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Shellfish Assessment & Groundfish Assessment Programs

Mapping Skate Nursery Sites in the Eastern Bering Sea

figure 5, click to enlarge
Figure 5.  Click image to enlarge.

In August 2009 we mapped four nursery sites in the eastern Bering Sea (Fig. 5) used for egg case deposition by the Alaska skate (Bathyraja parmifera).

We used the SeaBED autonomous underwater vehicle (AUV) (Fig. 6) developed by Dr. Hanumant Singh at Woods Hole Oceanographic Institution.

The SeaBED AUV produced high quality digital images of the seafloor while running transects across the nursery sites located in or near Bering, Pribilof, Pervenets, and Zhemchug canyons.

figure 6, Autonomous Underwater Vehicle
Figure 6.  The Autonomous Underwater Vehicle (AUV) SeaBED developed by Dr. Hanumant Singh of Wood's Hole Oceanographic Institution used to obtain images of the skate nursery sites.  Photo by Gerald Hoff.

The project, funded by NPRB, is designed to estimate Alaska skate production at nursery sites in the eastern Bering Sea. The digital images will enable us to estimate egg case densities at each site and couple it with a characterization of the habitat used for skate reproduction.

Preliminary analysis from images shows high densities of skate egg cases deposited directly on the seafloor over a relatively small area (Fig. 7 below).

The nursery site at Bering Canyon showed eggs broadly scattered on a soft muddy bottom while the other three sites were much more concentrated over more scoured, hard packed bottom substrates.

It is not clear why the heads of canyons are chosen for skate nursery sites however a relatively stable, warm bottom temperature, such as those found along the upper slope, may be an important factor in facilitating successful embryo development.

figure 7, AUV images of Alaska skate egg cases
Figure 7.  Images obtained by SeaBED (AUV) of egg cases of the Alaska skate (Bathyraja parmifera) at four nursery sites in the eastern Bering Sea:  A) Pervenets Canyon;  B) Zhemchug Canyon;  C) Pribilof Canyon;  D) Bering Canyon.

By Gerald Hoff

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