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Analysis of Minimum Size Limit for the Red King Crab Fishery in Bristol Bay, Alaska

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Low stock status of red king crabs Paralithodes camtschaticus in Bristol Bay prompted an industry initiative to reduce the minimum size limit for the commercial fishery from 165 mm in carapace width (CW) to 152 mm in CW. In terms of carapace length (CL), which is the metric used in data collection programs, this is equivalent to a reduction from 137 to 128 mm CL. The rationale was primarily to reduce potential nontarget handling mortality, which was suspected to contribute to the depressed stock status. Analysis of red king crab fishery observer data showed that the reduced size limit would initially increase catch rates of legals (under new size limit) by 10–41%, diminish total bycatch of nontarget red king crabs (sublegal and female crabs) by 9–33%, and reduce the fishing effort (pot lifts) needed to attain annual catch quotas by 2–27%. Yield-per-recruit analysis indicated that steady-state yield would decline 5–7% under the smaller size limit, assuming a 20% handling mortality rate. Size distributions of the catch would shift so that crabs of 128–136 mm CL would constitute 35% of the catch, yet the percentage of large males (>163 mm CL) in the spawning stock would barely increase from 6.1% to 8.2%. A population dynamics model revealed that there is a slightly higher probability of larger stock spawning biomass under the 128-mm-CL size limit than under the 137-mm-CL limit over a 50-year planning horizon. An economic break-even analysis showed that it takes 23 years for cumulative catch under the 128-mm-CL size limit to exceed cumulative catch under the 137-mm-CL size limit. At a 7% real interest rate, the reduced size limit takes 40 years to break even. Net benefits of the reduced size limit are larger and accrue more quickly if handling mortality rates are greater than 20%, whereas the reduced size limit never yields a positive economic benefit if handling mortality rates are 10% or less. The reduced size limit does not appear to be a cost-effective measure for red king crab resource conservation given likely values of handling mortality rates.
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North American Journal of Fisheries Management 20:307–319, 2000
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Copyright by the American Fisheries Society 2000
Analysis of Minimum Size Limit for the Red King Crab Fishery
in Bristol Bay, Alaska
G
ORDON
H. K
RUSE
*
Alaska Department of Fish and Game, Division of Commercial Fisheries,
Post Office Box 25526, Juneau, Alaska 99802-5526, USA
L
AURENCE
C. B
YRNE
Alaska Department of Fish and Game, Division of Commercial Fisheries,
211 Mission Road, Kodiak, Alaska 99615-6399, USA
F
RITZ
C. F
UNK
Alaska Department of Fish and Game, Division of Commercial Fisheries,
Post Office Box 25526, Juneau, Alaska 99802-5526, USA
S
COTT
C. M
ATULICH
Washington State University, Department of Agricultural Economics,
Pullman, Washington 99614-6210, USA
J
IE
Z
HENG
Alaska Department of Fish and Game, Division of Commercial Fisheries,
Post Office Box 25526, Juneau, Alaska 99802-5526, USA
Abstract.—Low stock status of red king crabs Paralithodes camtschaticus in Bristol Bay prompted
an industry initiative to reduce the minimum size limit for the commercial fishery from 165 mm
in carapace width (CW) to 152 mm in CW. In terms of carapace length (CL), which is the metric
used in data collection programs, this is equivalent to a reduction from 137 to 128 mm CL. The
rationale was primarily to reduce potential nontarget handling mortality, which was suspected to
contribute to the depressed stock status. Analysis of red king crab fishery observer data showed
that the reduced size limit would initially increase catch rates of legals (under new size limit) by
10–41%, diminish total bycatch of nontarget red king crabs (sublegal and female crabs) by 9–
33%, and reduce the fishing effort (pot lifts) needed to attain annual catch quotas by 2–27%. Yield-
per-recruit analysis indicated that steady-state yield would decline 5–7% under the smaller size
limit, assuming a 20% handling mortality rate. Size distributions of the catch would shift so that
crabs of 128–136 mm CL would constitute 35% of the catch, yet the percentage of large males
(
.
163 mm CL) in the spawning stock would barely increase from 6.1% to 8.2%. A population
dynamics model revealed that there is a slightly higher probability of larger stock spawning biomass
under the 128-mm-CL size limit than under the 137-mm-CL limit over a 50-year planning horizon.
An economic break-even analysis showed that it takes 23 years for cumulative catch under the
128-mm-CL size limit to exceed cumulative catch under the 137-mm-CL size limit. At a 7% real
interest rate, the reduced size limit takes 40 years to break even. Net benefits of the reduced size
limit are larger and accrue more quickly if handling mortality rates are greater than 20%, whereas
the reduced size limit never yields a positive economic benefit if handling mortality rates are 10%
or less. The reduced size limit does not appear to be a cost-effective measure for red king crab
resource conservation given likely values of handling mortality rates.
Red king crabs Paralithodes camtschaticus are
widely distributed on both sides of the North Pa-
cific from the Sea of Japan (Sato 1958) and British
Columbia (Butler and Hart 1962) north through
the Bering Sea to the Chukchi Sea (C. Lean, Alas-
* Corresponding author:
gordon
p
kruse@fishgame.state.ak.us
Received February 8, 1999; accepted November 16, 1999
ka Department Fish and Game [ADFG], personal
communication). These anomurans achieve max-
imum size (males) of 227 mm carapace length (CL;
Powell and Nickerson 1965) and ages greater than
20 years (Matsuura and Takeshita 1990),although
few crabs live longer than 15 years. After mating,
fertilized eggs are incubated externally on the fe-
male’s abdomen for nearly 1 year. Male crabs re-
cruit to the Bristol Bay (eastern Bering Sea) fishery
7–12 years after hatching, depending on temper-
308
KRUSE ET AL.
ature (Stevens 1990). The fishery cannot legally
retain females. The species supports some valuable
yet volatile fisheries in the Gulf of Alaska, Aleu-
tian Islands, and Bering Sea. In Bristol Bay in the
eastern Bering Sea, a fishery developed in the early
1930s and peaked in 1980 with a catch of 59,000
metric tons (mt) worth US$115 million exvessel.
Catches declined sharply in the early 1980s, and
no fishing was permitted in 1983, 1994, and 1995
due to low stock size. Nevertheless, even with a
short 4-d season in 1996, the fishery was lucrative;
3,800 mt were landed worth $34 million exvessel.
The rise and crash of many Alaska crab fisheries
prompted a plea to reevaluate management strat-
egies (e.g., Kruse 1993). Based on simulation stud-
ies (Zheng et al. 1997a, 1997b) the Alaska Board
of Fisheries adopted in 1996 a more conservative
harvest strategy to promote stock rebuilding and
long-term optimal harvest of Bristol Bay red king
crabs. Under the new strategy, a guideline harvest
level (GHL; i.e., annual catch quota) is calculated
by
GHL
5
E·N·W,
ave
(1)
where Eis annual exploitation rate, Nis number
of mature males (
$
120 mm CL) in the population
estimated by length-based analysis (Zheng et al.
1995a, 1995b), and W
ave
is average weight of legal
male crabs. No fishery occurs (E
5
0) when the
stock is below thresholds of 8.4 million mature
females and 6,600 mt of effective spawning bio-
mass. Effective spawning biomass is the estimated
biomass of mature females mated in a given year
based on sex ratio and male size distribution
(Zheng et al. 1995a, 1995b). When the stock is
above threshold but below the rebuilding target
level of 25,000 mt of effective spawning biomass,
E
5
10%. When the stock is greater than or equal
to 25,000 mt, E
5
15% to optimize the tradeoff
between long-term yield and variability in yield.
