September 20, 2005
"Skates deposit their eggs in specific nursery areas" said Hoff. "Virtually nothing is known about skate nursery grounds in Alaskan waters yet they are very important for the successful propagation of the greater than twelve species known to exist there."
Hoff is a Research Fisheries Biologist at the Alaska Fisheries Science Center in Seattle and also a graduate student at the University of Washington working on his PhD.
Skates are graceful elasmobranchs related to stingrays and sharks. Skates and sharks in general are large bodied, slow growing and slow to reproduce. They use distinct nursery grounds, returning annually en masse for long periods of time to mate, bear live young or deposit eggs on the bottom. Understanding the physical and biological characteristics of nursery grounds is important for the successful conservation of elasmobranch species.
"The goal of my research is to characterize the nursery sites and understand their biological and physical aspects in order to gain a better understanding of skate reproduction, habitat use, and nursery area dynamics and vulnerabilities. Successful skate reproduction may depend on secure nursery habitat."
Hoff focused on two study sites in the southeastern Bering Sea which are nursery grounds for the Alaska skate, Bathyraja parmifera, and the Aleutian skate, B. aleutica. The nursery grounds are both relatively small-about 5 square nautical miles.
"I returned to both sites about every 60 days for over a year," Hoff explained. "I want to determine the annual cycle of egg deposition and the duration of embryo development. I also want to understand more about predation on the developing embryos and newly hatched juveniles. I will spend about the next six months working on analyzing the data collected so far and synthesizing the results with what is known about skate reproduction."
Skates produce large, tough leathery egg cases that are deposited directly on the ocean floor. The egg cases are left to develop and hatch with little or no involvement of the adult skates. Many beachcombers have encountered skate egg cases worldwide as they often wash up on beaches during storms or after hatching. They are commonly known as 'mermaids purses'.
"The two species I am studying are taking about a year to develop inside the egg cases and hatch" said Hoff. "That is a long time to sit on the ocean floor."
Although the egg cases are quite tough and protect the developing embryo well, certain snail species are able to drill holes in the egg cases and dine on the large yolk before the embryo has a chance to develop. Also, several large predatory fish species prey on the newly hatched juvenile skates as they emerge from the egg cases.
"Understanding sources of mortality for skates helps scientists determine reproductive success and survival to adulthood which are important conservation aspects," said Hoff. "Hopes are that by the conclusion of this study we will have a greater understanding of skate nurseries and be able to draw some general conclusions about them".
Hoff's skate nursery research is funded by the North Pacific Research Board, the Essential Fish Habitat Division of the Alaska Region of NOAA Fisheries, and the Alaska Fisheries Science Center.
NOAA Fisheries manages a directed
fishery for skates in the Gulf of Alaska. From January through
the first part of September this year, about 2,520 metric tons
of skate have been caught in the Gulf. Skates are also caught
incidentally in the Bering Sea and Aleutian Islands.
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