OCS Study 

MMS 2006-005 

Fidelity of Red Snapper 

(Lutjanus campechanus) to Petroleum 

Platforms and Artificial Reefs in the 

Northern Gulf of Mexico 

Final Report 

U.S. Department of the Interior

Minerals Management Service 

Gulf of Mexico OCS Region 
OCS Study 

MMS 2006-005 

Fidelity of Red Snapper 

(Lutjanus campechanus) to Petroleum 

Platforms and Artificial Reefs in the 

Northern Gulf of Mexico 

Final Report 

Editors 

Megan B. Peabody 

Charles A. Wilson 

Prepared under MMS Contract 

1435-01-99-CA-30951-70414 

by 

Coastal Fisheries Institute 

School of the Coast and Environment 

Louisiana State University 

Baton Rouge, Louisiana 

Published by 

U.S. Department of the Interior 

Minerals Management Service New Orleans

February 2006 Gulf of Mexico OCS Region 
DISCLAIMER 

This report was prepared under contract between the Minerals Management Service (MMS) and 

Coastal Fisheries Institute, Louisiana State University (LSU). This report has been technically 

reviewed by the MMS and LSU, and it has been approved for publication. Approval does not 

signify that the contents necessarily reflect the views and policies of LSU or the MMS, nor does 

mention of trade names or commercial products constitute endorsement or recommendation for 

use. It is, however, exempt from review and compliance with the MMS editorial standard. 

REPORT AVAILABILITY 

Extra copies of the report may be obtained from the Public Information Office (Mail Stop 5034) 

at the following address: 

U.S. Department of the Interior 

Mineral Management Service 

Gulf of Mexico OCS Region 

Public Information Office (MS 5034) 

1201 Elmwood Park Boulevard 

New Orleans, Louisiana 70123-2394 

Telephone: (504) 736-2519 or 

 1-800-200-GULF 

CITATION 

Suggested citation: 

Peabody, M.B. and C.A. Wilson. 2006. Fidelity of red snapper (Lutjanus campechanus) to 

petroleum platforms and artificial reefs in the northern Gulf of Mexico. U.S. Dept. of 

the Interior, Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, 

LA. OCS Study MMS 2006-005. 64 pp.

iii 
ACKNOWLEDGMENTS 

The authors wish to thank Mr. Greg Boland in the Environmental Sciences Section of the 

Minerals Management Service in New Orleans for his guidance during this project We extend 

our appreciation to Chevron-Texaco Corporation, especially to the pilots, crews, and staff of 

South Timbalier 151 and the Leeville heliport. We also received tremendous support and service 

from the owner, captains and crews of Steve Tomeny Charters. We extend our deepest gratitude 

to Dave Nieland, James Cowan, James Geaghan, Yvonne Allen, Ed Chesney, Mark Benfield, 

and Larry Rouse for assistance in research direction, data analysis and critical review. Mark 

Miller, Andy Fischer, Scott Baker, Sean Keenan, Kevin Boswell and Lauren Mione assisted in 

field work. This research was supported by the Minerals Management Service, U.S. Department 

of the Interior through the Coastal Marine Institute of Louisiana State University under a 

cooperative agreement with Louisiana State University. 

v 
EXECUTIVE SUMMARY 

The habitat value of petroleum platforms for red snapper, Lutjanus campechanus, is poorly 

understood. However, it is widely recognized by both scientists and fishermen that the presence 

of platforms in the northern Gulf of Mexico (GOM) has affected the distribution of red snapper 

by the addition of hard substrate habitat. We evaluated the habitat value of standing and toppled 

platforms by monitoring the fidelity of red snapper to these structures with acoustic telemetry. In 

May 2003, 125 red snapper were captured with hook and line at several platforms in a 35-km2

portion of the South Timbalier oil and gas lease blocks, 50 km south of Port Fourchon, LA. 

Following anaesthetization with MS-222, an individually coded acoustic pinger was surgically 

implanted into the peritoneal cavity of each fish. After a short recovery period the red snapper 

were released at five platforms in the study area. Presences of individual snapper were recorded 

with omnidirectional acoustic receivers attached to seven platforms, and to one artificial reef, a 

toppled platform. Red snapper exhibited little movement between platforms in the study area. 

However, logistic regression showed a high initial fidelity to release location which subsequently 

decreased over time, thus site fidelity was found to be high in the short-term, but much lower in 

the long-term. This result differs from previous studies on red snapper fidelity that reported high 

fidelity over longer time spans. Red snapper recaptured outside of the study area showed little 

uniform directional movement. Estimates of instantaneous fishing mortality on this population 

were higher than those predicted by the most recent stock assessment. A Fourier analysis 

revealed a diel pattern of movement away from the structures at night, most likely for offsite 

foraging. Knowledge of red snapper fidelity to petroleum platforms will lead to more effective 

management of this species by clarifying both the specific function of these structures as habitat 

and their importance to the red snapper population in the GOM. 

vii 
TABLE OF CONTENTS 

Page 

ACKNOWLEDGMENTS .............................................................................................................. v 

EXECUTIVE SUMMARY .......................................................................................................... vii 

TABLE OF CONTENTS............................................................................................................... ix 

LIST OF FIGURES ....................................................................................................................... xi 

LIST OF TABLES.......................................................................................................................xiii 

1. INTRODUCTION ..................................................................................................................... 1 

1.1 Petroleum Platforms in the Gulf of Mexico........................................................................ 1 

1.2 Red Snapper at Platforms.................................................................................................... 2 

1.3 Red Snapper Fisheries......................................................................................................... 4 

1.4 Red Snapper Management .................................................................................................. 5 

1.5 Acoustic Telemetry ............................................................................................................. 6 

1.6 Objectives............................................................................................................................ 7 

2. METHODS AND MATERIALS............................................................................................... 9 

2.1 Study Site Description ........................................................................................................ 9 

2.2 Equipment ........................................................................................................................... 9 

2.3 Pinger Implantation........................................................................................................... 13 

2.4 Receiver Deployment........................................................................................................ 16 

2.5 Acoustic Range Test ......................................................................................................... 21 

2.6 Methods for Data Analyses............................................................................................... 21

3. RESULTS ............................................................................................................................... 25 

3.1 Storm Damage................................................................................................................... 25 

3.2 Thermocline ...................................................................................................................... 27 

3.3 Acoustic Range Test ......................................................................................................... 27 

3.4 Daily Location of Individual Red Snapper ....................................................................... 28 

3.5 Fish Recaptures ................................................................................................................. 34 

3.6 Fishing Mortality............................................................................................................... 37 

3.7 Logistic Regression........................................................................................................... 37 

3.8 Fourier Analysis................................................................................................................ 37 

3.9 Pinger Malfunction ........................................................................................................... 43 

ix 
TABLE OF CONTENTS (continued) 

4. DISCUSSION.......................................................................................................................... 45

4.1 Site Fidelity ....................................................................................................................... 45

4.2 Fish Recaptures ................................................................................................................. 48

4.3 Fishing Mortality............................................................................................................... 49

4.4 Diel Movement ................................................................................................................. 50

4.5 Pinger Malfunction ........................................................................................................... 51

5. CONCLUSIONS...................................................................................................................... 53

5.1 Management Implications................................................................................................. 53

5.2 Recommendations for Future Research ............................................................................ 56

6. BIBLIOGRAPHY.................................................................................................................... 59

x 
LIST OF FIGURES 

Figure Page 

Figure 1. Map of the study area in the MMS South Timbalier Blocks 128, 134, 135, 151, 

and 152. ............................................................................................................... 10 

Figure 2. The V8SC-2H acoustic pinger and Floy® internal anchor tag that were implanted 

into each tagged red snapper. ................................................................................ 11 

Figure 3. The VR2 acoustic receiver. ................................................................................... 12 

Figure 4. Foam-lined boxes and gill irrigation system used for the surgical procedure. ..... 14 

Figure 5. Scales were removed and an incision was made posterior to the insertion of the 

pelvic fin and midway between the pectoral and pelvic fins. ............................... 15 

Figure 6. Placement of the Floy tag and suturing of the wound........................................... 15 

Figure 7. Capture (a) and release (b) locations for tagged red snapper are shown with the 

corresponding percent of total fish captured and released at each location.......... 17 

Figure 8. Map of the final locations of acoustic receivers at seven platforms and one 

artificial reef in the study area............................................................................... 19 

Figure 9. Receiver deployment system including a weighted sled that moved along two 

guide cables and PVC pipe to which the receiver was bolted, hydrophone end 

down...................................................................................................................... 20 

Figure 10. Histogram of the total length frequency of tagged red snapper.. .......................... 26 

Figure 11. Daily locations of tagged red snapper RS 1 - 25. ................................................ 29 

Figure 12. Daily locations of tagged red snapper RS 26 - 50. .............................................. 30 

Figure 13. Daily locations of tagged red snapper RS 51 - 75................................................. 31 

Figure 14. Daily locations of tagged red snapper RS 76 - 97................................................. 32 

Figure 15. Map showing the starting and ending points for the seven red snapper that 

displayed movement between receivers. ............................................................. 33 

Figure 16. Map showing the starting and ending points for the seven red snapper that were 

recaptured at locations other than their release sites. .......................................... 36 

Figure 17. Predicted probability of presence at release location for both native and relocated 

snapper. ............................................................................................................... 40 

xi 
LIST OF FIGURES (continued) 

Figure 18. Periodograms for ST135-M (a) and ST128-R (b) showing the period and power of 

the sine functions describing red snapper movement away from the platforms. .. 41 

Figure 19. Actual values of pings/fish and LOESS predicted values of pings/fish for each 

hour of the day. The black bar below the x-axis denotes nighttime hours........... 42 

xii 
LIST OF TABLES 

Table Page 

1 Receiver deployment locations and dates............................................................................ 18 

2 Red snapper recaptured at locations other than their release sites. ..................................... 35 

3 Estimates of instantaneous fishing mortality rates from various combinations of tag 

reporting rate and instantaneous natural mortality rate (M)................................................ 37 

4 Results of the three logistic regression models: base, thermocline, and location. .............. 39 

xiii 
1. INTRODUCTION 

1.1 Petroleum Platforms in the Gulf of Mexico 

In 1942 the first petroleum platform (platform) was installed on the Outer Continental Shelf 

(OCS) of the Gulf or Mexico (GOM) (Pulsipher et al. 2001). Just over 30 years later, Dugas et 

al. (1979) claimed that there were so many platforms offshore that it was nearly impossible to 

find a place on the Louisiana coast without a view of at least a few. By 1997 almost 5,600 

platforms had been erected on the OCS, most off the coasts of Louisiana and Texas (Pulsipher et 

al. 2001). This extensive system throughout the northern GOM supplies 25% of the United 

States’ natural gas production and 10% of US oil production (Louisiana Department of Wildlife 

and Fisheries 2004). 

The substrate of the northern GOM is primarily muddy and sandy with limited natural hard 

bottom found far offshore (Render 1995); therefore platforms are virtually the only source of 

hard substrate close to shore (Stanley and Wilson 1990). Pulsipher et al. (2001) estimated that 

platforms have increased the total amount of reef habitat available by as much as 10 to 25 

percent, depending on the definition and estimate of natural reef habitat, though other estimates 

have been much more conservative (Stanley and Wilson 1997). The support pilings and cross 

members of platforms provide settling habitat for algae and pelagic larvae of many species of 

encrusting invertebrates. In addition, the platforms provide food and shelter for many species of 

reef fishes that subsequently attract larger predatory pelagic fishes (Gallaway et al. 1981; Bull 

and Kendall 1994; Fabi et al. 2002; Stachowitsch et al. 2002). The litter and shell hash that 

accumulates at the bottom of the platform also provides habitat for many organisms (Love et al. 

1999). Platforms have provided millions of square feet of solid substrate where little existed 

before (Dugas et al. 1979; Driessen 1986). Platforms on the continental shelf west of the 

Mississippi River may be especially important habitat because they rise above the nepheloid 

layer (zone of turbid water) (Render 1995). As a result of these various characteristics, platforms 

in the northern GOM serve as artificial reefs (Dugas et al 1979; Bohnsack and Sutherland 1985; 

Bull and Kendall 1994; Render 1995). A diverse community of reef and rocky bottom marine 

organisms now inhabit regions that were previously bare sandy and muddy bottom. 

Studies have shown that there is high diversity and biomass of fishes around platforms compared 

to the community that would inhabit the normally sandy and muddy substrate if platforms were 

not present. Platforms concentrate fish populations vertically and serve as vertical mixing 

grounds for normally stratified populations (Continental Shelf Associates 1982; Render and 

Wilson 1994). Sonnier et al. (1976) conducted a photographic survey of fish populations around 

platforms and artificial reefs on the Louisiana outer continental shelf and found 49 species 

associated with platforms. Continental Shelf Associates (1982) used underwater cameras to 

survey 13 platforms off the coast of Louisiana and identified 25 species; in another photographic 

survey Putt (1982) observed 35 species at Buccaneer Oil Field off the coast of Texas. From 

fishery logbook data Stanley and Wilson (1990) reported capture of over 46 different species 

around platforms off Louisiana. Wilson et al. (2003) used dual beam hydroacoustics to study 

fish biomass around natural reefs and standing and toppled platforms. They found that biomass 

around platforms was higher than that around natural reefs. 

1 
Federal and international law requires the removal of all decommissioned platforms to minimize 

hazards to navigation. Over time this removal of platforms could result in loss of recreational 

and commercial fishing opportunities, leading to user conflict and loss of tourism (Wilson et al. 

