NOAA Technical Memorandum NMFS-AFSC-20

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Marine Fisheries Service

Alaska Fisheries Science Center

August 1993

Use of Lower Minimum Size Limits to

Reduce Discards in the Bristol Bay Red

King Crab (Paralithodes camtschaticus)

Fishery

by

J. E. Reeves
NOAA Technical Memorandum NMFS

The National Marine Fisheries Service's Alaska Fisheries Science Center uses the

NOAA Technical Memorandum series to issue informal scientific and technical

publications when complete formal review and editorial processing are not

appropriate or feasible. Documents within this series reflect sound professional

work and may be referenced in the formal scientific and technical literature.

The NMFS-AFSC Technical Memorandum series of the Alaska Fisheries Science

Center continues the NMFS-F/NWC series established in 1970 by the Northwest

Fisheries Center. The new NMFS-NWFSC series will be used by the Northwest

Fisheries Science Center.

This document should be cited as follows:

 Reeves, J. E. 1993. Use of lower minimum size limits to reduce discards 

in the Bristol Bay red king crab (Paralithodes camtschaticus) fishery. U.S.

Dep. Commer., NOAA Tech. Memo. NMFS-AFSC-20, 16 p.

Reference in this document to trade names does not imply endorsement by the

National Marine Fisheries Service, NOAA.
NOAA Technical Memorandum NMFS-AFSC-20

Use of Lower Minimum Size Limits to

Reduce Discards in the Bristol Bay Red

King Crab (Paralifhodes camtschaticus)

Fishery

b y

J. E. Reeves

Alaska Fisheries Science Center

7600 Sand Point Way N.E., BIN C-15700

Seattle, WA 98115-0070

U.S. DEPARTMENT OF COMMERCE

Ronald H. Brown, Secretary

National Oceanic and Atmospheric Administration

Diana Josephson, Deputy Under Secretary

August 199 3
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INTRODUCTION 

METHODS . .

RESULTS . .

DISCUSSION

CITATIONS .

APPENDIX .

i i i

CONTENTS

1

5

9

11

15

17
“2

INTRODUCTION

It has recently been reported by Beers (1991) that bycatch rates

of red king crabs (Paralithodes camtschaticus) are substantial in

the eastern Bering Sea red king crab and Tanner crab

(Chionoecetes bairdi) fisheries. For example, in the 1990 

Bristol Bay red king crab fishery, the catch per potlift-of

females and sublegal males was 1.4 times the catch rate of legal

male crabs. During the 1990 winter Tanner crab fishery, red king

crab catch rates were 80% of the catch per potlift of legal-sized

Tanner crabs. These catch rates were estimated using data

collected from the catcher/processor portion of the fleet. If

 the samples are representative of the fleet as a whole, then it

can be estimated that the 262,181 potlifts exerted by the red

king crab fishery would result in the discard of 4.4 million red

king crabs, while the 711,137, potlifts of the winter Tanner crab

fishery would result in the discard of 11.5 million red king

crabs. The total estimate of discarded red king crabs, assuming

each--was handled only once, is about 16 million crabs. during

these two fisheries. This bycatch estimate represents over 40%

of the mature stock, as estimated by the 1990 National Marine

Fisheries Service Bering Sea trawl survey, and signals a

potential problem for the red king crab resource.

If these bycatch estimates are even approximately correct, they
immediately raise the question of the impact of discard

mortality. The question of the impact of discard mortality is in

need of study. While little information on the subject is

available, a study by Carls and O'Clair (1990) indicates that red

king crabs are adversely affected by exposure to cold air

temperatures, which may occur when crab pots are brought onboard

during winter fishing seasons. Additionally, some insight into

the question may be gained from the recent work of Shirley (pers.

 comm., University of- Alaska Fairbanks, 11120 Glacier Highway,

 Juneau AK) with Dungeness crabs (Cancer magister) in Southeast

Alaska. In aquarium experiments intended to simulate the

handling process or this species in local fisheries; delayed

mortality of up to several: weeks was observed. This mortality

was related to the number of times crabs were handled. Crabs

were subjected to handling one, two, three, and four times per

month, and then mortality rates were compared to crabs that had

not been handled. Proportionally increasing mortality rates were

observed, with a 100% death rate resulting from four simulated

-handling treatments. The unhandled (control) crabs had no

mortalities.

