Science Advisory Report  2016/034

Recovery Potential Assessment for White Hake (Urophycis tenuis): Population of the Southern Gulf of St. Lawrence

Summary

Biology, Abundance, Distribution and Life History Parameters

• Indices from research vessel (RV) bottom-trawl surveys indicate that adult abundance of White Hake declined by over 90% between 1985 and 2014 (about 3 generations). Although most of this decline occurred between 1985 and 1996, the index of adult abundance has continued to decline at a slower rate since then. RV indices of juvenile abundance tended to be higher between 1985 and 1992 than since 1993, though high indices occurred in several recent years.
• Age-aggregated indices from the sentinel bottom-trawl survey and the longline program both indicated substantial declines in biomass in the 2000s, with recent indices the lowest in the time series.
• Estimated spawning stock biomass (SSB) dropped sharply in the late 1980s and early 1990s, averaging 52,848 t during 1978 to 1982 and 6,516 t during 2009 to 2013, a decline of 88%. Estimated SSB in 2013 was 3,844 t, the lowest in the time series and a decline of 93% from the late 1970s and early 1980s. In contrast, estimated juvenile biomass and abundance fluctuated without trend over the 36-year time series.
• Area occupied by adults peaked at values near 25,000 km² in the early 1980s and declined to values near 5,000 km² in recent years, a reduction by about 70% from the late 1980s. Area occupied by juveniles remained fairly constant at 10,000 km² except for an increase to 10,000 km² in the late 1980s and early 1990s.
• In summer, White Hake are normally distributed in inshore waters at depths less than 50 m and offshore at depths greater than 100 m. The proportion of hake occurring in shallow inshore waters was 68% in the 1970s, 45% to 50% in the 1980s and 1990s, and 6% in the 2000s. This offshore shift in distribution is thought to result from increasing risk of predation by grey seals in the inshore.
• Spawning is known to occur in inshore areas in summer and thought to occur offshore in late winter and early spring. One inshore spawning area has not been utilized since the late 1990s. With adult hake now nearly absent in inshore waters, more losses of inshore spawning areas may be occurring.
• A dramatic contraction in the age composition of sGSL White Hake has occurred since 1971. Fish 10 years and older were commonly observed in the RV survey catches in the 1970s and 1980s, but no hake over 7 years of age have been observed in the survey since 1998.

Threats and Limiting Factors to the Survival and Recovery of White Hake

• The directed fishery for White Hake has been closed since 1995. Bycatch in commercial fisheries targeting other groundfish species has declined to very low levels, averaging 18 t annually in NAFO Div. 4T during 2010 to 2013. Based on population modelling, the instantaneous rate of fishing mortality (F) was estimated to average 0.002 for ages 4 and 5 and 0.033 for ages 6+ during 2010 to 2013.
• In the past two decades, the dominant source of mortality for White Hake has been natural mortality (M). For juveniles (ages 2 and 3 years), estimated M increased from 0.60 in 1978 to 1.36 in 2013 (45% to 75% annual mortality). For older ages, increases in M were even more extreme, from 0.4 in 1978 to an average value of 2.05 since 2000 for ages 4 and 5 (from 33 to 87% annually) and from 0.32 to 1.51 (from 27 to 78% annually) for ages 6 years and older.
• Natural mortality has also increased to extremely high levels in adults of other large demersal fishes in the sGSL ecosystem (e.g., Atlantic Cod, American Plaice, Thorny Skate, Winter Skate). Based on a review of evidence, increasing abundance of grey seals, an important predator of these fishes, is considered to be a major cause of these increases in M.
• Recruitment remains strong in this population of White Hake despite very low SSB. This reflects very high recruitment rates (recruits produced per unit of SSB) over the past 20 years.

Recovery Targets

• A sustained increase in SSB to or above 12,800 t, 40% of the SSB producing the maximum surplus production, is proposed as an abundance recovery target. Additionally, recovery would require an expansion in age structure to include substantial frequencies of fish older than 7 years, as observed in the mid-1980s and earlier.
• Estimated SSB in the most recent year (2013) is about 30% of the abundance recovery target with no chance of being at or above this target. Estimated SSB has been below the abundance recovery target since 1995.
• The return of hake to inshore waters of the sGSL, the areas where they predominantly occurred in summer from the 1970s to the mid-1990s, is proposed as a distribution target for recovery.
• The main threat and most important limiting factor to recovery of this population is the exceedingly high level of non-fishing mortality (M).

Projections

• Projections indicated that the declines in SSB and adult abundance are expected to continue under current conditions. Under these conditions, there is no chance of reaching the recovery targets even with no fishing. There was a 5% probability of extinction at 45 to 50 years into the projection. After 60 years, the probability of extinction was 19% or 38%, depending on the method used to project recruitment. An increase in fully recruited (6+) F to 0.24, the highest level observed since 2000, had a negligible effect on the population trajectory.
• Recruitment productivity has been high for the past 20 years. Under this condition, the probability of achieving the biomass recovery target was estimated to be 27% in 30 years after a reduction in M by 20%, 95% in 30 years after a reduction by 30%, and 51% in 6 years after a reduction by 40%.
• If recruitment productivity were to return to the lower levels observed in the 1978 to 1994 period, a 60% reduction in M would be required to allow SSB to increase above the abundance recovery target. The probability of achieving this target would be 77% after 30 years.

Allowable Harm Assessment

• There continues to be mortality of White Hake due to fishing activities but current (6+ F = 0.04) and recent (6+ F = 0.24) fishing mortality rates have negligible effects on the population trajectory because of the extremely high natural mortality rates experienced by this population.

This Science Advisory Report is from the January 14 to 16, 2015 meeting on the Recovery Potential Assessment – White Hake (Urophycis tenuis), population of the southern Gulf of St. Lawrence and population of the Atlantic and northern Gulf of St. Lawrence. 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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