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ANNUAL REPORT: 1998

TITLE:

PRINCIPAL INVESTIGATORS:

Richard D. Brodeur, Jeffrey M. Napp, and Miriam J. Doyle

Matthew T. Wilson, James D. Schumacher, Lorenzo Ciannelli, and Robert C. Francis

Our fundamental hypothesis was that the unique physical and biological conditions associated with the frontal regions around the Pribilof Islands provide an exceptionally good nursery habitat for age-0 pollock in the Bering Sea. To test this, we compared the abundance, size composition, growth, and condition of juvenile pollock at these fronts, on either side of the front, and at a control station near the Middle Shelf Front on the Bering Sea shelf away from the islands. The following key questions address aspects of this hypothesis:
 

1. Is the availability of food resources for juvenile pollock higher in these frontal regions relative to other habitats?
2. How are the small-scale (10-100 m) distributions of pollock and their prey related?
3. Are pollock selectively feeding upon any particular prey type or size or are they randomly feeding on whatever prey are available?
4. Are the growth rates, growth potential, and condition of juvenile pollock better in the fronts than outside of them?

SCIENTIFIC ACCOMPLISHMENTS:

In the fall of 1997, we conducted a successful cruise aboard the R/V Miller Freeman sampling around the Pribilof Islands for 10 days. In addition to our normal sampling, we used a Remotely Operated Vehicle to examine in situ the distribution and habitat dependencies of juvenile pollock, their predators, and prey. We also sampled the southeastern Outer and Middle Shelf Domains, including the Pribilof Islands during a summer research cruise aboard the Hokkaido University research vessel Oshoro Maru and are planning for a 12 day cruise in September aboard the chartered Russian research vessel Professor Kaganovsky.

A poster on the results of our research was presented at the Oceanography Society meeting in Paris in June 1998 which outlines the progress made in several of our studies relating frontal structure to juvenile distribution, growth and ecology. This poster can be viewed at the following URL: http://www.pmel.noaa.gov/programs/review98/fronts.jpg

The following are some results of analyses by the major topic areas of our research in 1997 and 1998.

Physical Oceanography- Temperature profiles along Transect A show the inner shelf well-mixed with the water column nearly isothermal (Fig. 1). The offshore part of the transects clearly show the stratified 2-layer system characteristic of the Middle Shelf Domain (MSD). The upper mixed layer was much shallower in 1995 than in 1994 and 1996 and the density differences between the layers were stronger. The deeper upper layer in the MSD during 1994 may also be a result of storm activity.

Nutrients and Chlorophyll- Nitrogenous nutrients (nitrate plus nitrite) were >1 mM below the thermocline in the stratified waters and throughout the water column in the transition and inner front area in all years (Fig. 2). Maximum chlorophyll-a concentrations (> 4 mg l^-1) were found in surface waters just offshore of the region where nutrients were > 1 mM. Moderate chlorophyll concentrations were measured in the stratified waters where nitrogenous nutrients were low.

Spatial Variability in Prey Concentration and Composition - Zooplankton displacement volumes for Transect A from the MOCNESS sampling were integrated throughout the water column and summarized each year for the three habitats. The patterns were consistent in 1994 and 1995 with the highest volumes found offshore and volumes at the front and inshore were much lower (Fig. 3). The high volume in offshore samples was due mainly to high abundances of copepods. The front had the highest volumes in 1996, but some of this difference may be due to using a smaller mesh size that year. This last year, we continued our collections of mesozooplankton in and around the Pribilof Island structural fronts in support of other projects in this study and in support of other SEBSCC studies. We have recently received the fall 1997 zooplankton data and are in the process of adding them to our relational database. These zooplankton data are an integral part of many of the recent SEBSCC manuscripts regarding the Pribiliof Island structural fronts.