The apparent high discard rates of king crab and the possibility.

of significant associated mortality provide a strong argument for

conducting experiments to estimate discard mortality, even if
only a rough estimate can be obtained. Knowledge of discard 

mortality rates can provide guidance on what approach should be

 taken to achieve a solution to the problem, and possibly improve

stock productivity. If discard mortality is low, then the most

prudent approach might be to maintain the status quo that is,

continue the current management strategy and await rebuilding via

a strong year &ass., If the mortality rate is high,. alternate

solutions having to do with changes in the selective properties

and operation of the gear, changes in time and area of the

fishery, or other modifications may well be preferable in order.

to increase stock productivity. It may be possible through

experimentation to design gear and gear operations to minimize

crab bycatch. Design, however, is complicated by the interaction

of the selective properties of mesh and escape port sizes, with

soak time and other factors affecting the attractive properties,

of baited pots. Thus, this type of experimentation could be

time-consuming and result in considerable costs to research

agencies and to the industry.. It may also be possible, to devise

time or area closures, but it is likely that the overlap of the.

legal and non-legal components of the stocks is substantial,

indicating, an ineffective solution.

Another management alternative to deal with high discard

mortalities could be to reduce discard mortality through the

3
retention of a larger portion of the catch normally thrown back;

Under this management strategy, the quota is reached sooner with

the same fleet, thereby shortening the fishing season and

reducing discards and unintended mortality on the stock of any

given year. This basic idea of keeping more of what comes up in

a pot could be realized through a lower size limit, or relaxation,

of the males-only restriction, or a combination of both

regulation changes. Discard reduction could prove to be an

effective approach to the problem, but not without perhaps

 substantial costs to the industry.

The purpose of this report is to provide a preliminary

examination of discard reduction via lower male size limits and

its effect on fishery characteristics through a simple computer

simulation of the 1990 red king crab males-only fishery in

Bristol Bay. Information from the 1990 fishery and NMFS research:

trawl survey was used to construct a model which was then

exercised under several lower size limits. Results of these

variations were then compared to simulated actual results of the

1990 red king crab season in terms of catches, average weights,

discards, and exploitation rates.
METHODS 

The 1990 Bristol Bay red king crab fishery was simulated using a

PC spreadsheet by applying observed rates of exploitation to

estimated stock to obtain a total. catch and average weight per

crab similar to the actual fishery. Discard mortality was

assigned two different values to simulate low and high rates, 25%

and 75%, and the number of crabs dying from the discard process

was calculated. Three alternate management options, represented

by three different minimum size limits on male crabs, were

applied to the stock and compared to the simulated actual fishery

having a 6.5-inch carapace width (CW) size limit: (1) lower the

minimum size limit to 6.0 inches CW, (2) lower it to 5.5 inches

CW, or (3) lower it to 5.0 inches CW. The harvest strategy for

this stock is to limit the rate of exploitation to 20% on mature

male crabs greater than 119 mm carapace length (CL)(ADF&G 1990,).

Thus, the simulated fisheries were terminated when this target

was attained.

Estimates of exploitation rates by size groups were obtained by

combining information on the size composition of landings and on-

 deck catches from the 1990 ADF&G Observer Program (Appendix Table

1). These catches were then compared to 1990 NMFS survey

5
estimates of abundance made prior to the fishing season (Appendix

Table 2). Using 5 mm grouping, the size composition of the 1990

landings was estimated by applying the dockside CL sample of the

landings (Griffin, pers. comm., ADF&G, Box 308, Dutch Harbor AK)

to the-total landed catch in numbers. 'Size composition of the

discard was estimated by applying the sample composition of ondeck catches to total discarded catch.. Total discarded catch was

estimated by multiplying catch rates for females and sublegal

males (10.1 and 6.5 crabs per potlift, respectively, from Beers

1991) by the total potlifts during the 1990 fishery (Griffin

1991). In applying the size 'composition of sublegal males to

total discards, 40% of the 135-,139 mm CL group was assumed to be

sublegal, assuming legal crabs are greater than 136 mm, and a

uniform distribution of crabs existed within this length group.

