Figure 4. Biomass trends of the offshore pelagic consumer guild in the eastern Bering Sea from 1979-1993 (after Livingston et al., 1994). Adult pollock dominate. Birds and mammals are represented by the top dotted line.
Upper trophic level species of the SE Bering Sea can be divided into three fairly distinct species groups or trophic guilds based on characteristics of feeding (Livingston et al., 1994). The first guild consists of an outer shelf group of fish, mammals, and birds that consume small pelagic fish, primarily juvenile pollock, and euphausiids. The second guild is an inshore group of fish, crab and other epibenthic fauna that consume mainly benthic infauna. These two groups represent a biomass of about 8-10 million metric tons each. The third guild is a smaller (~1.2 million tons), more ubiquitous group, dominated by cod and skates that feed on crab and fish. Walleye pollock dominate the biomass of the outer shelf pelagic guild and the species diversity of the guild is very low (Figure 4). For the pelagic guild a biomass-weighted effective number of species, or species diversity index (Suchanek, 1994), was nearly constant (1.0 - 1.2) between 1979 and 1993. In contrast, the benthic guild had a species diversity near 5.1.
Some ecologists speculate that a low species diversity within a guild could result in less year to year stability of the guild (Wilson et al., 1991, 1994). The instability results from interannual variations in the dominant species of the guild without compensation from other guild members. Given that walleye pollock represent a dominant species in a guild with low diversity, one might expect that pollock are a nodal or focal species in the outer shelf feeding guild and would play a central role in determining the general health of the Bering Sea ecosystem (Springer 1992; Livingston, 1993). If this hypothesis is true, then research that elucidates processes that influence interannual to decadal scale changes in the production of walleye pollock in the Bering Sea (Figure 5) provide information to the status of the outer shelf guild of the Bering Sea as well. This lack of ecological stability based on adult pollock and their primary food sources, juvenile pollock and euphausiids, is a major issue in understanding the present Bering Sea ecosystem.
Production of juvenile pollock is influenced by upper trophic level predation and by the spatial and temporal distribution of secondary productivity. We believe that density-dependent predation and environmental factors have influenced pollock recruitment since the 1960s. Contrast the low recruitment years 1986, 1987, 1988 in Figure 6 with the high recruitment years 1982, 1984 and 1989; all occurred at historical highs for population biomass. It is clearly necessary to move away from simple single species spawner-recruit models to view fisheries in an ecological context and to consider the full complexity of this approach (Ludwig et. al., 1993). Both the predator and food environment are important. These co-factors interact in unclear ways. Spatial overlap or lack of overlap is a clear issue.
The dominance of pollock shown in Figure 4 suggests that predation by marine mammals and birds on juvenile pollock is overshadowed by cannibalism by older pollock. There is evidence that pollock year class survival may be enhanced by separation of juveniles from adults. About 64% of the above average 1982 year class was found in the middle shelf regime at age 1, whereas only 15% of the below average 1987 year class was located in the middle shelf (Figure 7). Both these year classes were produced from similar spawning stock sizes (Wespestad, 1994). Simulated drifter tracks started in the outer shelf region in April go to inner and middle shelf areas in 1982, but are retained in the outer shelf region in 1987 (Ingraham and Miyahara, 1988).
Changes in availability of juvenile pollock and other forage fish to upper trophic level predators may also be the result of variations in environmental conditions among various habitats in the eastern Bering Sea (Quinn and Niebauer, 1995). Temperature has a profound impact on time to hatch and growth rate of larval pollock, and influences the amount of energy available to the pelagic and benthic guilds. Mortality estimates from FOCI research suggest a decrease of larval abundance by a factor of twenty for a temperature decrease of 4 °C; growth rate for the prey of larval pollock is also temperature-dependent. In the middle domain (50-100 m water depth), a cold pool exists in the bottom layer as a remnant of previous year's ice cover. The heat content and horizontal extent# of this pool vary greatly each year with minimum temperature between -1.5 to 3.0 &176;C. Over the outer shelf (100-180 m water depth) the intrusion of warm slope waters limits extremes in temperature. Thus, while temperature variations could limit survival in the middle domain, the outer domain probably provides a more stable environment. Data suggests that cold waters resulted in the small 1976 year class (Bailey et al., 1986), and will similarly influence the 1995 year class. Analysis of water temperatures in the period after spawning indicate that the largest year-classes of pollock occurred during the first year of a warm period (Bulatov, 1989).