Age-0 Pollock Density and Size Distribution - During mid-September of 1994 through 1996, sampling was conducted on Transect A in the vicinity of St. Paul. Our intent was to compare and contrast three habitats: at the front, inshore, and offshore of the front. Larger age-0 pollock and other nekton were collected using a 100 m² anchovy trawl containing 3 mm mesh in the codend. The overall catch composition of the anchovy trawl was numerically dominated (> 95% of total fish catch) by age-0 pollock in all three habitats. Age-0 pollock densities in the anchovy trawl were higher at the front in 1994 and 1996, but showed slightly higher densities inshore of the front in 1995 (Fig. 4), although the differences were not significant. The lowest densities were found offshore of the front, especially in 1994. In 1994, age-0 pollock were significantly larger in the offshore habitat than in the inshore and front habitats (P = 0.023 and P = 0.009), but no differences were found between the latter two habitats (P = 1.00). In contrast, there were no significant size differences among the habitats in 1995 and 1996.

Age and growth of juvenile pollock in relation to fronts - Analysis of otoliths for hatch-date distribution and daily increment structure has been carried out on age-0 pollock sampled during the fall of 1994 and 1995. The 1994 data indicate that hatching occurred from mid-April through early July with a peak occurring in early to mid June. A similar range of hatch dates was calculated for 1995 but the peak in hatching appeared slightly earlier, from the end of May through early June. Growth rates among the sampled populations have been estimated for 1994 and 1995 using regression analysis of age-length data. Some geographical distinction is noticeable in the age-length relationship, particularly during 1995 when a slight increase in size at age was apparent for fish sampled offshore relative to those at and inshore of the front (Fig. 5). The overall growth rate seems slightly higher in 1995 than in 1994. The possibility of variability in growth rates is being investigated further by back-calculating growth among individual fish using otolith microincrement measurements and an otolith length/fish length relationship.

Juvenile Pollock Feeding and Condition in Relation to Fronts - Dietary composition, feeding intensity, and condition index of age-0 walleye pollock Theragra chalcogramma, were examined for variations related to time of day, habitat, 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 habitats 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 (Fig. 6). 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)) overall. Year was not found to be an important factor in determining condition although the location with respect to the front was important (Fig. 7).

Spatial Variability in Nutritional Quality of Prey - During fall 1997 we began a pilot program to determine if there were differences among regions in the nutritional quality of a common prey item used by age-0 pollock, C5 copepodites of Calanus marshallae. The purpose of the project is to determine whether or not prey concentration is the only relevant variable for spatially explicit bioenergetic models of age-0 pollock or alternatively if regional differences in prey nutritional quality must be considered in addition to differences in prey density. Our first set of samples was collected around the Pribilof Islands on Transects A, B, and C. Copepodites of C. marshallae were significantly lighter (lower dry weights) on the offshore transect, Line B, than on Lines A and C. Concurrent samples for CHN and lipid analysis were also taken. The samples for CHN were analyzed and the data await statistical analysis; the lipid samples have yet to be analyzed. Collections of samples for dry weight and CHN content were repeated in the summer of 1998 across a much broader region of the shelf and will be analyzed this fall.

Bioenergetics Modeling - A bioenergetics model has been developed for juvenile stage of walleye pollock. The bioenergetics model includes physiological characteristics of juvenile pollock as well as environmental features of the study area. Some of the physiological features of juvenile pollock were estimated through laboratory experimentation during summer 1997, in Seward, Alaska. The experiments were carried out in cooperation with Dr. A.J. Paul from the University of Alaska, Fairbanks. Size and temperature dependencies of growth, consumption, egestion and excretion were studied for juvenile walleye pollock. Physiological parameters used in the model have been derived also from a synthesis of literature data, and tested by doing a sensitivity analysis. Environmental characteristics of the study area include temperature profiles and zooplankton distribution and density. Temperature profiles have been determined from CTD casts. Zooplankton distribution and density have been derived from the acoustic surveys in collaboration with Dr. Gordon Swartzman. Some preliminary results from the zooplankton analysis have been included in the bioenergetics model to produce growth potential map for juvenile pollock along Line A for the years 1994 and 1995. Growth potential areas are then compared with acoustically determined density and distribution of juvenile pollock, to study correlation between growth potential and fish distribution (Fig. 8). Growth potential results showed that on Line A, in 1994 there was a higher potential for growth than in 1995, due mainly to higher zooplankton biomass and a deeper thermocline. Geographical differences also appear along the transect line, with the inshore region showing the lowest growth potential.