This value, plus values for all smaller male groups, was used to

obtain the percent size composition of the male portion of the

on-deck catch. Exploitation rates were then calculated by 5 mm

groups for males and females by dividing estimated' catches by

estimates of the stock from the survey.

Landings in weight were underestimated by the initial simulation

of the fishery. To correct for this, the-weight-length exponent

used for estimating average weight per male crab for the survey

data was adjusted upward slightly from 3.174 to 3.181 to attain

6 
agreement with actual landings.

Using exploitation rates as calculated above, the fishery was

simulated under four different minimum size limits: 6.5, 6.0,

5.5, and 5.0 inches CW. The various size limits were simulated

by a sliding scale of size-specific selection factors (1.0, 0.4,

0.1, and 0.0; see Appendix Tables 3-6) determined- in the

following way. The predominant 5 mm size groups in the 1990.

landings ranged from 140 to 169 mm. Each of these groups

contained 2 million lb or more, and taken collectively comprised

87% of the catch in weight (Appendix Table 2). Thus, the smallest

of these groups, the 140-144 mm group, was considered the first

size group fully recruited to the landings, and was assigned

selection- factor of -1.0, as were all larger size groups. Size

groups smaller than this were assigned selection factors

calculated as the ratio of the size group-specific exploitation

rate to the average exploitation rate of groups with catches of 2

million pounds or more (0.37). The 6.5 inch CW size limit, when

converted to length in millimeters using a carapace

coefficient of 1.25 (Alverson 1980), falls into the

10 mm smaller than the -first fully recruited group.

width-length

size group

Thus, for

each 0.5 inch decrease in the simulated CW size limit, the first

fully recruited CL group was lowered by 10 mm:

7
Harvest strategy-for this stock dictates that 20% of the mature

males- (>119 mm) may be taken (ADF&G 1990). Thus, for each size

 limit simulation, this strategy was used to terminate the

fishery. The actual. value calculated for the 1990 fishery was

0.22, and the days fished and size-specific exploitation rates

for the season were adjusted until the value 0.22 on males >119

mm was achieved for each simulation. For each sex-size group,

the adjusted exploitation rate (u') = DF/12*u, where DF is. days

fished, set to achieve the target exploitation rate for each size

limit simulation, and u is the exploitation rate. under a 6.5 inch

size limit: -Catch (c) =u'*N and landings,.(l) = s*c, where N is

stock,, and, s is the selection, or sorting, factor. The unknown.

 discard mortality (dm) was assigned low (0.25) and high (0.75)

values-for each simulation. Discard mortality was applied to

each sex equally, and discard deaths were calculated -for each

group as (1-s)*dm*c. Calculations were summed over groups and,

along with average weight of landings and exploitation rates,

were compared among simulations. The 6.5 inch simulation was

considered the control to which other size limit strategies were

compared. 
RESULTS

As lower size limit strategies were employed, the number of days

fished and average weight of crabs in the landings decreased

(Fig. 1). Lowering the size limit increased the retention of

smaller crabs, which lowered the average weight of crabs in

landings. Additionally, as more crabs were included in landings,

the fishing rate required at higher size limits needed lowering

to stay within the 0.22 target value. As would realistically be

expected, the required reduction in fishing mortality was

attained. by reducing days fished. Landings in pounds overall

decreased, but they increased. slightly at the lowest size limit.

This increase was due to significant numbers of crabs less than

120 mm entering the catch; these crabs were not counted toward

attainment of the target exploitation rate. This ocurred to a

lesser degree with the 5.5 inch CW size limit simulation. Thus,

the lowest simulated size limit strategies, especially the

5.0 inch CW simulation, must be considered unrealistic in light

of an exploitation strategy based on males less than 119 mm.

As the size limit was lowered and more of the catch was retained

as landings, deaths due to discard mortality declined compared to

deaths due to landings (Fig. 2). Discard deaths which, at the

9
0.75 discard mortality level were higher than landings dropped

off substantially with the 6.0 inch CW size limit and more,

gradually thereafter. The magnitude of this and other trends

reflects the 1990 population size. composition: Although

untested, these trends are probably dependent on size composition

to a significant degree. 

