NOAA Technical Report NMFS 75

Codend Selection

of Winter Flounder

Pseudopleuronectes americanus

David G. Simpson

u.S. Department of Commerce

March 1989
NOAA TECHNICAL REPORT NMFS

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34. Additions to a revision of the shark genus Careharhinus: Synonymy of Aprionodon and Hypoprion, and description of a new species of Careharhinus (Carcharhinidae),

by J. A. F. Garrick. November 1985, 26 p.

35. Synoptic review of the literarure on the Southern oyster drill Thais haemastoma

floridana, by Philip A. Butler. November 1985, 9 p.

36. An egg production method for estimating spawning biomass of pelagic fish:

Application to the northern anchovy, Engraulis mordax, by Reuben Lasker (editor).

December 1985, 99 p.

37. A histopathologic evaluation of gross lesions excised from commercially important North Atlantic marine fishes, by Robert A. Murchelano, Linda DespresPatanjo, and John Ziskowski. March 1986, 14 p.

38. Fishery atlas of the northwestern Hawaiian Islands, by Richard N. Uchida and

James H. Uchiyama (editors). September 1986, 142 p.

39. Survey of fish protective facilities at water withdrawal sites on the Snake and

Columbia Rivers, by George A. Swan, Tommy G. Withrow, and Donn L. Park. April

1986, 34 p.

40. Potential impact of ocean thermal energy conversion (OTEC) on fisheries, by

Edward P. Myers, Donald E. Hass, Walter M. Matsumoto, David S. Peters, Michael

P. Seki, Richard N. Uchida, John D. Ditmars, and Robert A. Paddock. June 1986,

33 p.

41. A stationary visual census technique for quantitatively assessing community strucrure of coral reef fishes, by James A. Bohnsack and Scott P. Bannerot. July 1986, 15 p.

42. Effects oftemperarure on the biology ofthe northern shrimp, Pandalus borealis,

in the Gulf of Maine, by Spencer Apollonio, David K. Stevenson, and Earl E. Dunton,

Jr. September 1986, 22 p.

43. Environment and resources of seamounts in the North Pacific, by Richard N.

Uchida, Sigeiti Hayasi, and George W. Boehlert (editors). September 1986, 105 p.

44. Synopsis of biological data on the porgies, Calamus aretifrons and C. proridens

(Pisces: Sparidae), by George H. Darcy. September 1986, 19 p.

45. Meristic variation in Sebastes (Scorpaenidae), with an analysis of character

association and bilateral pattern and their significance in species separation, by Lochai Chen. September 1986, 17 p.

46. Distribution and relative abundance of pelagic nonsalmonid nekton off Oregon

and Washington 1979-84, by Richard D. Brodeur and William G. Pearcy. December

1986, 85 p.

·~7. Reproduction, maruration, and seed production of culrured species: Proceedings

of the twelfth U.S.-Japan meeting on aquaculrure, Baton Rouge, Louisiana, October

25-29, 1983, by Carl J. Sindermann (editor). February 1987, 73 p.

48. Widow rockfish: Proceedings of a workshop, Tiburon, California, December

11-12, 1980, by William H. LenalL and Donald R. Gunderson (editors). January 1987,

57 p.

49. Reproduction, movements, and population dynamics of the southern kingfish,

Menticirrhus amerieanus, in the northwestern Gulf of Mexico, by Stephen M. Harding

and Mark E. Chittenden, Jr. March 1987,21 p.

50. Preparation of acetate peels of valves from the ocean quahog, Aretiea islandiea,

for age determinations, by John W. Ropes. March 1987, 5 p.

51. Starus, biology, and ecology of fur seals. Proceedings of an international

workshop, Cambridge, England, 23-27 April 1984, by John P. Croxall and Roger

L. Gentry (editors). June 1987,212 p.

52. Limited access alternatives for the Pacific groundfish fishery, by Daniel D.

Huppert (editor). May 1987, 45 p.

53. Ecology of east Florida sea rurtles: Proceedings ofthe Cape Canaveral, Florida,

sea rurtle workshop, Miami, Florida, February 26-27, 1985, by Wayne N. Witzell

(convener and editor). May 1987, 80 p.

54. Proximate and fatty acid composition of 40 southeastern U.S. fmfish species,

by Janet A. Gooch, Malcolm B. Hale, Thomas Brown, Jr., James C. Bonnet, Cheryl

G. Brand, and Lloyd W. Reiger. June 1987,23 p.

55. Proximate composition, energy, fatty acid, sodium, and cholesterol content of

fmfish, shellfish, and their products, by Judith Krzynowek and Jenny Murphy. July

1987, 53 p.

56. Some aspects of the ecology of the leatherback rurtle Dermochelys eoriacea at

Laguna Jolova, Costa Rica, by Harold F. Hirth and Larry H. Ogren. July 1987, 14 p.

57. Food habits and dietary variability of pelagic nekton offOregon and Washington,

1979-1984, by Richard D. Brodeur, Harriet V. Lon, and William G. Pearcy. July

1987, 32 p.

58. Stock assessment of the Gulf of Menhaden, Brevoortia palronus, fishery, by

Douglas S. Vaughan. September 1987, 18 p.

59. Atlantic Menhaden, Brevoortia tyrannus, purse seine fishery, 1972-84, with a

brief discussion of age and size composition of the landings, by Joseph W. Smith,

William R. Nicholson, Douglas S. Vaughan, Donnie L. Dudley, and Ethel A. Hall.

