51 We
consider blue crab a data poor species in light of uncertainties over the age structure
in the population, uncertainties over key vital rates e.g., natural mortality, reproduction
and uncertainties in the reliability of the harvest time series. Therefore, we
recommend adoption of a target reference point equivalent to 0.75 U
MSY
. This
precautionary approach provides a buffer against uncertainty in estimates of
exploitation rates without foregoing substantial amounts of yield. This standard of 0.75
U
MSY
is used by several federal management councils, including the Mid‐Atlantic and New
England Fishery Management Councils as well as being an approach recommended by
National Standard 1 to the federal Magnuson Stevens Act 2006. This recommendation
implies a target exploitation reference point of U=0.255 based on the exploitation
of age‐0+ female crabs. Furthermore, based on the SSCMSA, the equivalent abundance
target reference point varies from 210 – 340 million age‐1+ female crabs for the
range of sex‐specific exploitation rates observed historically and currently in the fishery
0.6F
male
:F
female
1.2. We recommend adoption of an abundance target
reference point of 215 million age‐1+ females as a working value.
Based on these recommendations, we present a revised control rule for blue
crab in Chesapeake Bay in Fig. 6.15. The interpretation of the stock history from the
revised control rule Fig. 6.15 differs somewhat from that of the existing control rule
Fig 6.1. The revised control rule indicates that the crab population in Chesapeake Bay
experienced overfishing from 1998‐ 2004 and was technically overfished from 2001‐
2003. The existing control rule indicated that the crab population was experiencing
overfishing during the same period. In contrast, the existing control rule indicated that
the crab population had never been overfished – as a result purely of the empirically‐
derived overfished definition that was set at the lowest observed abundance recorded.
Importantly however, the revised and existing control rules do not differ in their
interpretation of the current population status. The revised control indicates that in
2009 the blue crab stock in the Chesapeake Bay was not overfished, nor was it
experiencing overfishing. More specifically, the exploitation rate in 2009 U
2009
=0.24 age
‐0+ female crabs was below the U
target
= 0.255. Also, the blue crab population in 2009
was above the overfished definition of 70 million age‐1+ females. The best estimate
of the abundance in 2009 N
2009
= 174.3 millon age=1+ female crabs was lower than
the target abundance. We note that the abundance of crabs in the winter dredge survey
of 2009‐2010 suggest that the population was above target abundance in 2010.
6.3. Alternative Assessment models
We explored two other assessment modeling approaches as a check on the
results produced by the SSCMSA model.
52 6.3.1.
Production modeling of Chesapeake Bay blue crab There
is a long history of the application of production models in fisheries stock assessment
Quinn and Deriso 1999. Simple equilibrium production models were developed
by Tang 1983 and by Rugolo et al 1997. These authors used estimated baywide
catch and nominal effort in a simple Schaefer 1954 production model. These efforts
produced an estimated maximum equilibrium yield of ~85 million pounds. However,
concerns over the reliability of the effort time series used as input to the models
and over the model fits themselves precluded use of these estimates for management.
No production modeling was included in more recent assessment efforts Miller
and Houde 1999, Miller et al. 2005. For this assessment we developed a simple production
model Assessment Working Paper 4. Both
production models examined provided credible yield‐based reference points.
The yields predicted by both models are comparable to observed values and comparable
with results of the SSCMSA. Abundance‐based reference points, however, were
more variable. Production model‐based overfished definitions were 255 million age
‐1+ crabs or 410 million crabs of all ages. In both cases, production model estimates were
greater than those predicted by the SSCMSA.
6.3.2. Catch‐Multiple Survey Analysis CMSA from 2005 Assessment
In the last assessment Miller et al. 2005, a Catch‐Survey Analysis CSA, e.g.,
Mesnil, 2003 was presented, modified to permit multiple fishery‐independent surveys
and to estimate both observation and process error. This model assessed the current
population status and its historical trajectory relative to biological reference points
estimated outside of the assessment model. Although this approach continues to be
valid, we view the fact that it relies on management reference points estimated external
to the model as a significant weakness. In the 2005 assessment, Miller et al. compared
output from this CMSA to reference points developed in an individual‐based yield‐per‐
recruit analysis.