Although the harvest rate in equation (1) is ap-
plied to mature male crabs, only legal male crabs
may be harvested and legal male harvest rate is
capped at 50%. The minimum size limit for red
king crabs in the Bristol Bay fishery is 165 mm
(6.5 in) carapace width (CW). Carapace width is
used for legal size determinations in the fishery
and is measured as the straight-line distance across
the carapace at a right angle to a line midway line
between the eyes to the midpoint of the posterior
portion of the carapace including the spines
(ADFG 1997). Carapace length is the ‘‘biological
measurement’’ defined as the distance from the
posterior margin of the right-eye orbit of the car-
apace to the center of the posterior carapace mar-
gin (ADFG 1990). Because CL is used in all as-
sessment, onboard observer, and dockside sam-
pling programs, CL was used in all analyses re-
ported here. Alternative size limits in CW were
converted to CL by CL
5
16.934
1
0.730 CW (r
2
5
0.95, P
,
0.001, N
5
805) for red king crabs
collected from Bristol Bay during 1986 and 1987
(R. S. Otto, National Marine Fisheries Service,
personal communication). The 165-mm-CW size
limit corresponds to 137 mm CL.
The coupling of harvest rate and minimum size
limit is critical to achieve management objectives
pertaining to yield optimization and resource con-
servation. In Alaska, size limits for red king crabs
were set based largely on market considerations
(Donaldson and Donaldson 1992) and the ability
to provide males at least one mating opportunity
before harvest (Otto 1985).
Merits of alternative limits have been consid-
ered for more than 2 decades (e.g., Alverson 1980;
Bibb and Matulich 1994). Interest in size-limit re-
duction to 128 mm CL (152 mm CW) stems from
conservation concerns about bycatch mortality of
discarded sublegal male crabs (Matulich and Bibb
1992; Reeves 1993; Thomson 1996). Proponents
contend that a reduced size limit will shift harvest
to smaller males, increase the number of larger
and reproductively more prolific males, reduce by-
catch mortality, promote more rapid stock rebuild-
ing, increase catch per unit effort (CPUE), and
reduce harvest operational costs.
Our goal was to analyze potential bioeconomic
impacts of an industry proposal to reduce the size
limit from 137 mm in CL (165 mm CW) to 128
mm in CL (152 mm CW), holding the number of
harvested crabs constant. We analyzed the pros and
cons of the reduced size limit with four related
analyses. (1) Observer data from the red king crab
fishery were analyzed to evaluate whether a re-
duced size limit will simply shift the bycatch prob-
lem to other components of the stock and to cal-
culate empirically derived estimates of immediate
changes in total bycatch, CPUE, GHL, and fishing
season length. (2) A yield-per-recruit analysis was
conducted to allow estimation of effects of size-
limit reduction in terms of this classical steady-
state solution to the size-limit problem and to fa-
cilitate direct comparison of our findings with pre-
vious analyses of this stock. (3) A simulation mod-
el of the Bristol Bay population was constructed
with stock, environment, and fishery dynamics to
estimate implications of size-limit reduction on
309
SIZE LIMITS FOR BRISTOL BAY RED KING CRABS
catch and likelihood of future fishery closures over
a 50-year planning horizon. (4) These simulation
results were distilled into a break-even analysis to
evaluate industry’s proposed policy as an invest-
ment in conservation. Sensitivity of our models to
uncertain handling mortality rates and discount
rates were investigated.
Methods
Analysis of fishery observer data.—At-sea ob-
servers collected data from the Bristol Bay red
king crab fishery during 1991–1993 and 1996; the
fishery was not opened in 1994–1995 due to low
abundance. All catcher–processors and at-sea pro-
cessors carry fishery observers; catcher–processor
catches were sampled when crab pots (typically
213
3
213
3
86 cm) were retrieved, whereas pre-
sorted deliveries from catcher-only vessels were
sampled on those vessels. Crabs in observed pots
were enumerated and identified to species, sex,
size, shell condition, and other attributes.
If 128–136-mm-CL male red king crabs tend to
be more closely associated with the bycatch of
females or smaller males (
,
128 mm CL) than with
the catch of larger (
$
137 mm CL) males, then a
reduced size limit may simply shift the bycatch
problem to another segment of the population. To
explore this, we calculated the correlation between
catches of 128–136 mm CL male crabs with fe-
males and with smaller (
,
128 mm CL) and larger
males (
$
137 mm CL). Also, we estimated the
magnitude of reduced bycatch and increased
CPUE (catch per pot) as a result of the shift in
definition of legal crabs. Crab CL (Lin mm) was
converted to weight (Win g; Balsiger 1974) with
2
4 3.160
W
5
3.614 · 10 · L. (2)
Mean weight of legal crabs was estimated and
GHL was calculated with equation (1) under the
two size limits. Last, the number of fishing days
needed to attain the GHL was calculated from es-
timates of daily fishing effort (number of pots
fished) and CPUE (catch per pot).
Yield-per-recruit analysis.—A yield-per-recruit
analysis examines the change in cohort biomass
with age from growth and mortality tradeoffs
(Beverton and Holt 1957; Ricker 1958). Yield is
affected by the age of entry to the fishery and
exploitation rate. Because red king crabs cannot
be aged, we constructed a length-based cohort
model to estimate yield per recruit. The initial
number of crabs in the cohort was set equal to
1,000 crabs at relative age 0 and having an initial
mean size of 105 mm CL and standard error of
6.0, which was consistent with Zheng et al.
(1995a). These sizes correspond to crabs that are
approximately two molt increments below legal
size in Bristol Bay (Stevens 1990). This ‘‘cohort’’
comprises crabs of various ages depending on in-
dividual growth histories (Stevens 1990), so we
kept track of relative, not absolute, cohort age.
We modeled growth identically to Zheng et al.
(1995a, 1995b). Growth was separated into two
components: growth increment (G) as a linear
function of premolt size (L) measured in CL (mm),
G
5
13.14
1
0.018L,
L
(3)
and molting probability (P) as a logistic function
of premolt size,
1
P
5
1
2
. (4)
L
2b
L
1
1a
e
Periods of slow (1980–1984, 1992–1993), medium
(1985–1991), and fast growth (1972–1979) asso-
ciated with shifts in molting probability occurred
during the past 3 decades (Balsiger 1974; Zheng
et al. 1995a). To explore the impacts of growth
changes on yield, estimates of parameters
a
and
b
were set for periods of low (20,584.9 and 0.077),
average (295,159.6 and 0.089), and high
(358,930.1 and 0.082) molting probabilities for
crabs of size 95–160 mm CL. Crab size measure-
ments were converted to weight estimates with
equation (2) for yield estimation.
Instantaneous natural mortality (M) of mature
males was equated to a long-term weighted av-
erage value of 0.3 (Zheng et al. 1997b), corre-
sponding to a 26% proportionate annual rate. Legal-
sized males were assumed to be fully catchable,
whereas sublegal-sized males were assumed to
have 50% catchability based on observed bycatch
rates in the directed fishery (Zheng et al. 1997a).
Nonlegal male crabs must be returned to the sea,
and an unknown number die from handling. We
assumed a handling mortality rate of 20%, and
examined the sensitivity of results to alternative
rates between 0% and 50%. Zero and 50% mor-
talities are unrealistic, so we effectively bracketed
the range of true values.
Simulation of stock rebuilding and fishery im-
pacts.—As a contrast to the steady state yield-per-
recruit analysis, we used a dynamic simulation
model of stock rebuilding developed by Zheng et
al. (1997b) to analyze the potential impacts of a
size-limit reduction on the red king crab stock and
the commercial fishery in Bristol Bay. Because
310
KRUSE ET AL.
average weight of landed legal crabs would decline
under the reduced size limit, adoption of the pro-
posal would lower the GHL, at least initially be-
fore conservation benefits accrue. Therefore, im-
pacts of the proposed policy on stock rebuilding
and fishery yield over time were of particular in-
terest. Our only change to the model was to ini-
tialize the stock with data from 1996 rather than
1994.
