-40 fm. 1 (0 8 0.0100 9 0. – 0.0165 0.0075

(0.0016)

1.333 

.1809) 25.6 0061

W. Gulf 

10-40 fm. 2 0.0005(0.0002)

1 00052 45.18 0.0002 – 0.0012

E. Gulf 

0- 5 0.0013 1.2 

( 0.00154 3 0. – 0.0039

32 
Table 11 (continued). Minimum density estimates (N km-2) for sea turtles by species in the 

Gulf of Mexico. Standard errors for each quantity are shown in parenthesis. %CV = Perc

coefficient of variation (100 * Standard Erro

ent

r / Estimate). 

Subregion Number

of Groups 

Group 

Density 

Average 

Group 

Size

Turtle 

Density 

Density 

%CV 95% Confidence Interval 

Hawksbill 

W. Gulf 

0-10 fm. 0 0 0 0

W. Gulf 

10-40 fm. 0 0 0 0

E. Gulf 

0-10 fm. 101 0.0260(0.004) 

1.634 

(0.144) 0.0425 18.41 0.0297 – 0.0608

E. Gulf 

10-40 fm. 38 0.0158(0.0046)

1.447 

(0.0043) 0.0229 31.89 0.0124 – 0.0423

Leatherback 

W. Gulf 

0-10 fm. 1 0.0003(0.0004)

1 

(N/A) 0.0003 139.66 0.00004 – 0.0026

W. Gulf 

10-40 fm. 2 0.0 5(0.0003) 0.0 5 49 3 0.0002 – 0.0013 00 1 (0) 00 .6

E. Gulf 

0-10 fm. 1 0.0026

(0.0027)

1 

(N/A) 0.0026 104.56 0.0004 – 0.0140

E. Gulf 

10-40 fm. 7 0.0029(0.0010)

1 

(0) 0.0029 34.82 0.0015 – 0.0057

W. Gulf 

0-10 fm. 7 0.0023

(0.0012)

1. 3 

(

14

0.1429) 0.0026 54.73 0.0009 – 0.0073

W. Gulf 

10-40 fm. 3 0.0008(0.0006)

1 

(0) 0.0008 81.50 0.0002 – 0.0032

E. Gulf 

0-10 fm. 89 0.0229

(0.0028)

1.337

(0.2827) 0.0306 24.59 0.0190 – 0.0495

E. Gulf 

10-40 fm. 41 0.0171

(0.00370)

1.463 

(0.1642) 0.0250 24.39 0.0156 – 0.0402

Unidentified Hardshell 

Table 12. Summary of survey effort by strata during Atlantic aerial survey. 

Subregion Boundary Stratum Area 

(km2

) 

Total Effort 

(km)

Total On-Effort 

Turtle Sightings

North 34- <36o N 3,986 560 22 

Central 31- <34o N 15,577 1378 109 

South 28- <31o N 4,651 304 37 

33 
Table 13. Miminum density estimates (N / sq. km) for sea turtles by species in the Atlantic. 

Standard errors for each quantity are shown in parentheses. %CV = Percent coefficient of 

variation (100 * Standard Error / Estimate). 

Stratum Number

of Groups 

Group 

Density 

Average 

Group 

Size

Turtle 

Density 

Density 

%CV 95% Confidence Interval 

North 21 0.0 1

(0.0 )

1.1 9

(0.0 25) 0.0 9 31 5 0.03425 – 0.1159 551675

42

78 62 .1

Central 91 0.0 1

(0.0143)

1.3 6

(0.0849) 0.1 3 16 3 0.0963 – 0.1817 97 62 32 .0

South 39 0.1885

(0.0404)

1.2821

(0.08172) 0.2417 22.35 0.1538 - 0.3796

Leatherback

0 0

Central 5 0.0053

(0.00371)

1

(

.4

0.2449) 0.0075 70.94 0.00207 – 0.0270

South 0 0 0 0

North 1 (N/A) 0.0026 148.75 0.00029 – 0.0230 0.0026

(0.0039)

1 

Central 6 (0.0032)

1 0.0066 51.37 0.0025 – 0.0176 0.0064 .036

(0.1170)

South 5 0.0242

( 0.0289 45.95 0.0118 – 0.0712 (0.0088)

1.2 

0.2000)

Loggerhead 

North 0 0 

Unidentified Hardshell 

34 
Table 14. Corrected density estimates for each turtle species including a proportion of the 

unidentified turtle density in the Gulf of Mexico and Atlantic surveys. The estimate for 

loggerhead in the North Atlantic subregion does not include hardshell density. Variance

stimates reflect the combined uncertainty in these parameters assuming uncorrelated values. 

%CV = Percent nt o tion tand or /

Subregion Proportion 

of Total sity

m2

) 

%CV 

Corrected 

De

95% CI 

Corrected Density 

e

coefficie f varia (100 * S ard Err Estimate). 

Corrected 

Den

(N k nsity

Loggerhead – Gulf of Mexico

West, 0-10 fm 0.902 0.0115 80.1 0.0030 – 0.0443 

.800 0.0 85.7

0.443 0.05

0.0452 

Kemp’s ridley – Gulf of Mexico 

West, 0-10 fm 0.032 0.0004 200.2 0.0000 – 0.0047 

0 0 

East, 0

0.0011 

West, 0-10 fm 0.064 0.0008 286.2 0.0000 – 0.0142 

West, 10-40 fm 0.200 0.0007 122.6 0.0001 – 0.0042 

0.017 0.0

0.150 0.0 6.1

Hawksbill – Gulf of Mexico 

West, 10-40 fm 0 027 1 0.0003 – 0.0278 

East, 0-10 fm 32 31.7 0.0295 – 0.0961 

East, 10-40 fm 0.493 36.0 0.0233 – 0.0880 

West, 10-40 fm 0 

-10 fm 0.065 0.0079 53.8 0.0030 – 0.0207 

East, 10-40 fm 0.013 64.4 0.0004 – 0.0035 

Green – Gulf of Mexico

East, 0-10 fm 021 76.1 0.0006 – 0.0075 

East, 10-40 fm 137 4 0.0060 – 0.0317 

West, 0-10 fm 0 0 0 

West, 10-40 fm 0 0 0 

East, 0-10 fm 0.473 0.0569 37.4 0.0285 – 0.1135 

East, 10-40 fm 0.344 0.0315 53.3 0.0122 – 0.0816 

Leatherback – Gulf of Mexico 

West, 0-10 fm 0.0003 139.66 0.00004 – 0.0026 

West, 10-40 fm 0.0005 49.63 0.0002 – 0.0013 

East, 0-10 fm 0.0026 104.56 0.0004 – 0.0140 

East, 10-40 fm 0.0029 34.82 0.0015 – 0.0057 

Loggerhead - Atlantic 

North 0.0630 30.3 0.0373 – 0.1137 

Central 0.1389 15.5 0.1035– 0.1864 

South 0.2707 20.7 0.1833 – 0.3997 

Leatherback – Atlantic

North 0 

Central 0.0075 70.94 0.00207 – 0.0270 

South 0 

35 
Figure 6. Survey area and tracklines during the NMFS 1992-1994 and 1996 Gulf of Mexic

aerial surveys for sea turtle abundance. 

33o

o

1992 a 6

1993

94

2

26o

2

30o

31o

32o

o 95o o 92o 91o 9o 88o 8 85o 8 80o

1992 a 6

1993

94

2

26o

2

30o

31o

32o

2

26o

2

30o

31o

32o

o 95o o 92o 91o 9o 88o 8 85o 8 80o

Latitude (N)

L e (E)

nd 199 19

3o

24o

25o

27o

28o

9o

98o 97o 96 94o 93 90o 8 7o 86o 4o 83o 82o 81o

nd 199 19

3o

24o

25o

27o

28o

9o

98o 97o 96 94o 93 90o 8 7o 86o 4o 83o 82o 81o

ongitud

36 
Figure 7. Survey area and tracklines during the summer 2002 Atlantic aerial surveys. 

26°

28°

30°

32°

34°

36°

38°

40°

42°

82° 80° 78° 76° 74° 72° 70°

Latitude (N)

Longitude (E)

26°

28°

30°

32°

34°

36°

38°

40°

42°

82° 80° 78° 76° 74° 72° 70°

26°

28°

30°

32°

34°

36°

38°

40°

42°

82° 80° 78° 76° 74° 72° 70°

Latitude (N)

Longitude (E)

37 
Figure 8. Geographic and depth strata used for density estimates in the Gulf of Mexico.

Eastern

Gulf of Mexico

Western

Gulf of Mexico

40 fa.

10 fa.

Eastern

Gulf of Mexico

Western

Gulf of Mexico

40 fa.

10 fa.

38 
Figure 9. Distribution of sightings for (A) Loggerhead, (B) Kemp's ridley, and (C) Green turtles 

in the Gulf of Mexico. 

A. Loggerhead Turtles

B. Kemp’s Ridley Turtles

C. Green Turtles

39 
40 

A. Hawksbill Turtles

B. Leatherback Turtles

C. Un-identified Hardshell Turtles

of sightings for (A) Hawksbill, (B) Leatherback, and (C) unidentified 

ardshell turtles in the Gulf of Mexico. 

Figure 10. Distribution

h
Figure 11. Sighting probability as a function of distance from the trackline for Gulf of Mexico 

turtles. The bars indicate scaled sighting frequencies within distance intervals and the line 

indicates the fitted half-normal curve. 

41 
Figure 12. Latitudinal strata used for density estimates in the Atlantic. The 10 fathom isobath is

shown. 

26°

28°

30°

32°

34°

36°

Latitude (N)

82° 80° 78° 76° 74° 72°

Longitude (E)

South

Central

North

26°

28°

30°

32°

34°

36°

Latitude (N)

82° 80° 78° 76° 74° 72°

Longitude (E)

South

Central

North

42 
Figure 13. Distribution of sightings for (A) loggerhead, (B) leatherback, and (C) unidentified 

hardshell turtles in the Atlantic. 

A. Loggerhead B. Leatherback

C. Un-identified Hardshell

43 
Figure 14. Sighting probability as a function of distance from the trackline for Atlantic turtles.

The bars indicate scaled sighting frequencies within distance intervals and the line indicates the 

ard rate curve. 

Adjusted Catch Rates

fitted haz

CPUE for those species and strata where data were available are presented for summer 

and winter in 

Table 15 and Table 16. When available, CPUE estimates from the Foundation study 

were used. Foundation data were available in the western Gulf of Mexico for loggerheads 

(summer Nearshore and Offshore, winter Offshore), Kemp’s ridley (summer Nearshore and 

Offshore), and green (summer Nearshore, winter Offshore). In the Atlantic, the Foundation 

study provided estimates of CPUE for loggerhead (summer Central and South), Kemp’s ridley 

(summer Central and South), green (summer Central and South), and leatherback (summer 

Central) 

If there were no Foundation CPUE data for a species in a given strata, estimates were 

obtained with one of two methods. The primary method used a “catchability” estimate for a 

species in a given strata, and we extrapolated to other nearby strata using the catchability 

estimate and relative abundance from the aerial surveys. In the absence of such information, our 

alternative method to estimate CPUE was to use the CPUE of that species from the most similar 

strata where we did have an estimate. 

44 
“Catchability” (Qi) was estimated as follows: 

Qi = Cij / Aij

Where Aij is aerial survey abundance index for species i in stratum j and Cij is CPUE in trawls 

for species i in stratum j. If there were no CPUE estimates for a particular stratum, we made the

assumption that Qi is the same as the nearest stratum with data, within a region (Gulf or 

Atlantic). We estimated Cik for a different stratum k, when no CPUE data were available, using 

the aerial survey abundance for that strata and the catchability from a nearby strata as: 

Cik = Qi * Aik

We estimated the CV for Qi using the following equations (Seber, 1982): 

CV (Qi) = sqrt (CV(Aij)

2

 + CV(Cij)

2

) 

CV (Cik) = sqrt (CV(Qi)

2

 + CV(Aik)

2

) 

For any species where we do have both a Cij and an Aij, we can estimate Qi and 

extrapolate to other strata. Qi could not be estimated for a species where Aij is unknown. In 

these cases, we assigned a nearby CPUE to the strata. 

In the Gulf of Mexico we have summer abundance (Aij) and CPUE estimates (Qi) in the 

estern Gulf of Mexico for loggerhead (Nearshore and Offshore), Kemp’s ridley (Nearshore), 

nd green (Nearshore). For the Atlantic, we only have abundance and CPUE estimates in the 

entral and South subregions for loggerheads.

ated using the catchability method in the Gulf of Mexico during the 

mme d

e

co.

e

here 

PUE estimates in the Foundation data, and we had only density estimates from the 

astern to

thwest Florida (Figure 10). Large portions of this 

w

a

C

CPUE was estim

su r for loggerhead (eastern Nearshore and Offshore), Kemp’s ridley (eastern Nearshore an

Offshore), and green (western Offshore, eastern Nearshore and Offshore). For leatherback in th

Gulf of Mexico, we applied the catchability estimate from the Atlantic to the abundance 

estimates of the Gulf of Mexico for all strata to estimate CPUE. The application of Qi between

regions was necessary because we had no data on CPUE for leatherbacks in the Gulf of Mexi

In the Atlantic, the catchability method only was used for loggerhead during the summer in th

North subregion. 

Due to a lack of data we were not able to estimate any CPUE for hawksbill turtles. T

were no C

e Gulf of Mexico. The absence of any data on the CPUE of hawksbill may be attributed

the fact that they associate with coral reefs or live bottom (NMFS and USFWS 1993). This 

association reflects their diet of sponges, which require hard-bottom substrate for attachment.