1987). In 1986 the Louisiana legislature enacted the Louisiana Fishing Enhancement Act (Act 

100-1986) that created the Louisiana Artificial Reef Program (LARP). This program was 

formed to retain at least some platforms as artificial reefs. The program also provided for 

coordinated siting of artificial reefs to avoid the proliferation of unmarked bottom structures that 

could be hazardous to net fisheries and others (Wilson et al. 1987). Since its inception LARP 

has used the well jackets of 85 decommissioned platforms to create 25 artificial reef sites off the 

coast of Louisiana (Louisiana Department of Wildlife and Fisheries 2004). 

Although artificial reefs are viewed as a useful fisheries management and enhancement tool, not 

enough is known about the life history and stock structure of fish that use these habitats to 

address specific management objectives with artificial reefs. Numerous studies have been 

conducted on a variety of artificial reef substrates. Stone et al. (1979) used automobile tires to 

construct an artificial patch reef mimicking nearby natural reefs in southern Florida. Within 

eight months of construction there were equal numbers of fish at the artificial and natural reefs, 

but greater species diversity on the artificial reefs during daylight hours as a result of pelagic 

visitors. Two years after construction, the artificial reef biomass was similar to that of the nearby 

natural reefs and the artificial reef had matured to a stage that supported fish cleaning stations 

where organisms such as French angelfish, Pomacanthus paru, and banded coral shrimp, 

Stenopus hispidus, were observed to clean larger fish such as jacks (Carangidae), and groupers 

(Serranidae). 

Buckley and Hueckel (1985) found that the density of fish on an artificial reef in Puget Sound 

was actually greater than the density on expansive natural rocky reefs due to what they deemed 

the “oasis” or “home base” effect in an otherwise bare sandy environment. In addition, the 

anglers fishing over the artificial reef retained almost two and a half times as many fish per hour 

as anglers fishing at nearby natural reefs. Bull and Kendall (1994) determined that three toppled 

platforms in the north central GOM were acting as recruitment sites for a variety of fish species. 

Szedlmayer and Shipp (1994) found that red snapper, Lutjanus campechanus, actually attained 

greater sizes at artificial reefs off Alabama compared to other nearby locations, although 

Strelcheck (2001) found red snapper length was negatively correlated to increasing reef 

densities, possibly due to less prey and more competition in greater densities of artificial reefs. 

Szedlmayer and Shipp (1994) also correlated the addition of artificial reef material to a rise in 

catch per unit effort (CPUE). 

1.2 Red Snapper at Platforms 

The historical distribution of red snapper is along the continental shelf from the Campeche Banks 

near the Yucatan Pennisula, throughout the GOM, around the southern tip of Florida, and 

northward along the Atlantic coast of the southern United States to Cape Hatteras (Rivas 1966; 

Nelson and Manooch 1982; Robins and Ray 1986). Lutjanus campechanus is usually found over 

sandy and rocky bottom, around reefs, and underwater objects at depths between 0 to 200 m 

(GMFMC 2001). 

Juvenile red snapper less than 200 mm fork length are found over bare, soft muddy and sandy 

bottoms where they are susceptible to capture in shrimp trawls. One to two years after hatch, 

2
juvenile red snapper begin to disappear from trawl samples and appear at reefs (Bradley and 

Bryan 1975; Gallaway et al. 1981; Gallaway 1984). Bull and Kendall (1994) found no age 

groups other than two to three year olds at toppled platforms designated as artificial reefs. 

Nieland and Wilson (2002) reported very few (0.68%) age-1 red snapper at platforms; the 

majority of the red snapper population (53%) was age-two fish. As red snapper grow older and 

larger, when predation becomes less of a threat, they are found more frequently over soft bottom 

and other lower relief structures such as shipwrecks, lumps, sea-bottom depressions, and natural 

reefs (Gallaway 1984; Nieland and Wilson 2002). 

Late juvenile and adult red snapper are generally considered to be reef-associated instead of reefdependent because they use reefs for some parts of their life history, but not for others; the 

dominant paradigm is that red snapper use the reef structure as protection from predators, but 

move off the reef to spawn and feed (Moseley 1965; Bradley and Bryan 1975; Stone 1978; 

Gallaway et al. 1981; Render 1995). Bradley and Bryan (1975) report that hook-and-line CPUE 

for red snapper at reefs off the coast of Port Aransas, TX, was best in the winter and lowest in the 

warmer months, corresponding to spawning season. Red snapper are believed to be 

opportunistic feeders and consume whatever is most readily available (Beaumariage and Bullock 

1976). Juvenile red snapper feed primarily on shrimp, crabs, other crustaceans and epifaunal 

benthic organisms; as size increases the diet of snapper incorporates fish with increasing 

prevalence (Bradley and Bryan 1975; Gallaway 1984). Some researchers have reported that the 

diet of red snapper does not tend to include rock or reef dwellers or members of the biofouling 

community (Gallaway et al. 1981; Gallaway 1984) engendering the hypothesis that red snapper 

move off reef to feed. McCawley (2003) found that to some extent red snapper did forage on 

water-column organisms above an artificial reef in Alabama as well as away from the reef on 

sand-associated organisms. 

The presence of red snapper at platforms has been widely documented. They have been listed as 

one of the most populous and important reef species taking shelter around these platforms 

(Hasting et al. 1976; Render 1995). Continental Shelf Associates (1982) found that red snapper 

were within the top nine species most frequently found around platforms, while Putt (1982) 

found red snapper to be within the seven most populous species around platforms in the 

Buccaneer Oil Field. In fact, in the Buccaneer Oil Field area red snapper dominated the benthic 

reef fish community to such an extent that they comprised 80% of the sample (Gallaway et al. 

1981). Commercial fishery logbooks show that red snapper are one of the most frequently 

caught species within the vicinity of platforms, and the same is true for the recreational fishery 

(Stanley and Wilson 1989; Stanley and Wilson 1990). Remotely operated vehicles have been 

used to videotape fishes around platforms, revealing that red snapper gather in large schools 

composed of mostly juveniles with some adults (Render 1995). In a study classifying and 

enumerating fish taxa after the detonation of a platform for removal, Nieland and Wilson (2002) 

found that 37% of fish mortalities collected were red snapper. Rademacher and Render (2003) 

again documented red snapper within the ten most frequently observed taxa around platforms. 

Clearly, platforms in the GOM are important habitats for red snapper (Nieland and Wilson 

2002). 

Many scientists and fishermen believe that the presence of platforms may have changed the 

distribution and abundance of many fish species. Bohnsack (1989) theorized that biological 

production of fish populations at artificial reefs would increase in areas where the artificial reefs 

3
are isolated from natural reefs, dependent upon the site-specific fidelity and life history of each 

species. In areas where artificial reefs are in close proximity to natural reefs, fishes may merely 

be attracted to the new habitat if benefits are to be gained. Whether northern GOM platforms 

form a production or attraction environment is highly disputed and depends on the specific 

species being discussed. As for red snapper, Render and Wilson (1994) and Render (1995) argue 

that, regardless of whether production or attraction is occurring, platforms have definitely 

affected the distribution of this commercially and recreationally important species. As early as 

1984, platforms constituted as much as 11% of total red snapper habitat in the GOM and 28% of 

habitat in the western GOM (Gallaway 1984). 

From the fishermen’s perspective, the presence of platforms has unquestionably influenced the 

availability of red snapper. Dugas et al. (1979) reports that before 1940, when platforms were 

not present, offshore sport fishing in Louisiana was almost nonexistent. They claim that the 

construction of platforms was the largest contributor to the growth of the now lucrative offshore 

sportfishing industry in coastal Louisiana, where “fishing the oil rigs” is a common expression. 

The majority of both the recreational and commercial fisheries center around platforms, partly 

due to their predominance, and partly due to the relative ease with which they may be located 

compared to natural reefs (Putt 1982). There is some concern that platforms may make legal size 

red snapper just above the minimum legal size more vulnerable to fishing mortality (by 

increasing catchability) and that these size and age groups may be harvested in larger proportions 

than the remainder of the population (Nieland and Wilson 2002). 

1.3 Red Snapper Fisheries 

The red snapper fishery is one of the most economically important fisheries in the GOM 

(Gallaway 1984; Wilson and Nieland 2001). It consists of highly lucrative commercial and 

recreational fisheries. A 1989 survey of recreational anglers and divers revealed red snapper to 

be among the most sought after fish (Stanley and Wilson 1989); little has changed in the past 15 

years. The net value of the Gulf-wide 2002 commercial red snapper catch (4.8 million pounds) 

was $10.7 million; the recreational fishery in 2002 landed an estimated 4.7 million pounds 

(Fisheries Information Network 2004). The commercial fishery uses a variety of gear types 

including rod and reel, bandit rigs, bottom long lines, and fish traps (to a very limited extent and 

exclusively in Florida) while the recreational fishery uses rod and reel and spear gun (Goodyear 

1995). Effort is concentrated at platforms in the northern GOM, as close to port as possible, 

although the commercial boats often travel farther offshore (50 km to 80 km) than do 

recreational fishermen (Gallaway 1984; Nieland and Wilson 2002). Recreational fishermen tend 

to have smaller boats and remain in water 30 to 45 m deep, less than 50 km offshore (Nieland 

and Wilson 2002). 

Red snapper also are taken incidentally in another lucrative fishery in the northern GOM. As 

juveniles, red snapper inhabit muddy and sandy bottoms, the same habitat used by shrimp; as a 

result, many juvenile red snapper are captured over smooth bottom by shrimp trawlers 

(Beaumariage and Bullock 1976; Goodyear 1995; Schirripa and Legault 1999; GMFMC 2001). 

Because most of these red snapper taken in shrimp trawls are very small they are of no current 

economic value. In 1988, shrimp trawl by-catch was first identified as a significant source of 

mortality in the red snapper population. In 1990, the Reef Fish Stock Assessment Panel 

(RFSAP) for the Gulf of Mexico Fishery Management Council (GMFMC) recommended closing 

4
the directed red snapper fishery because the by-catch in the shrimp fishery equaled the Allowable 

Biological Catch (ABC) for the species (GMFMC 2001). In the most recent red snapper stock 

assessment Schirripa and Legault (1999) state that the recovery of the red snapper fishery is 

more dependent on controlling the mortality inflicted by the shrimp fishery than on controlling 

the mortality derived from the directed red snapper fisheries. Thus, some consider red snapper to 

be undergoing recruitment overfishing because their movement to reefs as adults is limited by 

mortality in juvenile stages. This bottleneck in population growth may restrict the ability of 

artificial reefs to increase biological production (Bohnsack 1989). In recent years, much work 

has been done on methods to reduce the by-catch of red snapper in the shrimp fishery. By-catch 

Reduction Devices (BRDs) are required in nearly all shrimp nets in the GOM and continuing 

research is seeking to improve these designs both to minimize loss of shrimp and to maximize 

finfish escapement (GMFMC 2001). 

1.4 Red Snapper Management 

Goodyear (1995) detailed the history of the GOM red snapper fishery from the inception of a 

commercial fishery over 150 years ago, its early declines in the late 1800’s, the addition of a 

recreational fishery in the mid 1900’s, the conflict of red snapper as shrimp bycatch, to the 

current status of this heavily managed species. The highest commercial landings in the GOM 

occurred in the 1960s-1980s then began to decline; the recreational harvest declined both in 

number and biomass after 1983. Recruitment to the adult population subsequently declined 

during the mid 1980s, reached a low in 1987, but then rebounded to the highest level in ten years 

in 1990. Goodyear noted that a depletion of older snapper in earlier years may be inferred by the 

historical length frequencies which show a change in length composition towards smaller 

(younger) red snapper over time; this depletion of older, and more fecund, red snapper may have 

contributed to the decline in recruitment in the mid 1980s. Goodyear (1995) also described how 

the geographical concentration of landed red snapper has moved westward over the last few 

decades, to the extent that red snapper are now essentially commercially extinct in the eastern 

half of its historical range. 

In 1984 the GMFMC instituted management of the red snapper population with the Reef Fish 

Fishery Management Plan (FMP). Through the ensuing decades the GMFMC has tried various 

forms of management including minimum size limits, daily bag limits for the recreational 

fishery, commercial and recreational quotas, licenses for commercial boats, permits for charter 

and headboats, commercial trip limits, limited fishing seasons for both the commercial and 

recreational fisheries, and mandatory BRDs in shrimp trawls. The current regulations include a 

16 inch (approximately 41 cm) total length (TL) minimum size limit for the recreational fishery 

and a 15 inch (approximately 38 cm) TL minimum size limit for the commercial fishery. The 

recreational fishery is open from 21 April through 31 October and is allocated 4.47 million 

pounds, or 49% of the current Total Allowable Catch (TAC). The commercial fishing season is 

split into spring and fall sub-seasons with 4.65 million pounds, or 51% of the TAC, split between 

the two. The spring sub-season, opening in February, is allocated 2/3 of the commercial quota 

and the remaining 1/3 may be harvested in the fall, beginning in October. The seasons are 

limited to the first ten days of each month until the quota for that sub-season is filled (GMFMC 

2001). And although the commercial fishery is no longer the derby fishery it was in 1992 when 

the season closed after only 53 days, it still resembles a derby, or mini-derby, since fishermen 

rush to land as many trip limits as possible during the first 10 days of each month while the 

5
fishery is open (GMFMC 2001; Nieland and Wilson 2002). The GMFMC is currently 

considering establishing an Individual Transferable Quota (ITQ) system for the commercial red 

snapper fishery (68 Fed. Reg. 75,202 (December 30, 2003)). 

The red snapper stock assessment in 1999 showed the stock to be in the same condition as the 

previous assessment by Goodyear in 1995 (GMFMC 2001). After 20 years of management red 

snapper is currently overfished and still undergoing overfishing (Schirripa and Legault 1999). 

Although the adult population has apparently remained stable, the Fall Groundfish Survey and 

Summer SEAMAP Survey show recruitment to have increased through the 1990s (GMFMC 

2001). A high degree of uncertainty remains about the status and recovery of the red snapper 

stock. The 1998 RFSAP attributes this uncertainty to ambiguity in the shrimp trawl by-catch 

reduction levels, in the spawner-recruit curve, and in the constant catch vs. constant fishing 

mortality rate scenarios (GMFMC 2001). 