Simulated discard deaths by sex are, shown in Figure 3. The

decline in deaths with decreasing size limit was due to a greater

rate of decline for males, as opposed to females. Two factors 

are involved in the male process, that of conversion of discards

to catch, and fewer days fished. Only the latter factor is

involved in the decline in female discard deaths, and-this

explains the flattening out of discard deaths at lower size

limits.

The presence of discard mortality caused the target exploitation

rate to be, exceeded (Fig. 4), with the degree depending on the

level of discard' mortality; As the size limit waslowered, this

overshooting effect. diminished as a result of decreasing male 

discard deaths. The proportion of discard deaths and landings

(as noted above) that are less than 120 mm increased as the size

limit was lowered (Appendix Tables 3-6), and these did not enter

into the exploitation rate calculation; Their exclusion leads to
the formation of a plateau in the exploitation rate at lower size

limits. This pattern is an artifact associated with lowest

simulated size limits.

DISCUSSION

From the foregoing results,, the following conclusions may be

drawn. Under current harvest policy and stock conditions, and

assuming the discard mortality rate is greater than zero, a

minimum size limit lower than 6.5 inches CW will decrease the

number of deaths due to discards in the fishery. Such a. decrease

in discard deaths means that the actual exploitation rate on male

crabs greater than 119 mm will be closer to that dictated by

harvest strategy. The average size of crabs in the landings will

decrease and, given the same fishing effort as with the 6.5 inch

CW size limit, the seasonwill be shorter.

This study is limited in scope and the universality of these

conclusions is open to question. The magnitude, and perhaps the

direction,of trends upon which these conclusions are based is

dependent' on the stock size composition during the fishery.

Year-to-year variations in recruitment strength will cause

changes in size composition, which in turn will alter the

patterns found in this study. Thus, it would be useful to test

1 1
the generality of the above results through multi-year

simulations of the stock and fishery. Such simulations could

provide insight into the effect of year class fluctuations as 

well aschanges in other population parameters.

Another extension of this study' would 'be to examine, the. effects

 of retention of females. The limited pot bycatch data available

indicates that female red king crabs are predominant in the

discard. If discard mortality rates are high, it seems that

gains in production would be enhanced by the retention of

females, especially at higher stock levels.

This study emphasizes the need to

discard mortality for Bristol Bay

that the severity of the problem and the need for remedial action

'is directly related to the magnitude of discard mortality. It is

not clear, however, that the higher exploitation rates caused by

'higher discard mortality rates are actually retarding this

stock's' recovery.. But circumstantial evidence does exist that

the stock's decline and subsequent failure to significantly

rebound may be related to fishing effort. This is shown in

Figure 5A, where the total potlift trend in red king crab habitat

(east of 166" E long'.) is compared to the abundance of nonlegal

crabs.' It is possible that the buildup in effort. in the late

12 
1970s contributed to the decline in the early 1980s through

discard mortality. Further,' it is possible that the current

buildup in potlifts is contributing to the continuing depressed

state of the stock. The possibility that discards may be

affecting the stock provides additional impetus for evaluating

discard mortality.

The multispecies nature of the pot discard problem is becoming

apparent. As mentioned in the Introduction, if the estimate of

discards in 1990 is assumed to be representative of the total

fleet, then more red king crabs are thrown back in the eastern

Bering Sea Tanner crab fishery than in the target fishery. This

argues strongly for a multispecies, as well as a multiyear

approach in evaluating the red king crab discard problem. Figure

5B shows that potlift effort in red king crab habitat is

currently increasing after a period of decline and bottoming. As

in the previous buildup,. the composition of effort is made up

predominately of potlifts from-the Tanner and red king crab

fisheries. Thus, further studies on discard mortality impact

should be tailored to include probable interactions of the Tanner

crab fishery with the red king crab fishery. Studies similar to

this one should be conducted for Tanner crab, including the

concept of a multispecies or one-season fishery for these stocks.

