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Resource Assessment & Conservation Engineering (RACE) Division

Groundfish Assessment Program

Dynamics of Skate Nurseries in the Eastern Bering Sea

  photo of skate egg cases
Figure 1.  An Aleutian skate, Bathyraja aleutica, egg case (left) and an Alaska skate, Bathyraja parmifa, egg case (right). Not to scale.

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.

Map of skate
          nursery sites
Figure 2.  Location of skate nursery sites in the southeastern Bering Sea.

Frequency of embryo stages, see caption
Figure 3.  Frequency of embryo stages from the nursery sites during summer 2004 for the Alaska skate, Bathyraja parmifera, (top) and the Aleutian Skate, B. aleutica, (bottom). Stage 1 is newly deposited and stage 6 is at hatching.

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).

Seasonal sampling is being conducted at both sites, approximately once every 60 days, to determine annual cycles of egg deposition, duration of embryo development, and predation on developing embryos and newly hatched juveniles. Evidence to date suggests June and July have the highest nursery activity for the B. parmifera site (Fig. 4, below), as adults move into the site in mass and begin depositing eggs; however, seasonal sampling confirmed low-level egg deposition throughout the year. Duration of embryo development to hatching may be close to a year. The progression of embryo development indicates that eggs deposited in June 2004 may not hatch until May-June 2005.

Graph of skate abundance, see caption
Figure 4.  Abundance of the Alaska skate, B. parmifera, in the nursery site throughout the year. June and July are the most active months for skate reproduction at this site.

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


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