Science Advisory Report 2017/014
Recovery Potential Assessment for Winter Skate (Leucoraja ocellata): Eastern Scotian Shelf and Newfoundland Population
Biology, Abundance, Distribution and Life History Parameters
- The Eastern Scotian Shelf – Newfoundland (ESSN) population structure of Winter Skate was revised by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in 2015 to include the previous Eastern Scotian Shelf (ESS) population and the Newfoundland portion of the previously identified Northern Gulf and Newfoundland population. This new ESSN Designatable Unit (DU) population was assessed as Endangered, due to an estimated decline in abundance of 98% since the early 1970s and a decrease in distribution range.
- ESSN DU Winter Skate mature at older ages and larger sizes than Winter Skate in the Gulf of St. Lawrence (GSL) DU, but at sizes and ages similar to Winter Skate elsewhere along the eastern seaboard of North America.
- The ESSN DU range consists of Northwest Atlantic Fisheries Organization (NAFO) Divisions 3LNOP and 4VW, with the majority of Winter Skates located in Divisions 4VW.
- Based on annual area-specific DFO research surveys, biomass indices of all sizes of Winter Skate declined over 1996 to 2014 by 90% in Div. 3LNOP, and 92% in Div. 4VW. In Div. 4VW, the observed decline began in 1970, with a decrease by 99% during 1970 to 2014.
- Estimated biomass indices for all size groups in this DU indicated a 98% decline from a peak value of 48.6 kt in 1970 to 1.2 kt in 2015.
- Winter Skate on the ESS was broadly distributed on shelf waters at depths < 110 m and on the offshore banks, with concentrations on Middle Bank, Sable Island Bank, and Banquereau Bank in the 1970s and 1980s. Beginning in the 1990s, the distribution shifted from the Sable Island area to edges of the ESS and the average area occupied has decreased from 15,000 km² in the 1980s to about 4,000 km² by 2010 (a 74% decline).
- For Div. 4VW Winter Skate, estimated natural mortality (M) for adults (75+ cm) was low (10.5% to 18.1% annually) during 1970 to 1990 and increased to 36% since 1998. This population had a negative intrinsic rate of increase since 2005, i.e., the stock is not replacing itself.
Threats and Limiting Factors to the Survival and Recovery of Winter Skate
- There are no directed fisheries for Winter Skate in the ESSN DU. Winter Skate of this DU are incidentally captured in many fixed and mobile gear fisheries, including for groundfish, Thorny Skate (primarily in Subdivision 3Ps), shrimp, scallop, and surf clam. However, fishing mortality presently does not appear to be a limiting factor to Winter Skate survival and recovery.
- Winter Skate can only be retained in groundfish fisheries, but are usually discarded at sea. Based on low at-sea observer coverage (0% to 7% annually), annual discard estimates for these fisheries ranged from 0 to 63 t in Divs. 3LNOP, and 29 to 93 t in Divs. 4VW during 2005 to 2013.
- Based on estimates of landings, discards, and post-discard survival rates, maximum estimated exploitation rates during the 1970 to 2015 period in Divs. 4VW were 4% for small juveniles, 11% for large juveniles, and 17% for adults. Exploitation rates in 2011 to 2015 were 1.3% for small juveniles, 0.6% for large juveniles, and 0.41% for adults.
- Habitat is not a limiting factor to Winter Skate survival and recovery in the ESSN DU. There are no known anthropogenic threats that have reduced habitat quantity or quality for this population.
- Risk of extirpation has increased substantially for the ESSN DU population since the previous Winter Skate assessment by COSEWIC in 2005. At the DU level, estimated biomass declined by 98% over 1970 to 2015. For the Divs. 4VW component of this DU, adult abundance declined by 99% in this period. Despite closure of the Divs. 4VW skate-directed fishery and reductions in bycatch of skates in other fisheries, adult abundance in Divs. 4VW declined by 76% during 2005 to 2015. It is expected to continue declining even if there are no fishery catches. The estimated high natural mortality continues to be the most important limiting factor that places this population at risk of extinction.
- Predation by Grey Seal is a plausible cause of elevated natural mortality for Winter Skate in Divs. 4VW. Other predators, such as large sharks, may constitute an additional cause of the elevated mortality; although the abundance of large sharks appears to have declined to low levels in the Northwest Atlantic.
- Winter Skate life history characteristics of slow growth, late sexual maturity, and extended embryonic development result in low rates of potential population increase, decreased resilience and a limited possibility to recover from substantial declines, such as those induced by fishing or other causes.
- Candidate recovery targets for survival, size structure, distribution, and abundance are proposed. The survival target is defined by an intrinsic rate of population increase (r), or by an equivalency based on natural mortality (M). In terms of adult M, the population would increase and its risk of extinction would become negligible if current M (36% annually) was reduced by 50% (18% annually).
- The proposed size structure, distribution, and abundance targets cannot be realized until the current high natural mortality rate of adult Winter Skate has been substantially reduced.
- Modelling results indicated that the ESSN DU of Winter Skate is not viable under current conditions, due primarily to increases in adult natural mortality to unsustainably high levels. If the productivity conditions observed in 2005 to 2015 persist, this population is expected to decline to extinction, even with no fisheries catches.
- Assuming current productivity conditions, the Divs. 4VW component of this DU is expected to continue declining rapidly, with a 58% probability of decreasing below 50 t (a proxy for extinction) by 2040, even with no fishery-related losses.
Scenarios for Mitigation of Threats and Alternatives to Activities
- The lack of recovery and on-going decline of the Winter Skate ESSN DU is due to high natural mortality of adults. If this high natural mortality persists, any additional measures to further reduce the currently low fishing mortality will be ineffective in promoting recovery and reducing the high risk of extirpation.
- Potential measures to further reduce fisheries related losses of ESSN DU Winter Skate in scallop, groundfish, and shrimp fisheries are provided.
Allowable Harm Assessment
- Even with no fishery related losses (from retention and discarding), there is a greater than 50% probability that extirpation of the ESSN DU of Winter Skate would occur within 25 years if the productivity conditions of the 2005 to 2015 time period persist. At the level of the estimated exploitation rates during 2011 to 2015, there would be no noticeable impact on the projected population trajectory; although there would be a small increase in the probability of extirpation (58% to 65%) by 2040.
- Results were similar for the Divs. 4VW component of the DU based on a stage-structured model. If current conditions persist, the probability that adult biomass would decline below 50 t, a proxy for extinction, by 2040 is estimated to be 58% with no fishery catch, and 62% at 2011 to 2015 fishery exploitation rates.
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 population. 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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