To investigate the effect of both food and predation on the distribution and dynamics of juvenile pollock, a spatially and temporally explicit model is being prepared in collaboration with Dr. Peter Rand, an expert in the field of spatially-explicit bioenergetic models. Besides food and temperature, the model includes predation fields, which in turn are derived from bottom trawl collections and fish stomach contents, bird counts and diet information, and marine mammals distribution and predatory behavior.

Recent Anomalies in the Bering Sea Ecosystem - The Bering Sea Ecosystem appears to be undergoing major changes in key environmental factors (e.g. summer wind mixing, irradiance, spring storms). Concomitant with these changes have been major failures of the Bristol Bay sockeye salmon fishery, consecutive large-scale summer blooms of coccolithophores, new reports of whales feeding on the middle shelf domain, and high mortalities of some planktivorous seabirds (Short-tailed shearwaters). Our Habitat project allowed us to collect observations that will help determine the cause and effect of these large-scale ecosystem perturbations. Of paramount importance to SEBSCC is the effect these anomalies will have on the future recruitment of pollock to the fishery.

APPLICATIONS:

In addition to the publications and presentations listed in Appendix I, we initiated useful collaborations with several other SEBSCC studies including the Monitoring (Schumacher et al.), Modeling (Hermann et al.) and Acoustic (Swartzman et al.) projects. We also had close ties to the NSF-funded Inner Shelf Study and with Japanese researchers at Hokkaido University and Tohoku Fisheries Research Laboratory. We also received outside funding from NOAA's National Undersea Research Program to have a Remotely Operated Vehicle during our field work.

STEPS TO COMPLETION:

1. Analyze interannual variation in zooplankton and pollock biomass from Methot collections around the Pribilofs from 1994-1997 - submit paper to journal early in 1999.
2. Continue analysis of diel variation in zooplankton, pollock diets and feeding selectivity at the frontal region during 1996 - submit paper to ICES Journal of Marine Science in spring 1999.
3. Complete 1996 otolith analysis from Front A - submit paper on interannual variation in hatch-date distribution and growth of juvenile pollock in winter.
4. Revise and submit fronts acoustic and zooplankton manuscript by Napp et al. to Marine Ecology Progress Series by the end of the year.
5. Synthesize results of Pribilof Island zooplankton surveys 1994 to 1997 and produce a manuscript.
6. Complete sample and data analysis of prey nutritional quality project and produce an independent note.
7. Work with SEBSCC and NSF Inner Front Groups to publish Science manuscript and a series of 4 or 5 articles in Fisheries Oceanography regarding ecosystem change in the Bering Sea and the coccolithophore bloom.
8. Finish manuscript on laboratory derived bioenergetic parameters and submit to Journal of Fish Biology.
9. In order to complete the bioenergetic component of the project, more modeling effort is needed, together with refinement of field estimates of predator diet, distribution and density.
10. Use spatially-explicit model to examine areas of highest growth potential and test hypotheses about different migration or life history strategies.

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, Bering Sea. Proc. Int. Symp. on the Role of Forage Fishes in Marine Ecosystems, Alaska Sea Grant AK-97-01. p. 573-589.

Brodeur, R.D. 1998. Prey selection by age-0 walleye pollock, Theragra chalcogramma, in nearshore waters of the Gulf of Alaska. Env. Biol. Fishes 51:175-186.