In overview, we modeled both male (
$
95 mm
CL) and female (
$
90 mm CL) components of the
stock. Following Zheng et al. (1995a), stock–re-
cruitment data were fitted with a Ricker curve that
combined density-dependent stock effects with au-
tocorrelated environmental effects seemingly as-
sociated with dynamics of the Aleutian Low pres-
sure system (Tyler and Kruse 1996; Zheng and
Kruse, in press). Spawning stock was estimated as
effective spawning biomass. Male growth was
modeled identically to our yield-per-recruit anal-
ysis, using parameters for weighted average
growth during low, medium, and high molting pe-
riods. Female growth was modeled the same as
males but with the sex-specific and stock-specific
parameters reported in Zheng et al. (1995b). In-
stantaneous natural mortality for males was iden-
tical to that in the yield-per-recruit analysis, and
for females it was set equal to 0.47 (i.e., 37% per
annum). We assumed that groundfish trawls kill
200,000 red king crabs annually, the upper bound
on crab bycatch set for the Bering Sea groundfish
fisheries (NPFMC 1996). Handling mortality was
treated identically to our yield-per-recruit analysis.
Starting in 1996, we simulated the Bristol Bay red
king crab stock and fishery under status quo (137
mm CL) and reduced (128 mm CL) size limits over
a 50-year planning horizon under the current har-
vest strategy (equation 1). Each scenario was rep-
licated 500 times to ensure relative stability of the
summary statistics. We compared catch, probabil-
ity of future fishery closure (i.e., years when the
stock was below threshold), and probability dis-
tributions of future stock status with respect to the
rebuilding target level (25,000 mt) under the two
management alternatives.
Economic analysis.—The simulated catch tra-
jectories were translated into financial streams that
measure the direct economic benefits and costs of
the policy proposal—i.e., the present value of har-
vest weight changes over time. This type of anal-
ysis treats the proposed size-limit reduction as an
investment in conservation, which begs the ques-
tion, ‘‘When will the expected stream of benefits
equal and then exceed the expected stream of
costs?’’ An economic break-even analysis was
conducted to estimate how long it takes the crab
industry to recoup its investment in conservation
(reduced GHLs) before it benefits economically
from higher yields of healthier stocks. The analysis
compresses each 50-year catch trajectory into a
single, cumulative dollar amount that removes the
influence of time and interest from future dollar
values. The break-even analysis calculates present
value (PV) of ex-wholesale gross revenues from
the 50-year harvest streams with and without the
conservation policy. The lower size limit breaks
even and begins to yield PV in excess of that under
the current size limit when the net present value
(NPV) is zero:
NPV
5
PV
2
PV
5
0.
(128 mm CL) (137 mm CL)
(5)
The PV calculations are conditional on three pa-
rameters: ex-wholesale price, handling mortality,
and interest rate. Average ex-wholesale price of
processed red king crab is assumed to be $19.89/
kg for both harvest trajectories; product recovery
rate is 64% of live weight. Five handling mortality
rates (0, 10, 20, 30, and 50%) were examined to
reflect uncertainty of the estimates. Each mortality-
conditioned PV estimate was then calculated with
alternative real (i.e., inflation-adjusted) interest
rates ranging from 0% to 7% (removal of inflation,
2.0–2.8% at the time of analysis, essentially holds
wholesale crab prices constant over time). Al-
though the expected real interest rate for short-
term operating loans in the Bering Sea crab in-
dustry is 7% or less (about 9–10% nominal interest
rate), lower rates were also considered to illustrate
the sensitivity of reduced size-limit policy to more
conservation-oriented interest rates.
A second economic implication involves alter-
ing the catch-by-size distribution over time. The
reduced size limit initially will depress ex-
wholesale prices (and thus, exvessel prices), fol-
lowed by periods of rising and falling prices as
the average weight per crab fluctuates with red
king crab year-class strength. Precise incorpora-
tion of such price effects is not possible with avail-
able information. Wholesale price data do not exist
for the currently sublegal (
,
137 mm CL) crabs
and the influence of this smaller crab category on
the price of larger crab size categories (so-called
cross-price effects) are unknown (Matulich and
Bibb 1992). Moreover, the future catch-by-size
distribution depends on future changes in harvest
policy and effectiveness of stock rebuilding, which
is partly dependent on environmentally driven re-
311
SIZE LIMITS FOR BRISTOL BAY RED KING CRABS
T
ABLE
1.—Correlation coefficients (r) of catches of
128–136-mm-CL males with other male sizes (
$
137 mm
CL and
,
128 mm CL) and females from observed pots
during the commercial fishery (P
,
0.05*, P
,
0.01**).
Year N
Male size (mm CL)
$
137
,
128 Females
1991
1992
1993
1996
249
260
532
33
0.33**
0.58**
0.64**
0.53**
0.72**
0.60**
0.59**
0.38**
0.32**
0.07
0.08
0.37*
F
IGURE
1.—Mean catch per unit effort (CPUE) in
1991–1996 as (a) numbers of males in three size-groups
and all females and (b) total kilograms per pot of legal
male crabs under the 137-mm-CL size limit and under
the 128-mm-CL size limit; based on observed pot catch-
es during the 1991–1996 commercial fishery in Bristol
Bay.
cruitment cycles (Zheng and Kruse, in press). Re-
gardless, some quantitative insight into this issue
is gained by examining the NPV sensitivity to low-
er average prices. An ADFG cost-recovery fishery
in 1995 provides additional qualitative insight into
near-term revenue consequences of the lower size-
limit policy. The participating processor accepted
crabs of 128 mm CL or larger for the first delivery,
but because of adverse revenue implications, field
crews were asked to high-grade so that only crabs
147 mm CL or larger were landed during the sec-
ond delivery.
The one remaining economic consideration con-
cerns harvesting cost reductions due to higher
CPUEs attending higher retention rates and sub-
sequent stock rebuilding. Data do not exist to ad-
dress this issue.
Results
Fishery Observer Data Analysis
During 1991, 1992, 1993, and 1996, observers
were respectively deployed on 24, 17, 16, and 4
catcher-processor vessels and sampled 266, 280,
556, and 33 pots. Among pots that caught red king
crabs, a statistically significant relationship was
high (P
,
0.01) between catches of 128–136 mm
CL males and catches of smaller and larger males
in all 4 years (Table1). Correlations between 128-
and 136-mm-CL males and females were signifi-
cant (P
,
0.05) in 2 of 4 years.
Legal (
$
137 mm CL) males constituted 21–
68% of all red king crabs caught during 1991–
1996 (Figure 1a). If the size limit had been reduced
to 128 mm CL, bycatch rates of all nonlegal crabs
would have declined by 9–33% and catch rates of
legal males would have increased by 10–41%. In
terms of weight, catch rates would have increased
to a lesser degree (2–27%) because of lower av-
erage weight of landed crabs under the smaller size
limit (Figure 1b). These empirically derived esti-
mates of catch and bycatch rates only reflect ex-
pected initial changes associated with a reduced
size limit because they do not reflect cumulative
effects on stock size composition over time.
A reduction in size limit would have reduced
preseason GHL by 7–11% because of reduced
mean weight of legal crabs: from 8,100 to 7,538
mt in 1991; 4,635 to 4,145 mt in 1992; 7,560 to
6,777 mt in 1993; and 2,250 to 2,070 mt in 1996.
Mature male harvest rate was 20% during 1991–
1993 and 10% in 1996, when harvest strategy was
changed to equation (1) and our GHL estimates
preserved these historical rates. From increased
legal male CPUEs (Figure 1b), we estimated the
percent reduction in fishing effort (number of pot
lifts) needed to achieve the reduced GHL each year
under the 128-mm-CL size limit: 16.3% in 1991,
18.4% in 1992, 26.6% in 1993, and 1.5% in 1996.
