Research Document - 2013/033
Scotian Shelf Shrimp 2011-2012
By Hardie, D., Covey, M., King, M., and Zisserson, B.
Abstract
The Fisheries and Oceans Canada-industry survey stratified mean decreased by 18% for the second year in a row. The biomass estimate fell from 37,510 to 30,510 mt. This is consistent with the prediction of a lag between the complete mortality of the long-lived 2001 year class in 2010 and the recruitment of the next moderately abundant 2007-2008 year classes to the fishery in 2012-2013. Although commercial catch per unit efforts were stable (standardised) or increasing (Gulf) the distribution of catch rates are consistent with a declining resource, where areas of very high catch rates continue to decline while areas of low-moderate catch rates are stable or increasing. Spawning stock biomass also declined by about 22%, although the stock remains above the upper limit reference point (i.e. in the Healthy Zone). Although the total allowable catch was reduced to 4600 mt in 2011, this was relatively less of a reduction than the downturns in the point estimated of total and spawning stock biomass, so point estimates of total and female exploitation increased in 2011. Female exploitation increased to 20.3%, just over the removal reference point for this stock. Indices of production suggest that the population is currently composed of very few recruits from the 2010 year class, which is consistent with high spring sea surface temperatures that year. The abundance of age 2 shrimp also declined, as was expected based on low abundance of age 1 shrimp in the belly-bag in 2010. The abundance of age 4 shrimp increased by 50% in 2011, which is consistent with the increase in age 3 shrimp from length-frequency analysis in 2010 and the recruitment of the more abundant 2007 year class to the population. Size-based indicators (size at sex-transition, average maximum size, female size, count) show that the size of shrimp in the population is decreasing. This is consistent with end of the influence of the late-maturing year classes that followed the 2001 year class, and matured as larger than average females, and their replacement by smaller shrimp that are not delaying maturity, as is characteristic of less abundance year classes. The results of the ecosystem characteristic indicators suggest that conditions are currently not favourable for shrimp. Temperatures are increasing and the abundance of sympatric species is decreasing, while the abundance of predators known to have a negative influence on juvenile recruitments is increasing. The overall “traffic light”, summarising 25 stock health indicators, reverted from green to yellow when the 2010 data were finalised, and further reduced to red for 2011, largely due to negative values of production and ecosystem characteristics and the ongoing downturn of abundance, resulting in exploitation increases. Although the index of stock abundance remains in the Healthy Zone, the removal reference was exceeded, albeit barely, in 2011.
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