Science Advisory Report  2010/037

Assessment of redfish stocks (Sebastes fasciatus and S. mentella) in Units 1 and 2 in 2009

Summary

• This assessment follows two zonal reviews on redfish (Sebastes fasciatus and S. mentella) stock structure in Units 1 and 2. These reviews concluded that Units 1 and 2 correspond to a single biological population of each species. It was therefore recommended that Units 1 and 2 be grouped as a single biological unit for each of the two species and that they be assessed separately.
• From 2004 to 2009 (2009 data are preliminary) the average annual landings of the index fishery and by-catch in Unit 1 reached 622 t (mean annual TAC of 2,000 t). During the same period, landings and by-catch in Unit 2 averaged 5,229 t (mean annual TAC of 8,333 t). Industry reports that market conditions and by-catch limitations had a major effect on catches.
• The catch per unit of effort of bottom trawls in the Unit 1 index fishery declined between 2004 and 2009. Data for Unit 2 were not analysed.
• Recruitment mechanisms for both species are not well understood. Annual typical recruitment seems stable and low and has not resulted in an increase of the population.
• Strong year-classes of S. fasciatus, exhibiting the genetic signature of the southern edge of the Grand Banks population appeared in 1974, 1985, 1988, and 2003 in Units 1 and 2. These did not persist in Unit 1, but did contribute to the Unit 2 commercial fishery.
• The last strong S. mentella year-class from Unit 1 appeared around 1980 and expressed the genetic signature of Units 1 and 2. This cohort has supported the fishery for more than 20 years.
• The two species are distributed according to depth. Sebastes fasciatus is generally found in shallower waters than S. mentella. In summer surveys, S. fasciatus dominates water depths of less than 300 m, except at the Laurentian Fan where it is found in deeper waters. Sebastes mentella is mostly predominant in depths of more than 300 m.
• A unified survey index series was constructed for each species from 2000 to 2009. Over this period, estimates of S. fasciatus survey biomass appear stable and S. mentella has declined continuously. From 2000-2005, the biomass of S. fasciatus and S. mentella were similar, but in recent years, there are lower levels of S. mentella compared to S. fasciatus. The same trends are observed for the spawning stock biomass.
• Survey index of spawning stock biomass of S. fasciatus was estimated at 146,400 t in 2009. Since 2000 the average percentage of spawning biomass estimated in Unit 1 and Unit 2 are 18% and 82%, respectively.
• Survey index of spawning stock biomass of S. mentella was estimated at 115,400 t in 2009. Since 2000 the average percentage of spawning biomass estimated in Unit 1 and Unit 2 are 27% and 73%, respectively.
• Relative exploitation rates are estimated at 2% and 3% in Unit 1 and Unit 2, respectively, in 2009. Since the commercial catch can not be broken down by species, the exploitation rate cannot be estimated for each species.
• Given the relatively low level of biomass observed and the prospect of only typical low recruitment, it is recommended that the exploitation rate remains low for both species.
• In order to reduce exploitation on S. mentella, it is recommended to concentrate the exploitation in the shallower waters and along the edge of the outer shelf.

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