Reproductive biology of the Alaska skate Bathyraja parmifera, with regard to nursery sites, embryo development and predation

The Alaska Skate Bathyraja parmifera is an abundant elasmobranch species in the eastern Bering Sea, and vulnerable to directed fishing mortality. The reproductive biology of the species was studied with regard to nursery site selection and location, embryo development, and key events in the development process, including embryo and neonate mortality. A nursery site was located in the southwest eastern Bering Sea near Unimak Pass and was sampled once every 60 days during a 14-month period. The site was small in area (<2 km2) and possessed egg cases of the Alaska Skate at densities exceeding 500,000/km2. Embryo development time exceeded 3.5 years and multiple cohorts were developing concurrently within the nursery. Reproduction peaked during summer months, and during winter months peaks in hatching occurred. Predation sources to newly hatched skates included the Pacific Cod Gadus macrocephalus and the Pacific Halibut Hippoglossus stenolepis. Embryo development and its egg case underwent coordinated changes with the opening of the egg case to seawater closely tied to the development of the tail filament, which facilitates water flow through the egg case. Newly deposited skate egg cases were susceptible to predation by gastropod snails, which drill holes in the soft egg cases and prey on the large yolky mass. Qualitative and quantitative differences were evident between skate species and at different nursery site locations. Predation appears to be density dependent and nursery sites provide a benefit of lower predation rates at high egg case densities.