It is also possible that transport along and onto the shelf encourages juvenile pollock to migrate in a westerly direction passing through areas of particularly high primary, and presumably, secondary productivity; compare the location of age-0 juveniles (Figure 8) with that of age-1 (Figure 7). One high productivity area extends south from St. Matthews Island toward Cape Navarin (Sobolevski, et al., 1991). This region receives high concentrations of nutrients from the Bering Sea basin, in some cases probably via canyon upwelling. Juveniles that successfully transit the Bering Sea shelf westward can reach regions of high productivity.
We are now in a position to make the following assertions for a Southeast Bering Sea Regional Ecosystem Study:
RESOURCES: This proposal requests $500K for a start-up year in FY1996 and $1.5 M per year for five years beginning in Fiscal Year 1997. We also will request 50 days of NOAA Ship Miller Freeman and 30 days Class I vessel time per year.
SCIENTIFIC APPROACH: The approach is interdisciplinary and balances time-series measurements, process studies and models, phased over the life of the project. Modification of the emphases and details suggested in these approaches will depend on the outcome of the workshop and will be presented in the implementation plan.
The following sections outline three subprojects that comprise SEBSCC: 1) characterization of five biophysical domains, 2) studies of juvenile pollock in the ecosystem, and 3) a physical/IBM model and a MSVPA model of the shelf and slope.
The transport of new nutrients needed to support the lower trophic level productivity, and ultimately the juvenile pollock growth and survival, must flow from the deep basin onto the shelf. Temporal and spatial variations in such transport are poorly known, as are primary and secondary productivity during the late summer and early fall, at the time juveniles are present in the Bering Sea. The Pervenets Canyon along the central Bering Sea slope may be a particularly good source of nutrients as it differs from the other canyons along the shelf break by being wide with gently sloping walls. Most of the work done previously, particularly during the PROBES study, emphasized the early spring dynamics when only pollock larvae are present. Juvenile pollock in late summer feed on larger zooplankton and micronekton (Merati and Brodeur, 1995). Juvenile pollock undergo a diel vertical migration pattern that apparently follows that of euphausiids, their principal prey (Bailey, 1989). This pattern places them in the more productive surface waters at night when the risk of predation to visually-feeding marine birds and mammals is reduced.
We propose to contrast the temporal and spatial variability of the biological, nutrient and physical conditions between habitats. We will ask questions such as: What are the spatial distributions of larval, juvenile and adult pollock? What are the influences of sea ice and its conditioning of bottom temperature? What controls the variability of source waters in slope/shelf nutrient flux? What maintains the different stratification between the habitats?
Concept for a Field Program - We will sample the habitats in Figure 1. Observations will be made between April (ice permitting) and September. Sampling (Table 1) will include annual distribution surveys of spawning adults, larvae, and juvenile pollock; determination of primary and secondary productivity; assessment of prey and predators; measurement of the physical environment and nutrients; and study of important frontal regions which lie between habitats, to determine scales of variability. We will monitor seasonal predator food habits and energetics with index sites at the Pribilof Islands, outer shelf domains, and the middle shelf. From 1996 through 2001 we propose to maintain several moored biophysical platforms. Each moored platform will record meteorological (wind, temperature, irradiance, pressure), oceanographic (current, salinity, temperature), and biological (chlorophyll) data. A subset of sites will have ADCPs to measure ocean currents and indicate zooplankton biomass. Both shipboard surveys and platforms will continue time series begun in 1995 as part of BS FOCI. Other chemical-water column measurements will include 13C and 13N isotope content of phytoplankton, zooplankton, fishes and other higher animals. The 13C and 13N of zooplankton collected over deep basin pelagic waters appear to be lower than those collected from on shelf and continental slope waters (D. Schell, pers. comm.) The elevated ratios on the shelf may be the result of a vigorous supply of deep nutrients onto shelf waters via upwelling, and this may offer a useful signal to determine the importance of deep basin nutrients to shelf productivity.