Corresponding reductions in season length (round-
ed to 0.5 d) were from 7 to6din1991, 7 to 5.5
d in 1992, 9 to7din1993, and no change in the
4-d season in 1996.
312
KRUSE ET AL.
F
IGURE
2.—Equilibrium cohort biomass and relative
age for Bristol Bay red king crabs under scenarios of
(a) slow, medium, and fast growth with no harvest and
with a 15% mature male harvest rate provided under a
137-mm-CL size limit and assuming 20% handling mor-
tality; (b) medium growth with 10% and 15% mature
male harvest rates provided under a 137-mm-CL size
limit and assuming 20% handling mortality; and (c) me-
dium growth with a 15% mature male harvest rate pro-
vided under a 137-mm-CL size limit and assuming han-
dling mortality rates of 0, 20, and 50%. Crabs with a
mean size of 105 mm CL are relative age 0.
F
IGURE
3.—Yield (kg) per 1,000 recruits for Bristol
Bay red king crabs based on (a) 10% and 15% mature
male harvest rates under alternative size limits (115–145
mm CL) and assuming 20% handling mortality, and (b)
15% mature male harvest rate under two size limits (128
and 137 mm CL) and assuming 0–50% handling mor-
tality.
Yield Per Recruit Analysis
Cohort biomass at a given relative age declines
as growth rate decreases and as exploitation rate
or handling mortality increases (Figure 2). Faster
growth shifts age of maximum biomass to older
ages. In an unfished situation, biomass of an initial
cohort of 1,000 crabs peaks at 1,063 kg at relative
age 2 during years of fast growth, and 954 kg at
relative age 1 during periods of slow growth (Fig-
ure 2a). Under medium growth, cohort biomass is
maximized at 1,024 kg for relative age 2. Prose-
cution of a fishery reduces cohort biomass and
shifts peak biomass to younger ages (Figure 2a).
Cohort biomass is greatest at relative age 2 with
fast growth and greatest at relative age 1 with me-
dium and slow growth, under the current size limit
and 15% mature-male harvest rate assuming 20%
handling mortality. Compared with 15%, a 10%
harvest rate provides for greater cohort biomass at
older ages (Figure 2b). Not surprisingly, as han-
dling mortality increases, cohort biomass declines
(Figure 2c). Under the current size limit and the
15% mature male harvest rate, cohort biomass is
maximized at relative age 2 for handling mortality
rates less than 20% and at relative age 1 for higher
handling mortality rates.
Assuming a 20% handling mortality rate, a re-
duction in size limit from 137 to 128 mm CL would
slightly decrease yield per recruit by 7% under the
10% mature-male harvest rate and by 5% under
the 15% mature-male harvest rate (Figure 3a).
Over the range of size limits examined, larger
yields per recruit are obtained from larger mini-
mum size limits. Handling mortality reduces yield
per recruit (Figure 3b); the greater the mortality,
the greater the loss of yield. However,for probable
values of handling mortality rate (
#
45%), the cur-
rent size limit results in higher yields than the
reduced size limit.
Yield-per-recruit analysis allowed usto examine
the degree to which a reduced size limit shifts the
313
SIZE LIMITS FOR BRISTOL BAY RED KING CRABS
F
IGURE
4.—Size structure of the (a) equilibrium pop-
ulation and (b) catch for Bristol Bay red king crabs under
two different size limits (128 and 137 mm CL) with a
mature male harvest rate of 15% and sublegal handling
mortality of 20%.
F
IGURE
5.—Simulated (a) annual mean catch and (b)
probability of fishery closure for the Bristol Bay red
king crab fishery over 50 years under the current 137-
mm-CL and proposed 128-mm-CL size limits and as-
suming 20% handling mortality rate. Year 0 is 1996.
population size distribution to larger male crabs.
Because harvest rate is applied to the number of
mature males, legal male exploitation rate increas-
es with higher size limits because the catch is taken
from a smaller fraction of the male population
(Figure 3a). For the 128 mm CL size limit, harvest
is spread over a larger segment of the stock, shift-
ing the average size of crabs in the equilibrium
catch from 149.7 to 144.5 mm CL (Figure 4b).
Crabs greater than or equal to 137 mm CL decline
from 100% to 65% of the catch, replaced by males
128–136 mm in CL. However, due to natural mor-
tality, large (
.
163 mm CL) males, which predom-
inate collections of mating pairs off Kodiak Island
(Schmidt and Pengilly 1990), only increase from
6.1% to 8.2% in the equilibrium population (Fig-
ure 4a).
Simulation of Stock Rebuilding and Fishery
Impacts
Under the base scenario of 20% handling mor-
tality rate, annual mean commercial catch is slight-
ly higher for the 137-mm-CL than for the 128-
mm-CL size limit during the first 11 years (Figure
5). After year 16, annual mean catch for the smaller
size limit is 125–450 mt or about 1% to 5% higher
than that under the status quo. It takes 23 years
for cumulative catch under the 128-mm limit to
exceed catch under the 137-mm limit. Probability
of fishery closure is very similar for the two size
limits during the first 5 years, but subsequently
there is a slightly higher chance for fishery closure
under the current size limit (Figure 5). A close
examination of the probability distribution of ef-
fective spawning biomass, with respect to target
rebuilding level, shows that there is a slightly high-
er probability of higher biomass under the 128-
mm limit than under the 137-mm limit after 30
and 50 years (Figure 6).
Our results are sensitive to handling mortality.
With no handling mortality, theprobability of fish-
ery closure is identical under the two size limits,
but the fishery would be closed much less often
under the reduced size limit than under the current
size limit when handling mortality rate increases
above 20% (Figure 7). Annual mean catch is
slightly higher under the status quo than under the
reduced size limit when handling mortality rate is
0–10% during each of the 50 years (Figure 8).
However, with handling mortality rates greater
than or equal to 20%, annual mean catch is much
314
KRUSE ET AL.
F
IGURE
6.—Probability distributions of effective
spawning biomass of Bristol Bay red king crabs after
(a) 30 years and (b) 50 years (year 0 is 1996) for the
137-mm-CL and 128-mm-CL size limits. The vertical
line indicates the target level of 25,000 metric tons of
effective spawning biomass.
F
IGURE
7.—Probability of fishery closure for Bristol
Bay red king crabs over 50 years (year 0 is 1996) under
handling mortalities of (a) 0, (b) 10, (c) 30, and (d) 50%
and under the 137-mm-CL and 128-mm-CL size limits.
higher for the 128-mm-CL size limit than for the
137-mm limit after 15–20 years.
Economic Analysis
Two generalizations can be drawn from the
break-even analysis (Figure 9). First, larger inter-
est rates make future benefits less valuable because
they require a longer time to break even. Second,
higher handling mortalities yield greater economic
benefits from the reduced size limit. The reduced
size limit never pays if the handling mortality is
10% or less, regardless of interest rate. At the
policy-relevant 7% real interest rate, the conser-
vation policy takes 40 years to break even, assum-
ing that handling mortality is 20% and 19 years if
the handling mortality is 30%. Even at the unre-
alistically high 50% handling mortality rate, it
would take 13 years for the industry to realize any
positive net economic benefit. This long payoff
period raises a question about industry motivation.
If the industry’s conservation interest is altruism
or intergenerational wealth transfer, the appropri-
ate interest rate is 0% (i.e., after 50 years $1 is
equivalent to $1 today). The reduced size limit still
takes 22, 14, or 10 years to break-even under 20,
30 and 50% handling mortalities, respectively.
Thus, the 128-mm limit involves a protracted pay-
back period even in the absence of any positive
time value of money.