1.5 Acoustic Telemetry 

From the time of its inception in the late 1950s acoustic telemetry for fisheries research has 

evolved into a sophisticated and diverse science (Winter 1983). Researchers have used telemetry 

in many ways from assessing school fidelity and site fidelity (Klimley and Holloway 1999) to 

documenting the dynamics of spawning aggregations (Zeller 1998). The science of acoustic 

telemetry is relatively simple. Acoustic transmitters send out a high-frequency, ultrasonic “ping” 

that is then detected by a hydrophone. The ping is then either recorded by a data logger or 

relayed directly to a listening user. 

A wide variety of acoustic telemetry equipment is now used in scientific studies. Transmitters 

are available as simple ‘pingers’, which emit a signal at prescribed intervals, or as 

‘transponders’, which relay coded information after receiving a signal from sonar. Acoustic 

transmitters can be individually coded to differentiate among individual subjects by changing the 

frequency or the pulse repetition rate (Arnold and Dewar 2001). Transmitters may even include 

additional sensors to detect light intensity, water pressure, water temperature, and internal body 

temperature (Sibert 2001). Acoustic transmitters can be attached externally or implanted 

internally through surgery or oral insertion and have been implanted into fish as small as 40 

grams (Eklund and Schull 2001; O’Dor et al. 2001). 

Hydrophones can be deployed in fixed or mobile systems, and in single or multiple arrays. 

Fixed hydrophone systems can be used to investigate site fidelity or small-scale movement. 

Mobile hydrophone systems (typically deployed from boats or ships) may be used to track 

animals over longer distances (Arnold and Dewar 2001). Hydrophones can be mounted singly to 

detect presence or absence, or in arrays so they may triangulate and estimate a spatial location of 

the transmitter (Sibert 2001). They are available as omnidirectional, detecting in 360° around, 

and directional, which block out noise from most directions while focusing on a specific bearing 

(Winter 1983). 

Sound propagates very well in saltwater, but many obstacles to the acoustic signal may be found 

in the marine environment. Acoustic signals, or pings, can be deflected, obscured, and absorbed 

by solutes, suspended matter, plankton, fish, air bubbles, thermoclines, water turbulence, 

raindrops, wind, wave action, boat motors, submerged structures, and even biological noise like 

snapping shrimp (Winter 1983; Wolcott 1995). The range of detection for a 

6
hydrophone/transmitter combination depends partly upon the size, strength, and frequency of the 

acoustic transmitter (lower frequencies propagate further) and also the level of ambient noise in 

the environment (Klimley et al. 1998). 

Acoustic telemetry offers many benefits over direct observation and conventional tag and 

recapture techniques. In many situations, direct observation of subjects may not be possible or 

may alter the behavior of the subject (limitations of SCUBA and live diver disturbance) (Wolcott 

1995; Zeller 1997). Acoustic telemetry allows fishery independent data collection while 

avoiding possible bias from a subject’s reaction to the presence of a research vessel (Arnold and 

Dewar 2001). Continuous tracking technology (as opposed to discrete tracking) affords the users 

the ability to investigate diel movements, habitat utilization, and site residence times (Eklund and 

Schull 2001). Traditional mark and recapture studies, where fish are usually harvested 

permanently after recapture, are limited to collection of two data points: release location and 

capture location (Klimley et al. 1998). Most such studies are also dependent upon the 

cooperation and accuracy of fishermen for data collection. In some cases, the researchers 

perform recapture and subjects are released and recaptured multiple times (Watterson et al. 1998; 

Patterson et al. 2001). Even this type of mark and recapture study usually provides only vague 

descriptions of movement of a mobile subject (Hart and Summerfelt 1975). 

Acoustic tracking does have some drawbacks. The equipment is substantially more expensive 

compared with simple external tags used in traditional mark and recapture studies; acoustics 

transmitters cost hundreds of dollars apiece, and hydrophones cost thousands. Manual tracking 

studies, where the subject is followed by boat with the assistance of a directional hydrophone, 

are very demanding of manpower and funding, generally limiting studies to less than a week and 

risking disruption by rough weather (Klimley et al. 1998). 

In addition, most transmitter attachment methods are extremely invasive, requiring surgical 

implantation or gastric insertion. External attachment is possible, but may cause more 

complications for which the fish must compensate (Winter 1983). Externally attached tags may 

cause increased drag and affect swimming speed and energy expenditure. They may also cause 

abrasions or snag on objects and dorsally attached tags can disrupt balance. Internal 

implantation avoids many of the problems associated with external attachment (Winter 1983). 

Tags are generally placed below the center of gravity negating the previously mentioned balance 

problems. The best option for internal implantation is surgical, but these procedures are very 

invasive, take longer to perform, require more handling, require anesthesia and longer recovery 

periods, and bring the risk of infection especially in warm water (Winter 1983; Klimley et al. 

1998; Thorsteinsson 2002). Gastric implantation, by force-feeding, is another option, although 

tags are not permanently retained. Gastric implantation may affect the feeding frequency of the 

subject and tags can be regurgitated (Hart and Summerfelt 1975). 

1.6 Objectives 

Red snapper is one of the predominant species at platforms in the northern GOM. This species is 

the target of lucrative and heavily managed fisheries and the petroleum industry in the GOM is 

economically-important to many of the Gulf States and the U.S. as whole. Effective 

management regulations for the red snapper fisheries and the petroleum industry must include 

information about the habitat preferences of red snapper as they relate to platforms. It is evident 

that platforms and artificial reefs (often toppled platforms) act as some sort of habitat for red 

7
snapper in the northern GOM, but the importance of these artificial habitats to the red snapper 

population is unknown. In order to successfully manage this species we must consider its use of 

platforms as habitat and determine the level of importance of these platforms to the ecology of 

red snapper. Advances in acoustic telemetry have resulted in a valuable technology that permits 

continuous tracking of specimens and can yield more detailed and accurate information on red 

snapper usage of platforms. Therefore the overall objective of this study was to determine what 

could be learned about red snapper at platforms through acoustic telemetry. The null hypothesis 

was that red snapper do not show any movement away from or between platforms, residing at 

one platform on a continuous and long-term basis. The alternate hypothesis was that red snapper 

do exhibit movement between various platforms. More specific objectives were examined to 

determine how acoustic telemetry could be used to detect movement of red snapper and if any 

movement was found, to describe it. 

These specific objectives were as follows: 

• To develop a protocol for at sea surgical implantation of acoustic transmitters into red 

snapper. 

• To use acoustic telemetry to investigate the short- and long-term site fidelity of red 

snapper to petroleum platforms and artificial reefs on the OCS of the GOM off the coast 

of Louisiana, thereby evaluating the habitat value of these platforms. 

• To determine if uniform directional movement is undertaken by red snapper departing 

from the study area. 

• To estimate fishing mortality on the red snapper population. 

• To determine if any diel movement patterns are exhibited by red snapper around 

petroleum platforms and artificial reefs. 

8
2. METHODS AND MATERIALS 

2.1 Study Site Description 

Approximately 50 kilometers south of Port Fourchon, Louisiana, is a cluster of platforms often 

referred to locally as “The Circle,” due to the circular placement of 12 platforms around a salt 

dome (Figure 1). The area is part of the MMS South Timbalier Blocks 128, 134, 135, 151, and 

152 held primarily by ChevronTexaco Corporation, who first erected platforms there in the 

1960’s then again in the 1980’s. The area was chosen due to its unique configuration, the close 

proximity of 12 standing platforms and numerous artificial reefs within the 35 km2

 area, and the 

high frequency with which commercial and recreational fishermen visit the area. The closest 

platforms to this area are approximately eight km to the east in the South Timbalier 130 lease 

block. 

The majority of the platforms in The Circle are 6- and 8-pile structures with the exception of the 

ST151-Y complex that consists of four individual platforms connected by catwalks and provides 

living quarters for the field crew. The water depth in this area ranges from 30-42 meters and the 

large-scale current direction in this region is westerly (personal communication, Larry Rouse, 

Louisiana State University, 11 March, 2004). The study area is farther offshore than the normal 

extent of the hypoxic zone formed during the summer months by nutrient and freshwater input 

from the Mississippi River (Rabalais et al. 2002). The substrate in this region is muddy with the 

exception of the litter and shell hash that has accumulated within the immediate vicinity of the 

platforms. 

2.2 Equipment 

The telemetry system consists of transmitters and hydrophones manufactured by VEMCO LTD 

(100 Osprey Drive, Shad Bay, Nova Scotia, Canada, B3T 2C1). The transmitters were model 

V8SC-2H 4K pingers with a random pulse train delay of 150 to 300 seconds and predicted 

battery life of over 400 days (Figure 2). All pingers operated at 69 kHz, were 9 mm in diameter 

by 30 mm in length, weighed 5 grams in air, and each had a unique signal determined by the 

timing between the seven pulses in the train. 

The hydrophones were a component of a multipart system called a receiver, the VR2 model, 

which is a combination of an omnidirectional hydrophone, a receiver, an ID detector, a data 

logging memory, and a battery all housed in a submersible case 34 cm in length and 6 cm in 

diameter, weighing approximately 1.2 kg (Figure 3). The receiver memory is capable of storing 

300,000 detections and the replaceable batteries have a life of six to eight months. Data is 

downloaded to a laptop computer via a magnetic probe, allowing easy download in the field. 

In addition to the internal pinger, all red snapper were fitted with a Floy® internal anchor tag 

(FM-95W) (Figure 2). Printed on the internal and external sections of each tag were the tag 

number, an offer of a reward, and a 1-800 telephone number. Fishermen who returned the tags 

were given both $10 and an LSU Red Snapper Research fishing cap. Fliers publicizing our 

research efforts were posted in businesses near the coast; charter boat businesses known to 

frequent the study area were also notified of the project. In addition, newspapers in Baton 

Rouge, New Orleans, Lafayette, and Lake Charles, LA published articles about the project in 

their outdoors sections. 

9
Figure 1. Map of the study area in the MMS South Timbalier Blocks 128, 134, 135, 151, 

and 152. The inset in the upper right corner shows the location of the study 

area with respect to the coast of Louisiana. 

10
Figure 2. The V8SC-2H acoustic pinger and Floy® internal anchor tag that were 

implanted into each tagged red snapper. 

11
Figure 3. The VR2 acoustic receiver. 

12
2.3 Pinger Implantation 

On 21, 22, 28, and 29 May 2003, acoustic pingers were surgically implanted in the peritoneal 

cavities of red snapper captured at platforms in the study area. All specimens were caught with 

hook and line and slowly brought to the surface to minimize depth-related decompression 

complications such as over-inflated air bladders and distended stomachs. Only red snapper 

between 28 and 47 cm TL were retained for tagging. Any fish with either its stomach protruding 

from the mouth or its intestines protruding from its anus was returned to the water. A 16-gauge 

hypodermic syringe needle sterilized in a dilute providone iodine solution was used to puncture 

and deflate each fish’s air bladder through the sidewall of the body a few scales below and 

behind the tip of the opercular flap (personal communication, Edward Chesney, Louisiana 

Universities Marine Consortium, 20 March, 2003). All specimens were held in an aerated, flow 

through tank and monitored for any ill effects of handling; those red snapper not recovered and 

swimming normally in the tank were not retained for tagging. 

Red snapper showing no major trauma were moved to a tank with a solution of MS-222 (3-

aminobenzoic acid ethyl ester or tricaine methanesulfonate) at a concentration of 80 mg/l. 

(Anaesthetic concentrations and surgical procedures were tested and selected in the laboratory 

prior to the field operations.) During sedation, rough measurements of total length were 

recorded. These lengths were later converted to weight with formulas found in Wilson and 

Nieland (2001), for the purposes of comparison to pinger weight. After approximately five 

minutes, or when the specimens showed signs of adequate sedation, the fish were moved to a 

foam-lined plastic box for surgical implantation of the pinger (Figure 4). During surgery the 

gills were irrigated with a 50 mg/l solution of MS-222 pumped through tubes by a small 

submersible bilge pump. This water was caught within the box and flowed back through the 

system (Figure 4). The anaesthetic solutions were remade with fresh seawater twice daily or as 

needed. 

Scales were removed from an area three scale rows wide by six to eight scales long posterior the 

insertion of the pelvic fin to and midway between the pectoral and pelvic fins; the area was 

swabbed with diluted povidone iodine and a 2-3 cm incision was made with a disposable scalpel 

(Figure 5). The incision was made first through the integument, then through muscle layers, and 

finally through the lining of the peritoneal cavity. The acoustic pinger was inserted into the 

cavity and pushed slightly forward, away from the internal organs. The anchor end of the Floy 

tag was inserted at the posterior end of the incision and the wound was sutured with Ethicon 3-0 

absorbable chromic gut thread (Figure 6). The area was patted dry and the wound was sealed 

with Nexaband S/C Topical Skin Closure, a veterinary glue. The glue was allowed to dry for 

one minute and then the red snapper was moved to an oxygenated recovery tank. Upon 

recovery, usually 15-30 minutes, the red snapper were released into a 29 foot open-ended hoop 

net, the bottom of which was angled towards the platform. The hoop net was intended to protect 

the snapper from predation as they returned both to depth and to the protection of the platform. 

A remotely operated vehicle (ROV) was used to monitor the release of the first few tagged fish 

and confirm that they were able to exit the hoop net and orient towards the platform. Over the 

course of the surgical procedures 3% of the specimens did not recover from the surgery; the tags 

from these were removed and reinserted into new red snapper. Condition class upon release was 

recorded for all red snapper following the criteria 1) swam down immediately, 2) hesitated then 

swam down, and 3) floated at surface momentarily before swimming down. 

