Groundfish Assessment Program
Dynamics of Skate Nurseries in the Eastern Bering Sea
Skates are oviparous elasmobranchs that produce a relatively large,
tough, proteinaceous egg case containing a developing embryo (Fig 1).
The egg cases are deposited directly onto the seafloor, and the
embryo develops for many months before hatching
with little additional parental care. At hatching, juvenile skates are
highly mobile, able to feed, and resemble adults. Skates utilize
specific nursery sites and successful reproduction may be jeopardized
due to habitat disturbances during the long developmental period of
embryos and the vulnerability of large skate aggregations. Virtually
nothing is known about skate nursery grounds in Alaskan waters. The goal
of our research is to characterize two nursery areas for two of the most
abundant Alaskan skate species in the eastern Bering Sea, in order to
gain a better understanding of skate reproduction, habitat use, and
nursery area dynamics and vulnerabilities.
Nursery grounds for the Alaska skate, Bathyraja
parmifera, and the Aleutian skate, B. aleutica, were
identified in the southeastern Bering Sea (Fig. 2, above)
during a trawl study conducted in July-August 2004. The sites are
species-specific and relatively small in area (~5 square nautical miles
(nmi)), located approximately 10 nmi apart. Both sites indicated high
reproductive activity during the summer months, with high densities of
viable eggs and sexually mature male and female skates present. The
frequency of embryo developmental stages from each site indicated
multiple cohorts developing simultaneously with a low level of
continuous egg deposition throughout the year (Fig. 3).
Skate embryos and juveniles are vulnerable to predation in the early embryo stage and post-hatching. Early stage embryos are preyed on by an unknown snail species that drills holes in the newly deposited egg cases and consumes the developing embryo and yolk. Studies are under way to identify the snail species and estimate predation rate on skate embryos. In addition, newly hatched juvenile skates are consumed by two common piscivorous fishes: Pacific halibut, Hippoglossus stenolepis, and Pacific cod, Gadus macrocephalus. These two predator species may prove to be indicator species for skate nurseries and hatching events within the nurseries.
By Gerald Hoff