Why are forecasts of sea scallop biomass and fishing mortality uncertain?
Dvora Hart, NEFSC, Woods Hole
“Predictions are uncertain, especially about the future” Yogi Berra (?) Mid‐Atlantic
Updated CASA model estimates Georges Bank Biomass about 150,000 mt meats in 2009
2008 F MA: 0.38 GB: 0.18 Overall: 0.28 2009 F MA: 0.42 GB: 0.18 Overall: 0.30
Fishing mortality will need to be reduced by ~20% in 2010 to achieve a target of 0.24 Why is the estimate of fishing mortality higher than that projected in Framework 19?
Reason #1: Random error
Standard errors for (short‐term) projected fishing mortalities are estimated to be about 0.06, so that if F is projected to be 0.20, there is about a 5% chance that the fishing mortality will actually be greater than 0.30. Reason #2: Retrospective in assessment
Assessment models such as CASA compute a maximal likelihood estimate of biomass (and fishing mortality). This means that the reported biomass point estimate is “more likely” to be accurate than any other number.
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“Unlikely” events can and do occur Biomass in 2006 was estimated at 166,000 mt in SARC‐45. With the addition of 2+ years of data, the updated model estimates 2006 biomass at 151,000. The updated number is within the confidence interval estimated during SARC‐45, so that this result is not pathological. However, it implies that fishing mortality is higher than that estimated in 2007.
180000
160000
140000
Sarc45B 120000 UpdateB 100000
ass (mt) 80000 mm
60000 Bio
40000
20000
0 1980 1985 1990 1995 2000 2005 2010 Year Reason #3: Changes in the biomass/LPUE relationship
LPUE needs to be estimated to translate landings itinto days‐at‐sea. This has been done via a moddlel that estimates LPUE as a function of exploitable biomass. This relltiations hip appears to have chhdanged since the implementation of Amendment 10; vessels have become more effic ien t with the ir limited DAS. This lead to an underestimation of LPUE and F in the FFkramework 19 analilysis. The LPUE model has now been modified to reflect the changes since A10. Conclusion
Forecasts of fishing mortality hav e sfficientsufficient uncertainty so that an underestimate of about 0010.10 is possible, though not very likely.
Sufficient precaution in management is required to make the probability of future overfishing small