13
Figure 4. Foam-lined boxes and gill irrigation system used for the surgical 

procedure. 

14
Figure 5. Scales were removed and an incision was made posterior to the insertion of the 

 pelvic fin and midway between the pectoral and pelvic fins. 

Figure 6. Placement of the Floy tag and suturing of the wound. 

15
The capture locations were distributed throughout the study area as much as possible, dependent 

upon the availability of red snapper and time constraints. Most red snapper, however, were 

caught in the western part of the circle (Figure 7a). Seventy-two percent of the snapper were 

caught and released at the same location (hereafter referred to as native snapper), whereas 28% 

were caught and released at different locations (relocated snapper) in order to determine whether 

snapper would home back to the area from which they were removed and whether the relocated 

snapper were more likely to leave the release location than were native snapper (similar to 

Watterson et al. 1998; Patterson et al. 2001). As a result, the final release locations of red 

snapper were slightly more geographically balanced (Figure 7b). 

2.4 Receiver Deployment 

Acoustic receivers were deployed at seven platforms in the study area on 21 and 22 May 2003 

Additional receivers were deployed at an artificial reef and another platform later in the study. 

The receiver deployment locations and dates are listed in Table 1 and the final receiver locations 

are shown in Figure 8. 

The deployment system at platforms was such that the receiver would reside 10 to 20 meters 

below the water’s surface, but was retrievable from the lower deck of the platform. Two steel 

cables were attached to the lower deck of the platform at one end and to a cross-member 10 to 20 

meters below the surface at the other end. A weighted sled was attached to these cables with Ubolts and was able to freely move up and down the cables. Bolted to the sled was an antenna 

mount to which a one-meter long section of PVC pipe was attached (Figure 9). The receiver was 

bolted to the PVC pipe and the antenna mount was locked to hold the PVC pipe in a vertical 

position. A safety cable also attached the receiver directly to the sled. The hydrophone end of 

the receiver pointed down, and the PVC bent such that the pipe angled approximately 30 degrees 

from vertical, exposing more of the hydrophone to the sea floor. A third cable was attached to 

the sled and this was used to raise and lower the sled to and from the surface. Receiver data 

were downloaded monthly from June through December of 2003. ChevronTexaco. 

Corporation’s crew transport helicopters provided transportation to and from the platforms to 

download data and maintain equipment. 

16
a) 

b) 

Figure 7. Capture (a) and release (b) locations for tagged red snapper are 

shown with the corresponding percent of total fish captured and 

released at each location. 

17
Table 1 

Receiver Deployment Locations and Dates 

Platform name Latitude Longitude Date of Deployment 

ST135-M 28o 38.102’ 90o 16.914’ 05/21/2003 

ST151-I 28o 37.540’ 90o 16.334’ 05/21/2003 

ST128-R 28o 40.041’ 90o 14.724’ 05/22/2003 

ST134-S 28o 39.409’ 90o 14.132’ 05/22/2003 

ST151-Y 28o 36.970’ 90o 14.974’ 05/22/2003 

ST134-W 28o 37.630’ 90o 13.979’ 05/22/2003 

ST151-O 28o 36.926’ 90o 15.146’ 05/22/2003 

Artificial Reef 28o 38.215’ 90o 16.020’ 06/16/2003 

ST135-Q 28o 39.466’ 90o 16.929’ 07/29/2003 

18
Figure 8. Map of the final locations of acoustic receivers at seven platforms and 

one artificial reef in the study area. Receiver locations are circled and 

labeled; other structures of interest are also labeled 

19
Figure 9. Receiver deployment system including a weighted sled 

that moved along two guide cables and PVC pipe to 

which the receiver was bolted, hydrophone end down. 

20
The receiver on the artificial reef was deployed and retrieved by a SCUBA diver. This receiver 

was attached to a bracket chained directly to a beam of the toppled platform approximately 22 

meters below the surface and was checked every two to three months. The orientation of this 

receiver was in the horizontal plane, but it did not show less adequate detection than did the 

vertically oriented receivers. 

2.5 Acoustic Range Test 

An acoustic range test was performed on 11 March 2004 at ST151-I, a platform to which a 

receiver had been mounted at the beginning of the experiment. The test measured the detection 

range of the VR2 receiver and the V8SC pinger. During the test the sea state was calm and the 

weather clear. A pinger with a pulse train delay of 3-5 seconds was suspended from a boat at a 

depth of approximately 20 m below the sea surface. A VR2 receiver was also suspended from 

the boat to confirm that the V8SC was pinging as expected throughout the range test. The boat 

engine and all electronic equipment were powered off and the boat was allowed to drift with the 

current at approximately 1 knot. Multiple drifts were made toward and away from the platform, 

as the current allowed, to a maximum distance of 1200 meters. The path of the boat was tracked 

with a handheld GPS with the time synchronized to that on the platform receiver. The path was 

subsequently compared with the data file downloaded from the receiver to determine the 

maximum distance between the pinger and receiver for all the detections recorded by the receiver 

on ST151-I. 

2.6 Methods for Data Analyses 

Data analyses were performed on both fish recapture data and acoustic data. Fish recapture 

locations that differed from release locations were displayed on a map to determine if the red 

snapper undertook uniform directional movement. Fish recapture data was used to estimate a 

presumed fishing mortality with the following equations as described by Pine et al. (2003): 

 f=λu (1) 

where f is the tag return rate, or the number of tags returned out of the sample population, λ is the 

probability that a tag on a harvested fish is reported, or reporting rate, and u is the exploitation 

rate, and 

 u=F(1-exp(-F-M))/Z (2) 

where F is the instantaneous fishing mortality rate, M is the instantaneous natural mortality rate, 

and Z is the instantaneous total mortality rate (F+M). The actual reporting rate for this study was 

unknown; therefore, calculations were made with a range of reporting rates between 30% and 

100%. Calculations were also made with two values for instantaneous natural mortality: 0.1 and 

0.2. The former value is the natural mortality rate used for age 2-15+ snapper in the 1999 red 

snapper stock assessment (Schirripa and Legault 1999). There remains some degree of 

uncertainty in the scientific community as to the actual value of M (see Schirripa and Legault 

1999), therefore the latter value was used as a more conservative estimate of natural mortality. 

Acoustic data were used in analyses of site fidelity and diel movement. Short-term site fidelity 

(on a scale of weeks) was examined with plots of the daily location of each tagged red snapper to 

21
determine if specimens had moved among receiver locations. Fish recapture data (location and 

date) were included in these plots. Longer-term site fidelity (on a scale of months) was 

examined with a logistic regression performed by PROC LOGISTIC in SAS (SAS Institute Inc. 

2003). All red snapper removed from the population by fishermen were left out of this dataset 

because this removal was a forced absence. The same regression was run on two other data sets, 

one including a portion of the recaptured fish (those for which exact recapture location and date 

were known) and the other including all of the recaptured fish to determine if addition of these 

specimens would change the results of the regressions. An a priori decision was made to use the 

data set excluding all recaptured fish if there was little variation in the results; if the data sets did 

yield differing results, all would be reported. Red snapper from ST151-I and ST135-Q were 

removed from the regression dataset because of missing data due to receiver malfunction and late 

deployment. Time was divided into ten-day increments, called periods, beginning with the day 

of release for each fish. If a snapper was present within a given period, it was coded as category 

one; if a snapper was not present during the given period, it was coded category zero. Native and 

relocated snapper were treated as two different populations within the logistic regression. 

Location of release and potential for thermocline interference also were tested as variables of 

interest. A thermocline developed below the depth of the receivers in late July 2003 and is 

suspected of deflecting a substantial number of pings, causing a decrease in the ping detection 

rate (further explained in the results.) Periods falling within the estimated range of dates where a 

thermocline may have interfered with ping detection (periods 7-13) were coded category one and 

the remaining periods (0-6 and 14-20) were coded category 0. 

The null hypothesis of the logistic regression was that the probability of presence did not change 

over the duration of the study. The alternate hypothesis was that some or all of the variables 

(Period, Population, Location, and Thermocline) could be used to predict the probability of 

presence. Three models were formulated to assess different combinations of the variables. 

Period and Population and the interaction between the two were included in the Base Model as 

they were the two variables deemed most likely to affect the probability of presence. Location 

was not included in this model because it was confounding with population. All of the snappers 

released at platform ST151-Y were relocated specimens, approximately half the snapper released 

at platform ST128-R were relocated specimens and half were native, whereas nearly all the 

snapper released at platform ST135-M were native individuals. Any difference shown among 

locations was likely to also be affected by population; therefore, Location was left out of the 

Base Model to avoid multicolinearity. A second model, the Thermocline Model, added the 

variable Thermocline to the Base Model. This model was intended to determine if the 

interference of the thermocline could be used to predict a decrease in the probability of presence, 

regardless of whether this was an actual decrease in presence or a result of the acoustic pings 

being deflected and not detected by the receiver. A third model, the Location Model, added the 

variable Location to the Base Model to determine if this variable, in combination with 

Population, improved the model. 

Acoustic data was also subjected to Fourier analysis in Matlab (Student Version Release 13) to 

determine if there was a diel periodicity in red snapper movement. Platforms ST135-M and 

ST128-R were included in this analysis because they were the most highly populated platforms. 

This analysis was limited to data collected before the decrease in ping detection rate in late July 

2003 (at hour 1669.) Hours 0-263 of the data set were removed to allow a recovery period of 

two days for the last set of post-surgery red snapper released. The average number of pings per 

22
fish was calculated for each hour from hour 264 to hour 1668 at both ST135-M and ST128-R. 

This calculation was performed to normalize the total number of pings to the actual number of 

tagged red snapper present, accounting for the decrease in number of tagged snapper at each 

location through time. The average number of pings per fish was used as a proxy for distance 

from the platform, under the assumption that more detections would be received from fish closer 

to the platform than for fish farther from the platform because the strength of the acoustic signal 

decreases with distance transmitted. It is probable that some acoustic pings from red snapper in 

very close proximity to the platform were deflected by a cross member or support piling, which 

could also cause a decrease in the average pings per fish, but because red snapper tend to 

constantly swim around and through platforms it is unlikely that a great number of consecutive 

pings were missed. These time series were filtered by removing any data point more than two 

standard deviations from the mean. This filter was applied three times. The time series were 

then smoothed with a sliding box filter with a three-hour window, meaning that each data point 

was replaced with the average of itself and the points on either side of it (personal 

communication, Mark Benfield, Louisiana State University, 17 October, 2003). These time 

series were then subjected to a Fourier transform to decompose the times series into individual 

sinusoidal components, which when added together would yield the original signal. 

A second time series was calculated with the average number of pings per fish for every hour 

(0:00-23:00) of each day during the same time period used in the Fourier analysis. Data from 

platforms ST135-M and ST128-R were combined in this time series. These data were fitted with 

a local regression using the LOESS procedure in SAS (SAS Institute Inc. 2003), a nonparametric 

method for estimating regression surfaces where the regression surface is fitted to data points 

within a chosen neighborhood of each value of the independent variable, to determine if local 

regression could describe a pattern of diel movement away from the platform. 

23
25 

3. RESULTS 

During this study 125 red snapper were tagged and implanted with acoustic pingers. The 

limiting factor in the number of red snapper tagged was the number captured by volunteer 

anglers, not the length of time necessary to implant the pingers as was expected. The surgical 

tagging procedure lasted no more than ten minutes per fish including anaesthetization time. 

Tagged red snapper ranged in TL from 28 to 47 cm, with a mean length of 36 cm (Figure 10). 

The majority of tagged red snapper were smaller than legal size. Ages estimated from these 

lengths (using Wilson and Nieland 2001) indicate these fish were predominately in the 2 to 4 

year old age classes, with the possibility of some 5 year old fish. Weight was also estimated 

from these lengths (using Wilson and Nieland 2001) and compared to the pinger weight, 

revealing that the 5 gram pingers ranged from 0.3-1.6% of the body weight of tagged red 

snapper. This range is well below the limits given by Winter (1983) who recommended that 

pingers weigh less than 2% of the fish weight and by Wolcott (1995) who recommended a range 

of no more than 2-5%. ROV footage revealed that upon release red snapper had no difficulties 

navigating out of the hoop net and orienting towards the platform. Seventy-nine percent of the 

fish were assigned a release condition class of 1, fish that swam down immediately after being 

released, 19% were assigned a release condition class of 2, fish that hesitated then swam towards 

the bottom, and 2% were assigned a condition class of 3, fish that floated at the surface 

momentarily before swimming towards the bottom. Fish movement was examined in light of 

these data. There were no apparent differences in the behavior of condition class 1 and condition 

class 2 fish, although the fish assigned to condition class 3 were either never detected or 

disappeared within a few days of release. 

3.1 Storm Damage 

In late June 2003, Tropical Storm Bill moved through the study area, affecting some of the 

receiver deployment equipment. When the equipment was checked on 11 July, it was evident 

that the brass crimps used to form loops at the ends of cables had been eroded and weakened by 

electrolysis allowing the storm to break the crimps and pull some cables loose. The receiver and 

mounting apparatus was completely lost from ST134-W; the receiver at ST134-S had been torn 

loose, was damaged, and flooded. No red snapper had been released at either location, and when 

the data file from the latter location was downloaded, it contained no detections. The receivers 

and mounting apparati at ST128-R, ST151-Y, and ST151-O were unharmed. Some of the cables 

at ST151-I and ST135-M became detached during the storm, so the receivers were temporarily 

suspended from the cross beam until the cabling system could be repaired. The receiver from 

ST151-I was also flooded, but examination of the data file revealed that the receiver had actually 

ceased to collect data six hours after it was originally deployed on 21 May; the manufacturer 

accepted responsibility for this malfunction, damage, and data loss (personal communication, 

Glenn Coady, VEMCO Ltd., 23 July, 2003). The receiver deployment system was slightly 

modified to eliminate the brass crimps and avoid further issues with electrolysis. On 29 July the 

cabling systems at all platforms were repaired. On this date a receiver was also deployed at 

ST135-Q, replacing the receiver at ST134-W, because 19 red snapper had been released at 

ST135-Q and no specimens had been released at ST134-W; therefore, red snapper released at 

ST135-Q were only detected if they were still present at that site after 29 July. 
Total Length Frequency 

05

1015202530

28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 

TL ( )

commercial minimum size

recreatminimu

Figure 10. Histogram of the total length frequency of tagged red snapper (n=125). 