Otter trawls do not operate in the vicinity of coral reefs. When not on reefs, hawksbills are likely

to engage in directed movements between reefs. These directed movements would minimize 

their time over open bottom where trawls are likely to be operating. Additionally, aerial survey 

sightings of hawksbills are concentrated in sou

45 
area are closed to fishing (Everglades National Park), which would minimize any interaction 

Sources of Error 

sources

atherback turtles. Water clarity differs within and among subregions; it is less likely that these 

two species can be detected in turbid wate rn Gulf of Mexico). Also, there may be 

ze differences between subregions; larger turtles are more readily sighted. For example, green 

SCL=46.6 cm, 

E=0.11, n=160) than greens stranding in the western Gulf of Mexico (mean SCL=30.2 cm, 

SE=0.08, n=113).11 These pro d catch rates and therefore

estimates of interactions and mortalities for thes pecially inaccurate. We believe 

that for the eastern Gulf of M restimate the CPUE for 

ese two species when adjusting the empirical CPUE data from the western Gulf of Mexico 

ey

est to apply the unadjusted CPUEs from

nearby strata to strata with no empirical CPUE estimates. These estimates also may be biased, 

ed

es, ideally stratifying to minimize the variance 

ithin each subregion. We chose to minimize the number of subregions and use one overall 

stratific

s

PUE from an adjacent strata (Inshore estimates were assumed to be the same

s Nearshore) or season (almost all winter CPUE were assumed to be the same as the summer 

CPUE)

with trawls.

Foremost among the sources of error are the problems encountered with the sightability

of Kemp’s ridley and green turtles during aerial surveys. This is especially an issue in areas such

as the northern Gulf of Mexico or the Atlantic where none or few were sighted, but other

of information indicate their presence. There may be several sources of bias using aerial survey 

data for these two species that are smaller and have less contrast than the loggerhead or

le

rs (e.g., northe

si

turtles stranding in the eastern Gulf of Mexico 1999-2001 are larger (mean

S

blems render any estimates of adjuste

e two species es

exico, the direction of the bias is to ove

th

with the aerial survey data. Thus, our adjusted CPUE estimates for green and Kemp’s ridl

turtles likely are high. In the absence of other information indicating a difference in density 

among the subregions for these two species, it may be b

a

but the direction is unknown. 

 

Another source of error is the subregion divisions chosen. The divisions should be bas

on the known distribution patterns for each speci

w

ation scheme because of the dearth of data. Modifications of the boundaries between

subregions may yield different results. This may be especially true for the Gulf of Mexico that 

might have best been divided into three, not two subregions: western, northern, and eastern. 

Although we have utilized the best available data, there are still potential sources of error

in strata where we had neither CPUE from the Foundation nor the data to estimate a CPUE using 

our catchability method. In these cases we had to assume the CPUE for that particular strata wa

equivalent to the C

a

.

Applying the same CPUE to a given inshore strata based on the CPUE from nearshore 

strata, often extrapolated itself, assumes that turtle density is similar in inshore and nearshore

waters. For some species, such as green turtles we know that this generally is not true, except 

during their migrations, since this species is related with seagrass habitats in inshore waters, or 

when very small, with nearshore worm reefs (Hirth, 1997). Conversely, it is unlikely that a 

 

11 Wendy Teas, National Marine Fisheries Service, Miami, FL. Personal communication (E-mail) November 25, 

2002 to Sheryan Epperly, National Marine Fisheries Service, Miami, FL. 

46 
pelagic species such as the leatherback is as abundant in inshore waters as it is in nearshore or

offshore waters.

he same CPUE to a given strata in both the summer and winter implies that 

rtles are similarly distributed between seasons. Differences in distribution, and therefore 

CPUE,

the two

Applying t

tu

are likely to exist between seasons and strata, but no data exist to estimate a CPUE in 

some strata.

Additionally, our estimates of variance for the catchability method (expressed here as 

95% confidence intervals) are likely to be underestimated. This approximation assumes

variables are uncorrelated and the CV of the denominator is low for ratios (Qi). These

assumptions are violated here. Ignoring the covariance between the two results in an 

overestimate of the CV, and a large denominator produces an underestimate of the variance. 

With our data, we are likely to be underestimating the variance of our estimate. 

Table 15. Estimated catch rates of sea turtles captured in shrimp trawls in the Gulf of Mexico, 

March-November, by subregion and depth stratum. Cells with bold type represent CPUE 

measured by the Foundation, shaded cells were estimated using the catchability method, and 

cells in normal type with no shading are estimates applied from similar strata. Numbers in 

parentheses are the 95% confidence intervals. 

Area Species

/Subregion 

Depth 

Stratum Leatherback Loggerhead Kemp’s ridley Green

Gulf of Mexico (catch/hr)

Eastern Inshore 0.00101 0.0574 0.7327 0.0068 

(0 to 0.00416) (0 to 0.1664) (0 to 3.7383) (0 to 0.0486) 

 Nearshore 0.00101 

(0 to 0.00416) 

0.0574 

(0 to 0.1664) 

0.7327 

(0 to 3.7383) 

0.0068 

(0 to 0.0486) 

 Offshore 0.00112 

(0 to 0.00.390) 

0.0100 

(0 to 0.0498) 

0.1021 

(0 to 0.5265) 

0.0445 

(0 to 0.3122) 

Western Inshore 0.00012 

(0 to 0.00053) 

0.0124 

(0.0026 to 0.0247) 

0.0371 

(0.0181to 0.0598) 

0.0026 

(0 to 0.0077) 

 Nearshore 0.00012 0.0124 0.0371 0.0026 

(0 to 0.00053) (0.0026 to 0.0247) (0.0181to 0.0598) (0 to 0.0077) 

Offshore 0.00019 

(0 to 0.00069) 

0.0006 

(0 to 0.0014) 

0.0003 

(0 to 0.0009) 

0.0023 

(0 to 0.0169) 

Atlantic (catch/day) 

North Inshore 0.0141 

(0 to 0.0422) 

0.6325 

(0.3004 to 0.9645) 

0.1268 

(0.0563 to 0.2254) 

0.0141 

(0 to 0.0422) 

 Ocean 0.0141 

(0 to 0.0422) 

0.6325 

(0.3004 to 0.9645) 

0.1268 

(0.0563 to 0.2254) 

0.0141 

(0 to 0.0422) 

Central Inshore 0.0141 

(0 to 0.0422) 

1.3944 

(1 to 1.7887) 

0.1268 

(0.0563 to 0.2254) 

0.0141 

(0 to 0.0422) 

Ocean 0.0141 

(0 to 0.0422)

1.3944 

(1 to 1.7887) 

0.1268 

(0.0563 to 0.2254) 

0.0141 

(0 to 0.0422) 

South Inshore 0.0141 

(0 to 0.0422) 

1.7895 

(1.0479 to 2.5311) 

1.0175 

(0.4561 to 1.7719) 

0.0702 

(0.0175 to 0.1404) 

 Ocean 0.0141 

(0 to 0.0422) 

1.7895 

(1.0479 to 2.5311) 

1.0175 

(0.0563 to 0.2254) 

0.0702 

(0.0175 to 0.1404) 

47 
Table 16. Estimated catch rate of sea turtles captured in shrimp trawls in the Gulf of Mexico, 

December-February, by subregion and depth stratum. Cells with bold type represent CPUE 

easured by the Foundation and cells in normal type are estimates applied from similar strata. 

Numbe

m

rs in parentheses are the 95% confidence interval. 

Area Species

/Subregion 

Depth 

Stratum Leatherback Loggerhead Kemp’s ridley Green

Gulf of Mexico (catch/hr) 

Eastern Inshore 0.00101 

(0 to 0.00416) 

0.0574 

(0 to 0.1664) 

0.7327 

(0 to 3.7383) 

0.0068 

(0 to 0.0486) 

 Nearshore 0.00101 

(0 to 0.00416) 

0.0574 

(0 to 0.1664) 

0.7327 

(0 to 3.7383) 

0.0068

(0 to 0.0486)

 Offshore 0.00112 

(0 to 0.00.390) 

0.0100 

(0 to 0.0498) 

0.1021 

(0 to 0.5265) 

0.0445 

(0 to 0.3122) 

Western Inshore 0.00012 0.0124 0.0371 

(0 to 0.00053) (0.0026 to 0.0247) (0.0181to 0.0598) 

0.0026 

(0 to 0.0077) 

 Nearshore 0.00012 

(0 to 0.00053) 

0.0124 

(0.0026 to 0.0247)

0.0371 

(0.0181to 0.0598) 

0.0026 

(0 to 0.0077) 

 Offshore 0.00019 

(0 to 0.00069) 

0.0014 

(0 to 0.0041) 

0.0003 

(0 to 0.0009) 

0.0008 

(0 to 0.0024) 

Atlantic (catch/day) 

North Inshore 0.0141 

(0 to 0.0422) 

0.6325 

(0.3004 to 0.9645) 

0.1268 

(0.0563 to 0.2254) 

0.0141 

(0 to 0.0422) 

 Ocean 0.0141 0.6325 

(0 to 0.0422) (0.3004 to 0.9645) 

0.1268 

(0.0563 to 0.2254) 

0.0141 

(0 to 0.0422) 

Central Inshore 0.0141 1.3944 0.1268 0.0141 

(0 to 0.0422) (1 to 1.7887) (0.0563 to 0.2254) (0 to 0.0422)

Ocean 0.0141

(0 to 0.0422) (1 ) (0.05 254) (0 )

1.3944 

to 1.7887

0.1268 

63 to 0.2

0.0141 

to 0.0422

South Inshore 0.0141

(0 to 0.0422)

1.7895 

(1.0479 to 2.5311) (0 (0

1.0175 

.4561 to 1.7719) 

0.0702 

.0175 to 0.1404) 

Ocean 0.0141

(0 to 0.0422) 

1.7895 

(1.0479 to 2.5311) (0 (0

1.0175 

.0563 to 0.2254) 

0.0702 

.0175 to 0.1404) 

48 
Estimated Number Of Interactions With Shrimp Trawls 

We used effort data from 2001 and the CPUE data from the Adjusted Catch Rate section 

determine the number of interactions of turtles with shrimp trawls. Data on effort was 

ollected by State and NMFS port agents. The number of interactions with shrimp trawls was 

calcula r eac ies and strata using the equation

Interactions = CPUE sp ta, time, x eff , time 

Results on the number of interactions are presented in Table 17 and Table n the

Gulf f Mexico s ridley had the mos ns follo gerhea nd

leatherback. Interactions i c we or logg llowed s

ridle rbacks, and green. The number o actions for bills could

determined any a we CP

Sources of Erro

All the source or in aspe inate herein, 

resulting in very large conf rva as no

measurement as ed with nd theref not incorpo Thus, the ported is

an underestimat

The esti of the n of interac re subject to the same biases discussed in 

the Adjusted Catch Rates s ey w ned b PUE . If

we did not have a Foundati r w lc e e

CPUE from sim rata and e that wa sentative fo nknown str n one

hand, the take of all species wa er th of

shore vessels for a unit of effort (hours or days) likely is less than that of the ocean vessels. 

onversely, the density of all species except leatherbacks may be different (higher) than the 

ensity in ocean waters. 

Almost all the winter CPUE data was assumed to be the same as the summer CPUE due 

the lack of Foundation and aerial survey effort in that season. Applying the same CPUE to a 

given strata in both the summer and winter implies that turtles are similarly distributed between 

seasons. Differences in distribution, and therefore CPUE, are likely to exist between seasons and 

strata, but no data exist to estimate a CPUE in some strata. Our estimates of the number of

interactions were computed using the CPUE from similar strata when no other data were 

available. 

to

c

ted fo h spec :

ecies, stra ort strata

18. I

o , Kemp’ t interactio wed by log d, green, a

n the Atlanti re highest f erheads, fo by Kemp’

y, leathe f inter hawks not be

where bec use no data re available for UE.

r

s of err cts of the analysis thus far culm

idence inte ls. Many of the sumptions have error

sociat them a ore are rated. error re

e.

mates umber tions a

ection as th ere determi y multiplying C and effort

on CPUE o ere unable to ca ulate a CPUE, w had to apply th

ilar st assum s repre r the u ata. O

in inshore ters likely is ov estimated since e fishing power

in

C

d

to

49 
Table 17. Es Mexico 

nd Southeast U.S. Atlantic, March-November, 2001, by subregion and depth stratum. 

onfidence intervals (95%) are given in parentheses. 

timated number of sea turtle interactions in shrimp trawls in the Gulf of

a

C

Area Species

/Subregion 

Depth 

Stratum Leatherback Loggerhead Kemp’s ridley Green

Gulf of Mexico

Eastern Inshore 434 24641 

(0 to 1786) (0 to 71433) 

314537 

(0 to 1604798) 

2919 

(0 to 20863) 

N

(0 to 1088) (0 to 43504) (0 to 977349) 

1778 

(0 to 12706) 

earshore 264 15007 191559

 Offshore 517 

(0 to 1803) 

4622 

(0 to 23017) 

47189 

((0 to 243340) 

20567 

(0 to 144294)

Western Inshore 88 

(0 to 390) 

9113 

(1911 to 18153)

27267 

(13303 to 43950) 

1911 

(0 to 5659) 

 Nearshore 224 

(0 to 1025) 

23976 

(5027 to 47759)

71735 

(34998 to 115628) 

5027 

(0 to 14888) 

 Offshore 335 1057 52

(0 to 1216) (0 to 2467) 

9

(0 to 1586) 

4053 

(0 to 29782) 

Atlantic

North Inshore 174 

(0 to 521) 

7812 

(3711 to 11913)

1566 

(695 to 2784) 

174 

(0 to 521) 

Ocean 34 

(0 to 102) 

1529 

(726 to 2332) 

307 

(136 to 545) 

34 

(0 to 102)

Central Inshore 6 

(0 to 18) 

599 

(430 to 769) 

55 

(24 to 97) 

6 

(0 to 18) 

Ocean 380 

(0 to 1137) 

37575 

(26947 to 48201) 

3417 

(1517 to 6074) 

380 

(0 to 1137) 

South Inshore 9 

(0 to 26) 

1120 

(656 to 1584) 

637 

(286 to 1109) 

44 

(11 to 88) 

 Ocean 117 

(0 to 352) 

14909 

(8730 to 21088)

8477 

(3800 to 14762)

585 

(146 to 1170)

50 
Table 18. Estimated number of sea turtle interactions in shrimp trawls in the Gulf of Mexico 

and Southeast U.S. Atlantic, December-February, 2001, by subregion and depth stratum

Confidence intervals (95%) are given parentheses. 

