Science Advisory Report  2016/036

Assessment of the southern Gulf of St. Lawrence (NAFO Div. 4T) spring and fall spawner components of Atlantic herring (Clupea harengus) with advice for the 2016 and 2017 fisheries

Summary

• Atlantic herring in the southern Gulf of St. Lawrence are comprised of spring spawning and fall spawning components which are considered to be distinct stocks and as such assessed separately.

Spring Spawner Component (SS)

• The preliminary estimated landings of SS herring in 2014 and 2015 were 1,251 t and 1,190 t, respectively, from annual total allowable catch values of 2,000 t.
• A revised virtual population analysis model that incorporated changes in catchability in the fixed gear fishery was used in this assessment. The model provides a better fit to observations and no longer has a severe retrospective pattern.
• Catchability to the fixed gear increased to the mid-2000s and declined to an intermediate value since 2010.
• The estimates of Spawning Stock Biomass (SSB) at the beginning of 2015 and 2016 were 9,076 t (95% confidence interval: 5,686 – 14,282) and 9,659 t (95% CI: 5,530 – 16,294), respectively. The SSB has been in the critical zone of the Precautionary Approach framework since 2004 and the probabilities that SSB remained in the critical zone at the start of 2015 and 2016 were almost 100%.
• Estimated fishing mortality (F) declined below the reference level (F = 0.35) in 2010, reaching a low value of 0.08 in 2012. The fishing mortality rate in 2013-2015 of herring aged 6-8 years old averaged 0.18 (exploitation rate of 0.16).
• SSB at the start of 2017 and 2018 was projected to increase slightly at annual catches less than 500 t, remain roughly stable at annual catches of 1,000 t, but decline at catches of 1,500 t or more. However, uncertainty in projected SSB is high. Even in the absence of any removals of SS herring in 2016 and 2017, the SSB is expected to only increase slightly with a very high probability (90%) that the stock will remain in the critical zone.
• Poor recruitment, resulting from low recruitment rates, is constraining the rebuilding of the stock out of the critical zone. Declines in weight-at-age are also exacerbating the low levels of SSB.

Fall Spawner Component (FS)

• The preliminary estimated landings of the FS herring component in 2014 and 2015 were 29,214 t and 28,138 t respectively, from a total allowable catch of 35,000 t in 2014 and 40,000 t in 2015.
• Beginning in 2015, the FS herring assessment model incorporated the dynamics of three regional sub-stocks (North, Middle, South) which jointly comprise the NAFO Div. 4T stock. The catch options are evaluated at the level of the southern Gulf of St. Lawrence.
• Catchability to the fixed gear fishery was estimated to differ between regions and to have changed over time, being lowest with little variation in the North region in contrast to increases in the South region.
• For the southern Gulf of St. Lawrence, the median estimate of SSB at the start of 2016 is 165,000 t. The probabilities that the SSB was below the Upper Stock Reference level of 172,000 t at the beginning of 2015 and 2016 were 22% and 60%, respectively.
• The average fishing mortality rate on ages 5 to 10 for the FS has declined to an average of 0.19 since 2012 (an exploitation rate of 17%). The probability that the fishing mortality rate for ages 5-10 in 2015 exceeded F = 0.32 (the reference removal rate in the healthy zone; exploitation rate of 27%) is essentially 0.
• Poor recruitments at age 4 in the last three years, resulting from low recruitment rates, have contributed to the decline in SSB.
• The median of the SSB estimate at the start of 2017 and 2018 was projected to be close to the USR at annual catch levels in 2016 and 2017 less than 10,000 t but the probability of the stock declining into the cautious zone at the start of 2018 was greater than 50% at catch options above 10,000 t. At catches of 28,000 t (the catch in 2015) in 2016 and 2017, the probability of the SSB being in the cautious zone was estimated at 76.5%.
• At the 28,000 t catch level, the probability of the fishing mortality rate being above the removal rate reference was estimated at 42%.

This Science Advisory Report is from the March 15 and 16, 2016 regional science peer review meeting on the Assessment of stock status of Atlantic herring (Clupea harengus) from the southern Gulf of St. Lawrence (NAFO Div. 4T) to 2015 and advice for the 2016 and 2017 fisheries. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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