26 
3.2 Thermocline 

While repairing the cabling at ST135-M on 29 July, 2003 a diver sensed the presence of a 

temperature gradient (thermocline) at approximately the same depth as the cross member used to 

deploy the receiver. When the receiver was positioned below the cross member, before repairs 

on the cabling system, it was in relatively colder water. After the cabling system was repaired 

and the receiver was remounted as in the original design, it was moved upward into warmer 

water. At the exact time and date the receiver was moved above the thermocline a severe 

decrease in ping detection rate is evident in the data. Around the same date, other receivers also 

experienced decreases in ping detection rate. A detection is recorded by the receiver after all 

seven pulses in the pulse train are intercepted. If less than seven pulses are intercepted the 

receiver counts these pulses, but does not record a detection. From these data it is possible to 

estimate the minimum number of detections that should have been recorded if all seven pulses of 

each train had been intercepted, in other words, a theoretical number of detections. The ratio of 

actual detections to theoretical detections gives an estimate of the ping detection rate. The ping 

detection rate at ST135-M during the three weeks before the receiver was moved, when the 

receiver was below the thermocline, was 21.8%, whereas during the three weeks after the 

receiver was moved above the thermocline the ping detection rate fell to 5.5%. Over these same 

time intervals, the ping detection rate at the receiver on ST151-Y decreased from 22.2% to 9.3%, 

and the detection rate at the receiver on ST138-R decreased from 31.7% to 18.1%. On 28 July, 

the day before the receiver at ST135-M was moved above the thermocline, it received 324 

detections from 16 fish. On 30 July, the day after the receiver was moved above the thermocline, 

it received only 4 detections from 4 fish. 

VEMCO reported that a thermocline could dampen and deflect the acoustic signals (personal 

communication, Glenn Coady, VEMCO Ltd., 16 December, 2003). We conducted a test at a 

platform by concurrently hanging one receiver above the thermocline and one below the 

thermocline for half an hour; the receiver below the thermocline detected 20 times as many pings 

as the receiver above the thermocline. The ping detection rate of the receiver below the 

thermocline was 38.9% whereas the ping detection rate above the thermocline was 9.6%. These 

facts, along with the anecdotal information provided by the diver has led us to attribute this 

decrease in ping detection to the putative development of a thermocline between the receivers 

and the lower half of the water column where red snapper are predominantly found (Continental 

Shelf Associates 1982; Rademacher and Render 2003). Unfortunately, environmental data were 

not collected continuously throughout the study because of low frequency of visits to the study 

site and constraints on equipment transport to the oil field during data download trips. Historic 

cruise data (1994-2003) collected within 15 miles of the study area in the month of July shows 

evidence that thermoclines are common in this region at depths between 15 and 25 meters with 

temperature changes of up to eight degrees Celsius (data supplied by Nancy Rabalais, Louisiana 

Universities Marine Consortium, 11 February, 2004). Subsequent CTD casts from charter 

vessels during this study revealed that the water column in the study area was moderately mixed 

with no severe thermoclines by mid October 2003. 

3.3 Acoustic Range Test 

The maximum distance between the V8SC pinger and VR2 receiver for successful detection was 

between 30 and 180 m, dependent upon position of the pinger relative to the receiver. Therefore, 

tagged red snapper at distances greater than 180 m from the receiver were unlikely to be detected 

27
28

during the study; tagged snapper within 180 m of the receiver were likely to be detected unless 

the acoustic signal was deflected by a thermocline or blocked by cross-members or support 

pilings of the platform. This range of detection could vary with changes in the ambient noise 

caused by wind and wave action, water turbulence, and anthropogenic noise such as boat motors 

and drilling operations on the platforms. Therefore, a realistic estimate of the actual range of 

detection on any given day may be closer to 50-75 m. 

3.4 Daily Location of Individual Red Snapper 

The acoustic data was best illustrated by depicting the daily presence and location of each tagged 

red snapper (Figures 11-14). In general, presence vs. absence was highly variable with 

individual fish, receiver location, and time. Each horizontal line in Figures 11-14 is a different 

red snapper and the different shaded symbols represented different locations; therefore, a change 

in symbol within a line represents a change in location for that fish. The large X’s are located at 

the approximate date of recapture for any red snapper reported by fishermen and the X’s are 

labeled with most specific recapture location known. The fishermen did not always provide a 

specific date of capture or location, therefore the placement of the X is an estimation of the date 

of capture and the location is as specified by the fishermen. The vertical line at day 70 denotes 

the approximate date that we suspect the putative thermocline began to interfere with ping 

detection rate. The time period during which a receiver malfunctioned or was missing is denoted 

by dashed lines for all red snapper released at those sites (ST151-I and ST135-Q.) 

Ninety-seven of the 125 tagged red snapper were detected during the study. Seventeen of the 19 

snapper released at ST135-Q were no longer present when the receiver was deployed there in 

late July. Seven of the 17 red snapper released at ST151-I were not detected before the initial 

receiver failed or after another was deployed there in mid-July. The other four fish tagged but 

not detected could have had possible pinger malfunctions. As the receiver at any given platform 

was deployed in advance of the release of any tagged snapper at that location, it is unlikely that a 

functioning pinger was not detected at least once even if the snapper immediately swam away 

from the platform or was preyed upon. 

Red snapper movement between receivers appeared to be sparse and erratic. Seven of the 97 

detected fish moved to another receiver during the study with little evidence of homing behavior 

or uniform directional movement. A map displaying the geographical movement of these fish is 

displayed in Figure 15. Approximately three weeks after release, four red snapper from ST151-I 

moved to ST135-M, the next platform to the north, stayed at that platform for two to three 

weeks, then were not detected again (tag numbers RS 16, 17, 19, and 21; Figures 11 and 15). 

One of these fish (RS 19) was eventually caught at ST151-K (28° 36.977’ N, 90° 15.374’ W), a 

platform 2 km to the south of ST151-I. Another red snapper (RS 56; Figures 13 and 15) was 

released at ST151-Y, was detected at ST151-O (only 300 meters to the west) the next day, then 

was not detected again. RS 67 (Figures 13 and 15) was released at ST135-M, stayed there for 

approximately 45 days, and then moved to the artificial reef 1.5 km to the east. RS 67 stayed at 

the artificial reef for the next two months then was not detected again. RS 81 (Figures 14 and 

15) was released at platform ST135-M where it remained for over two months. Just before that 

snapper left the area, it was periodically detected at the artificial reef over a few days. 
Tags RS 1 - 25

123456789

10111213141516171819202122232425

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 

Day

Tag Number (RS #)

Thermocline moves below receiver depth

ST135-M

ST151-IST128-R No receiver

X Recaptured

Circl

Release 

Circl

Unknown

CirclCircl

29 Circl

Unknown

ST-151-K

ST-151-K

Circl

Circl

ST-151-K

Figure 11. Daily locations of tagged red snapper RS 1 - 25. Different symbols represent different locations, dashe

denote a period not covered by a receiver, X’s and labels denote when and where a red snapper was reand the vertical line at day 70 denotes the suspected interference of the thermocline with ping detectio
Tags RS 26 - 50

26272829303132333435363738394041424344454647484950

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 

Day

Tag Number (RS #)

Thermocline moves below receiver depth

Circle

ST-130

Circle

Unknown

No receiver

ST135-Q

ST135-M

ST128-R

X Recaptured

30 

Figure 12. Daily locations of tagged red snapper RS 26 - 50. Different symbols represent different locations, dashed lines 

covered by a receiver, X’s and labels denote when and where a red snapper was recaptured, and the vertical li

the suspected interference of the thermocline with ping detection. 
Tags RS 51 - 75

51525354555657585960616263646566676869707172737475

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 

Day

Tag Number (RS #)

Thermocline moves below receiver depth

Release 

Release Release 

Unknow

Unknow

ST-54

ST151-

ST135-M Artificial ReefST128-R

Recapture

d

X

+ ST151-

31 

Figure 13. Daily locations of tagged red snapper RS 51 - 75. Different symbols represent different locations, dashe

denote a period not covered by a receiver, X’s and labels denote when and where a red snapper was rec

the vertical line at day 70 denotes the suspected interference of the thermocline with ping detection. 
Tags RS 76 - 97

76777879808182838485868788899091929394959697

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 

Day

Tag Number (RS #)

Thermocline moves below receiver depth

Release 

Unknow

Unknow

Circl

Artificial Reef

ST135-M

Recapture

d

32 

X

Figure 14. Daily locations of tagged red snapper RS 76 - 97. Different symbols represent different locations, dashe

denote a period not covered by a receiver, X’s and labels denote when and where a red snapper was recthe vertical line at day 70 denotes the suspected interference of the thermocline with ping detection. 
Figure 15. Map showing the starting and ending points for the seven red snapper that 

displayed movement between receivers. RS 16, 17, 19 and 21 relocated from 

ST135-Q, RS 56 relocated from ST135-I, and RS 67 relocated from ST135-S. 

RS 81 was native to ST135-M. 

33
34

None of the relocated snapper showed any definitive homing behavior. Of the seven snapper 

that showed movement between receivers, six were relocated fish (RS 16, 17, 19, 21, 56, and 

67). Four were caught at ST135-Q and relocated to ST151-I (RS 16, 17, 19, and 21). The 

northward movement shown by these four fish to ST135-M and then subsequent disappearance 

could have been homing behavior to ST135-Q, which was north of ST135-M. However, RS 19 

was caught at ST151-K, farther to the south, and RS 21, which was the only one of these four 

snapper remaining in the population when the receiver was deployed at ST135-Q, was never 

detected there. RS 56 was captured at ST151-I and relocated to ST151-Y; there is no evidence 

that it homed toward ST151-I. RS 67 was also a relocated fish, originally captured at ST134-S 

and released at ST135-M. Although this snapper did move east to the artificial reef receiver, it 

was never detected at ST134-S. The only location from where red snapper were relocated that 

was not covered by a receiver was an artificial reef located between platforms ST128-X (28°

40.564’ N, 90° 16.121’ W) and ST128-R (Figure 8). Some snapper were also relocated from 

ST135-Q, which was not covered by a receiver until late July. 

3.5 Fish Recaptures 

Tags returned from recaptured fish provided information about red snapper movement within 

The Circle but beyond the limited acoustic confines of the receivers, and about large-scale 

movement of fish that left The Circle. Of the 125 tagged red snapper, at least 36 were recaptured 

by fishermen. Fourteen recapture reports came with platform specific information, 14 came with 

area specific information (for instance a lease block or general region such as The Circle), and 

eight came with no location information. Seven snapper were definitely recaptured at a site other 

than their release location. The release and recapture locations, approximate length at tagging, 

time at liberty, time since last detection, and distance and direction traveled for these seven red 

snapper are shown in Table 2; the release and recapture locations are geographically displayed 

on a map (Figure 16). The range in days at liberty was great, from 5 to 130 days, as was the 

distance traveled, from 2 to 25 km. Many of these red snapper displayed movement with an 

eastward component, though southward, northward, and westward components were also shown. 

In addition, one to two other snapper may have been recaptured and released by fishermen 32 km 

to the east of the study area. These reports were heard as rumors and were never confirmed. 

Of the 36 red snapper recaptured by fishermen, two were recaptured before Tropical Storm Bill 

in late June 2003. One was recaptured at the artificial reef between ST128-X and ST128-R and 

the other was recaptured at an unknown location. This second snapper was detected at the site of 

release up until the approximate date of recapture, so it is likely that it was recaptured at this site. 

Of the 34 red snapper that were recaptured after the tropical storm, six had definitely moved 

away from their release sites. 
Table 2 

Red Snapper Recaptured at Locations Other Than Their Release Sites 

Fish Number 

Release Location 

Recapture Location 

Length at Tagging (TL cm) 

Days at Liberty 

Days Since Last Detection 

Distance Traveled (km) 

Direction 

RS 98 ST151-I APCs 40 5 5* 5 NE RS 99 ST151-I ST151-K 38 59 58* 2 SE RS 18 ST151-I ST151-K 36 59 58 2 SE RS 23 ST151-I ST151-K 35 59 0 2 SE RS 19 ST151-I ST151-K 37 70 20 2 SE RS 31 ST128-R 

35 ST130 38 79 29 9 E 

RS 74 ST135-M ST54 36 130 44** 25 NW 

*Since no detections were made, the date of last detection was assumed to be the release date. **The last detection was considered to be when the fish was captured and released by a charterboat. 
Figure 16. Map showing the starting and ending points for the seven red snapper that were recaptured at locati

their release sites. The unconfirmed location where one or two other red snapper may have been rec

released east of the study area is also shown. RS 19 was last detected at ST135-M, but was ultimately different platform. 

36 
Table 3 

Estimates of Instantaneous Fishing Mortality Rates From Various Combinations of Tag 

Reporting Rate and Instantaneous Natural Mortality Rate (M). 

Tag Reporting Rate: 100% 80% 60% 40% 30% 

M=0.1 0.36 0.47 0.7 1.42 4.65 

M=0.2 0.38 0.5 0.74 1.54 6.7 

3.6 Fishing Mortality 

Estimates of fishing mortality ranged from 0.36 to 6.7 depending on the combination of natural 

mortality and reporting rate (Table 3). Reporting rates lower than 30% were impossible without 

forcing total annual mortality to exceed 100%. 

