Science Advisory Report  2008/024

Recovery Potential Assessment for Cusk (Brosme brosme)

Summary

• In May 2003, the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assessed cusk as threatened. They are currently being considered for listing on Schedule 1 of the Species at Risk Act (SARA).
• Cusk abundance has declined since the 1970s; however, there is insufficient data to determine the degree of decline and there is conflicting evidence on whether cusk abundance has continued to decline since the late 1990s.
• There has been no reduction in the range of cusk in Canadian waters. Cusk do not seem to have disappeared from any areas where they have been caught historically.
• Adult cusk prefer structured, hard bottom habitat and a depth range of 400 to 600m. The preferred habitat of juvenile cusk is not known.
• Given uncertainties associated with available abundance indicators, a recovery target for the abundance of cusk could not be precisely determined. With current information, a recovery target for cusk would likely be a stock of the size observed in the mid to late 1980s, prior to the large and abrupt decline in the indices of stock status available from that period. However, a reliable measure of the current magnitude of stock size relative to that period is not available. Accordingly, a practical interim strategy would be to promote measures that result in an increasing trend in cusk abundance.
• A reasonable spatial target would be to maintain the current distribution of cusk.
• According to the population models used in this assessment and assuming that unreported bycatch and discards remain constant, commercial landings in 4X of about 200 mt would result in a 75% chance of observing at least a 50% increase in biomass after 15 years (1 generation), while 4X landings of about 600 mt would result in only about a 54% chance of observing at least a 50% increase in biomass after 15 years. These landings would have to be increased by 1.43 times to be applicable to the entire management area (4VWX + 5Zc).
• Fishing is the only known major source of human-induced mortality for cusk. Canadian cusk landings from 4VWX and 5Zc have ranged from 790 metric tonnes (mt) to 1490 mt between 1999 and 2006. The vast majority of these landings are from the groundfish longline fishery in 4X and 5Zc. Cusk are also known to be caught as bycatch in some lobster fisheries. Cusk mortalities in 2005/2006 lobster fisheries were estimated to be a minimum of 226 mt in Lobster Fishing Area (LFA) 34 and 22 mt in LFA 41.
• Habitat does not appear to be, nor is likely to become, a limiting factor to cusk survival and recovery. There are no known threats that have reduced cusk habitat quantity or quality.
• There are a number of uncertainties associated with this assessment. For example, survival of released cusk may be overestimated in the bycatch analysis for lobster fisheries in LFAs 34 and 41 as it only considers immediate condition and not long-term survival. In addition, not all sources of cusk fishing mortality have been accounted for, i.e., potential bycatch in lobster fisheries outside of LFAs 34 and 41, and other trap fisheries.

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