13
Finally, the economic implications of the these results have not

been considered. Further studies ondiscard reduction through

lower size limits should examine various tradeoffs between

'industry perturbations. caused by smaller crabs, shorter or

multispecies seasons, and possible benefits of stock

rehabilitation. Also, costs of a solution such as the one

proposed in this paper should be compared to costs of gear

modifications or other potential solutions.

1 4
CITATIONS

Alaska Department of Fish and Game. 1990. Draft management plans

for Westward Region king crab stocks: Kodiak Island red king

crab, Bristol Bay red king crab, St. Matthew blue king crab,

Pribilof Islands blue king crab. March 17, 1990.

Alverson, D.L. 1980.

Alaskan red king

21st Avenue

An analysis of size limitation for the

crab. Natural Resources Consultants, 4055

West, Seattle WA, 57p.

Beers, D. 1991. A summary of data contained in the mandatory

crab observer database. Alaska Dept. Fish and Game, Reg.

Info. Rep. 4K91-14, 40p.

Carls, M.G., and C.E. O'Clair 1990. Influence of cold air

exposures on ovigerous red king crabs (Paralithodes

camtschatica) and Tanner crabs (Chionoecetes bairdi) and

their offspring, p. 329-343. In Proceedings of the

International Symposium on King and Tanner Crabs, Alaska

15
Sea Grant Report No. 90-04.

Griffin, K.L. 1991. Bering Sea area shellfish management report

 to the Alaska Board of Fisheries, p. 173-236. In Alaska

Department of Fish and Game We&ward region shellfish report

to the Alaska Board of Fisheries, March 1991.

16 
APPENDIX
*Days Fished

Figure 1. Landings, average weights, and days fished for simulations of

the 1990 Bristol Bay red king crab fishery, under different

minimum size limits (inches‘).
Figure 2. Discard deaths compared to landings for two levels of

discard mortality from simulations of the 1990 Bristol

Bay red king crab fishery under different minimum size

limits (inches).
Figure 3. Discard deaths by sex compared to landings for two

levels of discard mortality, A. 0.25 and B. 0.75, from

simulations. of the 1990 Bristol Bay red king crab

fishery under different minimum size limits (inches).

21
Figure 4. Rates of exploitation for males-greater than 119 mm,

 for two levels of discard mortality from simulations of

the 1990 Bristol Bay red king crab fishery under

different minimum size limits (inches).
A. Nonlegal Abundance vs. Fishing Effort

B. Composition of Fishing Effort

Millions of Potlifts

Figure 5. Bristol Bay red king crab (A) estimated-abundance of

nonlegal crabs compared to total potlifts east of 166"

E long. and (B) potlift composition by fishery.

2 3
Appendix Table 1: 1990 Bristol Bay red king crab fishery site composition-date from dockside and on-board observer sampling

programs.

CARAPAC E SAMPLIN G SAMPLIN G ESTIMATED NUMBER

LENGT H DOCKSIDE OBSERVE R DOCKSIDE OBSERVER LANDINGS DISCARD

GROU P MALE S MALES FEMALES MALES MALES FEMALES MALES MALES FEMALES

(mm ) ( N O S . ) (Nos.) (Nos.) (%) (%) (%) (MLns.) (MLns.) (MLns.)
Appendix Table 2. Survey and fishery data used for size-specific exploitation rate calculations

for use in simulations of the 1990 Bristol Bay red king crab fishery.
Appendix table 3. Simulation results for the 1990 fishery 6.5’ size limit,
Appendix Table 4. Simulation results for the 1990 fishery 6.0' size limit.

27
Appendix Table 5. Simulation results for the 1990 fishery 5.5’ size limit.

28 
Appendix Table 6. Simulation results for the 1990 fishery, 5.0' size limit.

29
RECENT TECHNICAL MEMORANDUMS

Copies of this and other NOAA Technical Memorandums are available from the

National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22167

(web site: www.ntis.gov). Paper and microfiche copies vary in price. 

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predation on commercially important prey species in the eastern Bering Sea from 1987 to 1989,

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10 KINOSHITA, R. K., and J. M. TERRY. 1993. Oregon, Washington, and Alaska exports of edible fishery

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1986, 53 p. NTIS No. PB93-158335.

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distributions for many commercially important fishes and shellfishes off the West Coast of North America,

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