3.7 Logistic Regression 

The acoustic data was edited in three different ways for use in the logistic regression. The first 

data set included fish not recaptured by fishermen, the second contained the same fish as the first 

dataset but also included recaptured fish for which recapture location and date were known, and 

the third dataset included all fish. These data sets yielded little variation in results; therefore we 

are reporting only the results of the regression on the first data set, containing fish that were not 

recaptured by fishermen. The results of the three logistic regressions models, Base (including 

the variables Period and Population), Thermocline (Period, Population, and Thermocline), and 

Location (Period, Population, and Location; see Materials and Methods, page 29, for model 

descriptions), are displayed in Table 4. All of the models were significant at alpha=0.05. The 

results of the three logistic regression models, including p-values and odds ratio estimates (ratio 

of the probability of an event occurring, or presence of a red snapper, and the probability of the 

event not occurring, or absence of a red snapper) are shown in Table 4. 

The Base Model was used to predict the probability of presence at release location for an 

individual fish. These predicted probabilities were graphed and are shown in Figure 17. Both 

populations (native and relocated) have been plotted with their 95% confidence intervals. The 

probability of presence decreases with time (Period), approaching zero at period 20. In the 

earlier periods, the relocated population has a lower probability of presence and the 95% 

confidence intervals do not overlap. During the later periods the relocated population has a 

higher probability of presence, but the 95% confidence intervals overlap during these periods. 

3.8 Fourier Analysis 

The Fourier analysis was conducted to determine if red snapper exhibit diel movement away 

from platforms. Fourier analysis decomposes a signal into many individual sine functions. 

Periodograms show the period (hours/cycle) and power, or magnitude, of each sine function. 

The resulting periodograms for ST135-M and ST128-R (the locations where the majority of 

37
tagged red snapper were released) are shown in Figure 18. The red snapper populations at 

ST135-M (Figure 18a) and ST128-R (Figure 18b) had very similar patterns of periodicity with 

the most powerful sine functions centered on a period of 24 hours/cycle. These 24-hour cycles 

represent a pattern of diel movement away from the receivers. 

The LOESS procedure revealed that red snapper are closest to the platform (the greatest number 

of pings, or detections, per fish) during the daylight hours, and farther from the platform at night 

(lower number of pings per fish.) The LOESS procedure also shows that the greatest movement 

away from platforms during the night occurs around sunset and just before sunrise. The actual 

distribution of pings per fish as a scatter plot with the predicted pings per fish overlaid as a local 

regression line is presented in Figure 19. 

38
39

Table 4 

Results of the Three Logistic Regression Models: Base, Thermocline, and Location. 

chi-square d.f. p-value Odds Ratio 

Estimate 

Base Model 876.41 3 <0.0001 

Period 295.99 1 <0.0001 0.71 

Population (native vs. relocated) 11.08 1 0.0009 3.62 

Period*Population 12.40 1 0.0004 

 

Thermocline Model 926.43 4 <0.0001 

Period 259.33 1 <0.0001 0.75 

Population 8.21 1 0.0042 2.92 

Thermocline (1 vs. 0) 50.12 1 <0.0001 0.24 

Period*Population 9.14 1 0.0025 

 

Location Model 882.36 5 <0.0001 

Period 292.55 1 <0.0001 0.70 

Population (native vs. relocated) 6.27 1 0.0123 3.06 

Location 5.69 2 0.0581 

ST135-M vs. ST151-Y 2.12 

ST128-R vs. ST151-Y 2.46 

Period*Population 10.94 1 0.0009 
Predicted Probability of Presence

0

0.20.40.60.8

1

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 

Period

Probability

Native Relocated Native Conf. Int. Relocated Con

Figure 17. Predicted probability of presence at release location for both native and relocated snapper. Dashed lines 

confidence intervals. Period= ten day increment. 

40 
a) 

b) 

Figure 18. Periodograms for ST135-M (a) and ST128-R (b) showing the 

period and power of the sine functions describing red snapper 

movement away from the platforms. 

41
Pings/Fish: Actual Values and LOESS Predictions

12345678

0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 2

Hour

Pings/Fish

Loess Predictions Actual Pings/Fish

-1:00 -2:00 -3:00 -4:00 -5:00 -6:00 -7:00 -8:00 -9:00-10:00 -11:00-12:00 -13:00-14:00-15:00-16:00-17:00-18:00-19:00-20:00-21:00-2

42 

Figure 19. Actual values of pings/fish and LOESS predicted values of pings/fish for each hour of the day. The b

below the x-axis denotes nighttime hours. 
3.9 Pinger Malfunction 

After pingers were returned by fishermen each were tested in the lab and the majority were 

pinging as expected. Initially any malfunction was attributed to the pingers being frozen inside 

the fish before being returned to our possession. In January of 2004, while using these tags to 

test acoustic receivers in the laboratory, we discovered that a great number of pingers that had 

been functioning properly when returned by fishermen were no longer functioning after storage 

in the lab for a few months. These pingers had been activated for a total of eight months (the 

original specification for these pingers predicted a battery life of over 400 days (personal 

communication, Dale Webber, VEMCO Ltd., February, 2003).) Thirty of the 32 pingers that we 

received from fishermen were not functioning properly or at all, so a portion of the tags were 

returned to VEMCO for testing. VEMCO found that the battery power was expended and 

informed us that the batteries may have come from a “bad” batch (personal communication, 

Glenn Coady, VEMCO Ltd., 25 February, 2003). Further testing revealed that some tags were 

still transmitting an acoustic signal, though the output was very low, indicating that the battery 

capacity had dropped off fairly recently (personal communication, Glenn Coady, VEMCO Ltd., 

26 February, 2003). It was impossible to differentiate between a red snapper leaving the study 

area and a pinger battery failing. Because most pingers returned by fishermen during the first 

few months of the study were operating correctly we made the assumption that most pingers still 

in the field were operating correctly during the first few months of the study, when the majority 

of the acoustic data were collected. Thus, we completed the analyses as planned keeping in mind 

that the acoustic data used to examine site fidelity may have been incorrect. 

43
4. DISCUSSION

Analysis of fish recapture and acoustic data revealed lower site fidelity and less uniform 

directional movement than shown by previous research on red snapper. The estimates of 

instantaneous fishing mortality were generally greater than the current value predicted by red 

snapper stock assessment. Red snapper at platforms exhibit diel movements away from the 

platforms during the night that may be explained by offsite foraging behavior. 

4.1 Site Fidelity 

The apparent site fidelity of red snapper is dependent upon the time scale being examined, 

whether days, weeks, months, or years. The daily locations of each fish were used to examine 

site fidelity on a scale of weeks and suggest low short-term site fidelity. The logistic regression 

was used to examine site fidelity on a longer-term scale, months, and predicts much lower site 

fidelity. Watterson et al. (1998) define site fidelity in terms of the percentage of fish recaptured 

at the site of release. For this study, we modified their definition to the percentage of red snapper 

acoustically detected at the site of release. 

Red snapper in this study displayed high short-term fidelity and relocated fish showed no 

evidence of homing behavior to original capture locations. The majority of tagged snapper did 

not exhibit movement between receiver locations on a daily or weekly, or in most cases, even a 

monthly basis (Figures 11-14). Although six of the seven red snapper that did move between 

receiver locations were relocated snapper, none showed evidence of homing back to their 

original capture location. Watterson et al. (1998) and Patterson et al. (2001) also relocated red 

snapper; Watterson et al. (1998) did not report any homing behavior and Patterson et al. (2001) 

reported only one out of 111 transported red snapper recaptured at its original capture site. None 

of the 35 relocated snapper in this study were detected or recaptured at the original capture site. 

The logistic regression used the acoustic data to predict the probability of an individual fish’s 

presence at the location of release. The Base Model was the simplest, testing whether or not 

Period (time) and Population (native vs. relocated) influenced site fidelity. The Thermocline 

Model tested the influence of the thermocline on the probability of presence of individual fish 

and the Location Model tested whether or not red snapper exhibited different site fidelity at 

different receiver locations. The results of the logistic regression suggest that the longer-term 

site fidelity of red snapper is very low, perhaps approaching zero, and much less than short-term 

site fidelity. 

The Base Model predicts that both Period (time) and Population (whether a fish was native or 

relocated) affect the site fidelity of red snapper. Odds ratios are used in logistic regression to 

describe the ratio of the probability of an event occurring and the probability of the event not 

occurring (i.e. the opposite event). For instance, the logistic regression Base Model gives an 

odds ratio estimate of 0.71 for the variable Period. This is interpreted as the odds of presence (or 

likelihood of presence compared to the likelihood of absence) decreasing by 29% for each unit of 

Period, or every ten days. This suggests that site fidelity decreases with time. The odds ratio 

estimate of 3.62 for Population is interpreted as the likelihood of presence for a native snapper 

being 3.62 times the likelihood of presence for a relocated snapper. In terms of site fidelity, 

45
native red snapper exhibit higher site fidelity than relocated red snapper. These results concur 

with the results of Watterson et al. (1998) who reported that 77% of recaptures of transported red 

snapper had moved from the release site whereas only 39% of recaptures of non-transported red 

snapper had moved from the release site. Patterson et al. (2001) also reported that non-capture 

release site (transportation of red snapper) increased probability of fish movement. Explanation 

for this phenomenon is elusive, though it may be related to environmental carrying capacity or 

red snapper behavior. Any given platform may only be able to support a given number of red 

snapper, and some locations may be more suitable than others. Charter fishermen who frequent 

The Circle quickly become knowledgeable of which platforms tend to yield the most red snapper 

(personal communication, Steve Tomeny Charters, 2003). Some platforms may provide better 

habitat than others because of quantity and/or quality of food, competition with other species for 

resources, and the risk of predation. Another possibility is if the environment at a platform is 

already ecologically balanced or saturated with red snapper, the habitat may not be able to 

support additional red snapper, forcing the exodus of a certain number of red snapper to regain 

the ecological balance. Red snapper could exhibit this sort of social structure or interspecific and 

intraspecific competition for shared resources; studies have shown these type of interactions and 

competition by other species of reef fish, including other lutjanids (Mueller et al. 1994; 

Overholtzer and Motta 1999; Munday et al. 2001). But the question remains, why are the new 

snapper, the relocated fish, those that leave the habitat? Are they driven away because they are 

not already established in the population at the platform? Do red snapper occupy such specific 

and limited niches at oil platforms and exhibit such rigorous social structure? The answers to 

these questions may lie in red snapper behavior and relationships and are not within the scope of 

this study, though they would make a very interesting study for an animal behaviorist. 

The Thermocline Model predicts a significant relationship between the existence of the 

thermocline and a lower probability of presence. The changes in the odds ratio estimates for 

Period and Population are minor, and do not change the qualitative interpretation of these 

variables. We do not believe that the presence of thermocline actually decreases the probability 

of presence of red snapper, but only decreases the probability that a receiver will successfully 

detect a tagged snapper. The odds ratio estimate (0.24) tells us that the likelihood of a detection 

decreases to 24%, approximately one quarter, of the original level while the putative thermocline 

is interfering with detection rate; this amount of decrease is proportionally similar to the 

decreases in ping detection rate described on pages 33 and 34. Care must be taken when 

interpreting these results for two reasons. First of all, the timing of the movement of the 

thermocline below the receiver depth in mid summer and subsequent dissipation in autumn are 

estimates because environmental data were not collected on a regular or continuous basis. 

Secondly, the estimated date for the movement of the thermocline below the receiver depth is 

shortly after a number of fish apparently left their sites of release. Therefore, Period and 

Thermocline may be somewhat confounding, or both attempting to explain the same variation. 

The Location Model did not vary much from the Base Model. The variable Location was not 

significant, though it was close, with a p-value just above alpha=0.05. The odds ratio estimates 

for Period and Population were again not very different from those predicted by the Base Model. 

The estimates of red snapper site fidelity resulting from this study differ from other studies 

conducted on the site fidelity of red snapper in the GOM. We found that red snapper exhibit 

high short-term site fidelity. They do not move from platform to platform on a daily or even 

46
weekly basis; following tagging red snapper remain at a platform for a few weeks to even a 

month or two. Longer-term, over a period of a few or more months, red snapper have low site 

fidelity; it is unlikely that a red snapper will remain at one location for more than a few 

consecutive months. 

Though our short-term site fidelity estimates are not anomalous, every other study, whether 

traditional mark and recapture or acoustic telemetry, has found much higher site fidelity in the 

long-term. Beaumariage (1969) tagged and release 312 red snapper off the coast of Florida. 

Fishermen returned approximately 26% of those fish, and all but eight (>90%) had been 

recaptured at their site of release after being at liberty for an average of 113 days. Beaumariage 

and Bullock (1976) reported that red snapper show definite specific reef residency in shallow 

water and that the only extensive movement occurred at reefs in water deeper than 15 fathoms, 

which may have been forage-motivated or in response to reproductive stimuli. Fable (1980) 

tagged 299 red snapper at natural reefs off the coast of Texas. Although the tag return rate for 

red snapper was very low (5.6%), only one red snapper moved from its release location (<6% of 

the tag returns.) This fish was at liberty for 162 days and moved only 5 km. In another study off 

the coast of Texas, this time at the Buccaneer Oil Field, Gallaway et al. (1981) found very high 

site fidelity for red snapper: none of the tags returned or noted during a visual SCUBA census 

were found at a location other than where the snappers had been tagged. A red snapper study at 

artificial reefs off the coast of Alabama again reported high site fidelity (Szedlmayer and Shipp 

1994). Of 1,155 tagged red snapper, 146 were recaptured by fishermen, although only 37 of 

those were accompanied by known recapture location. Fifty-seven percent of these fish were 

taken at the site of release and 76% of recaptures were within 2 km. Watterson et al. (1998) 

conducted a study over a period of two years, tagging 1,604 red snapper at artificial reefs off the 

coast of Alabama, and concluded that red snapper generally display high site fidelity. The 

researchers and fishermen recaptured 167 fish. Almost 80% of recaptures that were not at liberty 

during a major hurricane were recaptured at their site of release, although red snapper at liberty 

during a major hurricane had a much higher likelihood of movement. Strelcheck (2001) 

conducted a similar study in the same area, tagging 2,608 red snapper, and reported annual site 

fidelity to range from 49% and 72% dependent upon type of artificial reef. Patterson and Cowan 

(2003) continued the study by Watterson et al. (1998) and after tagging an additional 1,328 red 

snapper and collecting two more years of data, these authors estimated annual site fidelity to 

range from 24.8 to 26.5%. These results departed greatly from the results all of the previous 

work, though our current study departs to an even greater extent. 

