Research Document - 2006/004

Recovery potential assessment of 4T and 4VW winter skate (Leucoraja ocellata): Population models

By Swain, D.P., I.D. Jonsen, and R.A. Myers

Abstract

In May 2005, winter skate (Leucoraja ocellata) in the southern Gulf of St. Lawrence (sGSL, NAFO Div. 4T) and on the eastern Scotian Shelf (eSS, Div. 4VW) were designated as endangered and threatened, respectively, by the Committee on the Status of Endangered Wildlife in Canada. In this report, we model the population dynamics of 4T and 4VW winter skate using Bayesian state-space models. Data inputs were 35-yr time series of research survey catch rates for juvenile and adult length classes in each area, and estimates of fishery landings and bycatch. The models suggested that there have been changes in the mortality of winter skate, with juvenile mortality decreasing and adult mortality increasing from the 1970s to the 1980s and 1990s. Similar patterns were evident in both the 4T and 4VW areas, though the patterns were stronger in 4T. These changes in mortality appeared to reflect changes in natural mortality (M). For adults in the 4T area, the trend in exploitation rate was opposite to the trend in total mortality, and the estimated exploitation rate was very low after 1990. Possible causes of the trends in M were examined by modeling M as a function of potential explanatory variables such as predator abundance. In both the 4T and 4VW areas, adult M showed a significant positive relationship with grey seal abundance, suggesting that increased predation by seals may be a cause of the increased adult mortality. Population projections predicted a steady decline in the 4T population even in the absence of fishery removals. For the 4VW population, projections predicted a continued decline at recent (2002-2004) levels of removals, and a reduced decline or stability if there are no removals. These projections suggest that no recovery can be expected for either population at the current levels of adult natural mortality. Uncertainty around the projections was high, but in the case of the 4T population the 95% credible limits around the projected abundance over the next 10 years did not include recovery.

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