Figure 1. Three examples of catches from the beam trawl used by the scientists to capture juvenile flatfish. Each example shows a different level of animal abundance and ratio of fish to invertebrates. Photos by Dan Cooper.
The Recruitment Processes Program had an active summer field season this year. Activities started with the collection of marine zooplankton on the RACE Groundfish Assessment Program (GAP) cruises to the eastern Bering Sea shelf. Zooplankton biomass (food for walleye pollock, baleen whales, and seabirds) has paralleled (in recent years) changes in Bering Sea climate. J. Clark from the Recruitment Processes Program began collections during Leg 1 of the cruises, and P. von Szalay (RACE) and T. Buckley (REFM) were responsible for collections on the second and third cruise legs.
A joint Russian–U.S. cruise scheduled for August to examine the marine ecosystem of the Chukchi Sea was unfortunately cancelled due to problems with ship availability and was rescheduled for 2009. The cruise was part of the RUSALCA (Russian American Long-Term Census of the Arctic) program to understand how climate variability is affecting the marine resources of the Arctic.
During August, the Recruitment Processes Program collaborated with NOAA's Pacific Marine Environmental Laboratory (PMEL) on an examination of climate forcing and marine ecosystem productivity in the eastern Bering Sea (NPCREP–North Pacific Climate Regimes and Ecosystem Productivity study). PMEL chartered the research vessel Melville from the University of California San Diego to conduct physical and biological sampling over the eastern continental slope and shelf of the Bering Sea.
The 20-day cruise repeated sampling along a line of stations stretching from Bristol Bay to St. Lawrence Island, and found very high concentrations of zooplankton along the southern portion of the transect line. These stations are in the vicinity where AFSC scientists from the National Marime Mammal Laboratory (NMML) found and tagged a North Pacific Right whale (see NMML's Cetacean Assessment and Ecology Program article in this issue). Right whales are thought to eat only one or two types of planktonic prey, and one of the known prey types was the dominant zooplankter in the patches samples by scientists on the Melville.
To conclude our season, members of the Recruitment Processes Program returned to the eastern Bering Sea on the Miller Freeman to learn more about the nursery habitat of newly settled flatfish (Fig. 1). This cruise is part of our effort to understand the factors that are important in determining the transport and recruitment success of commercial (e.g., northern rock sole, Greenland halibut, Alaska plaice) and nontarget (arrowtooth and Kamchatka flounder) flatfish species in the eastern Bering Sea. The nontarget species are important predators of walleye pollock.
Early Life History of Greenland Halibut in the Eastern Bering Sea
Researchers from the Recruitment Processes Program and Oregon State University (OSU) are collaborating to examine transport of Greenland halibut (Reinhardtius hippoglossoides) eggs and larvae from spawning to potential nursery locations in the EBS. At one time, Greenland halibut supported a commercial fishery of up to 80,000 t, but catches have declined significantly since the 1970s. The reasons for the decline of Greenland halibut in the EBS are unknown, and we seek to determine whether recent atmospheric and hydrographic changes in the region have affected patterns of larval transport, dispersal, and survival of the early life history stage.
This ongoing project will assess five areas: 1) spawning locations, 2) egg and larval drift pathways, 3) egg buoyancy, 4) larval and juvenile feeding and growth patterns, and 5) vertical egg distribution. Results from the AFSC ichthyoplankton surveys (1982-2006) indicate that Greenland halibut larvae in the EBS have a long duration in the plankton and are subject to extended drift pathways.
The early stage Greenland halibut larvae were found off the continental slope, mostly below 500 m, and drifted northward during spring. Highest larval abundances were observed in March. Larval lengths ranged from 9 to 25 mm standard length (SL) during spring. Larvae were found throughout the water column, but highest concentrations were at 45 m depth. This vertical distribution pattern suggests that adult Greenland halibut spawn in very deep water (below 500 m) and eggs and larvae slowly rise after hatching.
In February 2008, scientists from the AFSC and OSU conducted an ichthyoplankton survey in the EBS that targeted Greenland halibut eggs and larvae. Objectives were to describe the distribution of Greenland halibut eggs and larvae over the slope and in Bering Canyon in winter and to obtain eggs for buoyancy studies. A total of 45 bongo and 64 MOCNESS tows were completed. Greenland halibut eggs about 4 mm in diameter were found consistently in small numbers throughout Bering Canyon and were used to make shipboard measurements of instantaneous specific gravity. Greenland halibut eggs ranged in age from early to late stage. Using an Egg Density Gradient Apparatus (EDGAR), we measured egg density, which ranged from 1.02429 to 1.02889 at experimental temperatures. Preliminary results indicate that older eggs may have a greater density than younger eggs.
