Science Advisory Report 2018/029

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 2018 and 2019 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 are assessed separately.
• Fishery dependent indices are an important component of the assessment. Indices such as the commercial gillnet CPUE, may not be proportional to abundance due to changes in catchability over time. For example, catch rates can remain elevated despite decreases in abundance (increased catchability) due to contractions in stock distribution and targeting of aggregations by fishing fleets, as well as due to improved fishing technology and fishing practices.

Spring Spawner Component (SS)

• The preliminary estimated landings of SS herring in 2016 and 2017 were 966 t and 1,189 t, respectively, from annual total allowable catch values of 2,000 t.
• A virtual population analysis model that incorporated changes in catchability in the fixed gear fishery has been used since the last assessment.
• The estimates of Spawning Stock Biomass (SSB) at the beginning of 2017 and 2018 were 11,744 t (95% confidence interval: 6,463 – 28,171) and 12,446 t (95% CI: 6,418 – 30,365), 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 beginning of 2017 and 2018 were over 90%.
• The average fishing mortality rates on ages 6 to 8 for the SS exceeded F0.1 (the removal reference level in the healthy zone, F = 0.35) during 2000 to 2011. F declined below F0.1 in 2012, reaching its lowest value of 0.19. The fishing mortality rate during 2015 to 2017 averaged 0.24 (annual exploitation rate of 0.21).
• Due to variable recruitment in recent years, projections were conducted under three different recruitment scenarios during the projection period: (1) high recruitment, (2) low recruitment, and (3) mixed recruitment.
• SSB at the start of 2019 and 2020 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 2018 and 2019, the SSB is expected to only increase slightly with a high probability that the stock will remain in the critical zone.
• Since 2009, the TAC has been set to 2,000 t annually. At a catch of 2,000 t, the probability of an increase in SSB ranges from 0% (low recruitment scenario) to 19% (high recruitment scenario) with only a 10% chance of exceeding the LRP even under the high recruitment scenario.
• Elevated fishing mortality, declines in weights-at-age, and variable but low recruitment rates are further impeding the rebuilding of the stock.

Fall Spawner Component (FS)

• The preliminary estimated landings of the FS herring component in 2016 and 2017 were 24,677 t and 20,523 t respectively, from a total allowable catch of 35,000 t annually.
• 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 Middle and South regions over the time series.
• For the southern Gulf of St. Lawrence, the median estimate of SSB at the start of 2018 is 112,000 t. The probabilities that the SSB was below the Upper Stock Reference (USR) level of 172,000 t at the beginning of 2017 and 2018 were 98% and 97%, respectively.
• The average fishing mortality rate on ages 5 to 10 for the FS exceeded F0.1 (the removal reference level in the healthy zone, F = 0.32) from 1994 to 2011 except in 2004, but declined from 2012 to attain the lowest levels in 2016. F averaged 0.20 during 2015 to 2017.
• Estimated abundances of age 4 herring at the start of 2017 and 2018 were very low, but with very large uncertainty.
• The median of the projected SSB at the start of 2019 and 2020 remains below the USR at all annual catch levels of 10,000 t or greater with a probability of at least 90%.
• At catches of 20,000 t (the catch in 2017) in 2018 and 2019, the probability of the SSB remaining under the USR in 2020 was estimated at 94%. At the 20,000 t catch level, the probability of the fishing mortality rate being above the removal rate reference was estimated at 46%. F0.1 is a removal reference for when a stock is in the healthy zone of the Precautionary Approach.
• Current retrospective patterns indicate that the assessment model may overestimate the exploitable biomass. Consequently, harvest options presented may be optimistic relative to attainment of management objectives.
• When a stock is below the USR (in the cautious zone), consideration should be given to increasing the SSB. A 5% increase in SSB by 2020 would only be likely (greater than 50%) at annual catches below 16,000 t.
• Elevated fishing mortality, during the mid-1990s to 2010, declines in weights-at-age, and low recruitment rates are contributing to declines in SSB, further impeding the rebuilding of the stock.

This Science Advisory Report is from the March 15, 2018 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-4Vn) to 2017 and advice for the 2018 and 2019 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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