Science Advisory Report 2016/059

Recovery Potential Assessment of Winter Skate (Leucoraja ocellata): Gulf of St. Lawrence population

Summary

Biology, Abundance, Distribution and Life History Parameters

• Winter Skate is endemic to shelf waters of the northwest Atlantic. Winter Skate in the Gulf of St. Lawrence (GSL) DU are characterized by maturation at a much younger age and smaller size than Winter Skate elsewhere.
• The early-maturing type of Winter Skate, characteristic of the GSL DU, does not appear to occur in the northern Gulf of St. Lawrence. This RPA considers the GSL DU to be restricted to Northwest Atlantic Fisheries Organization (NAFO) Division 4T. Within Division 4T, Winter Skate appear to be largely confined to the southern Gulf of St. Lawrence (sGSL), though there are rare reports in the St. Lawrence Estuary.
• Based on catches in the annual September Research Vessel (RV) survey, adult (≥42 cm total length) abundance of Winter Skate has declined by 99% since 1982.
• The RV biomass index for Winter Skate, all sizes combined, has declined by an estimated 99% between 1980 and 2014 (35 years, 3 generations). The biomass index from the Northumberland Strait survey, available since 2000, has declined by 78% over 15 years.
• Adult abundance estimated from a population model was high in the 1970s, at an average of 7.7 million fish, but declined to the lowest level on record in 2014 at 0.2 million fish, 3% of the average level in the 1970s.
• There has been an important contraction in the size distribution of adult Winter Skate. The average proportion of adults > 50 cm in length was 72% in the 1970s and 45% during 1995 to 2010.
• Estimates of natural mortality (M) from an age-structured model indicate that there has been a large increase in M of adults, from a median value of 10% annually during 1971 to 1977 to 63% annually during 1992 to -1998 and remaining high at 57% since 1999.
• Recruitment rates of Winter Skate have fluctuated without any trend, averaging 10.4 recruits per female spawner. Juvenile M has also fluctuated without trend around an average value of 57.6% annually.

Habitat and Residence Requirements

• During the summer, adult Winter Skate were widely distributed in shallow (< 50 m) coastal waters of the sGSL during the 1970s and 1980s but their geographic range contracted beginning in the late 1980s. For adults, the habitat occupied in summer has declined by 94%. During the winter, adult Winter Skate are broadly distributed on the Magdalen Shallows and the slope of the Laurentian Channel (approximately 40 to 200 m depth), however, there is no information available on possible changes over time in the winter distribution.
• Winter Skate distribution during the summer has now contracted to the western portion of the central Northumberland Strait, the only important area where Winter Skate is now detectable in the sGSL.

Threats and Limiting Factors to the Survival and Recovery of Winter Skate

• There has been no directed fishing for skates in the southern Gulf of St. Lawrence. However, skates, including Winter Skate, are incidentally captured in commercial fisheries directed at other species. Currently, all bycatch of skates must be released.
• The estimated fishing mortality of juveniles was below 0.1% annually in all years since 1971, averaging 0.006% annually over the last five years. For adults, estimated fishing mortality declined from an average of only 1.8% annually during 1971 to 1993 to 0.1% since 2011.
• Elevated adult M is the main cause of the on-going decline in abundance in this DU. The weight of evidence supports the hypothesis that predation by grey seals is the largest contributor to this high adult natural mortality.
• The summer distribution of Winter Skate is now concentrated in a small coastal area of the Northumberland Strait where predation risk appears to be relatively low. This area may therefore presently constitute a refuge for Winter Skate.

Recovery Targets

• Candidate recovery targets for survival, size structure, distribution, and abundance are proposed. The survival target is defined in terms of the intrinsic rate of population increase (r) or an equivalency in terms of natural mortality (M). In terms of M, the population would increase and the risk of extinction become negligible if current adult M (M = 0.85, 57% annually) was reduced by 80% (i.e., to M = 0.17, 16% annually).
• The size structure, distribution, and abundance targets cannot be realized until the natural mortality on adult Winter Skate has been substantially reduced.

Projections

• Assuming current productivity conditions and even with no fishery related losses, the Winter Skate population is expected to continue to decline rapidly and is almost certain to be extinct by mid-century.

Scenarios for Mitigation of Threats and Alternatives to Activities

• The lack of recovery and on-going decline of sGSL Winter Skate is due to the exceedingly high M of adult skate. If this high M persists, any additional measures to further reduce the already low fishing mortality will be ineffective in promoting recovery and reducing the high risk of extinction. Only actions which reduce the high M of adults are likely to be effective in reducing extinction risk. Reductions in grey seal predation on Winter Skate would be expected to reduce adult M.
• Possible mitigation measures to reduce harm to Winter Skate from fishing include: continued prohibition of retention of any skate species, priority be given to sorting and rapidly discarding skate catches to enhance post-release survival rates, and reduced fishing effort in areas and times with the highest potential of intercepting Winter Skate.

Allowable Harm Assessment

• The losses of juvenile and adult Winter Skate due to fishing were estimated to average 0.006% and 0.1% respectively over the last five years. Based on projections, fishing losses of these levels have negligible consequences for population trajectory, probability of recovery and risk of extinction.

This Science Advisory Report is from the January 19 to 21, 2016 meeting on the Recovery Potential Assessment – Winter Skate (Leucoraja ocellata), Gulf of St. Lawrence population and Eastern Scotian Shelf – Newfoundland. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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