Ciannelli, L., R.D. Brodeur, and T.W. Buckley. 1998. Development and application of a bioenergetics model for juvenile walleye pollock. J. Fish Biol. 52: 879-898.

Brodeur, R.D. 1998. In situ observations of the association between juvenile fishes and scyphomedusae in the Bering Sea. Mar. Ecol. Prog. Ser. 163:11-20.

Brodeur, R.D., M.T. Wilson, G.E. Walters, and I.V. Melnikov. 1998. Forage fishes in the Bering Sea: Distribution, species associations, and biomass trends. In: Loughlin, T.R. and K. Ohtani (eds.) The Bering Sea: Physical, Chemical, and Biological Dynamics. Univ. of Alaska Sea Grant (in press).

Stabeno, P.J., J. D. Schumacher, S. A. Salo, M. Flint and G. L. Hunt, Jr. 1998. The Physical environment around the Pribilof Islands. In: Loughlin, T.R. and K. Ohtani (eds.) The Bering Sea: Physical, Chemical, and Biological Dynamics. Univ. of Alaska Sea Grant (in press).

Vance, T.C., C.T. Baier, R.D. Brodeur, K.O. Coyle, M.B. Decker, G.L. Hunt Jr., J.M. Napp, J.D. Schumacher, P.J. Stabeno, D. Stockwell, C.T. Tynan, T.E. Whitledge, T. Wyllie-Echeverria, and S. Zeeman. 1998. Aquamarine waters recorded for the first time in the Eastern Bering Sea. EOS Trans. Amer. Geophys. Union 79: 121-126.

Brodeur, R.D., M.T. Wilson, and L. Ciannelli. In press. Spatial and temporal variability in feeding and condition of age-0 walleye pollock in frontal regions of the Bering Sea. ICES J. Mar. Sci.

Lang, G.M., R.D. Brodeur, J.M. Napp, and R. Schabetsberger. In press. Variation in groundfish predation on juvenile walleye pollock relative to hydrographic structure near the Pribilof Islands, Alaska. ICES J. Mar. Sci.

Swartzman, G., R.D. Brodeur, J.M. Napp, D. Walsh, R. Hewitt, D. Demer, G. Hunt, and E. Logerwell. In press. Relating predator and prey spatial distributions in the Bering Sea using acoustic backscatter data. Can. J. Fish. Aquat. Sci.

Brodeur, R.D., M. Doyle, J.M. Napp, P.J. Stabeno, J.D. Schumacher, and M.T. Wilson. 1998. Fronts and fish: Interannual and regional differences in frontal structure and effects on pollock and their prey (Oceanogr. In press).

Brodeur, R.D., and M.S. Busby. 1998. Occurrence of an Atlantic salmon Salmo salar in the Bering Sea. Alaska Fish. Res. Bull. 5(1).

Sugisaki, H., R.D. Brodeur, J.M. Napp. In press. Summer distribution and abundance of macrozooplankton in the western Gulf of Alaska and southeast Bering Sea. Mem. Fac. Fish. Hokkaido Univ.

Napp, J.M., R.D. Brodeur, D. Demer, R. Hewitt, P.J. Stabeno, G.L. Hunt, and J.D. Schumacher. MS. Observations of nekton, zooplankton, and seabirds distributions at tidally-generated shelf fronts in the eastern Bering Sea. (Submitted to Mar. Eco l. Prog. Ser.)

Swartzman, G., R.D. Brodeur, J.M. Napp, G. Hunt, D. Demer, and R. Hewitt. MS. Spatial proximity of age-0 walleye pollock to their plankton prey near the Pribilof Islands, Bering Sea, Alaska. (To be submitted to ICES Journal of Marine Science).

Schabetsberger, R., R.D. Brodeur, L. Ciannelli, J.M. Napp, and G.L. Swartzman. MS. Diel vertical migration and interaction of zooplankton and micronekton at a frontal region near the Pribilof Islands, Bering Sea. (To be submitted to ICES Journal of Marine Science).

