We have developed a hierarchy of target levels, designated to address sustainability, efficiency, and recovery scenarios.  Targets were derived from: 1) reported catches and effort in the commercial fishery, 2) statistics from fishery-independent surveys, and 3) knowledge of the biology of blue crab.  Targets that are recommended include population sizes, catches, and effort levels, as well as reference fishing mortality rates.  They are intended to be conservative and risk-averse.  (PDF contains 182 pages

Houde, Edward D.

Miller, Thomas J.

English

Aquatic Commons

Univer$ity of Maryland Center for Environmental Science R e f .  No. [UMCESICBL 99-100 
 hes sap eke Biol.;;$cal hboratog 
P. 0. Box 38 
SO~OIXIO~S, 20688-0038 
NAL REPORT 
Prepared Fg>r 
Living Resources Subcommittee 
Chesapeake Bay Program 
U.S. EPA 
41 8 Sewern Avenue 
Annapesfis, MD 21403 
Prepared & 
Thomas J. Miller 
and 
Edward 5. Houde 
5 March 1999 
This is Technical Series No. TS-177-99 d t h e  University of MayImd Center for Envkomental Science, 
Chesapeake Biological Laboratory 

Blue Crab Target Setting Miller and Houde 
EXECUTIVE SUMMARY 
The fishery for blue crab (CalZinectes sapidus) is the most valuable commercial fishery in 
Chesapeake Bay. In 1997, 69 million lbs. were landed with dockside value of $73 million. A 
large, but unquantified amount, is taken by recreational crabbers. Ecologically, blue crab is an 
important benthic predator. In turn, it is prey for several predatory fish. Sound management of 
this resource to insure a sustainable population is critical. It is important to establish targets to 
protect the resource and judge the efficacy of management actions. 
We have developed a hierarchy of target levels, designated to address sustainability, 
efficiency, and recovery scenarios. Targets were derived fi-om 1) reported catches and effort in 
the commercial fishery, 2) statistics fiom fishery-independent surveys, and 3) knowledge of the 
biology of blue crab. Targets that are recommended include population sizes, catches, and effort 
levels, as well as reference fishing mortality rates. They are intended to be conservative and risk- 
averse. Proposed targets will promote a sustainable and economically viable fishery, while 
protecting the ecological value of the blue crab in Chesapeake Bay. In the hierarchy, the first 
targeting level is one that designates population abundances and fishing mortality rates to insure 
sustainability of the resource. A second level of targeting insures that the maximum reproductive 
potential per crab is obtained over the long term. The final targeting level is one that recommends 
a population abundance and fishing mortality rate to insure a growing crab population. 
The assessment and analyses included in this report bear many similarities to those 
presented by the CBSAC Technical Committee in their stock assessment of blue crab. Our report 
differs, however, by concluding that the change initiated in 198 1 in Maryland's statistical 
reporting of commercial blue crab landings had a major effect on the landings time series. The net 
effect is that our adjusted statistics and analyses indicate that the blue crab has been fished more 
heavily, and that stock abundance has declined recently and is lower than the CBSAC Committee 
believed. The CBSAC Committee had concluded that stock abundance had increased in the 
1980s and then returned to average levels in the 1990s. 
A major difficulty in analyzing blue crab stock dynamics is the uncertainty associated with 
aging crabs and its potential effects on determining growth and mortality rates. Like the CBSAC 
Committee, we explored various scenarios of maximum age to derive estimates of a range of 
probable mortality and exploitation rates. We reconstructed a commercial landings time series 
based upon the adjusted landings, after 'correcting7 the Maryland statistics fiom 1945 to 1980. 
From the time series and from fishery-independent surveys, we estimated population abundances, 
biomasses, maximum sustainable yields (MSY), fishing effort to obtain MSY, fishing mortality 
rates, exploitation rates, yield per recruit, spawning potential per recruit, and relationships 
between spawning stock and recruitment. In a novel assessment of blue crab population 
dynamics, we developed a life-stage-based model in both aggregated and spatially-explicit forms 
to determine what life stages are most critical for future population growth and stability of blue 
crab in Chesapeake Bay. 
Page i 
Blue Crab Target Setting Miller and Houde 
Tier 1: Sustainability Limit 
We analyzed fishery-independent, fishery-dependent and life history information on blue 
crab in Chesapeake Bay. The spatial and temporal coverage of the data varied considerably. The 
winter dredge survey, supported by NOAA's Chesapeake Bay Stock Assessment Committee 