Although we did not simulate complex changes
in catch-by-size distribution and prices due to a
change in the size limit, we did attempt to gain
insight into the economic consequences of low-
ering the size distribution from the ADFG cost-
recovery fishery in 1995. Mean weight of live red
king crabs in deliveries under the 128-mm-CL and
147-mm-CL size limits were 2.4 kg and 3.1 kg,
respectively. Processed crabs were graded into five
size categories (Figure 10). The finished product
grading system measures weight in grams of a
cluster (i.e., three walking legs and a claw from
one side of a crab’s body), where M
5
300
2
499
g and L
5
500
2
699 g, 2 L
5
700
2
899 g, 3 L
5
900
2
1,099 g, 4 L
5
1,100–1,299 g, and 5 L is
1,300 g and greater. This system is commonly used
in the Japanese market, which is the primary mar-
ket for Bristol Bay red king crabs.
The 128-mm limit resulted in a substantially
smaller size distribution of processed product than
315
SIZE LIMITS FOR BRISTOL BAY RED KING CRABS
F
IGURE
8.—Estimated mean annual catch of Bristol
Bay red king crabs over 50 years (year 0 is 1996) under
handling mortalities of (a) 0, (b) 10, (c) 30, and (d) 50%
and under the 137-mm-CL and 128-mm-CL size limits. F
IGURE
10.—Size-grade distribution (M
5
300
2
499
g, L
5
500
2
699 g, 2 L
5
700
2
899 g, 3 L
5
900
2
1,099
g,4L
5
1,100–1,299 g, and 5 L is 1,300 g and greater)
of processed red king crabs from two deliveries made
during the ADFG cost-recovery fishery in Bristol Bay
during 1995: (a) a first delivery in which a 128-mm-CL
size limit applied, and (b) a second delivery when a 147-
mm-CL size limit applied.
F
IGURE
9.—Break-even analysis for the Bristol Bay
red king crab fishery under 50, 30, and 20% handling
mortalities. Plotted are the number of years needed for
the payoffs to equal the cost of the investment, i.e.,
PV
(128 mm CL)
2
PV
(137 mm CL)
5
0. Benefits never exceed
costs when handling mortality equals 0 and 10%.
the 147-mm limit (Figure 10). Average cluster
weight decreased more than a full grade from more
than3Lto2L.Price differentials among size
grades differ from year-to-year depending upon
market conditions. Generalizations from the par-
ticular market circumstances of the ADFG cost-
recovery fishery should be avoided. Nevertheless,
it is instructive to consider how reduced catch-by-
size might affect the magnitude of loss in just the
first year under the two size limits. The industry
would incur an expected first year loss of $1.7
million. On the one hand, high grading for crabs
larger than 147 mm CL (rather then 137 mm CL)
overstates the first-year loss estimate associated
with the proposed size-limit reduction. On the oth-
er hand, this estimate is understated because it only
reflects the diminished GHL due to lower average
red king crab weight in equation (1) rather than
any price drop. First-year losses would more than
double to $4.0 million if the smaller size distri-
316
KRUSE ET AL.
bution caused the average ex-wholesale price per
pound to decline $1 to $17.68/kg. If average price
fell to $15.47/kg, the industry would experience a
$6.4 million loss in the first year.
Discussion
Previous scientific recommendations on mini-
mum size limits for the Bristol Bay red king crab
fishery were based primarily on yield-per-recruit
analyses. This advice has been conflicting and am-
biguous. A chronology of published recommended
size limits in terms of millimeters CL (mm CW)
is 158 (193) by Hirschhorn (1966), 151 (183) by
Hirschhorn (1966) reanalyzed by Balsiger (1974),
150–155 (183–191) by Greenough (1972), 136
(163) by Balsiger (1974), 120–133 (142–159) by
Alverson (1980), 114 (133) by Reeves and Mar-
asco (1980), and 110 (127) by Reeves (1988). Dis-
crepancies are largely due to differences in growth
and mortality estimates. For instance, Balsiger
(1974) estimated growth of recaptured crabs
tagged during 1954–1961 and 1966–1969, where-
as Greenough (1972) used Weber and Miyahara’s
(1962) growth estimates from tagging during
1955–1957. Because large crabs tagged in 1955–
1957 grew faster than in other years, Greenough
estimated a larger optimal size than Balsiger.
Growth slowed in the 1980s and 1990s, so our
estimates of long-term mean growth are generally
less than those used in studies conducted in the
1960s and 1970s.
Compared with recent studies, pre-1980 inves-
tigations estimated lower Mfrom mark–recapture
studies, and correspondingly, larger size limits
were recommended. Studies in the 1980s were
based on an age–length key applied to survey size-
frequency data. Unfortunately, large growth var-
iability caused significant errors in resultant age
estimates from this method. Indeed, 50% of
Reeves’ (1988) age-specific annual Mestimates
were less than 0.01 and were deleted as being too
low, causing the average of the remaining values
to be a biased estimator of mean population M.
Our value (0.3), based on length-based analysis of
long-term survey data over 1972–1993, is inter-
mediate to published values (Zheng et al. 1995a).
Instantaneous natural mortality increased 4–5 fold
during 1980–1984 (Reeves 1988; Zheng et al.
1995a), but we held Mconstant because we ana-
lyzed long-term harvest policy and because we
cannot rule out that high Min the early 1980s was
fishing related. As a result of these combined
growth and mortality effects, our analysis indicates
that a size-limit reduction would diminish yield
per recruit for this fishery.
Our simulation model of stock rebuilding pro-
vides a somewhat different view of expected yields
over time under the two management alternatives.
Small red king crabs conserved by reduced han-
dling mortality eventually accumulate as larger
standing stocks generating higher average recruit-
ment, a biological feedback not considered by
yield-per-recruit analysis. Because of the relative-
ly slow growth and late maturity of red king crabs,
the industry would have to wait more than 2 de-
cades for the reduced size limit (at 20% handling
mortality rate) to yield cumulative catches that ex-
ceed those under the current size limit. This pro-
tracted biological payback period raises questions
concerning length of the economic payback for
what is ostensibly an investment in conservation.
A proper bioeconomic analysis would estimate
the net present value of economic surpluses (rev-
enues in excess of variable costs) under the two
management alternatives. Instead, this analysis fo-
cused on a partial economic metric—change in ex-
wholesale gross income—because cost data for the
fleet and processors are not available under the
current policy, let alone the proposed policy. Nev-
ertheless, qualitative insights are possible, as are
insights concerning potential policy bias.
The most notable economic effect of the reduced
size limit is that it takes considerably longer, if
ever, to provide a positive economic return than
to provide for increased cumulative catches. An
ancillary increase in harvesting efficiency (lower
production costs) due to higher CPUE should at
least partially offset diminished yields during the
rebuilding phase. However, higher CPUE and low-
er GHLs may come at another cost. Improved har-
vesting efficiency may jeopardize the ability to
manage this overcapitalized fishery in years of
very low abundance and short fishing seasons
(e.g.,
,
4 d). All other things equal, a reduced size
limit will result in an even shorter season. When
inseason data are inadequate to close the fishery
without exceeding the GHL, fishery managers
have little recourse but to err in favor of stock
protection by not opening the fishery at all. Such
increases in the probability of fishery closures
would lengthen the economic payback period un-
der the reduced size limit. Moreover, a size-limit
reduction will decrease the size-by-grade distri-
bution, which in turn will lower prices and, thus,
decrease gross receipts over some initial period of
time. This unquantifiable effect exaggerates the
aforementioned NPV implications. Anecdotal in-
317
SIZE LIMITS FOR BRISTOL BAY RED KING CRABS
formation indicates that the size distribution of
Russian red king crabs is decreasing, and high
grading has all but eliminated the largest size
grades. As a result, prices for medium, and small
size grades have reportedly softened. In summary,
it appears that the economic ramifications of the
size limit reduction would be deleterious to the
financial well being of the Bering Sea red king
crab industry.
