9/30/1997
| Principal Investigators: | Richard D. Brodeur Jeffrey M. Napp Alaska Fisheries Science Center Seattle, WA |
Miriam Doyle Joint Institute for the Study of Atmosphere and Ocean University of Washington Seattle, WA |
I. FIELD OPERATIONS 1996-97
| Miller Freeman | MF96-13 | Sep. 4-16 | Sampled three habitats on all four transects near Pribilof Islands. Also conducted intensive diel sampling at one site. |
| Wecoma | W97-06A | June 6-10 | Tested sampling gear to be used on subsequent Wecoma cruise. |
| Wecoma | W97-06C | July 1-14 | Conducted hydrographic, acoustic, plankton, and midwater trawl sampling in the vicinity of the Pribilof Islands for comparison with the fall surveys. Sampled all three habitats on four transects. |
| Miller Freeman | MF97-09 | Sep. 8-18 | Conducted hydrographic, acoustic, plankton, and midwater trawl sampling in the three habitats on all four transects near Pribilof Islands. Used a Remotely Operated Vehicle to observe habitat differences with respect to the fronts. |
II. LABORATORY AND DATA ANALYSIS
Zooplankton, Ichthyoplankton and Nekton (primarily age-0 pollock) from fall 1994 and 1995 cruises were analyzed by location of capture with respect to the frontal regions. These findings were written up and presented for an International Symposium on the Role of Forage Fishes in Marine Ecosystems held in Anchorage in November (see RESULTS).
All stomach samples of age-0 pollock have been analyzed for Line A for 1994-96 and analysis completed on stomach fullness, condition, and prey composition with respect to time, year and habitat.
Results have been written up in a paper which was presented at the ICES Recruitment Symposium in Baltimore in September (see RESULTS).
Selected stomach samples are being analyzed to determine genus and species of copepod prey. Results will be analyzed to determine prey selectivity.
Preparation of otoliths for microincrement analysis have been completed for the 1994 habitat samples. Age and hatch-date distributions have been determined and age-length relationships developed. Otoliths preparation from 1995 is ongoing and several stations are ready for age determination using OPTIMAS 5.2 image analysis software for measuring daily increments (see RESULTS).
Conducted laboratory experiments in Seward, Alaska on determining feeding and growth parameters of juvenile pollock needed for the bioenergetic model. The parameters examined included: 1) maximum consumption and growth versus temperature, 2) egestion and excretion rate, 3) maximum consumption and growth versus body size.
Consulted with UBC scientists who have worked on spatially-explicit bioenergetic models to come up with ways to adapt their techniques to our system. Work was completed on a bioenergetic model for age-0 pollock and was tested using available data from the Gulf of Alaska. This model can now be used for the Bering Sea juveniles as well.
Stomach contents from predators on age-0 pollock in the different habitats were examined and estimates of consumption rates for different years and habitats were made. The size of pollock consumed was compared to that available in the anchovy trawls at several locations to determine if size selectivity has taken place. Selectivity of predators was also examined for various fish and zooplankton prey.
III. SUMMARY OF RESULTS TO DATE
Using midwater trawl observations from September 1994 and 1995, we examined abundance, distribution, and size composition of age-0 walleye pollock in relation to the physical oceanography and biology of the structural front along a transect north of St. Paul Island. The frontal region occurred 12-20 km offshore during both years, but the thermocline was much shallower in 1995 ( 25 m) than in 1994 (45 m). The isothermal (< 8°C) inner domain had low chlorophyll, and relatively low, small (> 153 µm) and large (> 505 µm) zooplankton concentrations.
The highest chlorophyll and small zooplankton concentrations occurred at the front and were lower both inshore and farther offshore of the front. Large zooplankton (mainly euphausiids and cnidarians) were most abundant in the stratified offshore waters. By number, age-0 walleye pollock (Theragra chalcogramma) dominated the catches of midwater trawls (> 99 % and > 95 % in 1994 and 1995, respectively). Overall, however large medusae dominated the total biomass. Age-0 walleye pollock densities were the highest at the front or inshore of the front. Age-0 pollock densities averaged about three times higher in 1994 than in 1995. Age-0 pollock were significantly smaller in 1994 inshore and at the front (mean = 43.4 and 44.6 mm, respectively) than in 1995 (52.2 and 51.7 mm).