(CBSAC), is the only Bay-wide index of crab abundance that has been consistently collected. 
Unfortunately, this survey has been conducted for only 9 years and is not of sufficient duration to 
reach conclusions about the dynamics of blue crab in the Bay. Consequently, conclusions on the 
relationship between population abundance and exploitation must stem from analyses that 
combine data of differing resolutions to make Bay-wide inferences. 
A time-series intervention analysis and other lines of evidence supported our decision to 
adjust Maryland landings statistics for 1945-1980. Estimated baywide landings in those years 
were increased substantially as a consequence of the adjustment. The adjusted mean annual 
landings from 1945- 1996 were 84.6 million 1bs in contrast to 69.7 million lbs based upon the 
unadjusted landings. We believe that the long-term potential yield of the blue crab commercial 
fishery in Chesapeake Bay lies in the range 80-100 million lbs. Based upon,the adjusted landings 
data, there is little evidence that overall commercial landings of blue crab have changed much 
during the >50 years included in the landings time series. The upper bound of the long-term 
potential yield 'target' level is a risky choice, especially since information on possible trends in 
recreational fishery catches are lacking. A preferable commercial fishery target is near 80 million 
lbs. 
Effort in the commercial fishery has increased progressively over the years. Recent 
landings have only been maintained by a substantial increase in effort. Based upon our adjusted 
landings statistics, catch-per-unit-effort continued to decline, at least through the mid-1 980s. This 
observation indicates that stock abundance probably has been declining throughout the time series 
and supports our risk-averse recommendation of a target, long-term potential yield near 80 million 
lbs rather than 100 million lbs. 
Our estimate of MSY (99 million lbs), based upon the adjusted landings time series, also 
indicates that increases in commercial landings above levels in recent years may not be sustainable, 
especially when viewed with respect to present levels of effort and the declining catch-per-unit- 
effort. Effort in the fishery during the past 15 years has been greater than that required to obtain 
MSY. In fact, effort to obtain MSY (-450,000 pot hrs month-') has been exceeded by 
-100,000 pot hrs * month-' in recent years. MSY and its respective effort level should not be 
targets in a risk-averse blue crab fishery, but rather should be considered upper 'limits,' above 
which the stock is likely to decline. 
A target fishing mortality rate of approximately Fs0.9 may maintain blue crabs at the 
stock level believed capable of supporting the long-term potential yield from the commercial 
fishery. Our estimates of baywide abundance of potentially catchable crabs that can support that 
level of yield is -1 billion crabs with a biomass of -300 million lbs. Blue crab numbers have 
Page ii 
Blue Crab Target Setting Miller and Houde 
declined in the 1990s and exploitation rate from fishing has increased. Fishing mortality rates 
>1.0, now being observed, may not allow catches to be maintained at high and sustainable levels. 
Estimates of fishing mortality rate in the fishery indicate that F has ranged Gom 0.6 - 1.3, 
with sharp increases and highest levels in the most recent years. This supports our conclusion that 
increased effort has maintained landings at near-constant levels. The evidence points to a need to 
reduce effort to insure the sustainability of high yields in the fishery. An effort level and fishing 
mortality rate that would leave 10% of the virgin spawning potential in the population is often 
used to manage productive crustacean fisheries. Basing our sustainability target on this reference 
point, F,, is -0.9. This level of exploitation may insure that the stock will not collapse due to 
reproductive or recruitment failure, but it is too high to allow the fishery to be most productive in 
either a biological or economic sense. 
Tier 2: Efficiency Target 
Our analysis of the yield to the fishery for every blue crab that recruits to it, strongly 
suggests that the productivity of the fishery would improve if fishing effort decreased. The fishery 
is technically "growth overfished," meaning that the present fishing mortality rates are too high to 
obtain the maximum yield per recruit from the fishery. Although there is no evidence at present of 
a declining trend in recruitment, reducing effort would increase abundance and average size of 
crabs in the stock, in addition to increasing profitability in the fishery. Our analyses suggest that F 
shouId be reduced to -0.6 to maximize yield per recruit and lower still to maximize profitability. 
