Large Atlantic ray fished to the brink of extinction
August 20, 1998: Extinction of marine species is said to be rare (1). Yet according to a report published last month in Science magazine, the once common barndoor skate (Raja laevis), one of the largest skates in the northwest Atlantic, has come close to extinction without, until now, anyone noticing (2). If, say Jill Casey and Ransom Myers, the report’s authors, such a large and easily identified species has been allowed to disappear in an area that is well surveyed, it is probable that the survival of many lesser-known species is also in jeopardy.
In their study, Casey and Myers examined research vessel surveys of biomass (kg/m2) conducted since 1951 in the region from the southern Grand Banks of Newfoundland to southern New England (see Figure 1 in Casey and Myers). The data show that the populations of barndoor skate are decreasing at a rate corresponding to an exponential decay function (Figure 2 in Casey and Myers). Since the 1970s, the species has been caught on only two of the nine banks in the region, whereas all nine banks had catch in earlier surveys.
The analysis of data for the St. Pierre Bank, which is to the south of Newfoundland, shows the average number of barndoor skate fell from about 0.6 million in the 1950s to fewer than 500 in the 1970s (see Figure 2). Meanwhile, smaller skate species, namely the horny skate (Raja radiata) and the smooth skate (Raja senta), increased in biomass during the same interval. No barndoor skate have been caught on the St. Pierre Bank since the 1970s.
Direct biological information on the barndoor skate is scarce, but comparisons to its cousin, the “common” skate, yield estimates of age at maturity and maximum egg production of 11 years and 47 eggs/year, respectively (2). Thus, the instanteous mortality rate required to drive the species to extinction is 0.4/year, assuming mature and immature mortalities to be equal. This is less than the fishing mortality for cod (Gadus morhua) during the last 30 years in the surveyed regions. It is not surprising, therefore, that the barndoor skate has all but disappeared from those waters where it has been taken as a by-catch in the fisheries for cod and other species. Even the cod, an earlier maturing and much more fecund species, is no longer abundant in most of these waters.
One of the few remaining refuges for the barndoor skate may be an area just north of its reported range, at a depth of 1000 m (2). It was discovered purely by accident during surveys recently conducted for a new halibut (Reinhardtius hippoglossoides) fishery. Elsewhere, populations of the barndoor skate are most abundant on Browns Bank and Georges Bank, both of which have a seasonal closure to trawling.
Currently, no regulations governing the fishing of skates are in place. Perhaps, say Casey and Myers, the only hope for the long-term survival of the barndoor skate is to designate an area on all of the banks that is protected from trawling and is large enough to allow for a self-sustaining population. Pending such a plan, Raja laevis is likely to move ever closer to becoming extinct.