. 

ea Species

ubregion 

Depth 

Stratum Leatherback Loggerh Kemp’s rid

f of Mexico

(0 to 7 (0 to 3

136

(0 to 6978

1

(0 to 90

(0 to 16 (0 to 6

292

(0 to 1490 (0 to 193

2

222 

(0 to 77 (0 to 9

20

(0 to 1043 (0 to 6186

88

(0 to (46 to 44 ) (289 to 106

662 46 

(0 to 13

(0 to 18 (892 to 84 0) (5555 to 2050

12

) (0 to 264

8

(0 to 30 (0 to 1 (0 to 4

3

(0 to 107

ntic

h nshore

(0 to (13 to 4 (2 to 9 (0 to

(0 to (25 to (3 to 1 (0 to

(0 to (36 to (1 to (0 to

(0 to 17 (4164 to 74 ) (158 to 89

528 

(0 to 17

(0 to (122 to (53 to 20 (2 to 1

(0 to 1 (2701 to 6 5) (1176 to 45

2623 

) (45 to 36

181 

Ar

/S ead ley Green

Gul

Eastern Inshore 19

8)

1072 

106)

78

8)

27

7)

 Nearshore 40 

7)

2289 

634)

13

47)

71

8)

 Offshore 

3)

1982 

868)

231

27)

18

3)

Western Inshore 2 

9)

221 

1 8) 7)

 Nearshore 40 

2)

4252 

7

722

6

92

0)

 Offshore 85 

8)

624 

828)

134 

01)

57

0)

Atla

Nort I 1

2)

27 

2)

6 

)

1 

2)

 Ocean 1 

4)

53 

81)

11 

9)

1 

4)

Central Inshore 1 

2)

50 

64)

5 

8)

1 

2)

 Ocean 59 

6)

5807 

49 8)

59 

6)

South Inshore 2

5)

208 

294)

118 

6)

8 

6)

 Ocean 36 

09)

4613 

52 68 2)

51 
 Estimated Lethal Interactions In Shrimp Trawls 

Not all turtles will escape the shrimp trawls even though virtually all have turtle excluder 

devices (TEDs), but not all turtles retained will drown. TE not 100% effective regardless 

of the siz rtle vess TE en ough tentially

exclude turtles of all species and sizes. Turtles caught on these vessels are expected to escape. 

A prop f those ught on vessels using th open will ed in 

the trawls because th large an t a minimu eir surviv l be a fun n of the

duration of forced submergence and water temperat w w he turtles 

escaping the nets through TEDs, based on regulatio e cu es

mortali ose ret n the nets. 

TED Effectiveness

Ds are

e of the tu . Some els are using Ds with op ings large en to po

ortion o not ca e largest ing TEDs be retain

ey are too d, a m, th al wil ctio

ure. Belo e estimate t number of

ns in plac rrently, then timate the

ty of th ained i

S

struggle, and fail to escap o be ce y NMFS

al R 87, 1 ing 1 TE

hin 5 minu f being in ced into

2002 oub

rations, a turtles teste scaped.

12 In 2002 stud

Effectiveness of control TEDs 

ome turtles do not escape through TEDs even though they are small enough to fit 

through the openings. Some may be exhausted to the point that they do not explore the TED 

area or e. T rtified b , a TED design must be 97% 

effective in excluding sea turtles (Feder egister, 19 992). Dur 995-2002 D

certification trials, 250 captive-reared turtles were tested in nets with a control TED (Top 

Opening Super Shooter) and all but 11 escaped wit tes o trodu the

trawl.12 In 2000 and tests of the d le cover flap and of the 71” opening, in both top 

opening and bottom opening configu ll 70 d e a y

board the Georgia Bulldog, all 29 wild turtles encountered when using TEDs with large 

penings escaped (Table 19).13 These combined data indicate a TED effectiveness of 96.3%, 

hich probably is conservative because the test of a given animal was terminated if a captiveared turtle did not escaped within 5 minutes. Of those trials using large openings, 100% 

=99) of the turtles escaped. Thus, we are assuming that properly installed TEDs are at least 

7% effective in excluding wild sea turtles, assuming the turtles are small enough to fit through 

the openings. Thus, 3% of the interactions will result in turtles being retained in the trawl, 

regardless of the size of the turtles. 

 

a

o

w

re

(n

9

12 National Marine Fisheries Service, Pascagoula, MS. Unpublished Data. John Mitchell, National Marine Fisheries 

Service, Pascagoula, MS. Personal Communication (E-mail) November 12, 2002 to Sheryan Epperly, National 

Marine Fisheries Service, Miami, FL. 13 National Marine Fisheries Service, Pascagoula, MS Unpublished Reports. An evaluations of modified TED Flap

Designs on the exclusion of wild sea turtles off the Southeast Atlantic Coast May 13-17, 2002, 5p; Trip report for 

testing modified flaps for sea turtle exclusion off the Atlantic coast of Georgia & Florida, August1-5, 2002, 5 p. A

remote video camera attached to a shrimp trawl near the TED was used to obtain the observations. 

52 
Leatherback mo

TED openings can be modified to accommodate the escape of large leatherback turtles 

e

the

higher in the Atlantic, and they are used some in the 

shore waters of North Carolina.

onventional TEDs

TED opening dimensions differ between the Gulf and the Atlantic (Federal Register, 

e

exico

ces

use

s

to

p’s 

through existing TED openings in the Gulf. These data

dicate a TED effectiveness for loggerheads in the Gulf of Mexico of 25%. During the months 

f the study and in the same zones, 32.8% of the turtles that stranded and were measured were 

small enough (≤65.3 cm SCLstd) to fit through the 10 inch height opening of the Gulf TEDs.16 

Since 1998, 68-79% of the loggerheads stranding annually in the western Gulf (zones 13-21) 

 

dification of TED opening 

(Federal Register 1993, 1994, 1995). These modifications have been required during times when

the density of leatherbacks exceeded a defined threshold off Georgia or the Carolinas or when 

strandings exceeded background levels, such as off Florida or Texas. Some fishermen subjected 

to these periodic regulations now choose to use these TEDs year round. These large-opening 

TEDs should allow for the escape of all turtles, including mature leatherbacks.14 Current use of

these leatherback modifications is shown in Table 20. Based on the size of the offshore fleet, w

estimate that 7% of the fleet operating in the offshore waters of the western Gulf of Mexico is 

using the leatherback modification and 11% of the fleet operating in the offshore waters of

eastern Gulf is using it.15 Usage is much

in

C

1992). Height is measured simultaneously with width and is measured at the midpoint of th

straight-line distance of width (i.e., the width and height of a taut triangle is measured). Along

the Atlantic Coast these requirements are width ≥35 in and height ≥12 in. In the Gulf of M

these measurements are width ≥32 in and height ≥10 in. 

Not all turtles will escape the trawls even though virtually all have turtle excluder devi

(TEDs). Epperly and Teas (2002) identified the small size of TED escape openings as the ca

of significant mortality on loggerheads turtles before they were reaching the average size of 

maturation. Green turtles also were being retained, but at a lower rate than loggerheads. 

Virtually all Kemp’s ridleys, a smaller species, could escape through the existing openings. It i

assumed that all leatherbacks would be retained in the trawls. Strandings data were too sparse

evaluate hawksbills relative to the size of TED openings. 

 

Foundation data collected in the western Gulf of Mexico 1997-1998 demonstrate the 

problem. Seventy-five percent of the loggerheads captured but none of the greens and Kem

ridleys captured were too large to fit

in

o

14 Sheryan Epperly, National Marine Fisheries Service, Miami, FL and Peter Dutton, National Marine Fisheries 

Service, La Jolla, CA. Unpublished data. During the 2000 nesting season measurements were obtained from

leatherback turtles nesting in St. Croix, U.S. Virgin Islands (n=23) and Suriname, S.A. (n=30). An analysis (August

18, 2000) of width, body depth, and circumference indicated that all measured turtles could fit through the existing 

opening of TEDs equipped with the leatherback modification. 15 John Mitchell, National Marine Fisheries Service, Pascagoula, MS. Personal communiation (E-mail) October 30, 

2002 to Sheryan Epperly, National Marine Fisheries Service, Miami, FL. 16 Wendy Teas, National Marine Fisheries Service, Miami, FL Personal communication (E-mail) October 17, 2002

to Sheryan Epperly, National Marine Fisheries Service, Miami, FL. 

53 
have been deeper bodied than the 10 inch height opening of the Gulf TEDs.

17 The loggerhead 

strandings closely reflect the in-water population, based on size. It appears that in the Gulf, 

because of the relatively small TED opening size, few loggerhead turtles would fit through 

existing TED openings. Thus, the entire benthic population is being "sampled" and some

drowned without size bias. 

e

ectiveness percentage of 97.7% for loggerheads in the 

tlantic. During the same months and in the same area where the Foundation study was 

onducted in the Atlantic, 85.8% of the loggerheads stranding were small enough (≤79.8 cm

ugh the minimum opening size of TEDs in the Atlantic.16 In the Atlantic, 

ED openings are larger and most turtles do fit through the TEDs. Thus, although all size turtles 

are bei

of the mouth of the trawl or through 

e TED opening during haulback and it is never recognized that a turtle has been caught. In 

May an

2 were removed through the mouth 

Similarly, Foundation data collected in the Atlantic shows that 2.5% (5 of 201) of the 

loggerheads captured but none of the greens or Kemp’s ridleys captured were too large to fit 

through existing TED openings in the Atlantic. South Carolina Marine Resource Division 

fishery-dependent data collected aboard commercial trawlers working without TEDs during lat

May/early June through late July/early August, 2000-2001, off South Carolina and Georgia 

(n=277 tows) indicate that 1.7% of the turtles they captured (1 of 58) would be too large.18 The

combined datasets yield a TED eff

A

c

SCLstd) to have fit thro

T

ng sampled, not all are being retained and drowned - only the largest ones are. Thus, 

since strandings reflect only the mortalities, we would expect to see a size bias - a larger 

proportion of large turtles stranding than those turtles represent in the in-water population. 

Turtles that are retained in the trawl may slide out

th

d August of 2002, NMFS conducted a study in offshore waters of Georgia to observe the 

behavior of wild turtles encountering various sized TED openings13. A remote video camera 

attached to a shrimp trawl near the TED was used to obtain the observations. All wild turtles

encountering TEDs with large openings escaped. Twenty-one wild turtles were observed

encountering an opening which was 1 inch larger in height than the Gulf of Mexico minimum 

TED opening size; 5 did not escape after 10 minutes of their first encounter with the device. 

Upon haulback, 3 washed out of the TED at the surface and

of the trawl.

th

Effective May 20, 2002 all vessels fishing in South Carolina waters are required to use a

TED with an opening of 35” wide x 20” high.19 Based on measurements provided by the Sou

Carolina Department of Natural Resources for loggerheads nesting on Cape Island, SC (n= 87), 

virtually all loggerheads should fit through this size opening.20

s 

likely escape, based on the size of the TED opening. Elsewhere we assume that the fleet is using 

the minimum size openings required for the region. We therefore assume that, based on size, 

 

In summary, in South Carolina waters leatherbacks may be retained but all other turtle

17 Wendy Teas, National Marine Fisheries Service, Miami, FL. Personal communication (E-mail) August 15, 2002

to Sheryan Epperly, National Marine Fisheries Service, Miami, FL. 18 Phil Maier, South Carolina Department of Natural Resources, Charleston, SC. Personal communication (E-mail 

with data attached) October 9, 2002 to Sheryan Epperly, National Marine Fisheries Service, Miami, FL. 19 Code of Laws of South Carolina, Section 50-5-765. 20 Sally Murphy, South Carolina Department of Natural Resources, Charleston, SC. Personal communication (email with data attached) August 22, 2002 to John Mitchell, National Marine Fisheries Service, Pascagoula, MS. 

54 
green, hawksbills, and Kemp’s ridley all can escape through existing openings, but that not all 

loggerheads can. In the Gulf of Mexico 25% of the loggerheads may escape and in the Atlan

97.7% may escape. The consequence of being retained in the nets is that the turtle is forcefully 

submerged and possibly drowned. The likelihood of the interaction becoming lethal is a function 

of the duration of the tow an

tic

d water temperature. 

Sources of error

our

with

gia

TED Opening Turtle Escapes Turtle Captures 

Noticably absent is good information on the sizes of TED openings in use in each of

strata. There is error in our estimates of the proportion of the fleet using leatherback TEDs. We

assumed that the proportion of the fleet not using the leatherback modification of TEDs are 

pulling TEDs with the minimum sized opening. Many are pulling TEDs with larger openings, 

especially in the Atlantic. Conversely, in the Gulf a substantial number are pulling TEDs

openings smaller than the requirement, but within the tolerance allowed by enforcement 

personnel.21

Table 19. Escape of wild turtles through various TED openings during tests aboard the Geor

Bulldog, May and August, 2002. 

71-inch 9 0

Double Cover 7 0 

35” x 20” 13 0 

 

21 Jack Forrester, National Marine Fisheries Service, Pascagoula, MS. Personal communication October 30, 2002 to

John Mitchell, National Marine Fisheries Service, Pascagoula, MS. Gear Specialists with NMFS Harvesting

Systems Branch assisted State and Federal fisheries enforcement personnel in conducting TED compliance 

boardings during the 2002 Texas shrimp opening. All of the inspections were conducted in nearshore waters from

Galveston to Port Aransas. Thirty percent of the TEDs which were inspected had escape openings smaller than the

Gulf minimum size (32" x 10"). 

