Graduate Thesis Or Dissertation
 

Age, growth and maturity of the longnose skate (Raja rhina) for the U.S. west coast and sensitivity to fishing impacts

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  • In the past, sharks, skates and ray species have mainly occurred in incidental fisheries. Now they are increasingly being directly targeted due to the depleted status of traditionally targeted species. The life history characteristics of many elasmobranch species make them more sensitive to high rates of fishing mortality. The combination of these factors has resulted in the known local depletion of several elasmobranch species; including the barndoor skate, Raja laevis, of the Northwest Atlantic and the common skate, Raja batis, of Irish Sea. The longnose skate, Raja rhina, occurs commonly as incidental catch in trawl fisheries off the coast of California, Oregon and Washington. Commercial landings of skates, including R. rhina, have increased dramatically along the U.S. West coast. The objectives of this study have been to calculate accurate growth and maturity parameters for Raja rhina in order that the vulnerability of this species to fishing mortality can be assessed. The slow growth rates (k=0.04 to 0.06) and late ages-at-50% maturity (11 to 16 years with maximum ages between 15 and 22 years, depending on sex and region) which were calculated indicate that this species may be at great risk of depletion in the future. Survey biomass trends indicate that R. rhina populations have remained stable over the past twenty years. However, the possibility that biomass levels in the distant past were much higher than they are at present cannot be ruled out. It is recommended that the collection of fishery-dependent data for this species and other skate species begins immediately. This information, along with the results found in this are needed for a proper assessment for Raja rhina, so that the effects of current fishing pressures can be evaluated and regulated appropriately.
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