September 1987, 23 p.

60. Gulf Menhaden, Brevoorria patronus, purse seine fishery, 1974-85, with a brief

discussion of age and size composition of the landings, by Joseph W. Smith, Eldon

J. Levi, Douglas S. Vaughan, and Ethen A. Hall. December 1987, 8 p.

61. Manual for starch gel electropboresis: A method for the detection of genetic variation, by Paul B. Aebersold, Gary A. Winans, David J. Teel, George B. Milner, and

Fred M. Utter. December 1987, 19 p.

62. Fishery publication index, 1980-85; Technical memoradum index, 1972-85, by

Cynthia S. Martin, Shelley E. Arenas, Jacki A. Guffey, and Joni M. Packard.

December 1987, 149 p.

63. Stock assessment of the Atlantic Menhaden, Brevoorria tyrannus, fishery, by

Douglas S. Vaughan and Joseph W. Smith. January 1988, 18 p.

64. illustrated key to penaeoid shrimps of commerce in the Americas, by Isabel Perez

Farfante. April 1988, 32 p.

65. History of whaling in and near North Carolina, by Randall R. Reeves and Edward

Mitchell. March 1988, 28 p.

66. Atlas and zoogeography of common fishes in the Bering Sea and Northeastern

Pacific, by M. James Allen and Gary B. Smith. April 1988, 151 p.
NOAA Technical Report NMFS 75

Codend Selection

of Winter Flounder

Pseudopleuronectes americanus

David G. Simpson

March 1989

U.S. DEPARTMENT OF COMMERCE

Robert Mosbacher, Secretary

National Oceanic and Atmospheric Administration

William E. Evans, Under Secretary for Oceans and Atmosphere

National Marine Fisheries Service

James Brennan, Assistant Administrator for Fisheries
The National Marine Fisheries Service (NMFS) does not approve, recommend

or endorse any proprietary product or proprietary material mentioned in this

publication. ~o reference shall be made to NMFS, or to this publication furnished

by NMFS, in any advertising or sales promotion which would indicate or imply

that NMFS approves, recommends or endorses any proprietary product or proprietary material mentioned herein, or which has as its purpose an intent to cause

directly or indirectly the advertised product to be used or purchased because of

this NMFS publication.

11
Contents

Introduction 1

Methods 2

Results 5

Winter flounder retention rates in five codends 5

Comparison of observed and expected commercial length-frequencies

from three codends 6.

Yield-per-recruit 7

Discussion 8

Citations 10

iii

Codend Selection

of Winter Flounder

Pseudopleuronectes • amencanus

DAVID G. SIMPSON

Marine Fisheries Program

Connecticut Department of Environmental Protection

P.O. Box 248

Waterford, Connecticut 06385

ABSTRACT

Codend selection of winter flounder (Pseudopleuronectes americanus) in 76-127 mm mesh codends was examined from experiments conducted in Long Island Sound during the spring of

1986-87. The results show a slightly larger size at selection than

was found in earlier work as indicated by the selection factor,

2.31 in the present study compared with 2.2 and 2.24 from

previous studies. Diamond mesh was found to have a length at

50% retention about 1 cm longer (Lso =22.6 cm), and a selection range (3.4 cm) about 1 cm narrower, than square mesh

in 102-mm codends. Tow duration varied from 1 to 2 hours using

114-mm diamond mesh. As has been found in previous studies,

tow duration and Lso are positively related, with I-hour tows

averaging 24.6 cm and 2-hour tows averaging 26.6 cm. The importance of the slope of the selection curve was examined in

yield-per-recruit analyses by comparing knife-edge and stepwise recruitment. In all mesh sizes, stepwise recruitment provides a more conservative estimate of yield in the presence of

a minimum size limit. Differences in yield estimates between

the two models were generally small (1-7%), except in the largest

mesh size, 127 mm, where yield is overestimated by 10% when

assuming knife-edge recruitment.

Introduction _

The importance of mesh size, particularly codend mesh size,

in determining the length at first capture was documented

early in development of the otter trawl fishery (Todd 1906-

1908, Russell and Edser 1926, Herrington 1935). Codend

selection of winter flounder (Pseudopleuronectes americanus)

has generally been addressed as part of larger selection experiments on several species but targeting large species such

as cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) (Smolowitz 1983, McCracken 1963), or summer

flounder (Paralichthys dentatus) (Anderson et aI. 1983).

McCracken (1963) collected data in 1954 and 1955 on natural

fiber codends (untreated manila and tarred cotton) between

111 and 124 mm stretch mesh measure. Although the data

were insufficient for precise estimates oflength at 50% retention (Lso), the author suggested a selection factor, defined

as Lso divided by the stretch mesh size, of 2.0 for doublestranded manila twine.

Smolowitz (1983) examined winter flounder selection

in nylon-constructed codends of 114-mm nominal mesh

(103-mm stretch measure) and 154-mm nominal mesh

(133-mm stretch measure). Size selection in the 114-mm

mesh was evaluated using the covered codend method only,

while the 154-mm mesh codend was examined using both

covered codend and alternate haul methods. Covered codend

experiments gave lengths at 50% retention 2 to 3 cm shorter

than the alternate haul method for 154-mm mesh. Similar

results were observed for all species considered, including

the other flatfish species, yellowtail flounder (limandaferruginea) and American plaice (Hippoglossoides platessoides).

