Research Document - 2016/045

Recovery potential assessment of the Southern Gulf of St. Lawrence Designatable Unit of White Hake (Urophycis tenuis Mitchill), January 2015

By Douglas P. Swain, Luc Savoie and Sean P. Cox

Abstract

The Southern Gulf of St. Lawrence (SGSL) Designatable Unit (DU) of White Hake (Urophycis tenuis) was assessed as Endangered by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in November 2013. This DU consists primarily of white hake occurring in NAFO division 4T. Indices from the annual research vessel (RV) bottom-trawl survey of 4T indicate a 90% decline in adult abundance since 1985 (about three generations). Most of this decline occurred between 1985 and 1996, though the index continues to slowly decline. Sentinel indices available since 2003 (bottom-trawl survey) or 1995 (longline program) also indicate on-going declines, with recent values the lowest in the time series. The spawning stock biomass (SSB) estimated by population models declined from an average of 52,850 t in 1978-1982 to 3,850 t in 2013, a 93% decline. In contrast, juvenile abundance has fluctuated without trend since 1978. The area occupied by adult hake in the SGSL in September declined from about 25,000 km² in the early 1980s to 5,000 km² in recent years, but changed little over this period for juveniles. In summer and early fall, white hake in the SGSL occur either in shallow inshore waters or in deeper water along the slope of the Laurentian Channel and in the Cape Breton Trough. The proportion of hake occurring in inshore areas has declined over time, with hake virtually absent from these areas in recent years. This appears to result from increasing risk of predation by grey seals in inshore areas. These areas contain important spawning and feeding grounds for hake. The directed fishery for white hake has been closed since 1995, and fishing mortality has declined to negligible levels, F = 0.002 for ages 4-5 (0.2% annually) and F = 0.035 for ages 6 and older (3.4% annually) since 2006. In contrast, natural mortality has increased to very high levels, averaging 87% annually for ages 4-5 years and 78% annually for ages 6+ since 2000. Increasing predation by grey seals is considered to be an important cause of this increase in natural mortality. Recruitment remains strong in this population despite very low SSB. This reflects very high recruitment rates over the past 20 years.

SSB equal to 40% of the SSB producing the maximum surplus production of recruits is proposed as a recovery target. SSB has been below the target since 1995. The most recent estimate (for 2013) is 30% of the target with no chance of being at or above the target. Under current conditions, continued declines are projected, with no chance of achieving the recovery target and a 19-38% probability of extinction in 60 years, even with no fishing. Reductions in the instantaneous rate of natural mortality (M>) would provide some possibility of recovery. Under the current condition of high recruitment productivity, the probability of recovery was estimated to be 27% 30 years after a 20% reduction in M, 95% 30 years after a 30% reduction in M, and 51% and 100% 6 and 20 years after a 40% reduction in M. At the lower level of recruitment productivity observed in 1978-1994, a 50% reduction in M would be insufficient to permit recovery, and the probability of recovery would be 77% 30 years after a 60% reduction in M. At recent levels of fishing effort, catches associated with commercial fisheries directing for groundfish result in instantaneous rates of fishing mortality for fully-recruited fish of 0.24 (1998-2002) to 0.04 (2009-2013), and consequently have a negligible impact on the population trajectory. The current high levels of natural mortality are the main threat and the most important factor limiting the recovery of this population.

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