Research Document - 2011/061
The Last Traffic Light on the Scotian Shelf: Shrimp 2009-2010
By P. Koeller, C. Fuentes-Yaco, M. Covey, M. King, and B. Zisserson
Abstract
The DFO-industry survey stratified mean increased nearly 50% over the previous year to the second highest on record. The biomass estimate rose from 30,904 to 45,424mt, slightly lower than the record high 48,438mt of 2004. Although the increase can be accounted for, at least in part, by continued growth and recruitment to the survey trawl of the large 2001 year class, a concurrent increase in the standardized commercial CPUE, presumably also due to these factors, was only 10% above the previous year. This suggests that part of the survey increase was due to other, survey-related factors. Moreover, two independent abundance indicators (commercial standardized CPUE and shrimp catches in the snow crab survey), do not show the decreasing biomasses exhibited by the shrimp survey from 2004-2008, rather, they suggest continued high or even increasing abundances during this period. In addition, logbook data show that the area containing the highest commercial shrimp catch rates has remained large or increased in size during the same period, with no evidence of higher concentrations in smaller areas that are expected during a biomass downturn. Prior to the 2009 survey it was observed that the angle of attack of the Nordmore grate in the survey trawl had decreased significantly due to stretching of forward meshes, a change that probably decreased trawl efficiency and survey catches significantly. Consequently, it can be concluded that at least part of the decrease in the survey index from 2004-2008 was due to decreased efficiency of the survey gear, and part of the increase in 2009 due to repairs effected prior to the survey. On the other hand, shrimp catches from a third independent survey (July groundfish) parallel shrimp survey results, suggesting that the observed changes are real, possibly due to changes in availability associated with changing geographic and vertical distribution. Belly bag results, which are not affected by the Nordmore grate, indicate that the 2007 and 2008 year classes are strong, confirming that a new (third) biomass cycle is underway, probably stemming from the large number of females from the 2001 year class in those years. This is evidence that a large spawning stock biomass is important for the production of the large year classes on which the fishery depends. Like total biomass, SSB was also underestimated between 2004 and 2008 due to decreased survey trawl efficiency. The SSB estimate for 2009 is the second highest on record - there is currently no concern for recruitment overfishing. It appears that the 2001 year class, 8 years old in 2009, has still not completed sex change and is contributing significantly to the abundance (second highest of the series) of Age 4+ males. These will change sex and contribute to the SSB (females) in 2010. The decreasing trend in size at sex change and maximum size appears to have started to reverse as males from the 2001 year class are changing sex over a number of years, and have additional years to grow as males. Growth of the 2001 year class is clearly seen in the fishery which has reported a significant decrease in counts (i.e. increased average size) since 2007. In addition to good counts, the fishery experienced record catch rates in 2009. Largely in response to improvements associated with the biomass increase, the overall “traffic light”, summarizing 25 stock health indicators, reverted to green in 2009.
Despite decreasing survey biomass indices between 2004 and 2008, TACs were set at a high level for those years, largely based on the strength of the 2001 year class which began to recruit to the fishery in 2004. With the 2001 year class nearing the end of a “normal” life span (approximately 6 years) in this area, biomasses were expected to continue decreasing after 2008, consequently the TAC for 2009 was reduced to avoid an increase in exploitation rates. With 2009 exploitation estimated at 7.2%, the second lowest on record, it now appears that this was premature, due to an underestimate of the longevity of this year class and biased survey results. It is not clear if this year class, at Age 9, will support the fishery in 2010 at the higher TACs of 2004-2008, given uncertainties in natural mortality. Considering the smaller size of the succeeding (2002-2006) year classes it is reasonable to expect biomasses to decrease somewhat until the stronger 2007-2008 year classes begin to recruit significantly to the survey gear and fishery, probably in 2011-2012. However, the strength of the succeeding year classes were probably underestimated due to decreased survey trawl efficiency and/or other factors, consequently it is difficult to predict how much, or even if, biomasses will decrease in the interim. Considering the continually increasing catch rates since the 1980s, problems interpreting shrimp survey downturns, and recent results from crab and groundfish surveys, it appears that the stock has increased to a high and stable level, which may represent its carrying capacity under current environmental conditions. It is not clear at this time if the fishery has had any measurable affect on this biomass trajectory. It is possible that the stock can sustain harvest levels higher than those imposed to date. This can be confirmed through continued precautionary adaptive management, involving cautious TAC increases and rapid decreases in response to negative indicators, as was done in the past in this fishery, and as currently proposed as a longer-term management strategy and formal precautionary approach incorporating adaptive management, with basic decision control rules applied within limit reference points.
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