Knife-edge recruitment is often assumed when assessing

the effect of a codend mesh-size regulation on long-term yield

(Gulland 1963, 1976). Gulland (1963) showed that there is

little difference between the calculated yield assuming knifeedge recruitment and that estimated using approximations

to the selection ogive. Gulland concluded that only basic information on codend selection (i.e., the selection factor) is

required to evaluate changes in yield resulting from moderate

changes in codend mesh size or fishing rate unless a large

proportion ofthe yield comes from within the selection curve.

However, no minimum legal fish size was considered in these

comparisons so that all fish retained by the codend contributed to yield. Assuming knife-edge recruitment, without

minimum size limits, results in a more conservative estimate

of long-term yield than stepwise recruitment.

Several conditions exist today in the winter flounder fishery

which may diminish the utility of yield assessments assuming knife-edge recruitment, particularly when large mesh

sizes are considered, since a large proportion of the yield

comes from fish within the selection range. Winter flounder

are fully exploited through most oftheir range (NEFC 1986)

with evidence of growth overfishing in some areas (NEFC

1986, Gibson 1987). Winter flounder length-frequencies have

become truncated both inshore (Foster 1986) and on Georges

Bank (Gabriel and Foster 1986) as landings decline in the

largest market categories, "large" and "lemon sole."
71° 30' W

Connecticut

Rhode

Island

L--J

10 mi

Figure 1

Long Island Sound and area subdivisions for conunercial and survey catch data.

Finally, minimum fish size-regulations are enforced in many

areas, both inshore and offshore. Fish below the minimum

size that are retained by the trawl do not contribute to yield

but are subjected to high discard mortality (Jean 1963, Lux

1968, Powles 1969).

In Long Island Sound (Fig. 1) the length-frequency of

winter flounder is highly skewed to the left, with between

77% (Area 1) and 95% (Area 3) of the population (by number) below the minimum legal size (27.9 cm) at the height

of the spring fishery. The proportion of fish below 21 cm

ranges from 34% (Area 1) to 73% (Area 3) (by number) during the same period (CT Dep. Environ. Prot., Mar. Finfish

Surv., unpubl. data 1986).

The Long Island Sound fishery is highly dependent on

winter flounder near the minimum legal size with 72 % (by

weight) of the landed catch coming from fish between 28

and 32 cm (CT Dep. Environ. Prot., unpubl. data). Knowledge of both position and slope ofthe codend selection curve

is critical in managing this fishery, given the abundance of

small winter flounder in Long Island Sound and the dependence of the fishery on fish near the minimum size limit.

The codend selection experiments on winter flounder mentioned above (Smolowitz 1983, Anderson et al. 1983), using

codends constructed of synthetic material, concentrated on

large mesh codends that are generally too large for winter

flounder. The smallest ofthese codends (114 mm) examined

by Smolowitz (1983) is within the range considered reasonable for winter flounder (with a minimum size limit of27.9

2

cm), but resJ,llts for this mesh size are based on covered

codend experiments which tend to give smaller sizes at retention than the alternate haul method. Additionally, experiments

with all mesh sizes suffered from low sample sizes for fish

less than 21 cm. The low availability ofthese small fish was

particularly troubling in the 114-mm mesh codend experiments where only 50 fish, comprising the left half of the

selection curve, were taken in the codends and covers from

all replicates combined.

Because of the problems associated with these selection

experiments (large mesh size, low availability ofsmall fish)

on winter flounder and the declining average size of these

fish in many areas of the Northwest Atlantic, the Connecticut Department of Environmental Protection initiated selection experiments using a range of mesh sizes (76-127 mm

nominal mesh size) considered more appropriate for the

winter flounder fishery. These mesh sizes encompass the

range generally used in the Southern New England mixedspecies fishery and consequently reflect the mesh sizes to

which winter flounder are exposed in this region.

Methods _

Winter flounder (Pseudopleuronectes americanus) were

collected only during daylight hours to reduce changes in

catchabi1ity between day and night trawling during these experiments (Sissenwine and Bowman 1978). A total of 24
I-hour tows were conducted between 5 May and 8 May 1986

to examine the selection properties of four codends (76, 102,

and 127-mm diamond and 102-mm square mesh). From 27

April to 7 May 1987 a total of 21 tows (12 I-hour and 9

2-hour) were made to determine the selection properties of

a 114-mm diamond mesh codend and to evaluate the effect

oftow duration on codend mesh selection. All codend mesh

sizes cited in this document are nominal mesh size, the size

(metric equivalent) referred to by the manufacturer unless

otherwise noted.

All tows were made from the 12.8-m (42-ft) State of Connecticut research vessel James P. Galligan II using a combination sweep Wilcox "V" wing high-rise otter trawl.

Trawl specifications include a 15.2-m (50-ft) headrope and

an 18.3-m (60-ft) footrope. The net was constructed of No.

21 nylon with mesh size decreasing from 127 mm in the

wings and belly to 102-mm, with a 76-mm tailpiece. The

net was spread by 1.2-m (4-ft) steel "V" doors attached to

18.3-m (60-ft) ground wires, of9.5-mm (Ys") diameter, and

27A-m (90-ft) scissors. The top legs of the scissors were

6.35-mm (1,4") diameter wire, and the bottom legs were

51-mm (2") rubber cookies. The sweep had one strand of

6.35-mm chain in the wings and two strands of9.5-mm chain

in the belly. All tows were made at a speed of 2.8 to 3.0

knots over ground as determined by Loran C (Northstar

6000). The experimental codends were constructed of No.