Of course, catch and economic benefits are not
the only grounds on which management decisions
are made. The Magnuson–Stevens Fishery Con-
servation and Management Act of 1996 requires
fishery management plans to implement rebuilding
plans for overfished stocks and to include mea-
sures that minimize bycatch and mortality of un-
avoidable bycatch (NMFS 1996). Although Bristol
Bay red king crabs are not classified as overfished,
depressed stock levels persisted during the mid-
1980s to mid-1990s. A moderately high 1990 year-
class nudged the spawning stock over target levels
in 1998 (Zheng et al. 1998), but long-term stock
rebuilding remains an important fishery manage-
ment goal. Recently, the Bering Sea king crab fish-
ery was identified to have the second highest dis-
card ratio (number discarded per number landed)
of all fisheries worldwide (Alverson et al. 1994).
Indeed, we estimated that 32–79% of king crabs
caught in the Bristol Bay fishery were discarded
in 1991–1996 because of size and sex restrictions.
The benefit of bycatch reduction under the pro-
posed smaller size limit, however, is diminished
by the fact that females are illegal under both man-
agement alternatives.
The conservation benefits of reduced discards
depend on handling mortality rates. Lethal and
sublethal effects of handling on king crabs during
commercial fisheries have not been completely in-
vestigated (Kruse 1993), but 10–20% mortality
seems plausible. Handling does not inflict signif-
icant mortality under laboratory conditions when
crabs are promptly returned to the water with care
(Zhou and Shirley 1995, 1996). Also, low mor-
tality (deadloss) rates are typical of crabs held in
seawater tanks of fishing vessels for several days
before delivery to processing plants. However,
mortality estimates are not available for crabs re-
turned to the sea during commercial fisheries. Ex-
posure of red king crabs to extremely cold winter
air temperatures increases mortality and reduces
vigor and growth of the survivors (Carls and
O’Clair 1990). Studies on the added deleterious
cooling effects of winter wind are in progress. Fish
and amphipod (Anonyx spp.) predation on injured
crabs discarded during commercial fisheries are
suspected but uncertain sources of handling-
related mortality. Additional laboratory and field
studies are needed to fully resolve the potential
role of handling mortality on red king crab stocks.
The stock-recruit relationship used in our dy-
namic simulation model was based on a relation-
ship between male size and average number of
females mated per male. Small mature males (120–
124 mm CL) mate with one female, and large
males (
.
160 mm CL) mate with three females,
which was determined in laboratory studies by
Paul and Paul (1990, 1997). However, gains in
male reproductive potential from the reduced size
limit that are associated with a shift in catch dis-
tribution to smaller sizes appear to be very minor,
partly because accumulated natural mortality of
large, old red king crabs diminished the savings.
Moreover, males were seldom limiting to repro-
duction, even under the current harvest strategy,
which was designed to promote stock rebuilding.
Admittedly, if unattended molting females expe-
rience increased predation or cannibalism, then the
importance of mature males on population dynam-
ics may be underestimated.
There may be other less costly management al-
ternatives than reduced size limit to promote by-
catch reduction and stock rebuilding. New gear
designs are leading options. Zhou and Shirley
(1997) designed a pot that in laboratory experi-
ments reduced the catch probability of females and
sublegal males by more than 60% while increasing
the catch probability of legal males by more than
25%. Although the new pot did not perform to
expectations under higher crab densities experi-
enced during initial field trials in Bristol Bay
(Zhou and Kruse, Alaska Department of Fish and
Game, unpublished data), adjustments may result
in a pot that significantly reduces bycatch in the
fishery. A large (22.86 cm) stretch-mesh panel,
required in crab pots starting in 1996, shows prom-
ise. The percentage of nonlegal red king crabs in
observed pots during recent fisheries declined
from a mean of 64.5% during 1991–1994 to 31.8%
in 1996. (However, due to few participating catch-
er–processors and a short season, the sample size
in 1996 was only 9.5% of mean sample size during
1991–1994, so definitive conclusions of the effec-
tiveness of the large mesh are premature.) In ad-
dition to reducing total bycatch, handling mortality
rate should be minimized. Improved onboard sort-
ing procedures and educational programs may im-
prove crab survival. Adverse effects of winter
weather on discarded crabs are a lingering con-
318
KRUSE ET AL.
servation concern. Pending the outcome of on-
going investigations on cold wind chill effects,
season adjustments may be a cost-effective way to
reduce mortality rates of discarded crabs.
Should the size limit for the Bristol Bay red king
crab fishery be reduced from 137 to 128 mm CL?
Proponents are generally correct in many of their
contentions about the reduced size limit. Most no-
tably, bycatch would decline, CPUE would in-
crease, and spawning biomass and commercial
catches would increase slightly over a 50-year pe-
riod. On the other hand, a large industry invest-
ment in conservation would be recouped over a
very protracted pay-back period. Further, when
abundance and GHLs are low, higher catch rates
under the 128-mm-CL size limit increase the like-
lihood that stocks would be overharvested or that
the fishing seasons would be foregone due to man-
agement concerns about exceeding prescribed lim-
its. As a conservation measure, the reduced size
limit has merits, but examination of the full suite
of tradeoffs conducted in this integrated bioeco-
nomic analysis leads us to recommend no change
in minimum size limit for the Bristol Bay red king
crab fishery. In addition to maintaining the current
rebuilding plan with its schedule of conservative
harvest rates, other more cost-effective measures
should be explored to reduce total bycatch and
lower handling mortality rates to accomplish long-
term crab resource conservation objectives.
Acknowledgments
This study was funded by ADFG, Division of
Commercial Fisheries, and in part by cooperative
agreements from the National Oceanic and At-
mospheric Administration. We thank D. Pengilly,
J. McConaugha, and two anonymous reviewers for
helpful comments of a draft of this paper. The
views expressed herein are those of the authors
and do not necessarily reflect the views of NOAA
or any of its sub-agencies. Contribution PP-180 of
the Alaska Department of Fish and Game, Division
of Commercial Fisheries, Juneau.
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... It is a promising tool to enhance fisheries sustainability and has been an effective 74 management tool in other small-scale crustacean fisheries (Pine III et al., 2004;Liu et al., 2016;75 Miller, 1976). For example, Kruse et al. (2000) studied the size limit reduction policy proposed 76 by industry on red king crabs in Bristol Bay. Kruse et al. (2000) found that the proposed size 77 limit policy could reduce effort needed to capture the same annual quota. ...
... For example, Kruse et al. (2000) studied the size limit reduction policy proposed 76 by industry on red king crabs in Bristol Bay. Kruse et al. (2000) found that the proposed size 77 limit policy could reduce effort needed to capture the same annual quota. However, in the long 78 run the change was not considered cost-effective, because of a tradeoff in yield between handling 79 mortality rates. ...
... Minimum size limits are common in a many different fisheries (Pitcher and Hart, 1983;, although often established in conjunction with other measures (e.g. Kruse et al., 2000;Hutton et al., 2001). ...
... Foale and Day, 1997; Kruse et al., 2000;Hutton et al., 2001 8. Maximum size limit ...
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The International Workshop on CITES Implementation for Seahorse Conservation and Trade brought together over 40 participants from 9 countries, with representatives from CITES Parties, the CITES Secretariat, fisheries agencies, non-governmental organizations, industry, academia and public aquariums. The goal of the workshop was to assist countries in identifying sustainable management options for seahorse fisheries and addressing the Convention on International Trade in Endangered Species (CITES) permitting requirements for trade under the new CITES Appendix II listing of all seahorse species that goes into effect in May 15, 2004. The workshop was organized by Mexico and the United States, with logistical support provided by the International Fund for Animal Welfare.
