Research Document - 2012/032

Thorny skate (Amblyraja radiata) in the southern Gulf of St. Lawrence: life history, and trends from 1971 to 2010 in abundance, distribution and potential threats

By D.P Swain, H.P. Benoît, D. Daigle, and É. Aubry

Abstract

Thorny skate (Amblyraja radiata) has historically been the most abundant and widespread skate in the southern Gulf of St. Lawrence. COSEWIC (the Committee on the Status of Endangered Wildlife in Canada) will be conducting an assessment of the status of this species in Canadian waters in terms of its risk of extinction. This paper presents information on life history traits, trends in the abundance and distribution of this species, and threats to its persistence in the southern Gulf of St. Lawrence. Thorny skate in the southern Gulf are fairly slow growing, with a mean length of 43.8 cm for skates ten years of age. Estimated lengths and ages at 50% maturity are 49.7 cm and 12.3 years for females and 50.5 cm and 11.9 years for males. Abundance of adult skate (skates > 50 cm in total length) in the southern Gulf has declined steadily since the beginning of the time series in 1971. The estimated decline since 1971 is 95%, and there is no indication that this decline has ceased. In contrast to the adult sizes, juvenile skates (≤50 cm) increased in abundance in the mid to late 1980s, were at a high level from the late 1980s to the mid 1990s and then declined sharply, followed by a sharp increase in abundance since the mid 2000s. The high abundance of juvenile skates in the early to mid 1990s and the mid to late 2000s was entirely due to high abundance of skate less than 35 cm in length. Thorny skate were widely distributed throughout the southern Gulf in the 1970s and 1980s. A striking contraction in the distribution of both juveniles and adults occurred in the 1990s and the 2000s, with distribution now largely restricted to the slope of the Laurentian Channel and northeastern regions of the Magdalen Shallows. The area occupied by mature skates in recent years is only 10% of the area occupied early in the time series. There are no directed fisheries for skates in the southern Gulf. From 1991-2010, about 96% of the total catch is estimated to have been discarded at sea. Estimated discards declined from about 400-500 t in the early 1990s to about 40 t in 2009 and 2010. In the early 1990s, most of the discards were from the cod fishery; in the 2000s, most were from the Greenland halibut fishery. The abundance of grey seals in the southern Gulf has increased over the past 40 years and may contribute to the apparent high mortality of adult thorny skate.

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