84 braided nylon, while the control codend was No. 54

braided nylon. Codends were changed using a "zipper" attachment to the tailpiece of the trawl.

All winter flounder taken were measured to the nearest

1 cm total length. In 1986, 30 codend meshes were measured,

at random and when wet, after the last tow of the day for

that codend. Meshes were measured to 0.1 mm, stretched

with moderate thumb pressure opening the SPI 30-458-4

vernier stainless steel calipers. In 1987, 20 meshes were

measured wet, in the same manner as 1986, after six tows

(3 I-hour and 3 2-hour duration).

Selectivity of winter flounder was compared between a

51-mm (2 in) control codend and five experimental codends-76 mm (3 in), 102 mm (4 in), 114 mm (4.5 in), and

127 mm (5 in) diamond, and 102 mm (4 in) square mesh.

One experimental codend (1986 samples) or tow duration

(1987 samples) was examined each day. In 1986, six tows

were taken daily for four consecutive days, three tows using

the experimental (E) codend for that day and three using the

51-mm control (C). Each experimental tow was paired with

a control to determine a retention rate for each tow at 1-cm

length intervals. Tows were ordered EC, CE, EC to minimize the number of codend changes as well as temporal

variation between experimental and control tows. In order

to increase the number of experimental tows each day from

three to four in 1987, two experimental (114 mm-mesh) tows

were compared with a single control tow. One-hour tows

were then treated in two groups and ordered ECE, ECE.

Three 2-hour tows were made each day resulting in a single

group and ordered ECE.

3

The retention rate in each experimental codend tow was

expressed as a percentage of the control codend catch in each

1-cm length interval (LI) (Gulland 1976). An initial retention rate was calculated for each LI and experimental tow

separately, providing one observed retention rate at each LI

for every experimental tow made. From these initial retention rates, a three-point moving average (Anderson et al.

1983) was then calculated for each LI (>10 cm) in each experimental tow. All observations subsequent to the first total

length at which the three-point moving average reached or

exceeded 100% retention were excluded. The remaining

observation greater than or equal to 100% in each trial was

then set to 99 % to accommodate the linearized form of the

sigmoid models used to calculate initial parameter estimates.

An apparent increase in trawl efficiency with increasing mesh

size has been observed for some species by Clark (1963) and

Templeman (1963), suggesting that retention rates for large

codends could exceed 100% when compared with small

mesh. However, in the present investigation, the number of

winter flounder larger than the length at 100% retention for

the experimental codend did not differ significantly (using

chi-square) between the experimental and control codends.

This suggests that no increase in trawl efficiency with codend mesh size occurs for winter flounder and that an asymptotic retention rate of 100% is appropriate for this species.

A logistic growth curve (Eq. 1) was fitted to the winter

flounder retention data for all codends using nonlinear least

squares (Ralston and Jennrich 1979). Initial parameter estimates were derived using the regression techniques of

Ratkowsky (1983).

Percent retained = A/(l + exp(B + Y * TL» (1)

where Percent retained = percent retained in the codend

A = asymptote (100% retention)

exp = 2.7818

B = slope at the inflection point

Y = rate of exponential decay

TL = total length interval (em).

The codend selection factor (Jensen 1949) describes the

linear relationship between Lso and codend mesh size. A

codend selection factor can be calculated for any species from

experiments with a small number of codends (Gulland 1976).

The selection factor (b) (Eq. 2) for winter flounder was calculated using the mean of the stretch mesh measurements

for each diamond mesh codend. The 114-mm codend selection factor is based on I-hour tows only.

(2)

where b = codend selection factor

Lso = total length at 50% retention (mm)

m = codend mesh size (mm).
Ltj = Lmax (l - exp(-K(Age-to» (3)

Table 2

Parameter estimates and standard errors (SE) for the logistic equation, percent retained=A/(I+exp(B+Y*TL», relating winter

flounder retention rate and total length by mesh size, configuration,

and tow duration. Asymptote (A) equals 100%; slope at the inflection point (B) and rate of exponential decay (Y) estimated by

nonlinear regression analysis.

where Ltj = total length attained quarterly

Lmax = 402

exp = 2.7818

K = 0.43

Age = age at the midpoint of each quarter year

to = -0.485.

No. of

tow SE SE

Codend (mrn) pairs B of B Y of Y r 2

76 2 13.98 1.43 -0.79 0.08 0.96

102 (square mesh) 3 11.01 1.79 -0.51 0.08 0.78

102 3 14.92 1.68 -0.66 0.07 0.92

114 (l-h tows) 8 12.84 1.29 -0.52 0.05 0.83

114 (2-h tows) 5 8.78 0.81 -0.32 0.03 0.79

127 3 12.41 1.51 -0.44 0.05 0.88

(4)

where 10gIO Wt = 10gIO weight at length Ltj

a = -2.261

b = 3.226

Ltj = as above.