We have not developed the CMSA model further because of its inability to
provide integrated reference points. However, we updated the results of the CMSA in
this assessment for comparative purposes to allow us to assess the degree to which any
changes in the inferred status or trajectory of the stock reflect true underlying changes
or changes in the model framework Assessment Working Paper 5.
A comparison model run was conducted with parameters used in the base model
run of the 2005 assessment. These are: natural mortality M=0.9, ratio of survey
selectivities, L =0.4 for MD and VA trawls and L=0.5 for the winter dredge survey.
Likewise the ratio of observation to process errors was set to 0.65 for both trawl surveys
53 and
to 0.4 for the dredge survey. The CMSA model was still able to capture trends in the
abundance of age‐1+ females. However, comparison run results predicted considerably
higher exploitation fractions than are currently observed or estimated in the
fishery. These results led to control rule plot that suggested the blue crab population
is currently experiencing overfishing. CMSA results were not considered further.
6. 3.3 Alternative models and reference points. To
evaluate the effect of the changes in approach and data on inferred stock status,
we compare the following scenarios: a 2005 reference points with updated survey
averages and commercial data, b 2005 reference points with standardized survey
data and updated commercial data, and c the proposed new 2011 reference points
and data. We
have previously presented the updated version of the current control rule Fig.
6.1. This figure employs the winter dredge survey as an index of abundance for the
coming fishing year. It calculates an exploitation fraction, u, as the ratio of the total commercial
catch plus 8 for recreational harvest and the winter dredge abundance. Using
this approach, we infer that the crab population in the Chesapeake Bay is not overfished,
is close to or above target abundance and is not experiencing overfishing. To
assess the impact of the change of the survey time series on the overall conclusions, we
also completed an update of the CMSA model Miller et al. 2005 using the updated survey
indices and commercial data. We term this run the updated CMSA run. This run of
the model re‐estimated the trajectory of the crab population as far back as 1968. As with
the original CMSA presented in the 2005 assessment, the updated run generates exploitation
fractions early in the time series that are 1, with a large number 1.5. This
gives rise to concern about the performance of the model in the early years. However,
for the most recent years, the performance of the updated model is more reliable
Fig. 6.16. The model results infer that the population is not overfished, but was
experiencing overfishing in 2009 Fig. 6.16. The model also inferred that the population
was above target abundance and at or below target exploitation rate in 2006
‐07. Since then, the model results suggest exploitation has increased and abundance
has decreased. For
a final comparison, we also re‐ran the CMSA model but used the standardized
indices rather than the simple index means. Comparison of the results from
this model to the previous model will indicate the overall impact of the standardization
on inferences regarding stock status. We term this run the standardized CMSA
run. The results from the standardized CMSA run are provided in Fig. 6.17. As with
the updated CMSA runs many of the estimates of u 1.5 in the early part of the time
series. Again, more recent estimates are more credible. The standardized run
54 indicates
that the population is not overfished, and is above the interim target of 200 million
crabs. The standardized run indicates that the population is not experiencing overfishing.
The exploitation estimate for 2009 was u2009=0.46. The model infers that exploitation
rates have been 1 since 1994. Comparison
of the current control rule, the updated CMSA run and the standardized
CMSA run and the new sex‐specific reference point control rule proposed here
all indicate that the blue crab population in the Chesapeake Bay is not overfished. Three
of these models, including the new sex‐specific reference point control rule, indicate
that the population is not experiencing overfishing. The models differ to as to whether
they indicate that the population has achieved target abundance. Half of the four
models indicate that we are below target abundance, two indicate that we are above
target abundance. From
the comparison of models we present here we argue that evidence suggests
that the blue crab fishery in the Chesapeake Bay is operating in a sustainable manner.
All analytical results suggest that the population is in excess of abundance levels
that would give cause for concern. Current exploitation rates also appear sustainable.
55
8. Discussion and Recommendations