... Size restrictions on the clams harvested are an effective means of maintaining sustainable resource management and preventing juveniles from harvesting activities (Colteaux and Johnson, 2017;Rogers et al., 2010;Van Poorten et al., 2013). The establishment of a minimum size limit can help control the harvesting pressure and promote sustainable harvesting in various types of fisheries, such as clam (Schoeman, 1996;Spillman et al., 2008), fish (Braje et al., 2017;Van Poorten et al., 2013), crab (Kruse et al., 2000), lobster (McGarvey et al., 2015;Xu and Schneider, 2012), turtle (Colteaux and Johnson, 2017) and seahorse (Foster and Vincent, 2005). The results have shown that even when the clam-diggers claimed that they had not taken the smaller clams, suggests that the clam-diggers had no or little knowledge of the collected clams' maturity or the suitable size for sustainable collection. ...
Article
At every given site with conservation value, there is a dilemma between conservation and the use of natural resources. Here we describe the first in-depth investigation of the differences between perceived and actual behaviour in clam harvesting, with a view to determine a more sustainable approach to conserve a previously unprotected tourist and clam-harvesting site, Shui Hau, in Hong Kong. We assessed harvesting pressure by conducting a clam-harvest photographic survey and collected self-reported questionnaires in 2018. More than 90% of visitors per sampling day were assessed for the survey, resulting in 86 sets of completed questionnaires and clam harvest photographs, in which we counted the number of clams harvested and recorded the shell length of each clam. We found that the clam-diggers had high self-reported environmental attitudes and behaviour, and a higher new ecological paradigm score was associated with a better clam-digging related attitude and perceived clam-digging behaviour. The low correlation between self-reported behaviour and actual behaviour indicated that most clam-diggers had different concepts or definitions of the appropriate size of the clam harvest and the size of a mature clam, even though each digger reported that he/she had the highest level of environmental attitudes and behaviour. This differentiation may imply a transition problem between environmental attitudes and actual environmental behaviour. An effective management approach is needed to foster clam-diggers’ sense of biodiversity to embrace the long-term sustainable use of coastal resources.
... One possible complement (or alternative) to the use of quotas for managing trade in reptile skins is to implement limits on the size (minimum or maximum, or both) of specimens that can be captured and traded. Size limits are a commonly used management tool in fisheries and some terrestrial wildlife harvests (Fitzgerald et al. 1991;Kruse et al. 2000;Foster and Vincent 2005;Webb et al. 2012), but have been largely overlooked for management of trade in Southeast Asian reptile skins. Because trade is focused on whole skins (rather than pieces), we explore the applicability of using size limits to regulate the harvest and trade of snakes from Southeast Asia. ...
Article
ContextEach year, millions of reptile skins are commercially exported from Southeast Asia for exotic leathers. Quotas are commonly used to regulate this trade, but quotas are sometimes exceeded and do little to ensure harvest sustainability. AimsTo explore the relationship between the size of live pythons and their skins, and to assess whether skin measurements can be used to enforce the application of limits on the size of harvested snakes. Methods We measured the body size of three heavily harvested python species (Malayopython reticulatus, Python breitensteini and Python brongersmai) in Indonesia and Malaysia and examined the relationship with skin length, skin width, the size of the ventral scale and its adjacent dorsal scale. Key resultsMeasurements of 2261 pythons showed strong relationships between the size of live pythons and measurements made on their skins. Skins can be stretched 30% longer than the body length of snakes from which they came and inter-facility and country differences in stretching technique result in different relationships between the sizes of live snakes and the measurements made on their skins. Male and female Malayopython reticulatus differed in their skin dimensions relative to the size of the live snake, but these differences were minor. Conclusions Despite variations in stretching techniques, in functional terms, this variation is minor (maximum 3.5% relative to each mean measurement) and does not limit application of skin sizes for regulating trade within an acceptable level of error. However, differences in the stretched length of Indonesian and Malaysian skins were much greater (5.9% of the mean length of skins), and, thus, each country should apply its own limits and predictive tools. ImplicationsThe strong relationship between the skin size and the size of the live snake offers great potential for regulating trade by using skin-size limits. Inspection of the size of skins can be used to enforce harvest-size limits and focus harvesting away from sexes and life stages most critical for population persistence. This management tool has numerous advantages over current regulatory practices (quotas) and should be considered for management of trade in Asian reptile skins.
... In addition to estimates of growth and natural mortality rates, the relationship between market price per individual and product size is an essential input to models that aim to maximise revenue from invertebrate fisheries (e.g. Bunnell et al., 2010;Kruse et al., 2000;McGarvey et al., 2015). This relationship should also inform decisions about regulations on minimum size limits (Purcell et al., 2017), although such information is lacking in many fisheries. ...
Article
Market price trends of seafood can inform fishery management measures and strengthen the bargaining power of fishers. The four-sided sea cucumber Isostichopus badionotus and donkey dung sea cucumber Holothuria mexicana are heavily exploited in small-scale fisheries in the Gulf of Mexico and Caribbean for export to China. We recorded prices and sizes of the dried sea cucumbers from 41 lots in 28 shops in Hong Kong and Guangzhou, China. Market value ranged 132–358 US$ kg⁻¹ for I. badionotus, and 16–209 US$ kg⁻¹ for H. mexicana. The relationship between product length and price per kg was weak for both species, revealing large variability in the marketplace. Price per individual increased linearly with product length for H. mexicana, and increased nonlinearly for I. badionotus indicating that large specimens were disproportionately more valuable than small ones. Thus, the economic performance of fisheries, especially those for I. badionotus, could be optimised by strictly enforcing large minimum size limits. High market value identifies I. badionotus as a species of conservation concern. A large difference between reported prices received by fishers and market retail prices suggests substantial scope for upgrading value chains of small-scale export fisheries.
... Size limits generally require an individual be above a certain metric (minimum size limits) or between two metrics (slot limits). Size limits have been shown to be effective at increasing abundance in some species like the common whelk, Buccinum undatum (McIntyre, Lawler, & Masefield, 2015;Power & Power, 1996;Wilde, 1997), but ineffective in others including the red king crab, Paralithodes camtschaticus (Halliday & Pinhorn, 2002;Kruse, Byrne, Funk, Matulich, & Zheng, 2000;Nieland et al., 2007). Further, size limits can result in the targeting of one size/age class critical to the viability of a species, such as older reproductive adults. ...
... During moulting and mating, the females are dependent on a larger male to guard them. Females are at greater risk of predation or cannibalism immediately after moulting, when the new shell is soft, unless they are guarded by a male (Paul and Paul, 1990;Kruse, 1993;Kruse et al., 2000), and females avoid predation when males guard them for a long duration (Kruse, 1993). In the wild, the CL of the mating males averages ...
Article
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Hjelset, A. M. 2014. Fishery-induced changes in Norwegian red king crab (Paralithodes camtschaticus) reproductive potential. – ICES Journal of Marine Science, 71: 365–373. The introduced red king crab (Paralithodes camtschaticus) in the Barents Sea supports a valuable fishery in northern Norway. In this paper, I examine the effect of the increased harvest rate and the recently added female quota on the potential egg production of the stock. The size ranges of males and females in the period 1995–2011 were recorded, and estimated stock abundance of ovigerous females and established individual fecundity parameters from 2000–2007 were used to assess the reproductive potential of the stock from 1995–2011. The upper size ranges of males and females decreased throughout the period studied, presumably mainly due to fishing. The change in size composition among ovigerous females and functional mature males, and the reduced mean individual fecundity in the stock seem to have had a negative effect on the potential egg production of the stock.