55 
Table 20. Current ongoing use of TEDs with leatherback modifications.22 It is assumed that no

inshore boats are using the modification, except where noted, and that in the Gulf there is no 

usage in nearshore waters. 

P

State/Area 

ercent Using

Modified TED 

Texas, offshore ≤15% 

Louisiana, offshore ≤5% 

Mississippi, offshore ≤5% 

Alabama, offshore ≤5% 

Florida, west coast, offshore 15% 

Florida, east coast, ocean 70% 

Georgia, ocean 68%23

South Carolina, ocean 50%24

North Carolina, ocean 35% 

North Carolina, inshore ≤10% 

 Mortality As A Function Of Tow Time And Water Temperature

tuntz (1987) found a tistically significant relationship between tow

time and f sea turtles (r=0.98, p< 1) using data from 3 NMFS fishery observer 

rograms 1973-1984. They regressed average mortality over minutes fished broken into 

intervals of 30 minutes. The National Research Council (1990) revisited the data and found 

substantial differences in mortality between seasons using time intervals of ten minutes. They 

pointed out that Henwood and Stuntz assumed all comatose turtles survived and were treated as 

alive. The NRC noted that all resuscitated individuals do not survive. Furthermore it is assumed 

that comatose turtles likely would be returned to the water if an observer was not aboard to 

resuscitate it, surely resulting in the death of the turtle. We reanalyzed the dataset to estimate the 

mortality resulting from interactions between sea turtles and otter trawls. 

Following the recommendations of the NRC, we divided the data by season and 

combined dead and comatose turtles into one category to determine mortality. Per above 

discussions of effort and catch, seasons were defined as “summer” (March-November) and 

 

Henwood and S sta

mortality o 0.00

p

22 John Mitchell, National Marine Fisheries Service, Pascagoula, MS. Personal communication (e-mail) October 29, 

2002 to Sheryan Epperly, National Marine Fisheries Service, Miami, FL. Estimates are based on information

obtained from boardings with NMFS Law Enforcement and Coast Guard and from conversations with net shops, as 

well as estimates made by the staff of the Harvesting Systems Branch of the NOAA Fisheries Mississippi 

Laboratories. 23 Mark Dodd, Georgia Department of Natural Resources, Brunswick GA. Personal Communication (e-mail)

October 9, 2002 to Sheryan Epperly, National Marine Fisheries Service, Miami, FL. Estimate is based on a sample 

of approximately 90 interviews at 13 different docks in the state during 2001. 24 David Whitaker, South Carolina Department of Natural Resources, Charleston, SC. Personal communication (email) October 9, 2002 to Sheryan Epperly, National Marine Fisheries Service, Miami, FL. Estimate is based on

reports from fishermen and SCDNR staff.

56 
winter (December-February). Figure 15 and Figure 16 show mortality versus tow time in 

intervals of ten minutes. For ease of presentation, summer mortality was divided into three 

periods: 0 to 50 min, 51 to 200 min, and 201 to 610 min, which represent the 3 “stages” of the 

sponse curve. Winter mortality was divided into just two categories: 0 to 60 min and 61 to 420 

min. These divisions are log al breaks based on ortality low during short tows 

and high in long tows. 

We us to model tow time versus turtle mortality, and found the 

models fit the data well in both seasons (Table 21, Table 22, and Figure 17). The models 

accounted for 62% and 70% of the variation in summer and winter, respectively. The predictive 

ability of our n the association of the predicted probabilities with 

the observed r ). 

The findings here are consistent with and expand upon what has been reported by the 

NRC and Henwood and Stuntz. Specifically, tows of short duration have little effect on 

mortality, inte in a rapid escalation to mortality, and eventually reach a 

lateau of hig ll be high on long tows, but will not equal 100% as a 

rtle caught within the last hour of a long tow would likely survive. 

The NRC also acknowledged that if tow times were restricted to less than 40 min in the 

and current regulations

corporated those findings (Federal Register, 1992); previously, tow duration was allowed to be 

0 min e

e tow 

tow times have very 

igh mortality. Only animals captured within the last 60 minutes of a long tow are likely to have 

model 

Additionally, we followed the recommendation of the NAS and included all comatose 

turtles in the mortality calculations by assuming they were not resuscitated. We wanted to be 

conservative with our estimates of mortality. If some comatose turtles were resuscitated and 

survived, mortality would be slightly lower then we found in this analysis. 

re

ic the data with m

ed logistic regression

models was very good based o

esponses (Somers’ Dxy in Table 21

rmediate tow times result

p h mortality. Mortality wi

tu

summer and less than 60 min in the winter, few animals would drown,

in

9 (Federal Register, 1987). Our analysis of the same dataset differs, and indicates that th

stress of being captured in a trawl is greater in cold water than in warm water. For example, in

the summer the mortality associated with a 40 min tow is predicted at 3% whereas the sam

duration in the winter would yield a mortality of 5%. To achieve a negligible mortality rate 

(defined by NRC as <1%) the tows in both seasons would need to be less than 10 min. 

Sources of Error 

The largest source of error is the small sample size for very short tow times and long tow 

times. Short tow times are unlikely to cause mortality in turtles, but long

h

high survivorship. Data with more replicates of short and long tow time might produce a

that explains more of the variation. 

57 
Table 21. Logistic models of mortality.

Season Logistic Model1

 R-Square Deviance Somers’ Dxy

2 

Summer -4.6815 + 

0.0314x 

0.6161 Value=346.61, df= 

151, p<0.0001 

0.914 

Winter -4.7967 + 

0.0469x 

0.7007 Value=121.26, df= 

59, p<0.0001 

0.899 

1 wh is the tow duration 2

 Index of rank correlation between the predicted probabilities and the observed response

ere x

Table 22. Statistical evaluation of parameters in logistic regression models. 

Season Parameter Estimate Significance

Summer Intercept -4.6815 SE=0.1543, p<0.0001 

Coefficient (tow time) 0.0314 SE=0.0012, p<0.0001 

Winter Intercept -4.7967 SE=0.2858, p<0.0001

Coefficient (tow time) 0.0469 SE=0.00346, p<0.0001 

Figure 15. Tow durations and mortality (dead and comatose) of sea turtles during the summer. 

Summer Mortality

0

0 100 200 300 400 500 600 700

Time

20

40

60

80

100

% Dead

% dead

58 
Figure 16. Tow durations and mortality (dead and comatose) of sea turtles during the winter.

W orta

0

20

40

80

100

0 100 200 300 400 500

Time

%

% dead

inter M lity

60

 Dead

 

Figure 17. Logistic curves for sea turtle mortality.

0

0.4

0.6

0.8

1

0 100 200 300 400 500 600 700

Tow time

Proportion of dead turtles 

in a tow

Summer Mortality

Winter Mortality

0.2

 

Tow Times 

In 1992, a joint government/commercial research program between NOAA Fisheries 

Galveston Laboratory and the Gulf and South Atlantic Fisheries Foundation, Inc. (Foundation) 

was implemented to collect species-specific data from the U.S. southeastern shrimp fishery.

he primary objectives of this ongoing research effort are to provide and manage observercollected data on shrimp trawl bycatch, and to evaluate the effectiveness of bycatch reduction 

devices (BRD) in finfish reduction and shrimp retention during commercial shrimping operations 

in the Gulf of Mexico and southeastern Atlantic. Observer data are used to refine catch rate 

estimates by area and season. These data also can be used to characterize the fishery: number of 

tows per day, tow durations, number of nets, etc. This is not the same study used to obtain sea 

turtle CPUE estimates above. 

T

59 
A comprehensive description of program guidelines for the Cooperative Shrimp Bycatch

Characterization Study is presented in the research plan that was prepared by the Foundation 

under the direction of a Steering Committee composed of individuals representing industry, 

environmental, state, and Federal interests (Hoar et al., 1992). The intent of the sampling design 

is to evaluate BRD designs during commercial shrimp fishery operations. The sampling universe 

consists of all tows from all vessels shrimping in the south U.S. Atlantic and Gulf of Mexico. 

Parameters of interest are the differences between BRD and control nets as related to catch totals 

and size distributions of all species incidentally taken by the shrimp fleet. 

NMFS-certified observers collect data aboard commercial shrimp vessels for the 

evaluation of specific BRD designs as related to bycatch reduction criteria established for Gulf of 

Mexico and southeastern Atlantic. Comparisons of catch data for nets equipped with TEDs 

versus nets with TED/BRD combinations are conducted. Experimental and control nets are 

alternated from starboard to port outboard nets to reduce net and side biases. Detailed

easurement and written description of TED, BRD, net type, construction, installation, webbing, 

nd other associated gear characteristics are recorded at the start and end of each trip, or when 

made. For each tow, environmental parameters, bottom time (actual bottom

time) and operational a l catch weight, and 

counts, weights and target species are obtained from the experimental and control nets. All sea 

rtles are identified to species, measured, tagged, photographed and released. A subsample of 

approximately 70 pounds (32 kg) from each net (experimental and control) are processed, time 

permitting, for bycatch characterization. 

We examined data from 1997-2002. Observers monitored 88 trips ending in the Atlantic 

and 207 trips ending in the Gulf of Mexico. In the Atlantic, sampling effort included zones 27-

32, but was concentrated in zones 31 and 32. There was no sampling effort in inshore waters of

the Atlantic nor in the North subregion. In the Gulf, sampling was well distributed, occurring in 

the nearshore and offshore strata, in the eastern and western Gulf, summer and winter, but not in 

inshore waters. The location and season of a trip’s assignment was based on the location and 

date of the last tow of the trip. Individual days/tows, however, were assigned to strata based on 

the day/tow-specific information. These data were used to determine tow durations in each 

ion x depth stratum.

m

a

adjustments are

spects relative to each net are documented. The tota

tu

season x subreg

The distribution of tow times is given in Table 23. The proportion of animals dying in 

s computed by determining the mortality associated with each tow, based on the 

w’s duration and season, and weighted by the proportion of time represented by that tow to the 

total am

ll

each strata wa

to

ount of time towed in a particular stratum. Thus, these proportional mortality factors

(Table 24) can be applied to the number of turtles being retained in the nets.

Sources of Error 

This analysis assumes that the NMFS/Foundation dataset represents a random sample of 

the universe and is representative of the tow durations of the entire fleet. The extremely sma

number of vessels participating in the shrimp fishery observer program, in comparison to the 

number in the entire shrimp fleet, may yield biased results. It is assumed that the vessels 

60 
participating are representative of fleet at large with respect to times and areas fished and fishing 

power.

sing

w times in inshore 

aters are the same as sampled in offshore waters. This likely is not true, except perhaps in the 

large in rs to be

Season 

Sampling effort for BRD evaluation is conducted primarily in Federal waters in the Gulf

of Mexico. It is not known whether the characteristics of the fleet fishing in offshore waters 

differ from those fishing in nearshore waters. Once selected, a vessel must meet specific hou

and safety requirements before an observer can be placed on that vessel. Collectively, these 

factors may bias results in terms of extrapolation fleet wide. 

 

Furthermore, the small sample sizes in some subregion x season x depth stratum also are 

a source of bias. Lastly, in the absence of data, we must assume that the to

w

shore water bodies of North Carolina; we would expect tow times in inshore wate

shorter than those of vessels operating in the ocean or Gulf. This will have the effect of

overestimating the interactions and mortality in inshore waters.

Table 23. Distribution of tow time durations in summer (March-November) and winter 

(December-February). The proportion of tows made in each duration interval is given for each

subregion x depth stratum x season, and the number of tows is given in parentheses. Nearshore

is ≤10 fm and offshore is > 10 fm. Note that there was no effort in any inshore waters nor in the

North subregion of the Atlantic. 

Area Depth Summer Winter 

/Subregion Stratum 0-50 min 51-200 min >200 min 0-60 min >60 min 

Gulf of Mexico 

Eastern Nearshore 1.7 (6) 29.9 (106) 68.4 (242) 3.0 (1) 97.0 (32) 

Offshore 0.7 (5) 19.9 (150) 49.4 (597) 0.7 (1) 99.3 (133) 

Western Nearshore 1.1 (8) 27.8 (209) 71.1 (533) 0.0 100.0 (31) 

Offshore 0.6 (23) 15.4 (587) 84.0 (3196) 0.3 (1) 99.7 (375)

Atlantic 

Central Ocean 4.1 (13) 65.9 (211) 30 (96) 16.7 (5) 83.3 (25)

South Ocean 8.3 (1) 83.3 (10) 8.3 (1) 0.0 100.0 (52) 

61 
Table 24. The proportion of animals retained in trawls that likely drown, based on tow durations 

from NMFS/Foundation sampling data (bold type). In absence of data (normal type), it is 

assumed that the mortality rates in inshore waters are the same as in nearshore (Gulf) or ocean 

waters (Atlantic) and that the distribution of tow duration for the North subregion of the Atlan

ortality rates, are the same as for the Central s

tic,

and therefore m ubregion of the Atlantic.