Dietary composition, feeding intensity, and condition index of age-0 walleye pollock, Theragra chalcogramma, were examined for variations related to time of day, location, size of predator, and year. Stomach contents of pollock collected at a hydrographic front near the Pribilof Islands during September 1994 through 1996 were compared with those from pollock collected on either side of the front. Diets were dominated in all regions by small zooplankton, mainly copepods, pteropods, euphausiids, and chaetognaths, but fish and some epibenthic crustaceans were also consumed. Copepods and pteropods dominated the diet in all years and areas by number but the diet was more mixed by weight with chaetognaths, euphausiids, and fish (smaller pollock) also being important. Copepods were more abundant in the diet during the day. No significant day/night differences in weight composition were noted. Stomach fullness was highly variable by year and habitat and no significant differences were observed. Stomach fullness peaked at around sunset for fish <50 mm and at night for the larger fish, implying that feeding chronology changed with ontogeny. Age-0 pollock condition factor (Fulton s K) varied from 0.45 to 1.20 (mean = 0.767 ± 0.09 (SD)). Year was not found to be an important factor in determining condition although the location with respect to the front was important.
Figure 1 shows the hatch-date distributions for the fish taken at ten stations in the 1994 sampling area. Hatching occurred from mid April through early July with a peak occurring in June. Allowing approximately one month from spawning to hatching, at water temperatures of 1-3°C, it can be presumed that these fish were spawned from late March through early June with a peak in spawning during May. This spawning period coincides with the spawning period for pollock on the southeastern continental shelf and slope of the Bering Sea. Age in days after hatching ranged from 76 to 152 and were predominantly less than 120. Some geographical distinction is noticeable in the age-length relationship, particularly among the larger fish. The possibility of variability in growth rates was investigated using growth among individual fish backcalculated using otolith microincrement measurements.
Even though sampling was limited at and inshore of the tidal fronts around the Pribilofs, it seems from the data that fish were smaller and younger at these locations than those sampled offshore of the fronts.
IV. PUBLICATIONS COMPLETED OR IN PROGRESS
Brodeur, R.D., M.T. Wilson, J.M. Napp, P.J. Stabeno, and S. Salo. 1997. Distribution of juvenile pollock relative to frontal structure near the Pribilof Islands. Proceedings Forage Fishes in Marine Ecosystems. Alaska Sea Grant AK-SG-97-01.
Brodeur, R.D. In press. Prey size and type selection by juvenile pollock. Env. Biol. Fishes 51.
Brodeur, R.D. MS. In situ observations of the association between juvenile fishes and scyphomedusae in the Bering Sea. (In revision for Marine Ecology Progress Series).
Ciannelli, L., R. Brodeur, and T. Buckley. MS. Energetics and food consumption of age-0 walleye pollock using a bioenergetics model. (Submitted to J. Fish. Biol.).
Napp, J.M., R.D. Brodeur, D. Demer, R. Hewitt, P.J. Stabeno, G.L. Hunt, Jr. and J.D. Schumacher. MS. Observations of nekton, zooplankton, and seabird distributions at tidally- generated shelf fronts in the eastern Bering Sea. (Submitted to Mar. Ecol. Prog. Ser.).
Brodeur, R.D., M.T. Wilson, and L. Ciannelli. MS. Spatial and temporal variability in feeding and condition of age-0 walleye pollock in frontal regions of the Bering Sea. (Submitted to ICES J. Mar. Sci.).
Lang, G.M., R.D. Brodeur, J.M. Napp, and R. Schabetsberger. MS. Variation in fish predation on juvenile walleye pollock relative to hydrographic structure near the Pribilof Islands, Alaska. (Submitted to ICES J. Mar. Sci.).
V. PRESENTATIONS
October 1996- Distribution of juvenile pollock relative to frontal structure near the Pribilof Islands, Bering Sea. PICES Annual Meeting, Nanaimo, BC. (R. Brodeur, M. Wilson, P. Stabeno, J. Napp, and J. Schumacher - Recipient of Best Paper Award).
November 1996- In situ observations of the association between juvenile fishes and scyphomedusae in the Bering Sea. International Symposium on the Role of Forage Fishes in Marine Ecosystems, Anchorage, AK (R. Brodeur, poster).
November 1996- Distribution of juvenile pollock relative to frontal structure near the Pribilof Islands, Bering Sea. International Symposium on the Role of Forage Fishes in Marine Ecosystems, Anchorage, AK (R. Brodeur, M. Wilson, P. Stabeno, J. Napp, and S. Salo).
January 1997- FOCI ecosystem-related juvenile pollock studies. PFMC Ecosystem Workshop, Seattle. (R. Brodeur).
September 1997 - Spatial and temporal variability in feeding and condition of age-0 walleye pollock in frontal regions of the Bering Sea. ICES Recruitment Symposium, Baltimore, MD (R. Brodeur).