Conceptual Model of Relationships Between Pollock Recruitment and Biophysical Correlates in the Southeast Bering Sea

Bernard A. Megrey and Vidar G. Wespestad

[This work in progress is subject to change. Material herein may not be used without the permission of the authors].

The working conceptual model for Bering Sea walleye pollock (Figure 1) is adapted from a similar model developed for Gulf of Alaska walleye pollock. The model can be termed a switch or survival gauntlet model in that it represents successive conditions or switches that must be realized for the fish to survive. Each switch has a conditional probability of being set for survival or mortality. The probability is subject to spatial and temporal variability. For example, a "hatch switch" could be dependent on water temperature that varies in space and time. Switches can act on individuals, cohorts, or populations. This model has a type of dynamic termed supply dependent, multiple-life-stage control (Bailey et al. 1996).

Figure 1. Working conceptual model of eastern Bering Sea walleye pollock survival at different life stages. Relative mortality, important environmental processes and the life stages that they affect are indicated.

In our working conceptual model, most mortality takes place in the juvenile life stage, although mortality during the larval life stage could be important in some years. Two factors have been suggested as important mechanisms in regulating eastern Bering Sea pollock year-class strength. These are predation, primarily cannibalism of juvenile pollock by adult pollock (Laevastu and Favorite 1988), and environmental factors (Quinn and Niebauer 1995). Predation on pollock is greatest in the first and second year of life, and cannibalism has been shown to be a significant source of predatory mortality (Livingston 1991). It has been suggested that the intensity of cannibalism is primarily a function of the degree of spatial separation of adults and juveniles (Wyllie-Echeverria 1995, Wespestad et al. in prep.). Environmental factors enter our model through their impact on spatial overlap. Wind drift influences the distribution of animals prior to the juvenile life stage. The strength of vertical stratification of the water column during the juvenile life stage likely is also important. Our hypothesis is that strong year classes result when planktonic stages are transported shoreward and away from adults by near surface currents in spring (warm years). In cold years, the associated winds reduce the affect of this mechanism, and juvenile utilization of inshore regions is more limited. This results in similar distribution patterns of adults and juveniles, potential for more cannibalism, hence a weak year class. Even when adult and juvenile life stages coincide, stratification of the water column can effectively separate adults from juveniles. This "switch" works in opposition to the wind-drift switch. Wind mixing (turbulence) can influence year-class strength by affecting the ability of first feeding larvae to feed successfully. Its influence is mainly restricted to the yolk sac larvae and first feeding life stages. Climatic factors, such as atmospheric circulation dynamics that determine frequency and trajectory of storms, wind direction and intensity, ice extent and water temperature, can affect all life stages of pollock.

The spawner-recruit relationship for eastern Bering Sea pollock (Figure 2) reflects a moderate density dependence between the spawning stocks and recruitment, with reduced survival of recruits at high levels of adult abundance (Wespestad and Quinn 1997). Figure 2 adds evidence supporting our working conceptual model. Cannibalism is presumed to be the mechanism underlying density dependence. The spawner-recruit relationship indicates that several points are well above and below the fitted relationship. It has been shown that most of the points above the line (i.e., 1978, 1982, 1989) are associated with warm years. Warm years are characterized by strong shoreward wind drift, subsequent high spatial separation between adults and juveniles, low rates of cannibalism, and good recruitment. In these years, density-dependent (cannibalism) mechanisms are not in effect. Those data points close to the line are associated with cold years (Fritz et al. 1993). Cold years are characterized by weak shoreward wind drift and subsequent low spatial separation between adults and juveniles, high rates of cannibalism, and poor to average recruitment. In cold years, the cold pool temperature tends to alter distributions of both adult and juveniles tending to enhance coincidence. Thus, in cold years, density-dependent (cannibalism) mechanisms are important. Niebauer and Quinn (1995) also found a correlation with temperature that they suggested results from variation in the intensity of the Aleutian Low. They also found that the best fit occurred with a one year lag, which suggested that environmental effects were exerted, not in the first few months of life, but rather at later juvenile stages.

Figure 2. Eastern Bering Sea pollock stock and recruitment relationship.

The current conceptual model focuses on cannibalism and the physical processes that enhance or deter cannibalism. Other biophysical processes important to pollock recruitment (i.e., ice and spring bloom dynamics) are under examination but not yet included in the conceptual model.

References

Bailey, K.M., R.D. Brodeur, and A.B. Hollowed. 1996. Cohort survival patterns of walleye pollock (Theragra chalcogramma) in Shelikof Strait, Alaska: A critical factor analysis. Fish. Oceanogr. 5 (Suppl. 1): 179-188.

Fritz, L.W., V.G. Wespestad, and J.S. Collie. 1993. Distribution and abundance trends of forage fishes in the Bering Sea and Gulf of Alaska. pp. 30-43. In: Is it food?: addressing marine mammal and sea bird declines. Workshop Summary, Alaska Sea Grant Rept. 93-01. Univ. Alaska, Fairbanks, AK.

Laevastu, T. and F. Favorite. 1988. Fishing and stock fluctuation. Fishing Books Ltd. Farnham, GB. 239 p.

Livingston, P. A. 1991. Groundfish food habits and predation on commercially important prey species in the eastern Bering Sea from 1884-1986. U. S. Dept. Commerce, NOAA Tech Memo. NMFS F/NWC-207.

Quinn, T.J., II. and H.J. Niebauer. 1995. Relation of eastern Bering Sea walleye pollock (Theragra chalcogramma) recruitment to environmental and oceanographic variables. pp. 497-507. In: Beamish, R.J. [ed], Climate Change and Northern Fish Populations, Can. Spec. Publ. Fish. Aquat. Sci. 121, 739p.

Wespestad, V.G., L.W. Fritz and J. Ingraham. In prep. Spatial variation in juvenile pollock distribution, its utilization as forage by adults, and year class survival.

Wespestad V.G. and T.J. Quinn. II. 1997. Importance of cannibalism in the population dynamics of walleye pollock.. In: Ecology of Juvenile Walleye Pollock, Theragra chalcogramma. NOAA Technical Report, NMFS 126.

Wyllie-Echeverria, T. 1995. Sea ice conditions and the distribution of walleye pollock in the Bering and Chuckchi Sea shelf. pp 87-95. In: Beamish, R.J. [ed], Climate Change and Northern Fish Populations, Can. Spec. Publ. Fish. Aquat. Sci. 121, 739p.