Research Document - 2011/102
Scotian Shelf Shrimp 2010-2011
By D. Hardie, M. Covey, M. King, and B. Zisserson
Abstract
The DFO-industry survey stratified mean decreased by 18% from the previous year to a value slightly above the 2000-2009 (high productivity period) average. The biomass estimate fell from 45,424mt to 37,212mt. This downturn in the DFO-industry survey is consistent with decreases in other CPUE-based indicators (Gulf vessel Standardised commercial CPUE indicators) and with the prediction that the long-lived 2001 year-class would experience strong natural mortality in 2010. The moderate increase in the survey coefficient of variation and changes in the dispersion of the resource (decreased area of high abundance with approximately stable areas of low/moderate abundance) are also consistent with a biomass downturn. Spawning stock biomass declined as expected with the die-off of the 2001 year-class, although this appears to have been partially offset by the recruitment of late-maturing 4+ males (highly abundant in 2009) to the female component of the population in 2010. The abundance of Age 4 male shrimp was estimated to have doubled in 2010, which should further contribute to spawning stock biomass in 2011. Overall, the current downturn is consistent with predictions based on the expectation of a lag between the die-off of the 2001 year class and the recruitment of the strong 2007-2008 year classes to the fishery, which is expected to begin in 2011-2012. The expected contribution of the weaker 2002-2006 year classes remains uncertain; except for the increase in the 2010 index of Age 4 shrimp. The indices of total and female exploitation rates increased in 2010, as was expected given the increased TAC and biomass downturn. The total exploitation index (13.2%) remains below the conservative level experienced during the modern fishery (20%) which is the reference point for this stock. Size-based indicators (size at sex-transition, average maximum size, female size, count) continue to improve as the effects of the dominant 2001 year class diminishes. The belly-bag index of shrimp recruitment in 2010 was unexpectedly low given near-record high spawning stock biomass in 2009. Likewise, the decline in the Age 2 shrimp abundance index was unexpected given the high belly-bag index in 2009. The results of the Ecosystem characteristic indicators for 2010 provide somewhat equivocal signals. While cod abundance remains at a low level (low predation on shrimp), the abundance of the two species thought to correlate positively with shrimp abundance provide a conflicting signal (capelin increased while Greenland halibut continue to decrease). Similarly, the two temperature indicators provide conflicting signals; spring sea surface temperatures were high while bottom temperatures decreased on the shrimp grounds. As a result of this, and the uncertainties with the belly bag predictions in recent years, it is difficult to comment on recruitment in 2011 beyond the prediction of abundant spawning stock biomass in 2011. The overall "traffic light", summarizing 25 stock health indicators, remained green for 2010, largely due to the maintenance of high values of CPUE-based abundance indicators from survey and commercial data.
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