Area Season 

/Subregion 

Depth 

Stratum Summer Winter 

Gulf of Mexico

Eastern Inshore 0.8899 0.9842

Nearshore 0.8899 0.9842

 Offshore 0.9351 0.9885 

Western Inshore 0.9146 0.9826 

 Nearshore 0.9146 0.9826 

 Offshore 0.9588 0.9978 

Atlantic 

North Inshore 0.7303 0.8537 

Ocean 0.7303 0.8537 

Central Inshore 0.7303 0.8537 

Ocean 0.7303 0.8537

South Inshore 0.4055 0.9930 

Ocean 0.4055 0.9930

Estimated Mortalities

To est um rtal p st a tha

s would result in he animals being retained in the nets due to TED failure. Secondly, 

we u sage o erb pro

the re esca e the that a remain exce

atherbacks and loggerheads, could escape through the openings existing in conventional TEDs. 

ext we allowed all turtles, except leatherbacks, to escape from TEDs in South Carolina waters 

(defined as all of zone 32 and 48% of zone 3325). We assumed that no leatherbacks could escape 

through conventional TED openings. Those loggerheads remaining were subjected to the size 

test – 75% of the loggerheads in the Gulf would be retained and 2.3% of the animals in the 

Atlantic would be retained. The number of lethal interactions was based on the number of 

animals of each species retained – the sum of those retained as a result of the 3% TED failure 

and those too big to fit through TEDs. The proportion of animals retained in trawls that likely 

died, based on tow durations from NMFS/Foundation sampling data is given in Table 24 and the 

number of lethal interactions is given in Table 25 and Table 26. 

 

imate the n ber of mo ities in shrim trawls, we fir ssumed t 3% of all 

interaction t

sed data on the u f the leath ack TED modification and allowed that portion of

maining turtles to “ pe”. W n assumed ll turtles ing, pt 

le

N

25 Trips made in zone 33 and landed in South Carolina represented 48.2% of all days reported fished in zone 33. 

62 
Sources of Error 

All sources of error reported in this major section culminate herein. The estimates of 

error, and thus the confidence intervals are usually extremely large. Many sources are 

unaccounted for. 

back

ensities are high or strandings are elevated, the leatherback modification has been required. 

Except for the sam failure rate, al nimals should esc se estimates do not 

account for any possible requirement u

modification. 

Another factor that will result i stimate o ties a d to try nets. 

All try nets are not required to be equip EDs. Tu ght in ts will be

retained. If tow d re brief, mor ill be min tow s increase the 

probability of deaths increase. 

Table 25. Estimate interacti ting in sea ortali rch-November, 

2001. Confidence int vals (95%) are g arentheses

Sp

There is a bias that results in overestimates of moralities. At times when leather

d

e TED l other a ape. The

nder emergency rules to use the leatherback TED 

n an undere f mortali re relate

ped with T rtles cau these ne

urations a talities w imal. As duration

d number of ons resul turtle m ties, Ma

er iven in p .

a D ecies

erback d Kemp’s ley Gre

Mexico

81 8 7

Are

/Subregion

epth

Stratum Leath Loggerhea rid en

Gulf of

Eastern Inshore 3

(0-1572)

16426 

(0-47617)

304

(0-42367)

7

 (0-551)

 Nearshore 232 

(0-957) 

10004 

(0-29000)

5057

(0-25802)

47 

(0-335) 

Offshore 430 

(0-1497)

2912

(0-14502)

1317

(0-6789)

574 

(0-4026) 

Western Inshore 80 6282 744 52 

(0-354) (1317-12514) (363-1200) (0-154) 

 Nearshore 204 

(0-933) 

16527 

(3465-32922)

1958

(955-3157)

137 

(0-406) 

 Offshore 296 

(0-1077)

710 

(0-1656)

15 

(0-45)

116 

(0-849) 

Atlantic 

North Inshore 115 

(0-344) 

285 

(136-436)

34 

(15-61) 

4 

(0-11)

 Ocean 16 

(0-49)

50 

(24-76) 

7 

(3-12)

1 

(0-2) 

Central Inshore 4 

(0-12)

18 

(13-23) 

1 

(1-2) 

<1 

(0-<1)

 Ocean 125 

(0-373) 

917 

(657-1176)

75 

(33-133)

8 

(0-25)

South Inshore 4 

(0-11)

24 

(14-34) 

8 1 

(3-13) (0-1) 

 Ocean 15 

(0-46)

222 

(130-314)

103 

(46-180)

7 

(2-14)

63 
Table 26. Estimated number of interactions resulting in mortalities of sea turtles, DecemberSpecies 

February, 2001. Confidence intervals (95%) are given in parentheses. 

Area

/Subregion

Depth 

Stratum Leatherback Loggerhead Kemp’s ridley Green 

Gulf of Mexico

Eastern Inshore 18 

(0-76)

795 

(0-2306)

402 

(0-2052)

4 

(0-27)

 Nearshore 39 

(0-163) 

1699

(0-4925)

859 

(0-4382)

8 

(0-57)

Offshore 194 

(0-677) 

1316

(0-6552)

595 

(0-3067)

259 

(0-1819) 

Western Inshore 2 

(0-9) 

164 

(34 -327) 

19 

(9-31)

1 

(0-4) 

 Nearshore 39 

(0-178) 

3157

(662-6288)

374 

(163-603)

26 

(0-78)

 Offshore 78

(0-284) 

437 

(0-1279)

4 

(0-12)

11 

(0-32)

Atlantic 

North Inshore <1 

(0-1) 

1 

(1-2) 

<1 

(0-<1)

<1 

(0-<1)

 Ocean 1 2 

(0-2) (1-3) 

<1 

(0-<1)

<1 

(0-<1)

Central Inshore <1 2 <1 <1 

(0-1) (1-2) (0-<1) (0-<1)

Ocean 21 167 

(1 )

14 

(

2 

(0-64) 20-214 4-23) (0-5)

2

(0-5)

11 

(6-15)

4 

(2-6) 

<1 

-<1)

12

(0-35)

168 

8-238)

78 

5-136)

5 

-11)

in Trawls ippe

to e nge requ

ter consi n of pu commen

ments (F

ents NM promul g a final r at will r re TED

A ean and Ge

ped w atherba

ore wate f both re f m

≥ 20 inc eight. , assum

roperly omplia nd that la forcem

d be abl ape th

traw ed

South Inshore

(0

 Ocean 

(9 (3 (1

Estimated Mortalities Equ d with TEDs With Larger Escape Openings 

An advance notice of a proposed rulemaking, ffect a cha in TED irements, 

was issued by NMFS (Federal Register, 2000). Af deratio blic ts, NMFS

advertised a proposed rule to change the TED require ederal Register, 2001). In 

response to those comm FS is gatin ule th equi s used in

trawls in the tlantic oc in some orgia and South Carolina embayments and TEDs 

used in the Gulf of Mexico to be equip ith a le ck modification year round26. TEDs 

used in insh rs o gions will be required to have an opening that i easured 

simultaneously would be ≥35 inches in width and hes in h Thus ing that all

TEDs will be installed p , that c nce will be 100%, a w en ent will 

allow no tolerance of smaller openings, the expectation is that, based on the sizes of the TED 

openings, all turtles encountered in the ocean shoul e to esc e trawl nets and that all 

but the infrequently encountered leatherback will be excluded from l nets us in inshore

 

26 Robert Hoffman, National Marine Fisheries Service, St. Petersburg, FL. Personal communication (E-mail) 

October 15, 2002 to Sheryan Epperly, National Marine Fisheries Service, Miami, FL. 

64 
waters. There still will be TED failures (3%). Our estimates of mortality under the new rule a

based solely on this 3% failure rate (Table 27 and Table 28) of current interactions and the 

proportion of individuals that will die, based on tow times in each stratu

re 

m.

 

Table 27. Estimated number of intera esult turt ities, -November, 

2001, under new regulations. Confidence intervals (95%) are given in parentheses. 

pecies

ctions r ing in sea le mortal March

Depth S

atherbac oggerhead mp’s ridle Green

Gulf of Mexico

Eastern

(0 ) (0 (0- 7) (0 ) 

Inshore 11

-47

651 

-1886)

8304

4236

77 

-551

7

(0-29)

396 

(0-1149)

5057

-25802

47 

(0-335) 

 Offshore

(0

14

-50) (0-642) (0-6789) (0-4026) 

129 1317 574 

Western I 2

(5 (3 )

nshore 2

(0-11)

49

2-496)

744 

63-1200

52 

(0-154) 

N

(13 ) (9 )

earshore 6

(0-28)

655 

7-1304

1958

55-3157

137 

(0-406) 

Offshore 10

(0-35)

30 

(0-70)

15 

(0-45)

116

(0-849) 

Atlantic 

North Inshore 4

(0-11) (81 (15 ) (0 )

171 

-261)

34 

-61

4 

-11

1

(0-2) 

33 

(16-51) 

7 

(3-12)

1 

(0-2) 

<1 13 1 <1 

Area 

/Subregion Stratum Le k L Ke y

 Nearshore

(0 )

 Ocean 

Central Inshore

(0-<1) (9-17) (1-2) (0-<1)

 Ocean 8 823 75 8 

(0-25) (590-1056) (33-133) (0-25)

South Inshore <1 14 8 1 

(0-<1) (8-19) (3-13) (0-1) 

 Ocean 1 

(0-4) 

181 

(106-256)

103 

(46-180)

7 

(2-14)

65 
Table 28. Estimated number of interactions resulting in mortalities of sea turtles, December

ebruary, 2001 under new regulations. Confidence intervals (95%) are give

-

n in parentheses.

Species 

F

Area 

/Subregion

Depth 

Stratum Leatherback Loggerhead Kemp’s ridley Green 

Gulf of Mexico

Eastern Inshore 1 

(0-2) 

32 

(0-91)

402 

(0-2052)

4 

(0-27)

 Nearshore 1 67 859 8 

(0-5) (0-195) (0-4382) (0-57)

 O 7 58 595 259 

(

ffshore

(0-23) (0-290) (0-3067) 0-1819)

Western Inshore <1 

(0 )

7 

(

19 1 

-<1 1-13) (9-31) (0-4) 

Nearshore

(0 (2 (1 ) (0 )

1

-5) 

125 

6-249)

374 

63-603

26 

-78

3

0-9) 

19 

0-54)

4 

(0-12)

11 

0-32)

North Inshore <1 

(0-<1)

4 

(0-11)

<1 

(0-<1)

<1 

(0-<1)

Ocean <1

(0-<1)

1 

(0-2) 

<1 

(0-<1)

<1 

(0-<1)

Central Inshore <1

(0-<1)

1 

(1-2) 

<1 

(0-<1)

<1 

(0-<1)

Ocean 2 

(0

149 

(10 1)

 14 

(4 )

2 

-5) 7-19 -23 (0-5) 

South Inshore <1

(0 ) (

 

-<1 (2 ) (0 )

6 

4-9) 

 4 

-6

<1 

-<1

1

0-3) 

137 

-194)

78 

-13

5 

-11

 Offshore

( ( (

Atlantic 

 Ocean 

( (80 (35 6) (1 )

66 
Estimated Number of Individuals Interacting with Shrimp Trawls

Turtles not drowned in trawls may be caught again, i.e., it is likely that an individual 

turtle may interact with s w iv ggregated 

and although use of neritic habitat b i

sea turtle species appear to exhibit site fidelity, restricting their act s to preferred foraging 

areas. Immature hawksbills foraging on reefs have und t area ng from

0.01-0.21 km over an 11-16 d period (van Dam and , 1998).

turtles monitored using sonic telemetry for several in ins ers o d home

ranges betwe n 0.48-5.0 endo 83). Kem y tu owed using

radio and sonic telemetry ted the ging a to ar ng 30 km2

(Schmid, 2000). The 10-80 km foraging ranges estimated for juv oggerheads tracked 

using radio telemetry for 2-66 d (mean = 26.5 d) in a coastal bay are far larger than those found 

for other turtle species (Byles, 1988). However, mark-recapture data indicate that juvenile 

loggerheads in sub-tropical and temperate areas do exhibit site fidelity, as turtles often are 

recaptured at specific locations within a given year, as well as between years, aving 

undergone seasonal migrations (Avens et al., in review Byles, 198 During a rk-recapture 

study spanning four years, an average of 21% of juvenile loggerheads captured in Core Sound, 

North Carolina, and released near their capture locations were recaptured durin e same year in 

which they were initially tagged and at the same gen cation in which they were originally 

caught (Ave , in re Furthe re, between 4% and 21% of juvenile loggerheads 

tagged in North Carolina within a given year were recaptured in subsequent years, presumably 

after having migrated away from the capture area onths (Avens et al., in review). 

 

Site fidelity, or a preference for a specific home range, can also be inferred by the tendency 

of animals to return to restricted areas after being displaced from those locations (Papi, 1992). 

Such homing behavior suggests a stron ilecti iven ciall resources at

that site can be found elsewhere in the habitat, such as near the areas in which the animals were 

leased. Green turtles displaced from their feeding sites in Bermuda and followed using sonic 

telemetry exhibited a strong tendency to return to preferred feeding areas (Ireland, 1980). Markrecapture data show that juvenile loggerheads displaced from capture sites in sub-tropical and 

temperate areas will also return to their capture areas (Lutcavage and Musick, 1985; Byles, 

1988). Over the course of a four-year study, 17% of juvenile loggerheads displaced 15-20 km

from capture sites in the inshore waters of North Carolina were recaptured during the same year 

they were displaced in the same general area in which they were originally captured (Avens et 

al., in review). 

Other tagging studies indicate recaptures in shrimp trawls are likely. Of 68 loggerhead 

turtles tagged by a single contracted trawler fishing off central and northeast Florida, 1986-1991, 

and recaptured, 69% were recaptured in shrimp trawls, most within a year of initial capture 

(Schmid, 1995); 89% of the recaptured Kemp’s were caught in shrimp trawls. Henwood (1987) 

reported on the results of a relocation experiment which used a shrimp trawler in the vicinity of

Cape Canaveral. Over a 132 day period in 1980, 1097 loggerhead turtles were tagged and 

relocated several kilometers away from the capture site. Subsequent to release, 183 individuals 

were recaptured: 146 were captured once, 22 were recaptured twice, 11 were recaptured 3 times,

hrimp tra ls more than

y sea turtles

once. Shrim

s not well understood, individuals of various 

p trawling act ity is a

ivitie

been fo o inhabit s rangi

2 Diez Similarly, juvenile green 

months hore wat ccupie

e 6 km2 (M nca, 19

ir ra

Juvenile

ctiv ties

p’s ridle

eas ngi

rtles foll

restric from 5 to

2 

fo i ra

enile l

after h

; 8). ma

g th

eral lo

ns et al. view). rmo

during winter m

g pred on for a g site espe y if the

re

67 
3 were lded

40 recapture events (21.9%).