10gIO Wt = a + b(loglO Ltj )

Yield-per-recruit analyses were performed using Thompson and Bell's method (Ricker 1975) to assess the effect of

codend mesh size on equilibrium yield-per-recruit in the

winter flounder fishery. These analyses were performed

twice, first assuming knife-edge recruitment at the age corresponding to Lso and then using a stepwise approximation

to the selection ogive to simulate a gradual recruitment process. Four "steps" in recruitment were used each year based

on the retention rate corresponding to the mean length-atage at the midpoint of each quarter. Yield was also calculated

quarterly, from 1,000 recruits beginning at age-1 and terminating at age-12. All fish retained which were greater than

the minimum fish size limits enforced in Connecticut (25.4

cm recreational and 27.9 cm commercial) contributed to

yield. The trawl fishery comprised 65 % of fishing mortality, and recreational made up the remaining 35 % (NEFC

1986). Natural mortality (M) was held constant at M = 0.327

(Gibson 1987). The average weight at the midpoint of each

quarter was calculated using the von Bertalanffy length-age

relationship (Eq. 3), and the length-weight relationship of

Eq. 4 (NUSCo 1986).

Table 1

Sample sizes of winter flounder lengths for each control and experimental tow. One control tow was used for two experimental tows

in 114-mm mesh tow groups.

Winter flounder lengths (cm)

Tow

Codend (mrn) group Experimental Control (51 mrn)

76 1 96 139

2 145 258

102 (square mesh) 1 112 719

2 115 606

3 191 572

102 1 122 151

2 93 514

3 43 570

114 (l h) 1 143 57 617

2 102 58 737

3 125 126 470

4 95 34 311

114 (2 h) 1 137 73 1004

2 118 764

3 114 81 608

127 1 29 347

2 27 411

3 25 394

Selection curves based on data collected under controlled

conditions (vessel size, trawl configuration, tow duration,

geographic area, and time period) must reflect the size selection operating in the commercial fishery if they are to be

useful. To evaluate the usefulness of these selection curves

in describing the actual selection properties of different size

codends when operating in the fishery, data on commercial

length-frequencies were compared with length-frequencies

from trawl survey cruises conducted monthly in Long Island

Sound by the Connecticut Department of Environmental Protection using a 51-mm mesh codend. Commercial vessels

measuring 11.0 to 25.3 meters using 76, 102, and 114-mm

codends were sampled at sea on eight occasions (24 tows)

in areas 2 and 3 of the sound (Fig. 1) between March and

May, 1985-87. Survey length-frequencies were taken from

April (the earliest month sampled in the survey) and May

1986. Comparisons were made during this Spring period only

and examined separately by area to reduce the spatial and

temporal variability associated with seasonal movements. The

commercial length-frequency was expressed as the cumulative percentage of fish in each 1-cm interval for each mesh

size and area. Expected length-frequencies were generated

by multiplying the frequency in each I-em interval of the

survey distribution by the probability of capture determined

by the selection curve for the appropriate mesh size. This

expected frequency was then expressed as a cumulative percentage to allow comparison with the observed commercial

cumulative length-frequency.

4
76nm

...

...

102mm

127mm

10 15 20 25 30 35 40 10 15 20 25 30 35 40

FISh Length (em)

Figure 2

Selection curves for winter flounder in 76-127 mm diamond mesh codends, with plots of percent retention-at-length data.

Recruitment to the fishing grounds was assumed to be age-I.

Two fishing mortality rates (F) were used for comparison,

F = 0.38, and F = 0.551. These rates are the estimated optimum (when a Shepard stock-recruitment function is merged

with yield-per-recruit, and length-at-first-eapture = 280 mm)

and present fishing rates, respectively (Gibson 1987). For

simplicity, both fishing and natural mortalit~ were assum~d

to be constant throughout the year. Two dIscard mortalIty

rates (50 and 100%) were also used to evaluate the importance of this variable on yield-per-recruit.

Results _

A total of 11,916 winter flounder were captured in 45 tows

(I9 control, 26 experimental) during mesh selection experiments in 1986 (6,142 fish124 tows) and 1987 (5,774

fish/21 tows). Large catches in the control codends (Table

1) and a wide distribution in the length-frequencies (9-45 cm)

made an adequate assessment ofretention rates possible. Two

experimental tows (76 mm, and 114 mm 2-hour) were excluded from analysis due to poor net performance (76 mm)

and excessive mudding (114 mm 2-hour).

5

Winter flounder retention rates

in five codends

The logistic model (Table 2) provided significant fits (P <

0.01) to retention data from each mesh size examined (t-test

of correlation coefficient, Rohlf and Sokal 1969). Codend

size selection of winter flounder exhibited a fairly uniform

shift toward greater size at selection with the use of larger

codends (Fig. 2). The selection factor varied without trend

between 2.23 (127-mm codend) and 2.38 (114-mm codend)

with a mean of2.31 (Table 3). This selection factor is larger

than that reported for winter flounder by Smolowitz (1983)

who reported a range ofselection factors from 2.04 to 2.27,

and suggested an overall selection factor of 2.2. This selection factor is also larger than was found by Anderson et al.

(1983) who found the selection factor to vary from 2.13 to

2.36 with a mean of 2.24.

The selection range (the difference in total length between

75% and 25%, retention) varied between 2.8 (76-mm mesh)

and 5.0 cm (127-mm mesh). The selection ranges in the other

codends were 4.3, 3.4, and 4.3 for 102-mm square, l02-mm

diamond, and 114-mm (I-hour tow duration) mesh size

codends, respectively. A significant (r = 0.88, P < 0.05)

positive trend in selection range with mesh size was detected
Table 3

Codend mesh size (nominal) and mean measured stretch mesh dimensions of diamond mesh codends used in calculating the selection factor. Additionally, the winter flounder length at 50% retention (Lso)

and the calculated selection factor.

