Seasonal sea ice, the cold pool and gadid distribution on the Bering Sea shelf

Abstract

The variability in winter seasonal sea ice cover, its impact on the summer hydrography of the Bering Sea shelf, and the consequences to the demersal distributional patterns of walleye pollock (Theragra chalcogramma) and Arctic cod (Boreogadus saida) were investigated. Winter ice conditions acquired by remote sensing techniques were compared to the following summer's hydrographic conditions and fish distributions acquired from fisheries surveys between 1972-1993. Linkages among atmospheric, oceanic, and biological interactions occurred on two scales, an interannual mode of warm or cold conditions and multi-annual regimes of warm, cold or mixed conditions. The southernmost extent of sea ice as measured along meridian 169$\sp\circ$W (P$\sb{\rm S}$) can be used to identify warm or cold conditions on the shelf. Warm conditions occurred when P$\sb{\rm S}$ extended southward, between 60$\sp\circ$-$57\sp\circ30\sp\prime$N, bottom temperatures were 3.8$\sp\circ$-4.6$\sp\circ$C and the subsurface cold pool of water extended eastward, between meridians 170$\sp\circ$-166$\sp\circ$W. Cold conditions occurred when P$\sb{\rm S}$ extended southward, between $57\sp\circ30\sp\prime$-56$\sp\circ$N, bottom temperatures were 1.2$\sp\circ$-3.0$\sp\circ$C and the subsurface cold pool of water extended eastward, between meridians 163$\sp\circ$-158$\sp\circ$W. During the 20 year time series of climatic conditions, three regimes occurred: a cold regime prevailed from 1972-1977, a warm regime from 1978-1984, and a mixed regime from 1985-1991. Age-1 and age-2 and older walleye pollock were primarily in the outer domain during cold conditions and in the middle and inner domain during warmer conditions. Arctic cod were present during cold conditions. Shifts in distribution of these species have ecosystem-wide consequences and can occur on either interannual scales or on the time-scale of regime shifts. Changes in the level of piscivorous predation on age-1 pollock, including cannibalism, occurred on the annual scale while prey species of seabirds and marine mammals fluctuated on the regime scale. Warm or cold summertime conditions were predictable from the previous winter's ice extent, which provides basic information on the level of environmental variability that affects biological systems and can be utilized in modeling this system. The results predict that older pollock will be concentrated in the outer domain following winters with extensive ice, information which could be useful to the fishing industry.