Congruent shifts in sand eel abundance in western and eastern North Atlantic ecosystems

Abstract

It has been suggested, on the basis of model simulations and slowly accumulating empirical data, that changes in the structure of marine ecosystems may be caused as much by changes in the trophic levels as by environmental factors1,2. Support for this is found in recently observed shifts in species abundance of North Sea fish stocks, where large catches in the 1970s of small, fast-growing, opportunistic plankton-feeding fishes—sprat, sand eel and Norway pout—have been accounted for as the result of the replacement by these species of depleted herring and mackerel stocks3,4. This suggestion is difficult to confirm, as knowledge of the abundance of these ‘opportunistic’ species since the mid–1960s is largely limited to catch data for the North Sea5. The only sand eel report independent of commercial fisheries catch data is for larvae collected in 1948–68, just before the sharp decline in mackerel and herring stocks6. The area surveyed did not, however, cover all the major population centres in the North Sea. In support of replacement, we describe here evidence from the north–west Atlantic which indicates that population explosions of small, fast–growing sand eel can coincide with depletions of larger tertiary predators, including herring and mackerel in a continental shelf ecosystem. Sand eels serve as an important link in marine food chains, preying on secondary producers and constituting an important food of higher trophic level fish and marine mammals7,8.