Physical oceanography, food chains, and fish stocks: a review

Abstract

There are many examples in the literature of correlations between changes in physical oceanographic factors and changes in fish stocks. Commonly, the correlations hold for a few years, then break down. This does not necessarily mean that a correlation was invalid, but calls for a better understanding of the mechanism of interaction in all its complexity before we can understand correlations that change with time. In this review attention is focused on biological production processes as links between the physical phenomena and the fish stocks. There are two fairly distinct food chains in the plankton, one based on diatoms and the other based on bacteria and flagellates. Diatoms are consumed by mesozooplankton or by benthos (if they sink), and these organisms form the food of most commercially important fish stocks. Bacteria and flagellates are consumed by microzooplankton and enter a complex food web that is inefficient in supporting fish production. There is a pattern of events in the water column that favours diatom production, and hence fish production. It consists of a period of vertical mixing followed by a period of stratification. It was first described in connection with the spring bloom in temperate waters, but it is also found, for example, in estuaries, coastal upwelling systems, and tidal fronts. The most important forces causing vertical mixing are wind and tidal currents. Variations in wind strength from year to year provide plausible explanations for variations in fish productivity. Stratification is the result of either solar heating or freshwater input. Many changes in fish stocks can be seen to result from changing river runoff or influx of a layer of warm water. In recent years evidence has been accumulating for synchronous changes in widely separated stocks. Sardines in the Californian and the Peruvian upwelling systems have varied synchronously with the Far Eastern stocks. Three species of salmon in the north Pacific have varied synchronously over the last 60–70 years. Periods of El Niño, when Peruvian sardine catches were low, corresponded to periods of good catches of jack mackerel off Tasmania. In many cases, the mechanisms of such correlations are poorly understood. Yet it seems almost certain that global patterns of atmosphere–ocean interaction are in some way responsible. Stocks have varied synchronously in spite of widely different management patterns, and it looks as if physical factors, operating through marine food webs, are often the dominant forces for change in the fish stocks.