In

recaptured four times, and 1 was recaptured 5 times during the experiment. This yie

27 2

summary, it appears that at least 20% of the turtles involved in non-lethal interactions

subsequently will be recaptured. The number of individuals represented by the interactions can

be estimated as the sum of the number of lethal interactions and the number of individuals 

represented in the non-lethal interactions. The latter is estimated as the total non-lethal 

interactions reduced by the expected recapture rate (>20%). 

Sources of Error 

l the

The estimate of 20% likely is biased quite low because the studies above generally 

worked with just a single fishing operation and did not receive recapture information from al

fishing operations in the vicinity. Such a low estimate of the recapture rate would have the 

effect of overestimating the number of individuals actually interacting with the gear. 

 

27 National Marine Fisheries Service. Pascagoula Laboratory, Pascagoula, MS. Unpublished Data. Larisa Avens, 

National Marine Fisheries Service, Beaufort, NC. Personal Communication (E-mail) November 4, 2002 to Sheryan

Epperly, National Marine Fisheries Service, Miami, FL. 

68 
 Interactions In Other Gears And The Bait Shrimp Fishery 

Throughout the southeast region a variety of gear types other than otter trawls are used

commercially harvest table and bait shrimp. In this section we describe various gear types and 

level of use for which landing and effort data may not be recorded, or which may be combi

with data for otter trawls. The gear described is used almost exclusively within inshore

Additionally, all of the gear types described are exempt from

to

ned

waters.

federal TED regulations, although

there are some state TED requirements for specific gear. Skimmer trawls, butterfly nets, pusher 

stick rigs), and licensed bait shrimpers must adhere to tow time restrictions. 

Tow time limits for TED exempt gear are, 55 minutes from April1 to October 31 and 75 minutes 

from

tter Trawls Used for Bait Shrimp

head trawls (chop

 November 1 to March 31.

O

Otter trawls are the most common method of harvesting bait shrimp within the southeast 

U.S. coastal states. Table 29 provides bait shrimping effort by gear type for Florida and Georgia 

during 2001. Trawl types used for bait shrimping may vary from flat nets to high opening 

mongoose trawls, depending on the season and targeted shrimp species. State regulations vary 

with regard to the size of gear and time of fishing. Commercial bait shrimpers using otter trawls 

are exempt from TED use under the assumption that they are towed for short durations of time to 

ensure that the shrimp can be harvested alive. Under federal law, a bait shrimp trawl is allowed a 

TED exemption if the vessel has a valid state bait shrimping license on board, has no more than 

32 pounds of dead shrimp, and also has a container with circulating sea water system. 

Sea turtles are susceptible to capture in bait shrimp trawls, but are assumed to have a high 

rate of survival because of short tow durations. When high opening trawls are used to harvest 

bait shrimp, the potential for turtle capture may be greater. 

North Carolina 

Otter trawls are not used for harvesting bait shrimp in North Carolina28. 

South Carolina 

Otter trawls are not used to harvest bait shrimp in South Carolina waters29. 

Georgia 

Otter trawl use for bait shrimp is relatively small in Georgia with approximately 40 boats 

participating in the fishery, but it is Georgia’s third largest marine commercial fishery. The use 

of otter trawls in inshore waters for table shrimp is prohibited, but bait shrimping with the trawls 

is allowed. Participants must fish in defined zones in the upper reaches of the sounds and tidal 

 

28 David Taylor. North Carolina Division of Marine Fisheries, Morehead City, NC. Personal communication 

November 14, 2002 to Dale Stevens, National Marine Fisheries Service, Pascagoula, MS. 29 David Whittaker, South Carolina Department of Natural Resources, Charleston, SC. Personal communication (Email) October 18, 2002 to Sheryan Epperly, National Marine Fisheries Service, Miami, FL. 

69 
creeks. Thi shrimp

nursery grounds, and to facilitate law enforcement of the fishery. All vessels except one are 

small hand retrieve operations. The largest net which may be used is a 20-ft headrope length. 

Tow tim

in

a and northeast Florida. In the year 2000, a total of 180 bait 

shrimp licenses were sold in the state, however a significant portion of these vessels may be 

sing roller frame trawls. 

Bait shrimp are harvested throughout the inshore waters of Alabama exclusively with 

otter tr

drope len

ouisiana

the number of licensed bait shrimpers 

Louisiana were available.

Texas 

Commercial bait shrimping in Texas is conducted almost exclusively with otter trawls 

waters. The season is open year round and restricted to daylight hours from

August in a range of

om the 25-ft to 40-ft. headrope length. In the year 2000, a total of 1,363 bait shrimping 

ere issued in the state of Texas. Many fishermen who target table shrimp also hold a 

bait shr

 

s was enacted primarily to establish protected no trawl areas within penaeid

es and fishing effort are relatively consistent throughout the year with a small peak in 

activity in May-June and August-October.30 In a 1998 to 2001 study of the fishery, tow times 

ranged from 4 minutes to 52 minutes, with an average of 21 minutes (Gaddis et al., 2001). 

Florida 

Otter trawls are used to harvest bait shrimp in Florida state waters. Trawl size is limited 

to no more than 60 feet as measured around the perimeter, or leading edge of the trawl. Areas

which otter trawls are used for bait shrimp are not clearly defined, but it is presumed that they 

are common to the panhandle are

u

Alabama 

awls. Bait shrimping is restricted to designated areas. A licensed bait shrimper may tow 

one trawl with a hea gth no greater than 16-ft. For the year 2000, a total of 44 resident 

bait shrimp licenses were issued for the state of Alabama (Bloom, 2001). 

Mississippi 

Bait shrimping in Mississippi is restricted by area and is conducted almost exclusively in 

river mouths and bays. A licensed bait shrimper may tow one trawl with a headrope length no 

greater than 16-ft. In the year 2000, a total of 46 bait shrimp (boat) licenses were issued. 

L

Otter trawls used to harvest bait shrimp in Louisiana are limited in size to 25-ft. as 

measured along the headrope and 33-ft. as measured along the footrope. Bait shrimp may only

be taken during daylight hours. No information regarding

in

and in all inshore

15 through March 31. Trawl size is regulated with the majority of nets with

31 fr

licenses w

imping license. 

30 Mark Dodd , Georgia Department of Natural Resources, Brunswick, GA. Personal communication (E-mail)

October 23, 2002 to Sheryan Epperly, National Marine Fisheries Service, Miami, FL 31 Texas Administrative Code, RULE §58.164, Shrimping Inside Waters--Commercial Bait Shrimping

70 
Table 29. Bait shrimp effort in 2001 (trips), reported by gear type, in Southeast U.S. waters. 

Data reported only for Florida and Georgia 

Gears 

Otter Butterfly

Beam/

Roller Hand 

Zone Trawl Net Trawl Gears32

25 0 234 6105 0 

26 0 0 0 0 

27 0 0 0 7 

28 7 79 0 3 

29 0 54 0 33 

30 658 0 0 4 

31 2089 0 0 0 

32 144 0 0 0 

 

Roller Frame Trawls

ctangular metal frame with a slotted roller along the entire lower portion of the frame (Ault et 

he gear is designed to reduce bottom damage by rolling over rather than dragging 

through r

es, and also to prevent algae and debris from entering the net and 

rushing live catch. A 1989 Florida statute requires that these bars must be spaced, not more 

inches apart. In federal waters and in those states that do not regulate roller frames, 

federal regulations allow a 4-inch spacing between the bars. In a fishing configuration, the roller 

frame trawl has a vertical opening of approximately 2 to 2.5-ft34. It is unlikely that a sea turtle 

would become entrapped within a roller frame trawl due to the required deflector bars positioned 

across the trawl mouth. Slow moving turtles, caught in the path of the gear may become

impinged against the frame for a short period and or be overrun by the gear. 

In Florida, roller frames are allowed throughout the state but only are used in areas with 

agrass and hard bottom. Roller frame gear may be found in the Big Bend area from St. Marks 

 

Use of the roller frame trawl appears to be limited to Florida with no other southeast 

states reporting this gear type. The gear is used to harvest table and bait shrimp. Relatively 

unchanged since the 1960’s, the roller frame trawl design consists of a net attached to a 

re

al., 1997). T

the substrate. Most vessels pull two frames simultaneously and winches are used fo

gear retrieval. Frames sizes range between 10 and 16 ft. with reports of frames as large as 30 ft.

in length from the St. Marks area33. Deflector bars are fixed across the mouth of the frame to 

help exclude non-target speci

c

than 3-

se

32 Cast nets and dip nets. 33 Lionel LaForce, National Marine Fisheries Service. Personal communication, November 7, 2002 to John

Mitchell, National Marine Fisheries Service, Pascagoula, MS. 34 John Mitchell, National Marine Fisheries Service, Pascagoula, MS, observation during gear evaluation dives,

1995. 

71 
to the Pasco/Pinellas county line for food and live bait; Pine Island and the southwest region fo

live bait; sparse use in the Florida Keys for live bait and Biscayne Bay for live bait. A 1997 

study, which evaluated roller frame use for b

r

ait shrimp in Biscayne Bay, reported 25 vessels 

sing this gear, with average trawling time of 25 minutes (Ault et al., 1997). 

Beam Trawls

u

The advent of the e in t rly 1900’s led to the use of beam trawls in 

some areas of the southeast U.S. shrimp fishe ut ha

conventional trawling gear (Hein an eier, 1 5). Beam rawls c be described as a shrimp

trawl net, which is attached at the m th to a r d pole, b m or fr e to maintain spread. No 

trawl boards or spreading d es ar sed. U f a bea rawl in the Southeast U.S shrimp

trawl fishery is reported only in Texas. Texas lists the beam traw

commercial bait and table shrimp harvest in inshore wate only. The gear is restricted in size to 

no more than 25-ft in total width. A beam traw ay be used as a try net in Texas and is limited 

to 5-ft in total width.35 Use a be inim ay be limited to 

pproximately 15 vessels operating in the Corpus Christi area. Vessels were observed in this area 

wing a rectangular frame with an attached trawl. The frames had a vertical opening of from 6 

le, large poly-floats along the top. The vessels were observed towing the gear 

water, so that the floats and the top of the frame were exposed at the 

surface. The trawls were not rigged with TEDs.36 These trawls may be used on a very limited 

basis a

ector

gasolin engine he ea

ry, b ve since been replaced with larger, 

d M 99 t an

ou igi ea am

evic e u se o m t

l as an allowable gear type for 

rs

l m

of am-type trawl in Texas is m al, and m

a

to

to 8 feet and multip

dead astern, in shallow

nd for harvesting bait shrimp only.37 

If used for harvesting table shrimp, beam trawls would have to be fitted with defl

bars, spaced no more than 4-inches apart, across the mouth of the trawl. If used for harvesting 

bait shrimp and thus TED exempt, the gear could potentially capture sea turtles. Assuming that

the gear is hauled with the required tow time limits, which apply to bait shrimpers, turtle survival 

rates should be high. 

Skimmer Trawls

Developed in coastal Louisiana in the early 1980’s, the skimmer trawl has gained 

widespread popularity throughout the southeastern U.S. shrimp fishery. Skimmer trawls are 

pushed by the vessel rather than towed. The trawls are always fished in pairs, from the sides of 

the vessel. An advantage of the skimmer trawl over an otter trawl is that they are more 

maneuverable, especially in small bays and bayous and can fish more selectively, i.e. along

hannel edges. An additional advantage of the gear is that while retrieving the codend, the frame, 

 

c

35 Texas Administrative Code, RULE §58.164, Shrimping Inside Waters--Commercial Bait Shrimping 36 Lionel LaForce, National Marine Fisheries Service. Personal communication October 29, 2002 to John Mitchell, 

National Marine Fisheries Service, Pascagoula, MS. 37 James Nance, National Marine Fisheries Service, Galveston, Texas. Personal communication (E-mail) November 

5, 2002 to John Mitchell, National Marine Fisheries Service, Pascagoula, MS. 

72 
or mouth of the trawl, remains in a fishing configuration, thus little effort and time is required to

dump the catch. 

The trawl is held open by a metal framework and is fished on the bottom. A skimmer 

awls consist of an “L” shaped frame constructed from metal pipe with a shoe or skid on the 

outboard leg. The outboard wing edge and headrope of the trawl is attached around the 

frame. The inboard wing edge of the trawl is sewn to a line suspended from the 

frame and terminates at a weight or bullet, which, when deployed, rides slightly off the bottom.

A chain e

he

na,

h

from TEDs under the assumption that the trawl 

ags were typically retrieved at intervals that would not be fatal to sea turtles. The incidental 

capture

The only southeast Atlantic state reporting skimmer trawl use is North Carolina. 

Introduced to North Carolina in the early 1990’s (Hines et al., 1993), the skimmer trawl has 

choice for the commercial harvest of shrimp and recently has been tried for 

arvesting crab. Skimmers are used most often for white shrimp during mid-summer to fall in 

Pamlic % of

ay 

Bay only. It has been reported that in the near future, the allowable use area will be enlarged to 

 

tr

perimeter of the

ed footrope and tickler chain are used. When fishing, the outboard shoe rides along th

bottom, allowing the trawl to rise and fall with the bottom contour. The vertical height of t

skimmer trawl varies depending on the target shrimp species, and may be as much as 12-ft in 

overall height. 

Skimmer trawls are used exclusively in inshore waters in all states where the gear is 

allowed. Originally designed to catch white shrimp by fishing the entire water column, today 

skimmers may also be rigged with low opening nets and are used to target brown shrimp38. 