October 1996- Distribution of juvenile pollock relative to frontal structure near the Pribilof Islands, Bering Sea. PICES Annual Meeting, Nanaimo, BC. (R. Brodeur with 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 with M. Wilson, P. Stabeno, J. Napp, and S. Salo).

November 1996- Prey selection and food consumption by age-0 walleye pollock in nearshore waters of the Gulf of Alaska. International Symposium on the Role of Forage Fishes in Marine Ecosystems, Anchorage, AK (poster, R. Brodeur with L. Ciannelli).

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 with M. Wilson and L. Ciannelli).

September 1997- Variation in groundfish predation on juvenile walleye pollock relative to hydrographic structure near the Pribilof Islands, Alaska. ICES Recruitment Symposium, Baltimore, MD (G. Lang with R. Brodeur, J. Napp, and R. Schabetsberger).

February 1998- In situ observations of the habitat dependencies of juvenile fishes in the Bering Sea. Western Groundfish Conference, Monterey, CA (poster, R. Brodeur).

February 1998- The 1997 Eastern Bering Sea shelf-wide coccolithophorid bloom: Ecosystem observations and hypotheses. Ocean Sciences Meeting (AGU/ASLO), San Diego, CA (poster, J. Napp with C. Baier, R. Brodeur, J Cullen, R. Davis, M. Decker, J. Goering, C. Mills, J. Schumacher, S. Smith, P. Stabeno, T. Vance, and T. Whitledge).

May 1998- Fronts and fish: Interannual and regional differences in frontal structure and effects on pollock and their prey. Oceanography Society Meeting, Paris, France (poster, R. Brodeur with M. Doyle, J.M. Napp, P.J. Stabeno, J.D. Schumacher, and M.T. Wilson).

July 1998- Juvenile pollock distribution, growth and ecology in the Bering Sea. International Workshop on Ecosystems of the North Pacific Ocean, Seattle, WA. (R.D. Brodeur, M.S. Busby, J.M. Napp, and M.T. Wilson).

July 1998- Biophysical factors that affect pollock survival and prey production over the southeastern Bering Sea Shelf. International Workshop on Ecosystems of the North Pacific Ocean, Seattle, WA. (J.M. Napp, C. Baier, A.J. Kendall, and J.D. Schumacher).

August 1998- The role of pre-recruit walleye pollock in the Bering Sea and North Pacific Ecosystems. International Marine Science Symposium on Ecosystem Dynamics, Hakodate, Japan (R. Brodeur, Invited keynote presentation).
 
 

Fig. 1. Comparison of temperature sections on Transect A during 19 September 1994, 10-11 September 1995, and 11-12 September.
concentrations along Transect A.
 
 

Fig. 2. Distribution of nitrogenous nutrients (M NO₂ and NO₃ combined) and chlorophyll-a (g l^-1) (no data available for 1994)
 
 
 

Fig. 3. Displacement volumes by habitat and year for zooplankton collected in MOCNESS tows integrated over the water column. Mesh size is 505 m in 1994 and 1995 and 333 m in 1996.
 
 
 

Fig. 4. Density and size distribution of age-0 pollock by year and habitat.
 
 

Fig. 5. Growth curves for age-0 pollock showing size at age for collections from different positions relative to the front.
 

Fig. 6. Stomach contents of age-0 pollock by year and location with respect to the frontal region.
 

Fig. 7. Comparison of the mean ( 1 standard deviation) condition factor (W/L³) for each year and habitat and the overall means for each year (dashed lines).
 
 

Fig. 8. Distribution of zooplankton (upper panel), juvenile pollock (center panel), fish growth potential (lower panel), and temperature (isotherms in upper an center panels) for Line A, 1994. Zooplankton and juvenile pollock distributions are acoustically determined, growth potential is derived from a spatially explicit bioenergetics simulation.