Science Advisory Report 2021/033

Impact of an expanding Redfish (Sebastes spp.) fishery on southern Gulf of St. Lawrence White Hake (Urophycis tenuis)

Summary

• The southern Gulf of St. Lawrence (sGSL) White Hake Designatable Unit (DU) has been assessed as Endangered by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC).
• sGSL White Hake and Redfish have largely overlapping distributions, as White Hake have shifted their distribution almost exclusively to the deep waters of the Laurentian Channel which is the same distribution as Redfish.
• An expanding Redfish fishery could have an impact on the sGSL White Hake population due to strong overlap in their spatial distribution.
• Spawning stock biomass (SSB) of the sGSL White Hake population declined rapidly in the late 1980s and the 1990s. A moratorium on directed fishing was established in 1995. No signs of recovery have occurred since then despite negligible fishing mortality. Estimated SSB was 7,400 t in 2019, 13% of the average level in the early to mid 1980s.
• The failed recovery of this population is due to extremely high natural mortality. Estimated natural mortality of adult White Hake has been 80-85% annually for the past 20 years. Predation by Grey Seal appears to be a major cause of this high mortality.
• Recruitment has fluctuated without trend since 1978 despite the decline in SSB. This reflects an increase in recruitment rate, the number of age-2 recruits produced per unit weight of SSB. The average rate was ten times greater for recruitment in 2000-2019 compared to 1980-1994. The estimated recruitment rate in 2019 is the highest on record though the uncertainty in this estimate is high.
• At the current high level of natural mortality the sGSL White Hake population persists only because of the unusually high recruitment success since the year 2000. If recruitment rate were to decrease a decline leading to local extinction would be expected at the current level of natural mortality.
• The spawning stock of this population consisted of ages 4 to 10 years and older in the past. It now consists mostly of 4-year-old fish. A spawning stock consisting of a single reproductive cohort represents a high risk to this population.
• The sGSL White Hake population was projected forward 25 years assuming that productivity would remain at recent levels. SSB was estimated to decline by 38.7% with no catch and 39.3% with annual bycatch of 20 t, the recent level. With annual bycatch of 150 t to 350 t, SSB was estimated to decline by 43% to 48%. With bycatch of 500 t to 1,500 t, SSB declined by 53% to 70%.
• The probability that SSB would be less than 2,000 t at the end of the 25-year projection (a 73% decline) was estimated to be 22.8% with no catch and 23.3% at the recent bycatch level (20 t). With bycatch of 150 t to 350 t this probability was 26% to 30%. With bycatch of 500 t to 1,500 t, this probability increased to 33% to 49%. For a population that had an SSB near 60,000 t in the early 1980s, an SSB near 2,000 t represents a very high risk of local extinction.
• There is clear evidence that the Redfish fishery will overlap with the sGSL White Hake stock. Available data suggests this interaction risk varies with time of year, depth, and geographic location.
• sGSL White Hake bycatch in Redfish catches is lower at depths greater than 380 m and in area of 4T outside of the Cape Breton Trough. Deeper than 350 m, the proportion of White Hake is 34% sGSL DU and 66% Atlantic DU.
• Based on a small scale experimental and index Redfish fishery and multi-species bottom-trawl surveys, we calculated an average estimated bycatch rate of 10.5%. This estimate was not observed to be gear dependent.
• In considering the development of a future Redfish fishery, the bycatch rate may be lower for various reasons that could reduce bycatch of White Hake, including improvement in the fishing technology, fishing season (timing), location of the fishing grounds and species interaction.

This Science Advisory Report is from the February 17, 2021 regional advisory meeting meeting on the impacts of increases in fishing effort on White Hake (Urophycis tenuis), Southern Gulf of St. Lawrence Population. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

Science Advisory Report 2021/033 (PDF, 4,973 KB)

Accessibility Notice

This document is available in PDF format. If the document is not accessible to you, please contact the Secretariat to obtain another appropriate format, such as regular print, large print, Braille or audio version.