Winter Skate (Leucoraja Ocellata)
- Assessment Summary
- Executive Summary
- COSEWIC History, Mandate, Membership and Definitions
- List of Figures
- Species Information
- Population Sizes and Trends
- Limiting Factors and Threats
- Special Significance of the Species
- Existing Protection or Other Status
- Summary of Status Report
- Technical Summary
- Acknowledgements, Authorities Contacted, and Information Sources
- Biographical Summary of Report Writer
COSEWIC Status Report
Southern Gulf population
Eastern Scotian Shelf population
Georges Bank-Western Scotian Shelf-Bay of Fundy population
Northern Gulf-Newfoundland population
Winter skate, Leucoraja ocellata (Mitchill 1815), belong to the Class Chondrichthyes, Subclass Elasmobranchii, Order Rajiformes and Family Rajidae (Figure 1). Other common names include big skate, eyed skate, spotted skate, and raie tachetée.
From Scott & Scott (1988).
Skate are found worldwide in the ocean from shallow waters to great depths. There are over 200 species. In Canada, there are 13 Atlantic species, 7 Pacific species, 1 Arctic species and 3 Arctic-Atlantic species (Coad, 1995). Skate are recognized by their flattened disc shape. The head, body and greatly enlarged pectoral fins are strongly depressed as a rhomboid or subcircular disc (Coad, 1995). The five gill openings and mouth are ventral. The spiracles are on the upper surface and enable clean water to be passed over the gills for respiration. The tail is a long structure with longitudinal lateral folds, slightly depressed in cross-section, having one or two small caudal fins on the posterior portion (Scott and Scott, 1988). There are two small dorsal fins, about equal in size and shape, set far back on the tail, and two pelvic fins appearing as separate limb-like structures, fan-like in females but modified to form claspers in males (Whitehead et al., 1984). The upper surface is usually light to dark brown and the underside white to grayish in colour (Whitehead et al., 1984).
In the Gulf of Maine, winter skate can be distinguished from other skate species, with the exception of little skate (Leucoraja erinacea), by a combination of the following characters: the snout is very round and obtuse; the midbelt of the disc and tail bears three or more rows of thorns from the shoulder region to the origin of the first dorsal fin, with the mid-row disappearing with growth; and the upper surface is patterned with round dark spots and generally with one or more eye spots near the rear corner of the pectoral fins (Collette and Klein-MacPhee, 2002). However, specimens that lack these spots resemble little skate so closely when small (<35cm) that a distinction cannot be accurately made by visual inspection. (Note that this difficulty in distinguishing winter skate from little skate will not affect the abundance trend estimates reported here because the latter are presented for the mature part of the population only, excluding skate less than 35 cm in length.) Winter skate have more tooth rows in the upper jaw (but the numbers vary with size) and more spines (usually >21) on the midline of the tail than do little skate (Collette and Klein-MacPhee, 2002). Winter skate are also distinguished by their wedge-shaped anterior contour, by the lack of large, conspicuous thorn-like spines behind the shoulders and on posterior part of the tail, and by the dense prickled lower surface (except extreme tip) of the tail (Scott and Scott, 1988). The colour of the upper surface is light brown and the lower surface is usually whitish, sometimes with irregular brownish blotches on the posterior portion of the disc and tail (Scott and Scott, 1988).
There is reason to believe that a single designation would inadequately or inappropriately reflect the status of the winter skate throughout its Canadian range; declines evident in the southern Gulf and eastern Scotian Shelf are not evident in waters further south. In addition, there are significant spatial differences in life history traits that are inextricably linked to individual fitness and, thus, population growth rate. The assessment of the species as separate Designatable Units is further supported by spatial discontinuities in distribution.
The distribution of winter skate in Canadian waters is characterized by three concentrations, each of which is separated by a spatial disjunction in the locations in which skate have been captured by fisheries-independent research surveys (Figure 2). These areas of concentration occur in the southern Gulf of St. Lawrence, eastern Scotian Shelf, and, more broadly, in the Bay of Fundy, western Scotian Shelf (notably Browns Bank) and on the Canadian portion of Georges Bank. These concentrations are separated by areas in which exceedingly few skate have been caught. These areas of negligible skate concentration range between approximately 330 and 450 km in size, distances considerably greater than the maximum lifetime distances that have been documented for winter skate and a closely related species. Winter skate tagged in the Gulf of Maine prior to 1950 moved a maximum of 190 km from their site of release (Whitehead et al., 1984). A recent study of a related species of skate in the North Sea, the thornback ray (Raja clavata), indicated that the annual migration cycle, based on conventional and electronic tagging experiments, was restricted to an area 1° latitude X 1° longitude in area (i.e., approximately 110 km X 110 km) (Hunter et al. submitted). Thus, it appears as though the distances separating the concentrations of winter skate in Canadian waters exceed the distances across which winter skate are likely to disperse.
Available data indicate that these spatial differences in the concentrations of winter skate are associated with spatial differences in heritable life history traits. The most compelling evidence for separate consideration comes from differences in size and age at maturity. Winter skate generally mature at a length of 75 cm (Simon et al., 2003), but in the southern Gulf of St. Lawrence size at maturity is closer to 50 cm and they do not attain the same maximum size that they do elsewhere (McEachran & Martin, 1977). Limited aging data suggest that age at 50% maturity for skate on the eastern Scotian Shelf ranges between 7 and 8 years; maximum age has been estimated to be 20 to 30 years (Simon and Frank, 2000; Sosebee and Terceiro, 2000). A study undertaken in the Gulf of Maine indicates an age at maturity of 12-13 years and maximum age estimates of 18-19 years (Sulikowski et al., 2003).
Additionally, skate in the southern Gulf of St. Lawrence appear to differ from other populations in regard to morphological traits such as the number of tooth rows, body size and jaw size, but the degree to which these characters provide reliable metrics of population identity has not been unequivocally demonstrated (McEachran & Martin, 1977). No genetic research has been undertaken to compare winter skate populations from different areas in Canadian waters.
Finally, as the available data on trends in population abundance suggest (see below), the status of winter skate appears to differ throughout its Canadian range. Skate in the southern Gulf and on the eastern Scotian Shelf have shown dramatic reductions in abundance since the early 1970s; in contrast, skate in the most southerly part of its Canadian range appear to have been relatively stable in abundance over the time period for which data are available.
Thus, on the basis of observed differences in spatial distribution, life history traits, inferred and observed constraints on individual movements, and spatial differences in abundance trends, winter skate in Canadian waters can be divided into four Designatable Units (DUs), the boundaries of which can be delineated by Northwest Atlantic Fishery Organization (NAFO) divisions (Figure 3): 1) Southern Gulf population (NAFO division 4T); 2) Eastern Scotian Shelf population (NAFO divisions 4VW); 3) Georges Bank-Western Scotian Shelf-Bay of Fundy population (NAFO divisions 4X5Ze); and 4) Northern Gulf-Newfoundland population (NAFO divisions 3NOP4RS) (Figure 4). Rather than being distinguished by known or inferred differences in biology, behaviour and life history, the fourth of these DUs identifies winter skate in waters at the northern edge of the species’ range where they are rare.
The boundaries are delineated by heavy lines. The depth contour is at 200 metres.
- Date Modified: