Document de recherche 2013/079
Évaluation du stock de pétoncles (Placopecten magellanicus) de la zone « a » du banc de Georges et du secteur « nord » du banc de Brown
Par P.B. Hubley, A. Reeves, S.J. Smith et L. Nasmith
Résumé
This research document describes the analytical assessment of scallops (Placopecten magellanicus) on Georges Bank ‘a’ and Browns Bank ‘north’, where the majority of landings from the offshore scallop fishery occur. Total reported landings in 2012 were 4,001 t and 475 t for Georges Bank ‘a’ and Browns Bank ‘north’, respectively. The analytical methods for this assessment are based on the 2009 framework of Georges Bank scallops (Jonsen et al. 2009), along with a number of improvements in the areas of growth, mortality and observation error. Annual dredge surveys using stratified random design, where the strata are defined by the historical abundance indices, are conducted on Browns and Georges banks every May and August, respectively.
Both commercial catch rate and survey indices show increases in fully-recruited biomass for Georges Bank 'a' from 2011 to 2012, and are well above the long-term median. The survey abundance indices of pre-recruits and recruit size scallop are comparable to the long-term median. The distribution pattern indicates that pre-recruit and recruit size scallops are mainly found on the northern half of Georges Bank ‘a’, while fully-recruited scallops are found throughout the survey area. The overall condition factor for Georges Bank ‘a’ in August 2012 was 16.07 g/dm3 (meaning that on average a scallop with a 100 mm shell would have an 16.07 g meat). This was an increase from 2011 and above the long-term mean of 15.36 g/dm3. Spatial patterns in condition seemed consistent with bottom temperatures collected during the survey. New growth parameters derived from recent age data were used for calculating growth terms in the model and to redefine the recruit size class to include scallops between 85 and 95 mm shell height.
Fully recruited biomass for Georges Bank ‘a’, estimated to be 23,400 t in 2012, increased from the 2011 estimate (21,370 t) and was above the 26-year median biomass of 14,810 t. Recruit biomass, estimated to be 4,685 t in 2012, declined from the 2011 estimate (7,657 t), but was still above the 26-year median biomass of 2,995 t. The 2013 interim Total Allowable Catch (TAC) of 4,000 t is expected to result in an exploitation rate of 0.16, and incoming recruitment is expected to be above the median. Harvest scenarios ranging from 2,000 to 6,000 t are all predicted to yield increases in commercial biomass for 2013 with a probability of decline ranging from 0.36 to 0.48.
The commercial catch rate in 2012 was at the long-term median (1991-2009). In 2012, the survey index for pre-recruits was similar to what it was in the early 2000s and well below the 21-year (1991-2011) median. The 2005 year class is now mainly fully recruited to the fishery and survey indices of recruit and fully recruited scallops were near their respective 21-year (1991-2011) median levels. The overall condition factor for Browns Bank north in May 2012 was 11.83 g/dm3. This was an increase from 11.03 g/dm3 in 2011, but is still below the long-term mean of 12.62 g/dm3. The spatial pattern in condition on Browns Bank was less consistent with temperature and depth than it was on Georges Bank. Recent age data yielded similar growth parameters to what was used in previous assessments.Fully recruited biomass for Browns Bank ‘north’, estimated to be 5,950 t in 2012, increased slightly from the 2011 estimate (5,504 t) and is approximately equal to the 21-year median biomass of 5,807 t. The 2013 interim TAC of 750 t will result in an exploitation rate of 0.13. Biomass is expected to remain relatively stable, however, there is little indication of strong year classes in the near future. Harvest scenarios ranging from 200 t to 1000 t were examined and all had moderate (0.40-0.56) probability of decline in commercial biomass for 2013. Biomass change ranged from 9 to -5 % for the range of catches considered here.
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