Research Document - 2016/061

Population modelling results for the assessment of Atlantic herring (Clupea harengus) stocks in the southern Gulf of St. Lawrence (NAFO Division 4T) to 2015

By D.P. Swain

Abstract

For assessment purposes, Atlantic herring in the southern Gulf of St. Lawrence are treated as two independent components, spring and fall spawners. Spring spawning herring were assessed using a virtual population analysis (VPA) which allowed for time-varying catchability (q) to the gillnet fishery. Model estimated spring spawner spawning stock biomass (SSB) has been in the critical zone of the Precautionary Approach (PA) framework since 2004. The median estimate of SSB at the start of 2016 (9,660 t) was less than 45% of the limit reference point (LRP = 22,000 t). The estimated instantaneous rate of fishing mortality (F) on aged 6 to 8 year old spring spawner herring declined below the reference level (F = 0.35) in 2010 and averaged 0.18 in 2013 to 2015. Based on projections with catch in 2016 and 2017 varying between 0 t and 2,500 t annually, the probability that SSB will be less than the LRP in 2018 varies between 90 and 95%, the probability that average F for ages 6 to 8 will exceed the reference level (0.35) in 2017 varies from 0 to 70%, and the probability of a 5% decline in SSB between 2016 and 2018 varies from 23 to 65%.

Fall spawning herring were assessed as separate populations in three spawning regions (North, Middle, South). The population model allowed for time-varying catchability (q) to the gillnet fishery, with different trends in q permitted between regions. Estimated SSB has been declining in all three regions in recent years, though it remains at a relatively high level in the North. Summed over all three regions, SSB at the start of 2016 is estimated to be about 165,000 t, just below the upper stock reference (USR) of 172,000 t, at a probability of 60%. Estimated F (ages 5 to 10) declined substantially in recent years in the North region but has remained higher in the Middle and South regions. Averaged over all regions, ages 5 to 10 F declined from about 0.4 in 2008 to 0.19 in 2012 with essentially zero chance of having exceeded the removal rate reference level of F = 0.32. The median projected value of SSB at the start of 2018 remained below the USR at all catch levels between 10,000 and 50,000 t, with the probability of being below the USR varying from 57% at 10,000 t to 87% at 50,000 t. The probability that ages 5 to10 fishing mortality rate will exceed F = 0.32 in 2017 is estimated to increase from 0% at catches of 10,000 t to 97% at 50,000 t.

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