Within the last decade, an increasing number of inshore fishermen in North Carolina, Louisia

Mississippi and Alabama have either fully converted their vessels from otter trawls to skimmer 

rigs, or switch out their gear on a seasonal basis. The TED exemption likely has caused many 

Louisiana fishermen to covert their gear to skimmer trawls (Horst and Holloway, 2002). 

Skimmer trawls are exempt from TED regulations and must be fished in accordance wit

tow time restrictions. Skimmers were exempted

b

 of sea turtles in skimmer trawls has been documented in North Carolina (Coale et al., 

1994; NMFS unpublished data39). Because skimmers are typically rigged to fish higher in the 

water column, the potential for turtle capture may be greater than a lower opening otter trawl. 

North Carolina 

become the gear of

h

o and Core Sounds, south to New River inlet. It is reported that as many as 30 to 40

vessels fishing in these waters are using skimmer trawls. Skimmer trawl use in Core Sound m

be as high as 90%.40

 

Florida 

Skimmer trawls are listed as an allowable gear type for shrimp harvest in Apalachicola 

38 Jack Forrester, National Marine Fisheries Service, Pascagoula, MS. Personal communication October 29, 2002 to

John Mitchell, National Marine Fisheries Service, Pascagoula, MS. 39 Joanne Braun-McNeill, National Marine Fisheries Service, Beaufort, NC. Personal communication (E-mail) 

October 24, 2002 to Sheryan Epperly National Marine Fisheries Service, Miami, FL. 40 Parks Lewis, North Carolina Division of Marine Fisheries, Morehead City. Personal communication November 

14, 2002 to Nick Hopkins, National Marine Fisheries Service, Pascagoula, MS. 

73 
include most of the northwest region of Florida.41 Skimmer trawls are restricted to 500 square 

feet of mesh area with a maximum of two nets per vessel. 

Alabam

o

timates 

Mississippi does not license commercial shrimp vessels by gear type. Estimates on 

skimm ear

is

ls,

Skimmer trawls are used extensively throughout inshore Louisiana. Commercial license 

records

me 

Skimmer trawls are not an allowable gear for harvesting shrimp in Texas state waters. 

 

a

An increasing number of skimmer trawls are being used in Alabama inshore waters. N

delineation by gear type is made when licensing commercial shrimp vessels, however es

range that between 40 to 60% of inshore vessels are using skimmer trawls on a regular or 

seasonal basis

42. Skimmer trawls must adhere to otter trawl size restrictions, or no more than 50-

ft. of overall trawl headrope length. This equates to two skimmer trawls with a frame size no 

larger than 25-ft. 

Mississippi 

er trawl use range from 60 to 75% of the vessels operating in inshore waters.43 In the y

2000, there were 577 commercial shrimp licenses issued for vessels 45-ft. in length or less. Th

figure may include some offshore vessels that would not use skimmer trawls. Skimmer traw

like otter trawls must adhere are restricted in size to no more than 50-ft. of overall trawl headrope

length. This equates to two skimmer trawls with a frame size no larger than 25-ft. 

Louisiana 

indicate they are increasing in favor over otter trawl gear. In the year 1992, the number 

of individuals holding resident skimmer trawl licenses totaled 1,836. For the year 2000, this 

number was recorded as 3,655. Skimmers are restricted in size with a maximum allowable fra

width of 16-ft. and a maximum frame height of 12-ft. 

Texas 

Butterfly Nets

Butterfly nets, sometimes called “wing nets” consist of a square metal frame that forms 

the mouth of the net. Webbing is attached to the frame and tapers back to a codend. The nets can

be fished from a stationary platform or a pair of nets can be attached to either side of a vessel. 

The vessel is then hored in a tidal current to capture emigrating shrimp, or the nets are pushed 

rough the water by the vessel (Hein et al., 1995). In Louisiana, some shrimpers use them

a wharf or platform attached to the shore in man-made passes, bayous, or canals (Horst 

et al., 2

 

anc

th

singly on

002). The primary difference in fishing butterfly nets and skimmer trawls, is that the 

former is not fished on the bottom. Butterfly nets are typically set, or pushed so that the top of 

41 Bill Teehan, Florida Fish and Wildlife Conservation Commission, Tallahassee, FL. Personal communication

November 14, 2002 to Dale Stevens, National Marine Fisheries Service, Pascagoula, MS 42 Drew Hopper, National Marine Fisheries Service, Pascagoula, MS. Personal communication November 12, 2002

to John Mitchell, National Marine Fisheries Service, Pascagoula, MS 43 David Burrage, Mississippi State Coastal Research & Extension Center, Biloxi, MS Personal communication 

November 14, 2002 to John Mitchell National Marine Fisheries Service, Pascagoula, MS

74 
the frame, and net are exposed above the surface of the water. As with skimmer trawls, the cat

may be picked up and dumped without raising the entire ne

ch

t out of the water. Vessels fishing

ith butterfly nets typically operate in the deeper parts of rivers, channels and canals, avoiding 

loping banks. 

re.

orth Carolina

rfly net use is minimal in North Carolina. Approximately 2 or 3 individuals may be 

actively

Butterfly nets are not used to harvest shrimp in Georgia.45

Florida

fly

a & Mississippi

Louisiana 

t as popular as they once were, butterfly nets are still in use in Louisiana. Many 

shermen have converted their gear to the more versatile skimmer rigs. In the year 2000, the 

numbe

 

w

gear contact with the s

Butterfly nets are exempt from TED regulations and must be fished in accordance with 

regulated tow times. Like skimmer trawls, the gear is capable of incidental sea turtle captu

Because the gear is fished of the bottom, in deeper parts of channels, the chance of turtle 

interaction with this gear may be somewhat less than skimmer gear. 

N

Butte

using this gear44 

South Carolina 

Butterfly nets are specifically outlawed in South Carolina. 

Georgia 

Florida lists “wing nets” as an allowable gear type. They are allowed for commercial 

food harvest in Biscayne Bay and live bait harvest in Volusia County (inshore waters). Butter

nets, like skimmer and otter trawls, are restricted to 500 square feet of mesh area with a 

maximum of two nets per vessel. 

Alabam

We found no reports of butterfly nets being used in Alabama or Mississippi. 

While no

fi

r of individuals holding resident butterfly net licenses totaled 1,337. Although not 

confirmed, butterfly nets are likely restricted in size similar to that of skimmer trawls with a 

maximum allowable frame width of 16-ft. and a maximum frame height of 12-ft. 

Texas 

Butterfly nets are not listed as allowable gear for harvesting shrimp in Texas. 

44 Don Hesselman, North Carolina Division of Marine Fisheries, Morehead City, NC. Personal communication (Email) November 14, 2002) to John Mitchell, National Marine Fisheries Service, Pascagoula, MS. 45 Mark Dodd , Georgia Department of Natural Resources, Brunswick, GA. Personal communication (E-mail)

October 23, 2002 to Sheryan Epperly, National Marine Fisheries Service, Miami, FL 

75 
Channel Nets

Channel nets may take different forms depending on the region or area fished. In general

they are funnel-shaped, stationary nets that are set in high flow channels, canals and rivers to 

atch emigrating shrimp. The mouth of the net is spread by attaching it to poles, stakes, anchors 

or buoy

s

d across the float line

eadrope), to no more than 80-ft in total length. 

he southeast U.S. shrimp fishery. Channel nets have been

ocumented to capture sea turtles

46. South Carolina has enacted a TED requirement for channel 

n below). This rule is the only TED requirement for fixed shrimping gear 

among

arolina

e reported.

47 Landings data 

rovided by the North Carolina Division of Marine Fisheries report a total of 185,567 pounds of 

nded by channel nets in the year 2001.

n any 

size to no more than 80-ft. in total float line length. No channel 

et when set may be unattended for more than twenty-four hours. A hard or soft TED must be 

ithin specified areas. TEDs are not required in channel nets that are set 

in Nort

labama and Mississippi

No channel net fishing for shrimp is reported for Alabama or Mississippi. 

 

c

s. The net terminates in a codend, much like an otter trawl, and is emptied by lifting it 

into a boat or taking the bag to shore if possible. No reports on the dimensions of channel nets 

were obtained in the preparation of this report, however, South Carolina fishing regulation

governing the use of this gear restrict the mouth of the net, as measure

(h

Channel nets are not described under the federal TED requirement, presumably due to the

minimal use of this gear in t

d

nets (see descriptio

the southeast coastal states.

North C

Approximately 15 to 20 individuals fishing channel nets ar

p

shrimp la

South Carolina 

The state issues no more than a total of sixty licenses for the use of channel nets i

one year. Each net is restricted in

n

used in a channel net set w

h Santee Bay.

Georgia 

No channel net fishing for shrimp is reported from Georgia48 

Florida 

Channel nets are not listed as allowable gear for harvesting shrimp in Florida. 

A

46 Joanne Braun-McNeill, National Marine Fisheries Service, Beaufort, NC. Personal communication (E-mail) 

October 24, 2002 to Sheryan Epperly National Marine Fisheries Service, Miami, FL 47 Parks Lewis, North Carolina Division of Marine Fisheries, Morehead City. Personal communication November 

14, 2002 to Nick Hopkins, National Marine Fisheries Service, Pascagoula, MS. 48 Mark Dodd , Georgia Department of Natural Resources, Brunswick, GA. Personal communication (E-mail)

October 23, 2002 to Sheryan Epperly, National Marine Fisheries Service, Miami, FL 

76 
Louisiana 

Although once used in Louisiana, channel nets are not an allowable gear for the harvest 

of shrimp. 

Texas 

Channel nets are not listed as an allowable gear for harvesting shrimp in Texas. 

Cast Nets

Cast nets are conical shaped nets, which are thrown from the shore or a boat to cover a 

bottom

eter of a 

et and out of a thimble at the apex of the cone. From the 

one, the lines are spliced to form a single line that is used for retrieval of the net. As the net is 

shrimp are entangled within the folds of the net, and are shaken out at the 

surface

There have been no reported turtle interactions with cast nets from any states. While the 

e capture exists with this gear, because the gear is retrieved almost 

immed

rimp in North Carolina.

rolina

recreational cast net

shery called the "shrimp-baiting fishery" exists. One report estimates that approximately 

it shrimp licenses are issued annually. Cast netters place bait balls (fish meal and mud) 

on the b th a total 

f 10 poles per boat per day allowed. The season is restricted to a 60-day period between 

15.

eorgia

Commercial and recreational cast netting for food shrimp is allowed. A limited access 

system was enacted in 1998, which allows a maximum of 200 commercial licenses and restricts 

net radii to 8 ft for recreational users and to 12 ft for commercial users. Commercial license 

holders may possess no more than 60 quarts of heads-on shrimp per individual boat/day. 

Restrictions on modifications to cast nets which improved their fishing efficiency may have 

reduced the number of participants in this fishery (Gaddis et al., 2002). 

 

 area where shrimp may be located. When the net is deployed, it covers a circular area on 

the bottom, the size of which is determined by the circumference of the net. The perim

cast net is weighted with multiple lead weights to maintain bottom contact. The net is cinched 

into a closed position through a series of lines attached radially along the net perimeter and 

running through the interior of the n

c

cinched together,

. Cast nets used to commercially harvest food shrimp may be as large as 12 ft 

(commercial) in radius (Gaddis et al., 2002). 

possibility of turtl

iately after deployment it is unlikely that a turtle would be harmed by the gear if caught.

North Carolina 

Cast net landings data for 2001 report a total of 289 pounds. This indicates minimal use 

of cast nets for food sh

South Ca

All cast netting in South Carolina is in estuarine areas. A large

fi

14,000 ba

ottom and then cast on the bait for shrimp49. Poles must mark bait deposits wi

o

September 1 and November

G

49 David Whittaker, South Carolina Department of Natural Resources, Charleston, SC. Personal communication (Email) October 23, 2002 to Sheryan Epperly, National Marine Fisheries Service, Miami, FL 

77 
Florida

t nets are not listed as allowable gear for the commercial harvest of shrimp in Florida, 

labama or Mississippi. 

Louisia

Cast nets are an allowable gear for the commercial harvest of shrimp. No information on 

size limits of the nets was found. In the year 2000, 580 resident dip net/cast net licenses were 

all holders of this license were cast netters as dip nets are used in Louisiana for shad 

nd herring.

, Alabama & Mississippi

Cas

A

na

issued. Not

a

Texas 

Cast nets are not listed as allowable gear for the commercial harvest of shrimp in Texas.

Other Gear

Dip Nets 

Dip nets can be described as a hand-held net consisting of a mesh bag suspend

circular, oval, or rectangular rigid frame attach

ed from a

ed to a handle. The incidental capture of sea 

rtles would be rare if at all, and only with juvenile animals. Dip nets are not reported as gear 

used fo

, and

ugh boats may be used as well.

eines vary in overall length and depth usually covering the entire water column of the area 

line of the seine may be floated in order to keep the panel taught. This is 

especia h

d for 

. Sea

ot been recorded. 

ach seines are listed as one of the gear types within shrimp landings database for North 

Carolin

ercial 

til

 

tu

r harvesting shrimp in North Carolina, South Carolina, Georgia, Mississippi, Alabama, 

Louisiana or Texas. Florida lists dip nets as allowable gear for recreational shrimping

restricts the size of the net to no larger than 96 inches around the perimeter. 