Stretch mesh

Codend size measure Selection

(nominal) (mm) Lso factor

76 74.7 177 2.36

102 98.7 226.1 2.29

114 103.9 246.9 2.38

127 126.7 282.1 2.23

--

Mean 2.31

76

I [J II:

§ [J

D.

26 Square 0- --

DIamond 0-

10 16 20 26 30 36 40

FIsh Length (em)

Figure 3

Selection curves for winter flounder in I02-mm diamond and I02-mm

square mesh configuration codends, with plots of percent retention-atlength data.

by (-test of correlation coefficient (Rohlf and Sokal 1969).

An increase in selection range with mesh size has also been

shown for several important species in the Northeast Atlantic (Robertson and Stewart 1986).

Diamond and square mesh codend configurations were

compared using 102-mm mesh codends. The square mesh

codend retained smaller fish than the diamond mesh, though

the difference was small (0.9 cm at Lso ). At the same time,

the selection range is larger in the square mesh codend configuration than in the diamond mesh (as indicated above)

resulting in a smaller size at selection at retention rates less

than 75 %, while the size at 75 % selection or greater is similar

to the 102-mm diamond mesh (Fig. 3).

The effect of tow duration on codend selection was to increase Lso by 2 cm (Fig. 4). The Lso was 24.6 cm in I-hour

tows and 26.6 cm in 2-hour tows. Clark (1957) and Beverton (1964) observed similar increases in size at selection with

increased tow duration.

75

I

II:•

J

Hlotr 1 --

2-hou' 2 - - -

2

Figure 4

Selection curves for winter flounder in 1 and 2-hour tows using 114-mm

mesh codend..~, with plots of percent retention-at-length data.

When comparisons were made with l02-mm mesh in Area

3, a bias toward underestimating the proportion ofthe population less than 24 cm was apparent. The observed median

length in this case was 1.5 cm smaller than expected (22.5

cm observed, 24 cm expected). The observed and expected

catch-compositions were in good agreement above the median length. Fish between 16 cm and 21 cm made up a larger

than expected proportion of the commercial catch as evidenced by the steeper slope in the observed cumulative

length-frequency in this region of the curve (Fig. 5).

FIsh Length (em)

10 16 20 26 30 36 40

6

Comparison of observed and expected

commercial length-frequencies

from three codends

Agreement between observed and expected commercial

length-frequencies from 76, 102, and 114 mm mesh codends was generally good (Fig. 5). Commercial lengthfrequencies are based on 24 tows from which a random

subsample of 2,826 winter flounder were measured (Table

4). The best agreement between observed and expected

catches was seen in Area 3 using 76-mm mesh codends where

little deviation occurred.

Commercial catches in 102-mm mesh were compared with

expected catches in two different areas. In Area 2, 50% of

the commercial catch was less than 26.8 cm, 1.3 cm larger

than expected (25.5 cm). The expected length-composition

at first slightly underestimated the proportion each length increment contributed to the catch; then at approximately 23

cm, the contribution of each length increment was slightly

overestimated.
Area 2

114mm

Area 2

102mm

/ '" /

I

I

I

I

I

I

'" / '" I

I

I

I

I

I

I

I

I

/

'" '"

Area 3

76rml

Area 3

102mm

I

I

I

I

I

I

I

I

'"

Obe.-

Pred. - - -

5

75

10 15 20 25 30 35 40 45 10 15 20 25 30 35 40 45

FISh Length (em)

Figure 5

Observed and expected commercial length-frequency distribution from Areas 2 and 3 of Long Island Sound using 76, 102,

and 114-mm mesh codends.

Two vessels were sampled on three occasions using

I14-rom mesh codends. The observed median length was approximately 24 cm, 2.5 cm shorter than was expected (26.5

cm). Observed and expected catch-compositions diverge

between 16 cm and 25 cm, then slowly converge thereafter.

A systematic bias toward underestimating the proportion of

smaller fish «25 cm) in the catch was apparent (Fig. 5).

Yield-per-recruit

The effect of codend mesh size on yield-per-recruit was determined at two levels of discard mortality (50 and 100%) and

two fishing rates (F = 0.38 and F = 0.551) using the Thompson and Bell method (Ricker 1975). Yield-per-recruit was

calculated first, assuming knife-edge recruitment at the age

corresponding to Lso for each codend, then using a stepwise

approximation to the selection ogive (Table 5). Knife-edge

recruitment consistently overestimated the yield-per-recruit,

although the differences from stepwise recruitment were

generally less than 5%. The largest difference, 10%, occurred in I27-rom mesh at the higher levels of discard and

fishing mortality. No difference in yield could be detected

between 102-rom square mesh and 102-rom diamond mesh

Table 4

Characteristics of commercial sea samples used in comparing observed and predicted winter flounder length-frequencies from trawl

vessels.

Codend Vessel Avg. tow No. of Sample

(nun) Area size (m) time (h) tows size

76 3 11.6 2.0 2 188

102 2 11.0 2.0 4 383

102 2 12.2 1.0 5 360

102 3 25.3 3.5 2 234

102 3 25.3 2.5 2 393

114 2 15.2 2.5 4 485

114 2 21.9 2.5 2 304

using knife-edge recruitment. This is because the small increase in Lso (l cm) from the former to the latter mesh configuration occurs within the quarter-year time interval used

in computing yield.