... Size limits generally require an individual be above a certain metric (minimum size limits) or between two metrics (slot limits). Size limits have been shown to be effective at increasing abundance in some species like the common whelk, Buccinum undatum (McIntyre, Lawler, & Masefield, 2015;Power & Power, 1996;Wilde, 1997), but ineffective in others including the red king crab, Paralithodes camtschaticus (Halliday & Pinhorn, 2002;Kruse, Byrne, Funk, Matulich, & Zheng, 2000;Nieland et al., 2007). Further, size limits can result in the targeting of one size/age class critical to the viability of a species, such as older reproductive adults. ...
Conference Paper
Background/Question/Methods An increasing global trade in a wildlife commodity presents complex challenges for resource managers. With the sharp decline of Asian turtle populations, commercial harvest and farming of US species has risen to fulfill this demand. One of the most commonly targeted species is the snapping turtle, Chelydra serpentina, where annual export eclipsed a million live individuals for the first time in 2012. As this figure does not include individuals butchered and exported as meat, it is clear that snapping turtles are facing unprecedented levels of harvest pressure, as well as supporting a growing farming industry. We examine the commercial export rates of snapping turtles, and harvest rates in the Mid-Atlantic region of the US (from the states of Maryland, Virginia, and North Carolina). Results/Conclusions Harvest in Virginia increased nearly ten-fold from 2002 to 2012 while Maryland averaged 125,873 lbs harvested from 2007 to 2012 and North Carolina experienced a doubling of harvest from 2011 to 2012. The sharp increases in snapping turtle take raises concerns over the long term sustainability of this slow-growing, long-lived species. Regulations on snapping turtle harvest vary greatly among states in regards to open/closed commercial harvest, season length, acceptable capture methods, and bag limits. We suggest that development of regional management plans, and re-assessment of current regulations, would provide a cost-effective method of improving snapping turtle conservation while maintaining a sustainable harvest.
... When a population is identified as overfished, fisheries managers must also develop a plan to rebuild the stock to B MSY within some specified time period . In order to support the establishment of rebuilding plans, biologists frequently develop a series of recovery trajectories, which define the recovery process quantitatively under various regulatory alternatives (Ianelli and Hastie 2000;Kruse et al. 2000;Mori et al. 2001;Caddy and Agnew 2003;Powers 2003;Dulvy et al. 2005;Nowlis 2005). This is done by projecting the population that is estimated in an assessment model forward through time based on a range of different fishing mortality rates. ...
... Most previous studies focused on total catch, but the proportion of sublegal and female crabs caught is also of interest to both managers and fishers. Handling mortality may be high for some of these fisheries, potentially jeopardizing the health of future stocks (Zhou and Shirley 1996;Kruse et al. 2000). If fewer unwanted crabs are caught and returned to the sea, handling effects should be minimized and stock health should increase. ...
Article
1 The Regional Information Report Series was established in 1987 to provide an information access system for all unpublished divisional reports. These reports frequently serve diverse ad hoc informational purposes or archive basic uninterpreted data. To accommodate timely reporting of recently collected information, reports in this series undergo only limited internal review and may contain preliminary data, this information may be subsequently finalized and published in the formal literature. Consequently, these reports should not be cited without prior approval of the author or the Division of Commercial Fisheries.
Article
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A modifiable harvest rate constrained by a minimum spawning abundance (threshold) is currently used to set the annual harvest level for Bristol Bay red king crab, Paralithodes camtschaticus. A length-based simulation model was constructed to evaluate effects of recruitment, natural mortality, and handling mortality on this harvest strategy. Evaluation criteria included mean yield, stability of yield, harvest opportunity, and stability of spawning stock. Optimal mature male harvest rates were strongly negatively related to handling mortality. For any given harvest rate, handling mortality is a key factor influencing optimal thresholds. The current harvest strategy produces a high mean yield and low variability in yield under low handling mortality scenarios, but the population is at high risk of collapse with a high handling mortality. Given uncertainties of recruitment, natural mortality, and handling mortality estimates, we recommend reducing mature male harvest rate from 20 to 15% and maximum legal male harvest rate cap from 60 to 50%. If handling mortality rate is greater than 30%, then we recommend increasing the threshold fi-om 6600 to 11 000 metric tons of effective spawning biomass. Our recommended harvest strategy produces a mean yield similar to the current harvest strategy and safeguards against recruitment overfishing.
Article
The abundance of Bristol Bay red king crab Paralithodes camtschaticus has fallen to historically low levels in recent years. Concerns about depressed spawning stock levels and economic hardships associated with fishery closures in 1994 and 1995 prompted reevaluation of the status quo harvest strategy and investigation of alternative strategies to rebuild the stock. Using a length-based model initialized with the 1994 population abundance, we simulated future effects of seven alternative rebuilding strategies on this stock. Strategies ranged from the status quo strategy through increasingly restrictive harvest strategies culminating in complete fishery closure until the stock is rebuilt. Statistics on catch, variation in catch, effective spawning biomass, probability of rebuilding, probability of fishery closure, and present exvessel value were collected for comparisons. Sensitivities of the harvest strategies to natural mortality, handling mortality, stock-recruitment relationship, and measurement error were examined. Reducing the status quo harvest rate greatly shortened rebuilding time and enhanced long-term catch. The most conservative strategy achieved a 50% probability of rebuilding the stock to a target 25,000 t of effective spawning biomass in 12 y. In terms of catch and present exvessel value, the status quo strategy performed best among the seven strategies over a short-term (≤20 y) planning horizon, whereas strategies with a mature male harvest rate of 50-75% of the status quo level performed best over longer planning horizons. Scenarios with lower natural mortality, higher handling mortality, or a more density-dependent stock-recruitment relationship favor the implementation of more conservative strategies. Our analysis of population and fishery dynamics leads us to recommend a 25-50% reduction in harvest rate to rebuild this depressed stock.
Article
Most multiparous red king crab Paralithodes camtschaticus (Tilesius, 1815) hatch their eggs during the brief spring diatom bloom; then, they molt and breed during a period of =20 days. This study examined the percentage of cleaving eggs in clutches of multiparous red king crab mated to males 140-204 mm carapace length (CL). Ten males had access to three or four females, and the intervals between individual matings ranged from 1 to 22 days. There was no obvious relationship between breeding success and the time interval between matings. Fertilization of the first females' clutch was successful for all 10 test males, with 97-100% of the eggs initiating division. Nine males bred their second potential mate; one did not. That female ovulated with another male, so she was fertile. Egg division rates ranged from 86 to 100% for the second matings. All 10 males fertilized the third female, with 5-100% of the eggs starting division. Only 66% of the males fertilized a fourth clutch, and egg division rates were 79-100%. One female fourth in line to be bred extruded a clutch in the presence of the test male, but none of the eggs divided. Two of the fourth mates had to have fresh males put in with them before they ovulated. The results suggest that most male red king crabs ≤140 mm CL can fertilize three mates during the brief period when most multiparous females breed.
Article
A modifiable harvest rate constrained by a minimum spawning abundance (threshold) is currently used to set the annual harvest level for Bristol Bay red king crab, Paralithodes camtschaticus. A length-based simulation model was constructed to evaluate effects of recruitment, natural mortality, and handling mortality on this harvest strategy. Evaluation criteria included mean yield, stability of yield, harvest opportunity, and stability of spawning stock. Optimal mature male harvest rates were strongly negatively related to handling mortality. For any given harvest rate, handling mortality is a key factor influencing optimal thresholds. The current harvest strategy produces a high mean yield and low variability in yield under low handling mortality scenarios, but the population is at high risk of collapse with a high handling mortality. Given uncertainties of recruitment, natural mortality, and handling mortality estimates, we recommend reducing mature male harvest rate from 20 to 15% and maximum legal male harvest rate cap from 60 to 50%. If handling mortality rate is greater than 30%, then we recommend increasing the threshold from 6600 to 11<|>000 metric tons of effective spawning biomass. Our recommended harvest strategy produces a mean yield similar to the current harvest strategy and safeguards against recruitment overfishing.