Avis scientifique à l'appui du plan de rétablissement du stock de merluche blanche (Urophycis tenuis) dans le sud du golfe du Saint-Laurent, zone 4T de l'OPANO

Abstract

High mortality is responsible for the decline of the southern Gulf of St. Lawrence White Hake (sGSL Hake). As natural mortality increased, fishing mortality of older fish rapidly increased in the 1989 to 1992 period. Spawning stock biomass (SSB) started to rapidly decline in 1989. The persistence of a low production-low biomass state since the early 1990s is an indicator of serious harm to the stock productivity. High natural mortality associated with predation by Grey Seal has prevented the stock from recovering since the fishery moratorium. A review of the SSB reference points identified a new Limit Reference Point (LRP) based on a proxy for BMSY (0.4BMSYproxy). The proxy was defined as the mean SSB in a high biomass high production period (years 1978 to 1982). A candidate Upper Stock Reference point at 0.8BMSYproxy and a Target Reference Point at the BMSYproxy were proposed. The stock has been in the Critical Zone (below the LRP) of Fisheries and Oceans Canada (DFO) Precautionary Approach framework since 1992. The chosen rebuilding target was SSB having a 75% probability of being at or above the LRP. An additional measure to ensure a low likelihood of the stock returning to the Critical Zone was proposed that included the stock being at or above this level for 5 consecutive years with the 75% likelihood. Even in the absence of fishing mortality, SSB is projected to continue declining under prevailing conditions (current recruitment and natural mortality rates). A considerable natural mortality decrease along with current high recruitment rates is required for the stock to rebuild within a 40-year time frame. The stock is vulnerable to declines in recruitment rates, which could lead to a rapid decrease in SSB below 1,000 tonnes (t). Projections showed that at 100 t and 1,000 t of annual bycatch, SSB in 10 years would be reduced by 3.3% and 17.6% compared to no fishing, respectively. The main sources of bycatch are fisheries targeting Greenland Halibut, Redfish, Atlantic Halibut, and Witch Flounder. Bycatch of Hake could be minimized by: o Shifting the eastern latitude limit north from Cap Gaspé (48.75°N) to Les Trois Ruisseaux (48.92°N) for the Greenland Halibut fishery. o Restricting fishing to a minimum fishing depth of 300 m for the Redfish and Witch Flounder fisheries. o Temporal fishing restrictions in an area near Cape Breton for the Atlantic Halibut fishery. Few commercial groundfish fishing trips are currently covered by at sea observer leading to small sample sizes and highly uncertain estimates of incidental fishing mortality. As sGSL Hake and other groundfish overwinter in deeper waters, additional caution is warranted for fisheries operating from November through March, especially those operating within the Laurentian Channel and NAFO Subdivision 4Vn. Analyses of certain life history traits indicate that Hake in the St. Lawrence Estuary may not be biologically part of the sGSL stock which has consequences for stock management in the context of rebuilding. Additional measurable objectives of the rebuilding plan include; (i) increase the proportion of larger Hake and Hake aged 5+ to averages observed historically (37% in the 1970-1980s), (ii) restore the presence of Hake greater than or equal to 45 cm in the inshore waters of the sGSL during September according to their historical distribution prior to the late 1990s. Rebuilding progress will be tracked using the interim indicator derived from an annual survey and from stock assessment models. The periodic review of the rebuilding plan should be set to the 5-year stock assessment cycle with an interim update at the halfway point. Objectives should be revised and models should be updated if stock productivity or external factors influencing stock dynamics change.