Age, growth, and demography of the big skate, Raja binoculata, from the northeast Pacific, with a revised model using histology

Megan V. Winton

Email: mwinton@mlml.calstate.edu

I am primarily interested in elasmobranch life history theory, demography, and fisheries management. Before coming to Moss Landing, I graduated with a B.S. in biology from Emory University and worked as an intern and technician with the Shark Research Center at Mote Marine Laboratories and the Shark Population Assessment Group at the NMFS Southeast Fisheries Science Center, Panama City Laboratory.

M.S. Thesis:

Although skates have only recently become the target of commercial fisheries along the western coast of the United States, they have long been a major component of fisheries bycatch.  Despite their commercial importance and the documented decline of skate stocks due to fishing pressure in the North Atlantic and North Sea (Brander 1981; Casey and Myers 1998; Dulvy et al. 2000; Frisk et al. 2002), the detailed biological information needed to enact effective management schemes is lacking for most skate species. Given that many skates possess characteristics that may make them especially vulnerable to exploitation, such as slow growth, late maturity, and low reproductive rates, the accurate assessment of species’ longevity and growth rates are indispensible in developing sustainable management plans.

The big skate, Raja binoculata, is a reproductively unique member of the family Rajidae. Generally found at depths of less than 100 m in continental shelf waters ranging from the Bering Sea to southern Baja, California, the species is the largest skate in the eastern north Pacific, reaching maximum reported total lengths of up to 240 cm (Ebert 2003). Unlike most rajid species (which produce only one embryo per egg case), R. binoculata may produce up to eight embryos per egg case, potentially making it the most fecund of all oviparous elasmobranch species (Ebert et al. 2008).

Raja binoculata is taken primarily as bycatch in groundfish trawls throughout its range but has become a target species in the last decade. In 2003, as a result of improved market prices for skate and the closure of the cod fishery, a directed skate fishery developed in the Gulf of Alaska. From 2003 to 2005, over half of all Gulf of Alaska directed skate catch was comprised of only two species, the big skate and the longnose skate, R. rhina (Gburski et al. 2007).

The focus of my thesis research is to determine if the Raja binoculata population off the coast of California has changed over the past several decades by comparing age and growth estimates from specimens collected in the 1970s with those collected in the past decade. Results using traditional aging methods will be compared with those produced using an alternative histological method, which may result in higher age estimates than previously reported.

The results of the aging portion of the study will be used in conjunction with the results of an age validation and reproduction study conducted on a captive R. binoculata population to construct stochastic demographic models. The results of these models will be used to determine annual rates of population growth, compare growth rates over time, determine whether age composition has changed over time, and assess the population’s vulnerability to commercial exploitation.

Additional Research:

Age, growth, and demography of the roughtail skate, Bathyraja trachura, from the eastern Bering Sea, with a revised model from the west coast of the United States using histology.

A similar project will focus on the age, growth, and demography of the roughtail skate, Bathyraja trachura, from the Bering Sea. Although a directed fishery for B. trachura does not currently exist in Alaskan waters, there is the potential for such a fishery to develop as populations of many targeted groundfish species decline. Baseline biological data produced for current populations will be invaluable in determining the effect harvesting may have on fish exhibiting similar life history traits and will help provide guidelines for developing fisheries.

Literature Cited:

Brander, K. 1981. Disappearance of common skate Raja batis from the Irish Sea. Nature 5801:48-49.

Casey, J.M., and Myers, R.A. 1998. Near extinction of a large, widely distributed fish. Science 28:690-692.

Dulvy, N.K., Metcalfe, J.D., Glanville, J., Pawson, M.G., and Reynolds, J.D.  2000.  Fishery stability, local extinctions, and shifts in community structure in skates. Conservation Biology 14(1):283-293.

Ebert, D.A. 2003. Sharks, rays, and chimaeras of California. University of California Press, 284 pp.

Ebert, D.A., Smith, W.D., and Cailliet, G.M. 2008. Reproductive biology of two commercially exploited skates, Raja binoculata and R. rhina, in the western Gulf of Alaska. Fisheries Research 94:48-57.

Frisk, M.G., Miller, T.J., and Fogarty, M.J. 2002. The population dynamics of the little skate Leucoraja erinacea, winter skate Leucoraja ocellata, and barndoor skate Dipturus laevis: predicting exploitation limits using matrix analyses. ICES Journal of Marine Science 59:576-586.

Gburski, C.M., Gaichas, S.K., and Kimura, D.K.  2007.  Age and growth of the big skate (Raja binoculata) and longnose skate (R. rhina) in the Gulf of Alaska.  Environmental Biology of Fishes 80:337-349.