That level may be viewed as an 'efficiency' target. 
IfF declined to -0.6, the spawning potential of blue crab would increase to -20% of that 
in the virgin, unfished population fiom a level that is -40% in the most recent years. Although 
blue crab is resilient to reductions in adult stock and no trend in recruitments is apparent, we 
found evidence in our stock-recruitment analysis that low adult stocks only infrequently were 
associated with high recruitments. In this sense, reducing present F fiom -1.0 to -0.6 or lower 
may increase average recruitment levels and contribute to the stability of the fishery. 
There also may be ecological advantages derived from implementing a reduction in F. 
There is dear evidence of a decreasing trend in the average size of crabs collected in the fishery- 
independent Calvert Cliffs pot survey. Reducing F would cause the size structure of the current 
population to shift to favor larger crabs. This would produce a population more closely 
resembling the historical size structure and is likely to have two benefits. Increases in average size 
of adult crabs will increase reproductive potential because there is an allometric relationship 
between fecundity and female size, and some evidence of an allometric relationship between male 
size and sperm production. Larger crabs may also be more capable of multiple matings. 
Additionally, increases in average size would increase the predation pressure that blue crab exerts 
in the benthic community. If blue crab is a keystone predator that controls benthic community 
structure, allowing its historical size-distribution to redevelop may have important community- 
wide effects. 
Page iii 
Blue Crab Target Setting Miller and Houde 
Tier 3: Recovery Target 
The third targeting level in our hierarchy is usefbl in the context of possible fbture needs to 
manage for population recovery in the event of precipitous, unexpected declines of blue crab. 
The stage based model results indicated that abundance of mature females, proportion of age 1 
crabs recruiting into the fall fishery, and overwinter mortality on pre-recruit crabs were likely to 
exercise the most control on potential growth of the blue crab population. Management actions 
directed at reducing mortality of those life stages are likely to have positive results with respect to 
population growth rates of blue crab. Though preliminary, modeling results indicated that F 
would have to be reduced substantially from present levels to insure increase in population growth ' 
rate. The stage based model indicated that F<0.5 would be required to insure a positive rate of 
population increase. This target F may not represent the ideal long-term level of exploitation, 
since the population clearly has withstood higher rates of exploitation. However, it is a level of F 
that guarantees high probability of positive population growth and can be regarded as a 
contingency level to foster recovery should the blue crab stock require such management action in 
the hture. 
Management Options and Monitoring Needs 
Management options that may be effective to attain goals and targets, determined in part 
from stage based-modeling of blue crab, to reduce the fraction of age-1 crabs that reach 
recruitable size and become vulnerable to the fishery include increases in size litflits during the fall 
fishery, or reducing effort in late-summer and fall peeler pot fishery. Reductions in fishing effort 
in the winter dredge fishery also may be beneficial to stabilize long-term abundance in the 
population, although this approach alone would not be sufficient to achieve such stability. In our 
analysis, an overall reduction in fishing mortality of approximately 30% from present levels would 
benefit the fishery and would be protective of the long-term ecological benefits that accrue fkom a 
healthy and resilient blue crab population in Chesapeake Bay. 
Continuous monitoring of the blue crab stock is essential to know its status and to develop 
effective management actions. The fishery has been essentially healthy, although productivity of 
the stock and quality of the fishery may be improved by reducing fishing mortality. The temporal 
and spatial variability in elements of the fishery and the spatially-explicit nature of the crab 
population must be understood better to improve modeling of the stock and its management. 
Programs to determine trends and variability in the recreational fishery need to be instituted, and 
the recreational landings eventually must be considered in stock assessments of blue crab in 
Chesapeake Bay. 
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Blue Crab target setting: final report

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Blue Crab target setting: final report

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