Mortality estimates are used to examine the role of environmental conditions on larval survival, to construct life history models, and to evaluate hypotheses concerning the relationship of larval size and survival rates. We believe that combining otolith (Bailey and Macklin, 1994) methods to determine cohorts, with Lagrangian methods of marking a patch of larvae and monitoring changes in abundance as it drifts, offers an accurate estimate of mortality (Talbot, 1977; Yoklavich and Bailey, 1990; Hill, 1991). Field results indicate that retention mechanisms operate to maintain larval patches (Hinckley et al., 1993).
We hypothesize that the unique physical and biological conditions associated with the frontal regions around and to the northwest of the Pribilof Islands provide a rich nursery habitat for juvenile pollock. Further, we hypothesize the importance of intermittent advection of nutrient-reach deep Bering basin slope water onto the shelf leading to intense new summer production. To test this, we will compare the abundance, size composition, growth, and condition of juvenile pollock at these fronts compared with those on either side of the fronts.
A factor in the survival of age-0 pollock in the Bering Sea may be the high abundance of large medusae in the summer. There is some evidence that the largest medusae are capable of feeding on small age-0 pollock (Hamner, 1983). At the very least, the high biomass and prodigious feeding capability of these medusae make them potential competitors with juvenile pollock and other small fishes for the available food. There is also some evidence that juvenile pollock may derive benefit by associating commensally with these abundant medusae, obtaining shelter from predators and possibly food from these hosts (Van Hyning and Cooney 1974, Hamner 1983).
Research Activities
Individual Based Models (IBM) have been successful in the OPEN, Georges Bank and Shelikof Strait FOCI programs (Werner et al., 1994; Hinckley et al., 1995; Hermann and Stabeno, 1995). We propose including lower trophic level interaction (NPZ). The IBM follows the trajectories through space of individual fish using the hydrodynamic model flow fields. The IBM is a probabilistic and mechanistic model which includes development, behavior, feeding, bioenergetics, growth and mortality for each life stage. Processes are driven by physical factors (temperature, salinity and turbulence) derived from the hydrodynamic model, and by prey levels derived from the NPZ model. The addition of the NPZ model will help address issues of match/mismatch of larval feeding to the timing of the spring phytoplankton bloom. Predation pressure exerted by other species will be incorporated into the IBM. Circulation of the Bering Sea shelf and slope should be calculated using an eddy resolving, free surface, hydrostatic primitive equation model which accurately resolves mixed layers and shelf break processes. Model hindcasts can include assimilation of data from cruises, the biophysical platforms and drifters.
We will ascertain the most important parameters that govern the year-to-year variability of food and predation relative to physical processes and the location of shelf fronts. We intend to establish links between success of the juvenile pollock, interannual variability, and spatial and temporal shifts in forcing. Combined observations and simulations will suggest which pollock spawning sites yield lowest mortality for present and projected physical forcing. Sensitivity analyses will yield insights into the fundamental predictability of pollock dynamics in this region.
We will use the output of the IBM/trophodynamic model to parameterize a spatially explicit model of upper-trophic level predators and their predation on juvenile pollock and alternate sources of prey. This second model, known as MSVPA (multispecies virtual population analysis) will include pollock and major predators of pollock and will be used to address ecological stability issues and assess the influence of various management schemes on long-term species abundances. Questions that we can ask are: What is the importance of juvenile pollock versus euphausiids in the diet of the pelagic guild based on different food switching scenarios? Can we quantify predation pressure on juvenile pollock? What is the stability of the present ecosystem? We will make use of the facilities at the University of Alaska Arctic Region Supercomputer Center. As the causal mechanisms are determined as a basis for survival, statistical models (Megrey et al., 1995) will be used to assess the robustness of the SEBSCC indices to biological and physical environmental fluctuations.