Seines 

Seines are single panels of mesh webbing, which are typically moved through the water 

by persons walking along a channel bank or shoreline, altho

S

being fished. The top

lly important with large seines. Seines are primarily used for harvesting fish, althoug

shrimp may be a bycatch of this gear. Mesh size will vary according to target species. If use

shrimp, the mesh size would likely be less than 2-inches in stretched length. Sea turtle captures 

are possible with seine gear, and depending on the length, depth and mesh size of the panel, a 

turtle could become entangled by creating a “pocket” or by folding the panel around itself

turtle captures in seine gear have n

Be

a, however there were no recorded landings for 2001. In South Carolina, haul seines are

legal for commercial fish harvesting with less than 24 individuals participating in the fishery.50

They also can be used to harvest shrimp in Georgia. Florida lists a beach or haul seine as 

allowable recreational shrimping gear. Seines are not listed as allowable gear for the comm

harvest of shrimp in Alabama and Mississippi and Texas. Louisiana issued seine licenses un

1995 for the harvest of fish only. 

50 David Whittaker, South Carolina Department of Natural Resources, Charleston, SC. Personal communication (Email) October 23, 2002 to Sheryan Epperly, National Marine Fisheries Service, Miami, FL 

78 
Pots and Traps 

Traps or pots designed to capture shrimp are not commonly used. The traps are 

constructed of wire mesh similar to that used for crab pot construction. The most common design 

similar to that of a blue crab trap with inverted cones used to entrap the shrimp. Traps may be 

rked with a line attached to a float at the surface. Traps most likely are baited with 

fishmea

d fish pots has been documented. 

Crab pots are listed as one of the gear types within shrimp landings database for North 

Carolin

harvest is illegal in South Carolina. There are no reports of this gear being used in Georgia, 

Florida, Alabama, Mississippi, Louisiana and Texas. 

usher head trawls (chopstick rigs) 

roduced in Louisiana the early 1980’s by Vietnamese fishermen, the chopstick rig 

consist

w

ists

ls are listed as TED

empt, but must be fished in accordance with tow time restrictions. Like skimmer and butterfly 

usher head trawls are capable of incidentally capturing sea turtles, thus turtle survival is 

depend

s 

t to which skimmer trawls are being used in 

e southeast U.S. shrimp fishery is warranted. 

is

staked or ma

l. Turtles may be attracted to the bait and investigate the pot. There have been 

observations of sea turtles damaging crab trap pots, presumably to get to the bait51 Turtle 

entrapment in a pot or trap used to harvest shrimp is not likely, but entanglement in the buoy 

lines of crab, lobster, an

 

a, with a total of 105 pounds landed for the year 2001. Trap and pot gear used for shrimp

P

Int

s of a rigid or flexible frame and net that is attached to a pair of long poles mounted to the

bow of a boat. At the bottom of each pole is a skid or shoe to allow the frame and net to follo

the contour of the sea floor. The gear is pushed out in front of the vessel, allowing exceptional 

maneuverability. As with butterfly and skimmer nets, the trawl bag is retrieved and dumped 

without raising the entire net. This gear was designated illegal in Louisiana in 1984, but ex

today among approximately 4 to 6 vessels in Mississippi.52 Pusher head traw

ex

trawls, p

ent on adherence to tow times. 

Sources of Error 

Skimmer trawl use in the inshore shrimp fishery of Louisiana, Mississippi, Alabama, 

Florida and North Carolina is clearly on the rise. Because this gear is TED exempt, and 

enforcement of tow time limitations difficult, an accurate estimate of effort for this gear become

increasingly important. Although our estimates of skimmer trawl use were obtained through 

interviews with state fisheries managers, researchers and fishermen we feel they may be 

conservative. A more thorough appraisal of the exten

th

 

51 Jeff Gearhart, North Carolina Division of Marine Fisheries, Morehead City NC. Personal communication 

February 1998 to John Mitchell National Marine Fisheries Service, Pascagoula, MS. 52 David Burrage, Mississippi State Coastal Research & Extension Center, Biloxi, MS. Personal communication 

November 14, 2002. to John Mitchell National Marine Fisheries Service, Pascagoula, MS. 

79 
Discussion 

of all 

er

Loggerheads and leatherbacks will be the beneficiaries of the new regulation; their 

mortali

rn North Atlantic used estimates of annual 

rvival rates to evaluate the proposed regulation (NMFS SEFSC, 2001). Extending the TED 

ltaneously decreasing mortality on pelagic oceanic 

loggerh

mpt 

. The next approach was to allow everything to enter the 

trawl, but separate the target species from the bycatch near the codend. This is the philosophy of 

is strategy works and it will be extremely difficult to achieve a higher TED 

ffectiveness than the 97% currently realized. There is some indication, however, that the larger 

openin ced

mer 

Assuming a stable population, we do expect the new regulations to result in increased 

interactions, mostly because more turtles will escape and be available for subsequent recapture. 

As sea turtle populations begin to recover, the rates of interactions also will increase. It is 

extremely important to minimize the impact of those interactions to facilitate the growth

turtle populations. The new TED regulation will reduce mortalities significantly – 71% ov

current levels of mortality (Table 30). 

ties should be reduced 94% and 97%, respectively, over current levels of mortality. 

Under current regulations, only small juvenile loggerheads are able to escape through 

conventional TED openings and it is unlikely any leatherbacks escape. A recent stock 

assessment of the loggerhead populations of the Weste

su

benefit to the larger turtles while simu

eads would result in a positive population growth rate for the beleaguered northern 

subpopulation. This bycatch analysis does not alter that conclusion. 

There are but a few other means to reduce mortalities in shrimp trawls further. Catch 

cannot be controlled with current net designs and the current position of TEDs. The first atte

to reduce the bycatch of sea turtles in shrimp trawls was to install a panel of large mesh webbing

over the mouth of the trawl. The panel allowed shrimp to pass and excluded most sea turtles. 

The panel was heavy, though, and caused the mouth of the trawl to be drawn inward, reducing 

trawl efficiency and shrimp catch. Also, turtles sometimes became entangled in the large meshes

of the panel (Oravetz and Grant, 1986)

modern TEDs. Th

e

gs may improve upon the effectiveness. Turtle interactions and mortalities can be redu

further by decreasing effort and/or fishing power. Also, turtle mortalities in try nets, skim

trawls, beam trawls, channel nets, butterfly nets, and otter trawls used for bait shrimp can be 

reduced with the installation of TEDs, and less so, by reducing the allowed tow times. 

80 
able 30. Estimated annual number of interactions between sea turtles and shrimp trawls in the 

ulf of Mexico and southeast U.S. Atlantic shrimp fishery for food shrimp, estimated mortalities 

Estimated Estimated Estimated 

T

G

with current regulations, and estimated mortalities with proposed regulation. Confidence 

intervals (95%) are given in parentheses. Note that these estimates are subject to many sources 

of error, many unquantified. Please refer to the individual sections on Sources of Error

throughout the report. 

Species 

Interactions

With Current 

Regulations 

Mortalities 

With Current 

Regulations 

Mortalities 

With Proposed 

Regulation 

Leatherback 3090 

(0-11274) 

2311 

(0-8719) 

80 

(0-296) 

Loggerhead 163160 

(56139-337023) 

62294 

(6680-162419) 

3948 

(1221-8498) 

Kemp’s ridley 747205 

(61996-3362865) 

19972 

(1633-90074)

19972 

(1633-90074)

Green 48239 1440 

(204-300346) (3-8408) 

1440 

(3-8408) 

81 
Research Recommendations 

e new

ed action that prompted this bycatch analysis. To estimate takes, 

information on catch rates and on the effort of the fleet is required. With this information, it is 

ossible to predict the number

Evaluation of the impact of a proposed action on the protected species requires (1)

current information on the status of populations and stocks, (2) accurate estimates of sea turtle 

take currently, and (3) estimates of the number to be taken under the proposed action. Th

TED rule is the propos

p of turtles taken (mortality) by species and size of individuals 

under current and proposed TED regulations. Mortality rates are needed to determine the effect 

that the proposed ons wil pu s

For s he North only mation on status of stocks is 

from nesting beaches where numbe re coun lly. This sea turtles is

acceptable fo ales, but tells us nothing about the status of hatchlings, juveniles, 

subadults or adult males, and pr ation on distribution and abundance away from

the nesting b rom a ma nt point of ny measurable reduction in 

nesting turtles would be an Aafte tor tha ected earlier

life stages 10 rs before. Curre pulation gr s measured only on nesting 

beaches and these measurements figure prominently i ation of mortality rates used in 

sea turtle stock assessments (TEWG, 1998, 2000; NMFS SEFSC, 2001). Measures from other, 

younger life stages, are desirable, which means we must assess the in-water populations, both for 

population trends and to obtain demographic information critical to the models. 

The in-water surveys must be conducted long-term. Such surveys are costly and labor 

intensive, and could only be accomplished with an agency commitment to support them for the 

long term. NMFS currently funds a few area specific monitoring programs using gill nets, pound 

nets, and a trawler in a limited area; these studies are relatively inexpensive given the turtle 

encounter rates and are of value in monitoring trends in specific areas; however, it is the sum of 

trends from many such programs that will be required to assess the growth rates of the 

populations. To assess the status of a population or stock, the surveys should cover the entire 

range of distribution for that stock; this can be multiple surveys as mentioned above, or larger, 

comprehensive surveys. 

There are few means to conduct a comprehensive monitoring effort of sea turtles in the 

water with a single method and over the large geographic range. Two may be feasible. The first 

is random sampling with a trawler over the range of all species of interest. This type of survey 

would be expensive and time consuming and would probably require several years to cover the 

area. The advantage of this type of survey is having each captured turtle on deck for accurate 

identification, measurement, genetics, etc. The major disadvantage is expected low catch rates 

over large areas of the Gulf of Mexico and the associated lack of precision of estimates. This is a 

common problem when dealing with rare, threatened, and endangered species. 

The second possible approach is aerial surveys, but it cannot be used alone. Aerial 

surveys have proven effective in sighting some sea turtle species in the water, and such surveys 

can cover large areas in a relatively short time frame. These surveys can provide an index of 

regulati l have on the po lations or stock

cur nfor

of listed species.

ea turtles in t Atlantic, the rent i

r of nests a ted annua index of

r nesting fem

ovides no inform

eaches. Also f nageme view, a

r-the-fact@ indica of something t adversely aff

-30 yea ntly, po owth i

n the estim

82 
abundance, at least for leatherb to distribution patterns over

e population range. These surveys are less costly and labor intensive than trawl surveys. 

Results from aerial surveys are more accurate when performed by a well-trained and experienced 

team o

ral 

fisheries

anagement in the southeast, and without such programs NMFS would not be able to provide 

inform

fishery need to

e included in the study. The total effort of the fleet is needed, also, across all areas, gears and 

interes

r

e high 

f openings in TEDs being used, especially in inshore waters. TED 

sting of wild animals, as documented by underwater video, should continue to measure TED 

effectiv

acks and loggerheads, and insights in

th

f observers. The major drawback of aerial surveys is that identifications cannot be 

verified, demographic information cannot be obtained, and there is a size bias in our ability to 

sight smaller turtles. This size bias leads to underestimation of smaller species such as Kemp=s 

ridleys. This bias may be resolved by concurrent trawl surveys and aerial surveys in seve

areas, and the in-water surveys may also provide the needed demographic data. 

Long term surveys would provide the baseline information needed to monitor future 

trends in sea turtle abundance. Long term monitoring programs are the cornerstone of

m

ation required for effective management of many fish species. The need for monitoring

fish stocks is a concept that must be applied to management of endangered and threatened 

species if the agency is to meet mandates of the Endangered Species Act. Failure to monitor

stocks places NMFS in a position of conducting Sec. 7 Consultations with little or no data to 

support the biological opinions. 

In addition to the need for long term monitoring, information on the rate of turtle 

encounters by the shrimp fleet also is needed. For shrimp trawls, this information is best 

obtained through a Anaked net@ study similar to what was done by the Foundation study, but it 

needs to have a more rigorous experimental design to adequately sample all areas. Future 

opinions on the shrimp trawl fishery will need such results. All gears used in the

b

ts (commercial and recreational). If the naked net study is not completely random then 

information on the characteristics of fleet, as related to the rate of sea turtle capture, is needed fo

each strata. 

As with the long term monitoring, these surveys would be costly and time consuming. 

Unfortunately without this information, any bycatch analyses are fated to obtain the sam

errors and huge confidence intervals that we realized here. Also, before any future bycatch 

analyses are undertaken, it is essential that information be collected on compliance with TEDs 

regulations and the sizes o

te

eness. 

 

83 
Acknowledgments

lly

ig Brown, 

arina Burgos, Guy Davenport, Jack Forrester, Linda Hardy Bernstein, Nick Hopkins, Drew 

Hoppe

odd,

,

Lewis, Dee Lupton, Lees Sabo, and David Taylor (North 

arolina Division of Marine Fisheries); Lindsey Parker (University of Georgia Marine Extension 

Service lf & 

who

nony

This report is the result of the endeavors of a large number of people who provided 

assistance, data, and advice on very short notice and always with encouragement. We especia

appreciate the efforts of Joshua Bennett, Lisa Belskis, Joanne Braun McNeill, Cra

M

r, Lionel LaForce, Frank Patella, Joe Serafy, Dale Stevens, and Wendy Teas (National

Marine Fisheries Service); Mark Schexnayder (Louisiana State University Marine Extension 

Service); David Burrage (Mississippi State Coastal Research and Extension Center); Rick 

Wallace (Auburn University Marine Extension Service and Research Center); Bill Teehan 

(Florida Fish and Wildlife Conservation Commission); Homer Bryson, Julie Califf, Mark D

Jim Page (Georgia Department of Natural Resources); Nan Jenkins, Phil Maier, Sally Murphy

Ray Rhodes, and David Whitaker (South Carolina Department of Natural Resources); Jeff 

Gearhart, Don Hesselman, Parks

C

), and all the state and federal port agents. We also gratefully acknowledge the Gu

South Atlantic Fisheries Foundation, Inc. for the use of their data. We wish to thank those

reviewed this report: Alex Chester, Mark Dodd, Gabe Gaddis, Linda Hardy Bernstein, and Joe

Powers. 

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