The relative increases in yield-per-recruit associated with

increasing mesh size were similar using knife-edge and

7
Discussion _

Table 5

Effect of codend mesh size restrictions on yield-per-recruit and a

comparison of differences in yield-per-recruit estimates based on

assumptions of knife-edge (KR) and stepwise (SR) recruitment.

stepwise recruitment except when progressing from 114 mm

to 127 mm mesh, particularly at the higher fishing and

discard mortality rates. In this case a 19 % increase in yield

is expected assuming knife-edge recruitment, while a more

conservative 11 % gain, is estimated from the stepwise

recruitment model.

The results of this study indicate a slightly larger selection

factor (2.31) for winter flounder than the 2.2 found by

Smo10witz (1983) or the 2.24 found by Anderson et al.

(1983). A substantially larger LSD is estimated in this study

using 114-mm (4.5 in.) nominal mesh than was found by

Smolowitz (1983) using the same size mesh (LSD = 247 mm

versus LSD = 208 mm). Much of this disparity is probably

due to differences in experimental design, with Smolowitz

(1983) based on the covered codend method, while this study

and Anderson et al. (1983) employed the alternate haul

method.

The codend mesh measurement is an important component in the calculation of the selection factor, and differences

in this measurement can result in very different selection factors. The mean stretch mesh measure ofthe 114-mm codend

(103.9 mm) was only slightly larger than the 103 mm reported by Smolowitz (1983), even though different methods

were used in measuring mesh size [vernier calipers under

moderate thumb pressure in this experiment, and an ICES

gauge with 4 kg pressure used by Smolowitz (1983)]. The

small difference in stretch mesh measure «0.01 %) between

these methods for codends of the same nominal mesh size

is insufficient to alter conclusions about the selection factor

because of differences in measurement methods.

The logistic curve has been suggested to describe the relationship between fish length and percent retention (Holden

1971) and has been used to describe this relationship for haddock (Melanogrammus aeglefinus) and whiting (Merlangius

merlangus) (Robertson and Stewart 1986). There are some

distinct advantages to this method of curve-fitting over fittingby-eye as is commonly done in codend selection studies. The

fit of the curve to the data can be objectively evaluated, and

standard errors about the parameter estimates can be calculated. Another advantage is that the logistic equation can

easily be incorporated into yield-per-recruit, spawning stock

biomass-per-recruit, or other models to simulate a gradual

or stepwise recruitment into a fishery in order to evaluate

the effect that gradual recruitment into the fishery may have

on these assessments.

The significant increase in selection range with mesh size

detected in this study has been observed by other researchers

for haddock (Melanogrammus aeglefinus) (Clark et al. 1958,

Holden 1971) and whiting (Merlangius merlangus) (Holden

1971). Jones (1963) examined factors affecting the shape of

selection curves. He found the number of escape attempts

and the range of mesh shapes in a codend to be among the

important factors. The increase in selection range with mesh

size observed with winter flounder in the present study is

likely due to the. increasing range of possible mesh openings

that accompanies an increase in mesh size as the codend fills,

changing its shape from elongated to bulbous.

The larger LSD in 102-mm diamond mesh compared with

102-mm square mesh is probably due to the laterally compressed shape of flounder which allows greater escapement

through the elongate diamond meshes. It appears that the

greater selection range observed in square mesh may be due

to the greater range in mesh shape possible in square mesh

versus diamond mesh. It is generally felt that square mesh

has a smaller possible range in mesh shape (Robertson and

Stewart 1986). However, during the first 15 minutes or so

ofthe I-hour tow duration used in this study, there is relatively little weight in the codend to restrict the meshes from being stretched into an elongate shape by a fish squeezing

through the mesh. By the time the square mesh shape has

been set by the accumulating weight in the codend, the

meshes have ranged in shape from completely elongate, as

when a flounder squeezes through the mesh, to square. Diamond mesh is naturally elongated at the beginning of each

tow, then gradually is stretched into a less elongated diamond

shape, around and just forward of the accumulating catch,

never reaching the square mesh configuration. The result is

the broader selection range ofsquare mesh evident in Figure

3, where larger fish are better able to escape from square

mesh early in a tow; but as the square mesh shape becomes

set by the weight of the catch, fish of all sizes are retained

at an increasing rate.

Yield (kg)

Knife-edge Stepwise KR/SR

76 0.095 0.093 1.02

102 (square) 0.101 0.099 1.02

102 0.101 0.100 1.01

114 0.104 0.103 1.01

127 0.110 0.106 1.04

76 0.082 0.079 1.04

102 (square) 0.092 0.088 1.05

102 0.092 0.091 1.01

114 0.098 0.096 1.02

127 0.110 0.103 1.07

76 0.097 0.095 1.02

102 (square) 0.106 0.103 1.03

102 0.106 0.105 1.01

114 0.111 0.110 1.01

127 0.121 0.115 1.05

76 0.078 0.074 1.05

102 (square) 0.093 0.087 1.07

102 0.093 0.091 1.02

114 0.102 0.099 1.03

127 0.121 0.110 1.10

100

100

Discard Codend

mortality mesh

F (%) size (mm)

0.551 50

0.38 50

8
Increasing tow duration in experiments with a 114-mm

codend acted to increase Lso by 2 cm. Several other investigators have observed similar results from increasing tow

duration (Clark 1957, 1963; Beverton 1964; Pope and Hall

1966). The selection range also increased with tow duration,

from 4.3 cm to 6.6 cm. The positive relationship between

Lso and tow duration can be attributed to fish making

repeated attempts to escape and thereby having a greater

probability of escaping in a longer tow (Pope et al. 1975).

