Science Advisory Report 2008/055

Assessment of Northern Shrimp on the Eastern Scotian Shelf (SFA 13-15)

Summary

• Total biomass decreased 20% from last year and 36% since the all-time high of 2004. The DFO-industry survey index remains higher than the long-term (1982-2008) average, but is now just below average for the current survey series (1995-2008).
• The SSB (spawning stock biomass of females) has decreased 29% from last year and 50% since the all-time high of 2004. SSB remains higher than the long-term average, but is just below average for the current survey series.
• SSB and total biomass are expected to decrease further as total mortality of the 2001 year-class (yc) females increases and recruitment to the fishable biomass declines due to the weaker 2002-2006 year-classes.
• Commercial catch rates (CPUEs) have been fluctuating at a high level since 2002 and unlike survey results show no signs of a rapid decline. This divergence of survey and CPUE indices was seen as the previous strong group of year classes (1993-1995) declined. However, the area of highest catch rates remains large and the stock is still widely distributed.
• At 15% total exploitation was above average (13%) in 2008. Female exploitation increased to 20%, above the average (16%) and near the maximum (21%) observed. Exploitation rates for SFA 14, which has recently contributed the bulk of the catch, were the highest on record for this area (26%). Exploitation of the smallest (<19mm CL) shrimp continued above average in 2008, which is a conservation concern due to the lower recruitment in recent years.
• Fishers continued to experience difficulty in avoiding small shrimp but this problem declined from last year presumably due to growth of the 2001yc.
• The long-term decreasing trend in length at sex change and maximum size continues. A short-term increase in length at sex change during the last 2 years is probably associated with delayed sex transition and an additional year(s) of growth of 2001yc males.
• Belly-bag results indicate a strong 2007yc, consistent with the establishment of a pulsed recruitment pattern at a frequency about equal to the species’ lifespan (6-7yr) in this region.
• Decreasing shrimp sizes and pulsed recruitment may be cumulative fisheries effects that erode population stability and reproductive capacity. These factors should be considered in the development of a long-term harvesting strategy.
• With recruitment of the weaker 2002-2006 year-classes, total and SSB biomass will probably continue to decrease in 2009, the rate of decline depending on the total mortality of the 2001yc. A TAC reduction will be necessary in 2009 to prevent exploitation rates from increasing further.

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