Research Document - 2011/042
Browns Bank ‘North’ Scallop (Placopecten magellanicus) Stock Assessment
By B. Hubley, A. Glass, A. Reeves, J. Sameoto, and S.J. Smith
Abstract
This research document describes the first analytical assessment of scallops (Placopecten magellanicus) on Browns Bank ‘north’ and the first formal assessment since 1998. There was no fishery in 2009 and a modest total allowable catch (TAC) of 200 t and interim TAC of 500 t were set for 2010 and 2011, respectively, as a large pulse of recruitment has begun to reach commercial size.
The analytical methods developed for the assessment framework of Georges Bank scallops were applied to this stock with some minor modifications (Jonsen et al. 2009). Modifications included adding a natural mortality term on the recruits and incorporating spatial and annual variability in the shell-height meat-weight relationship, expressed as a condition factor. This analysis indicated that the overall annual condition factor in 2010 of 11.05 g/dm³ was well below the long term median (12.3 g/dm³ from 1991-2009).
The fishery on Browns Bank north has been variable with catches that range from 0 to 2007 t and operating at different times of the year. In order to properly line up removals with the survey which occurs in late May, the fishery data was summarized by survey year (June to May) instead of calendar year.
The current scallop population on Browns Bank ‘north’ is dominated by a large cohort ranging is shell height between 70 - 110 mm. These scallop which straddle all three size classes (pre-recruit, recruit and fully-recruited) have begun to enter the fishery and are concentrated on the northern and southern areas of the bank.
Fully recruited biomass, estimated to be 9096 t in 2010, increased from the 2009 estimate of 5069 t due to the highest recruit biomass since 1991 estimated to be 5077 t in 2009. Continued strong recruitment in 2011 will result in a fully-recruited population dominated by younger scallops (95-105mm) and higher exploitation rates at this time could result in a loss of potential yield.
There have been three major recruitment events on Browns north since 1991, each leading to a peak in commercial biomass that has essentially sustained the fishery until the next recruitment. When the first event occurred in the mid 1990s exploitation increased sharply as these scallops reached commercial size leading to biomass falling to early 1990 levels until the next recruitment event (increases in biomass in 1999 and 2000 were largely due to improved condition at this time). As a consequence of high exploitation, landings greater than 1000 t resulting from this recruitment event lasted only two years. During the next recruitment event in the early 2000s, exploitation remained low at first giving the stock time to grow, which allowed for four years where landings were greater than 1000 t.
The 2011 interim TAC of 500 t corresponds to an exploitation rate of 0.04 and an increase in fully-recruited biomass of 43% to 13,090 t assuming no change in condition factor from 2010. Harvest scenarios ranging from 100 t to 1,000 t were examined and all were predicted to yield increases in commercial biomass for 2011 with low probability of decline.
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