The reason for the increase in selection range is less clear,

but examination of the scatter plot for 2-hour tows (Fig. 4)

reveals greater variability in retention rates as fish length increases. Given enough time, the greater vigor of larger fish

(>30 cm) appears to allow many of them to squeeze through

the meshes.

An increase in the weight of the catch in a tow can act

to lower Lso (Clark et al. 1958, 1963), although several

papers indicate little or no effect of catch weight on size at

selection (McCracken 1963, Hodder and May 1964, ICES

1965, Pope and Hall 1966). Pope et al. (1975) noted that

this relationship has only been observed in covered codend

experiments where cover masking may playa role in lowering Lso. The weight of the catch had no apparent negative

effect on Lso in these experiments between 1 and 2-hour

tows as evidenced by the increase in Lso with tow duration

noted above.

Comparisons of observed and expected commercial winter

flounder length-composition are dependent on a general

seasonal pattern of availability and as such are subject to error

due to daily fluctuations in length-composition. Such comparisons were possible for winter flounder because their

movements are not related to size, at least for adult fish (Saila

1961). Very small fish, at least to 10 cm in length, usually

occur near shore (Warfel and Merriman 1944), so fish below

this size were excluded from these comparisons.

The comparisons which showed the best agreement were

from small vessels with average tow durations of 1 to 2 hours,

while larger vessels with average tow durations of2.5 to 3.5

hours characterized the data sets in which agreement was

generally not as good (Table 4).

Vessel size and trawl size alone have been shown not to

significantly affect selectivity (McCracken 1963, Pope and

Hall 1966), and tow duration has already been shown to be

positively related to Lso. However, Pope et al. (1975) suggest that a light trawl aboard a small vessel is likely to have

a different selection factor than a large trawl aboard a large

vessel.

It appears that the amount of chain attached to the footrope

may be an important difference between small and large

vessels in terms of observed and expected length-eomposition

in this study. The small vessels uniformly used a single strand

of 9.5 mm (% in) chain in the wings, with two strands in

the belly. The large vessels generally used two strands of

9.5 mm (% in) chain in the wings with an additional length

of 9.5 mm or larger chain, up to 19.1 mm O~ in), in the

belly. Sea samplers noted fish from these tows were frequently coated with mud and in poor condition. In addition to

9

occasional clogging, the poor condition of fish in some tows

may have reduced their escape response. The effect ofsweep

characteristics in mud bottom areas on both size selectivity

and discard mortality needs further examination.

Yield-per-recruit assessments based on stepwise recruitment were found to be more conservative than assessments

based on knife-edge recruitment. Stepwise and knife-edge

yield calculations are equal when the products of abundance

and weight on either side of the 50 % retention point on the

selection curve are equal. A modest difference (2 to 7 %) occurs in the smallest mesh sizes considered (76-mm diamond

and 102-mm square) reflecting the effect of partial fishing

mortality on the numerous, rapidly growing age 1+ fish

(Table 5). Little difference in yield (1 to 3%) occurs in

102-mm and 114-mm codends. The 3% difference in yield

in 114-mm mesh codends is due to the escapement of some

fish greater than the minimum legal size which is accounted

for in the stepwise yield assessment but not the knife-edge.

The greatest difference exists in 127-mm mesh comparisons

(4 to 10%), particularly as discard and fishing mortality rise.

Differences in 127-mm mesh appear to be caused by both

ignoring discard mortality and overestimating yield in knifeedge calculations.

Estimated increases in yield with increasing mesh size are

generally similar between stepwise and knife-edge models

below 127-mm mesh. However, there is a greater disparity

when increasing mesh size from 114 mm to 127 mm is considered. Again, differences are most notable at the high

discard and fishing mortality rates which probably best

characterize the fishery at present. A 19% increase is forecast

in knife-edge yield, while 11 % (stepwise yield) is a more

realistic expectation.

The increase in yield associated with an increase in mesh

size is clearly an important management consideration, influencing both the ultimate choice of mesh size and the speed

with which that mesh size is to be introduced into the fishery.

To evaluate short-term as well as long-term changes in yield,

information on both the Lso and the shape of the selection

curve is required. The short-term effect of a mesh-size

restriction on landings cannot be evaluated at all without this

information when the minimum legal size falls within the

codend selection curve. At the same time, the relative

increase in long-term yield expected from increasing mesh

size beyond 114 mm can be greatly overestimated unless

the gradual recruitment of fish into the fishery, caused by

the codend selection process, is incorporated into yield

assessments.
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1983 Codend mesh selectivity in the Long Island spring trawl fishery

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Hole Lab., Natl. Mar. Fish. Serv., NOAA, Woods Hole, MA 02543,

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Beverton, R.J.H.

1964 The selectivity of flap-type topside chafer. Int. Comm. Northwest

Atl. Fish. Redbook 3: 132.

Clark, J.R.

1957b Effective escape area of the codend. ICNAF/ICES/FAO

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1958 Summary of gear selection information for the Commission area.

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1986 Estuarine winter flounder, p. 41-44. In Report on the third NEFC

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1986 Georges Bank winter flounder, p. 78. In Report on the third

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10

NEFC (Northeast Fisheries Center)

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