The studies reviewed thus far have all been traditional mark and recapture techniques. Only one 

other study has used acoustic telemetry, and yet still concluded that red snapper display limited 

movements and exhibit high site fidelity (Szedlmayer 1997). This research was conducted at 

artificial reefs in the northeastern GOM off the coast of Alabama. Twenty-three red snapper 

were tagged with ultrasonic transmitters and release sites were acoustically scanned on a 

monthly basis for a period of nearly 20 months. Twelve red snapper remained at the site of 

release, seven moved distances less than one km, and four were not located after release. The 

authors concluded that the repeated locations of the tagged red snapper at the same artificial reef 

suggested high site fidelity. 

It is important to note that none of the previously mentioned studies have taken place at 

platforms off the coast of Louisiana. Although Gallaway et al. (1981) also conducted their 

47
research at platforms, it is important to remember that since 1981 the number of platforms in the 

OCS region of the GOM has risen by almost 145% (Pulsipher et al. 2001). It is conceivable that 

red snapper in the north central GOM, off the coast of Louisiana, exhibit much lower site fidelity 

than in other regions of the GOM because of the proximity and plethora of available habitat in 

the form of platforms. Frazer and Lindberg (1994) showed that stone crabs (genus Menippe) 

showed lower residency, or site fidelity, at more closely spaced artificial reefs. This relates to 

the ‘resource mosaic hypothesis’ proposed by Lindberg et al. (1990) which states that as reef 

spacing decreases, so does access to prey on the soft-bottom around each reef. The foraging 

haloes, or areas of depleted prey, at closely spaced reefs may overlap and cause a disproportional 

depletion of resources. At closely spaced reefs, foraging fish may be forced to travel greater 

distances to locate a suitable amount of prey, possibly encountering new habitat. Platforms are 

sometimes closely spaced, within a few hundred meters of each other; therefore red snapper may 

encounter numerous other suitable habitats on foraging forays. Because platforms offer such 

high relief, vertical habitat, they may support higher densities of fish than natural reefs (e.g. 

Wilson et al. 2003); therefore the foraging haloes at platforms may be even larger than would be 

found at natural reefs, forcing platform red snapper to forage at greater distances where they may 

lose sensory orientation to the platform, thus losing the ability to find their way back to that same 

platform. 

4.2 Fish Recaptures 

The fish recapture data, although not always as precise and complete as was desired, lends itself 

to the conclusion that red snapper movement does not appear to have a uniform direction. Red 

snapper that were at liberty longer moved greater distances, as though they gradually dispersed 

from their area of origin over time. There does not appear to be a pattern associating TL at time 

of tagging with distance moved; the largest of the seven snapper that showed movement traveled 

one of the shorter distances and the snapper that moved the greatest distance was one of the 

smallest of the seven. Contrary to these results, other studies have found evidence of uniform 

directional movement. Beaumariage (1969) found a general pattern of eastward movement of 

red snapper tagged off the Florida Panhandle. Watterson et al. (1998) and Patterson et al. (2001) 

found overall eastward movement of red snapper tagged off the coast of Alabama. The area to 

the east where many of these fish were captured is also one that supports the highest recreational 

landings in the northern GOM; therefore this apparent eastward directional movement could 

simply have been a result of higher fishing pressure (Watterson et al. 1998). All three of these 

studies were conducted over much longer periods of time, had much larger sample sizes than the 

current study, and were conducted in the GOM east of the Mississippi River. 

Watterson et al. (1998) and Patterson et al. (2001) found that hurricane events increased the 

probability and distance of red snapper movement. Tropical Storm Bill potentially affected fish 

movement in this study and passed through our site one month into the study. No departures of 

tagged fish coincided with the storm and since the majority of tags were returned after this storm 

(94.4%) it is difficult to determine if the storm increased the probability of fish movement in this 

study. 

These estimates of red snapper movement must of course be taken as conservative. It is probable 

that not every recaptured red snapper was reported by fishermen and also possible that some 

snapper lost their external tags and were not reported when recaptured. 

48
4.3 Fishing Mortality 

The various combinations of natural mortality and tag reporting rates yielded a wide range of 

estimated instantaneous fishing mortality (F) values, ranging from 0.36 to 6.7. The most recent 

red snapper stock assessment panel report (GMFMC 1999) predicted a current F ranging 

between 0.292 and 0.474 (dependent upon the value of the steepness parameter of the stock 

recruitment curve.) The smaller value is less than any of our estimates, though the larger value, 

0.474, almost precisely matches the value resulting from the combination of a 0.1 natural 

mortality and a tag reporting rate of 80%. The stock assessment panel report also predicts that F 

at Maximum Sustainable Yield (MSY) ranges from 0.097 to 0.118, values much lower than any 

of the F estimates in this study. 

Although many tagging studies have discussed the potential bias due to tag reporting rates, few 

have attempted to directly predict these rates. Green et al. (1983) surreptitiously implanted tags 

during creel surveys in Texas; only 29% of the tags were returned. A study conducted in the 

western tropical Pacific Ocean found that the overall average reporting rate was 59% (Hampton 

1997). Heifetz and Maloney (2001) estimated tag reporting rates for sablefish in the northern 

Pacific Ocean to be 28%, although the rate varied greatly by region and year, with more recent 

years having higher reporting rates. Another study was undertaken to estimate a tag reporting 

rate for recreational red drum fishermen in South Carolina and Georgia (Smith and Woodward 

1999 in Latour et al. 2001); estimates ranged from 60% to 80%. Most of these tag reporting 

rates are lower than the 80% reporting rate that in combination with natural mortality of 0.1 

yielded a value close to the stock assessment value. 

As in many tagging studies, we did not directly estimate the tag return rate, but instead used a 

range of tag return rates to calculate fishing mortality. It is likely that the reporting rate for this 

study varied temporally and spatially. For the first four months of our study only the recreational 

red snapper fishery was open, the participants in which may be more likely to take note of a tag 

and report it. In October 2004 the commercial red snapper fishery opened and the recreational 

fishery closed. The pace on a commercial boat is generally much more intense and tags are more 

likely to be overlooked. Only two tagged fish (5% of the total returns) were reported from the 

commercial catch, and neither report was from the actual fishermen. A fish market and a 

restaurant reported tags after the snapper had been sold and shipped to Nashville, TN. In 

addition, We believe the tag reporting rate for red snapper recaptured in the study area may have 

been higher than for red snapper recaptured outside of the study area. A large proportion, 

approximately 75% (Steve Tomeny Charters, personal communication, 6 April 2004), of the 

fishing pressure in The Circle is exerted by Steve Tomeny Charters, a charter boat business with 

which we worked very closely; it was responsible for 53% of our tag returns and we expect that 

their reporting rate approached 100%. As the tagged red snapper dispersed from the area with 

time they may have been more likely to be caught by non-charter fishermen who may have been 

less likely to report the tag. In fact, only non-charter recreational fishermen reported tagged fish 

that were recaptured outside of the study area. 

In light of these results, we believe that a tag reporting rate of 40-60% is appropriate for this 

study. Regardless of the level of natural mortality used to calculate F, 40% and 60% reporting 

rates yield rather high fishing mortality estimates (0.7-1.54) in comparison to the 1999 stock 

assessment predictions. We must remember that the red snapper population sampled in this 

49
50

study is certainly not representative of the entire population of the GOM. These red snapper 

were of a limited size range and were tagged and recaptured primarily at platforms off the coast 

of Louisiana. Also, it may be possible that this area, because of its popularity with charter and 

private fishermen, may have a higher fishing mortality than the general red snapper population in 

the GOM. 

Although the estimated values of instantaneous fishing mortality may be higher than expected, F 

may yet have been underestimated for a variety of reasons. The data used in this analysis are 

from a six-month period; collection of a full year’s data likely would have resulted in more tag 

returns. In addition, as stated before, tagged fish recaptured by commercial fishermen were 

likely to go unnoticed and unreported. Tag shedding may also have resulted in an 

underestimation of recaptures. Patterson et al. (1999) used similar tags in red snapper and found 

tag retention for red snapper at liberty 200 days (the approximate length of this study) was 87-

96%. Any such increase in the tag return rate (f, Equation 1) would increase the exploitation rate 

(u, Equations 1 and 2), resulting in larger instantaneous fishing mortality rates (F, Equation 2). 

4.4 Diel Movement 

The results of the Fourier analysis and the LOESS procedure show that red snapper undergo a 

diel movement away from the receivers and platforms at night. The most likely explanation for 

the nocturnal movement away from the platform is feeding behavior. As stated previously, red 

snapper feed on benthic organisms, most likely moving off reef, or platform as the case may be, 

to feed. Other studies have reported that red snapper exhibit nocturnal feeding behavior. 

Moseley (1965) found that the diet of red snapper includes a high percentage of mantis shrimp, 

which is known to burrow into the sediment during the day and emerge at night and become 

vulnerable to predation. Beaumariage and Bullock (1976) also found a predominance of squid 

and mud-burrowing shrimp in the gut contents of red snapper and believed that to be evidence of 

nocturnal feeding behavior. Moseley (1965) and Bradley and Bryan (1975) both reported that 

night fishing for red snapper was much more productive than during the day, potentially 

signifying nocturnal feeding behavior. Recent research conducted at artificial reefs off of 

Alabama suggests that red snapper actually feed continuously during the 24-h cycle but exhibit a 

diel shift in prey categories; during the day red snapper foraged above the reef on water-column 

organisms and at night snapper foraged away from the reef on sand-associated organisms 

(McCawley 2003). Combining the theories that red snapper feed off platform and feed 

nocturnally may explain why red snapper appear to move away from platforms at night: to feed 

on benthic organisms in the surrounding area. 

Although the overall trend in our study shows a diel pattern with less presence at platforms 

during the night compared to the day, two troughs are depicted by the LOESS procedure, just 

before sunrise and at sunset, suggesting that red snapper may exhibit some sort of crepuscular 

movement. Crepuscular movement and nocturnal feeding behavior are not mutually exclusive. 

Fish exhibit crepuscular feeding behavior to take advantage of the overlap of diurnal and 

nocturnal organisms during the twilight periods as diurnal organisms settle into nighttime 

inactivity and nocturnal organisms initiate activity; during these times, predators are maximally 

active and successful (Helfman 1986). Red snapper may take advantage of this overlap in 

potential prey by venturing on intense feeding forays at dusk and dawn, with intermittent feeding 
forays throughout the night. This behavior would explain the pattern shown by the LOESS 

procedure. 

4.5 Pinger Malfunction 

The premature loss of battery power in the pingers causes some lack of confidence in the 

acoustic data used to examine site fidelity. It is impossible to determine whether a fish was truly 

absent from an area or if the pinger lost power. The majority of pingers we retrieved from 

fishermen functioned through the end of the summer. Most of the apparent movement of red 

snapper away from the study area occurred during the early-mid summer and we believe that the 

pingers in those red snapper functioned beyond the summer months due to VEMCO’s conclusion 

in February 2004 that the battery capacity appeared to have decreased recently. Some pingers 

functioned for much longer and may still exhibit the expected battery life. The site fidelity 

portions of this study have been compromised by the thermocline and pinger malfunction, but 

the data were analyzed as planned because the extent of these complications is not certain. The 

analysis of diel movement, estimates of fishing mortality, and fish recapture data were all 

unaffected by the pinger malfunction. 

51
5. CONCLUSIONS 

Based on analysis of fish movement shown by acoustic data we conclude that on a short-term 

time scale (a few weeks) red snapper appear to exhibit high site fidelity but as the time scale 

increases (over a period of months) red snapper appear to exhibit lower site fidelity. Site fidelity 

on longer time scales, for instance on an annual basis, would likely appear even lower. In other 

words the probability of a red snapper remaining at one location decreases with time. Tagged 

red snapper showed little movement between receiver locations while in the study area, but as 

time increased the probability of a tagged fish remaining at its release locations decreased. Some 

of this decrease could be due to recaptures not reported by fishermen or by pinger battery failure. 

Relocated red snapper exhibited lower site fidelity than did native snapper, but these relocated 

fish did not display any homing behavior. The acoustic data also revealed diel movement away 

from the platforms at night, most likely for offsite feeding. 

Based on the fish recapture data we conclude that red snapper in this region of GOM do not 

appear to display uniform directional movement. The tropical storm that passed through the 

study area did not appear to affect the probability or magnitude of red snapper movement. The 

estimates of instantaneous fishing mortality derived from the fish recapture data were higher than 

current red snapper stock assessment prediction, but are not unreasonable. 

Even if the acoustic data used for the site fidelity analyses is incorrect due to pinger malfunction, 

rough estimates of site fidelity can be derived from the movement of red snapper between 

receivers and from fish recapture data. Thirteen of the 125 tagged red snapper moved away from 

their original release location over a period of six months. Therefore a maximum estimate of site 

fidelity for six months is 90%. Assuming that red snapper would continue to disperse from the 

study area during the next six months, a maximum estimate of annual site fidelity would be 80%. 

It is highly unlikely that all red snapper movements were documented by acoustic data or fish 

recaptures, especially if pinger batteries failed, therefore actual site fidelity is likely to be much 

less than these maximum estimates. 

5.1 Management Implications 

One of the most controversial topics in artificial reef theory, and also very important to fishery 

managers if artificial reefs are used as a management tool, is the question of attraction vs. 

production. James Bohnsack first introduced this idea in 1989. He hypothesized that artificial 

reefs could either “provide additional critical habitat that increases the environmental carrying 

capacity and eventually the abundance and biomass of reef fishes” (production) or “attract fishes 

as the result of behavioral preferences but do not significantly increase total fish biomass” 

(attraction). Increased production is one of the underlying rationales behind establishment of 

artificial reefs, but if attraction is actually the case, reef fishes could be subjected to increased 

catchability, thus increased fishing mortality (Bohnsack 1989). 

Attraction and production are not necessarily mutually exclusive; Bohnsack (1989) describes 

them as opposite extremes along a gradient. Numerous factors contribute to the placement of an 

artificial reef or fish population along this gradient such as local habitat availability, the level of 

fishery exploitation, the various life history and behavioral characteristics of each species, and 

whether a population is limited by habitat or recruitment (Bohnsack 1989). Application of these 

53
criteria to red snapper in the GOM may lead one to believe that red snapper lie more toward the 

attraction end of the gradient. The only criterion that points toward production is the sparse 

availability of local reef habitat, at least in the western GOM (Gallaway 1984). The red snapper 

population is heavily exploited and overfished and, according to Bohnsack, such a population is 

less likely to experience increased production. It is also likely that the population is recruitment 

limited due to bycatch mortality in the shrimp fishery rather than habitat limited, again pointing 

towards attraction. Production is more likely for species that are highly faithful to and dependent 

upon reefs. Red snapper are reef-associated, not reef-dependent, and as this study has shown, do 

not exhibit high long-term site fidelity. As mentioned previously, Goodyear (1995) describes a 

shift in the red snapper population center from the eastern GOM in the mid-1900s to the western 

GOM in the late-1900s. It is conceivable that this shift could have been a result of attraction to 

new habitats provided by platforms. But is it feasible that absolutely no additional production 

resulted from the presence of platforms? This answer is likely no, placing red snapper 

somewhere in the middle of the gradient Bohnsack describes. Of course, any increase in 

production could be balanced or negated by an increase in fishing mortality if the catchability of 

red snapper has increased because platforms make them easier to locate. But, if the bottleneck in 

population growth caused by the shrimp fishery were eliminated and if fishing pressure were 

reduced, platforms might function more as sites of production. As we have shown, there are 

many uncertainties and questions left to be answered in the attraction vs. production debate. 

Unfortunately, due to the complexity of the issue, the question of attraction vs. production may 

never be definitively answered for red snapper in the GOM. Consequently, managers should not 

assume that the GOM red snapper population is growing due to the additional habitat created by 

platforms. 

The concept of marine reserves, or no-take marine protected areas, arose in the 1960’s, 

developing into a popular tool for fisheries managers in the 1970’s and 1980’s (Alder 1996; 

Bohnsack 1996; Shipp 2003). Many authors have detailed the immense and varied benefits of 

marine reserves (Bohnsack 1990 in Roberts and Polunin 1991; Ticco 1995; Amos 1998; 

Coleman et al. 2004). Reserves protect the spawning stock biomass within the area, therefore 

providing a recruitment source for surrounding areas and supplemental restocking of fished areas 

through emigration. These protected populations are insurance against management failures in 

fished areas and protection against fishery stock collapse. It is important to remember that all 

vulnerable life stages must be protected as must all habitats and prey species (Carr and Reed 

1993). Reserves can also be used to reduce user conflicts and protect habitat (Coleman et al. 

2004). 

Some groups have suggested the use of marine reserves for red snapper in the GOM. In light of 

the results of this study, we do not believe reserves would be an effective management tool in 

this case. A NOAA Technical Memorandum (Plan Development Team 1990) used red snapper 

as an example of a reef fish in a model to show the effect of fishing on a population: reduction in 

average age and total fecundity per individual. They subsequently recommended placing 20% of 

available reef habitat in marine reserves while utilizing traditional management in the remaining 

80% of habitat. The authors believed the system would preserve the genetic composition of the 

reef fish stocks and protect older fish that are more valuable as egg producers. Unfortunately 

they conducted the study under the assumption that reef fish are sedentary and display high site 

fidelity, stating that marine fishery reserves “are ideally suited for reef fishes” for these reasons. 

54
We have shown this to not be the case for 2-4 year old red snapper, at least those using the 

habitat provided by platforms. 

Yet again in 1996, GOM red snapper were used in a simulation model to demonstrate how a 

combination of effort reduction and establishment of 15-19% of the available habitat as marine 

reserves could be used to protect a fishery (Holland and Brazee 1996). One of the key 

assumptions for the use of red snapper was limited movement of adults. Bohnsack (2000) also 

uses GOM red snapper to show how protection of 30% of the all age and size classes with 

marine reserves is a better tool than minimum size limits. He does explain that the more mobile 

the species the more habitat area must be protected to achieve the requisite 30%. His graphs 

show that for a moderate mobility species, almost 40% of fishing grounds should be closed and 

for a higher mobility species, almost 60% of fishing grounds should be closed. We believe red 

snapper fall into the moderate or higher mobility group. 

The GMFMC has established three marine protected areas intended to reduce the fishing 

pressure on red snapper in the GOM. None were designated for the purpose of increased 

production and none were “no take” reserves; only one seems to have been effective in meeting 

its objectives. Coleman et al. (2004) offer a review and evaluation of these marine reserves; a 

summary follows here. The first reserve was the Reef Fish Stressed Area, designated in 1981. 

This reserve was created to reduce the intense recreational fishing pressure in the near shore reeffish populations, especially red snapper (GMFMC 1981) but the implemented gear restrictions 

were aimed at the commercial sector and rarely used gear types (Coleman et al. 2004). Overall, 

the objective was not addressed by the regulations and recreational fishing effort does not appear 

to have been reduced. The second reserve was the Alabama Special Management Zone (ASMZ), 

designated in 1993, that was intended to decrease recreational vs. commercial user conflicts on 

artificial reefs off the coast of Alabama by limiting commercial access to these areas (GMFMC 

1993). Although formal evaluations of the effectiveness of the ASMZ have not been conducted, 

it appears to be meeting the objective (Coleman et al. 2004). The third reserve is the seasonal 

closure of the recreational and commercial fisheries as was described in the Introduction. The 

objective was to reduce overfishing of red snapper, but the result has merely been a temporal 

shift of effort into the open seasons (Coleman et al. 2004). 

Some scientists believe that dense networks of reserves should be established as protection 

against management failure without assessing whether or not the reserves are likely to function 

as desired (Roberts 1998). If reserves do not produce the predicted results they could actually 

promote management failure by instilling a false sense of security. Coleman et al. (2004) give 

examples of marine protected areas in the GOM that have both failed and succeeded, often due 

to the level and depth of planning before establishment of the reserve. Reserves that do not 

contribute to fishery welfare are a waste of time, effort, and money and restrict fisheries 

unnecessarily (Carr and Reed 1993). Red snapper are not good candidates for marine reserves, 

as some have previously believed. They are highly mobile and exhibit little long-term site 

fidelity; therefore management efforts should be focused on other methods. Shipp (2003) arrives 

at this same conclusion, citing Watterson et al. (1998) and Patterson et al. (2001) as evidence that 

red snapper do exhibit movement away from tagging sites, especially under the influence of 

tropical cyclones. 

55
One fisheries enhancement tool with which some of the Gulf States have experimented is the use 

of decommissioned platforms as artificial reefs (Louisiana Department of Wildlife and Fisheries 

2004; Texas Parks and Wildlife Department 2004). The intent of these artificial reef programs is 

not necessarily to rebuild fish populations, but rather to maintain local habitat and fishing 

opportunities. By 1997, 1,645 platforms had been removed from the GOM; the number of 

platforms on the OCS is predicted to decline 29% from 1999-2023 as yearly removals exceed 

installations by an average of 44 per year (Pulsipher et al. 2001). This decrease in easily 

available fishing opportunities may cause economic stress in the commercial and recreational 

fishing industries. Although red snapper do not display high site fidelity to one specific platform 

they do move from one platform to another. Red snapper are attracted to platforms for some 

reason, and therefore probably derive some sort of benefits from them. The removal of a large 

portion of these platforms may have potential to hinder the recovery and rebuilding of this 

heavily exploited population and must be taken into consideration and thoroughly researched in 

the future. 

In addition, many of the new installations are expected to be located in deeper water, whereas 

most of the removals will be platforms with closer proximity to shore and at lesser depths 

(Pulsipher et al. 2001). Many of these near shore removals will be platforms that were easier and 

safer for fishermen to access. Commercial fishermen, who must fish for their livelihood, will 

likely follow the habitat farther and farther from shore, spending more of their time and income 

and taking greater risks. Recreational fishermen and recreational divers may choose to change 

their habits rather than spend more time and income and take these same risks. According to 

Hiett and Milon (2002) over 20% of all recreational fishing trips and almost 94% of dive trips in 

Texas, Louisiana, Mississippi and Alabama are to platforms. The economic impact to coastal 

counties in these regions from fishing and diving at platforms is $324.6 million, $164.1 million 

of that in personal income, and these industries support 5560 full time jobs (Hiett and Milon 

2002). The same study interviewed for-hire operators (charter and headboat) and dive shop 

owners and found that 85% of for hire operators and 100% of dive shop owners feel their 

industry would be damaged if platforms are removed after decommissioning. For these reasons, 

we believe the Gulf States should expand their artificial reef programs and allow more of the 

decommissioned platforms to remain as valuable habitat for red snapper in the northern GOM. 

The red snapper fishery in the GOM is clearly important to the economy of the Gulf States, as is 

the oil and gas industry. Although red snapper do not show high site fidelity to platforms, they 

do use them as important habitat. The question of whether artificial reefs are sources of 

production for red snapper or sites of attraction is yet unanswered, and is likely to remain so. It 

is important to remember that the catchability of red snapper at platforms may be higher than at 

natural hard bottom because of the ease with which fishermen may locate platforms; an increase 

in catchability would likely lead to an undesirable increase in fishing mortality, therefore this 

subject must be investigated. Neither red snapper nor platforms appear to be good candidates for 

marine reserves, but the artificial reef programs in the Gulf States are beneficial to the red 

snapper fisheries. 

5.2 Recommendations for Future Research 

The mass tagging procedures developed in this study were extremely successful. It was possible 

to tag as many fish as could be captured by 10-12 people fishing at all times. Onboard 

56
anaesthetization and post-surgery recovery eliminated the costly and time-consuming need to 

transport fish back to shore for surgery. MS-222 is appropriate for rapid anaesthetization of red 

snapper. The combination of sutures and veterinary glue adequately closed and sealed the 

wound; examination of fish recaptured and returned whole revealed that the wounds healed well 

enough that the scars on most fish were barely visible and scales were beginning to regenerate. 

Half an hour of post-surgery recovery time in an oxygenated tank was sufficient for fish to return 

to a conscious and alert state. The hoop-net protected the fish from immediate predation and 

guided them to depth and to the platform. 

The GOM offshore environment is often severe and it is difficult to maintain research equipment 

in this setting. Storms are often harsh and prolonged, especially during hurricane season and the 

winter. Equipment must be designed to withstand the wind, waves, and currents generated by 

these storms. Research undertaken at platforms may face additional difficulties such as metal 

degradation by electrolysis. Oil and gas companies deal with this problem by deploying large 

anodes on the underwater portion of the platforms, but these are eroded through time and must 

be periodically replaced. When deploying gear underwater on platforms great care should be 

taken to preemptively combat electrolysis by deploying additional large anodes or using only 

stainless steel of the same series so as to avoid loss of and damage to equipment. 

Another way to eliminate the risk of electrolysis is to deploy equipment independently of 

platforms. Although deploying the receivers on the platforms allowed for easy retrieval of 

equipment, a receiver moored on a buoy a short distance away from the platform would have not 

only eliminated the risk of electrolysis, but would also have reduced the acoustic shadow caused 

by the support pilings and cross-members of the platform which may have shielded some pings 

from detection. An array of receivers moored around the platform would reduce the potential for 

acoustic shadowing to an even greater extent. 

The effect of a temperature gradient on an acoustic signal can be considerable, depending upon 

the strength of the signal, distance traveled, and degree of temperature change. A thermocline is 

a likely culprit for the decrease in ping detection rate the receivers experienced two months into 

the study. Deeper deployment would likely have avoided or lessened this decrease in the rate of 

ping detection. Future studies should deploy receivers deeper to lessen the probability of a 

thermocline developing between the tagged red snapper and the receivers and should collect 

environmental data on a regular basis; ideally, temperature data loggers would be attached to the 

receiver to constantly collect data. Future studies should also attempt to continuously or 

regularly measure the ambient noise in the environment that could interfere with the reception of 

an acoustic signal. 

Due to the malfunction of the pinger batteries, the acoustic data may not have represented true 

presence and absence of red snapper in the study area. Re-examination of the site fidelity 

objectives and hypotheses is warranted and these portions of the study should be conducted again 

if time and finances allow. 

The next step in assessing the habitat value of platforms for red snapper is to determine if 

platforms increase the catchability of red snapper. This information is crucial to the successful 

management of the red snapper population in the northern GOM. If the catchability is increased, 

platforms may not be as beneficial to the red snapper population as is currently thought by many.

57
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