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Climate Variability & Marine Fisheries

Reduced Abundance of Zooplankton and Temperate Fish Off Southern California During Two Decades of Warming

Ocean productivity off southern California has declined during the last 25 years. This is the period of increased sea surface temperatures along the US west Coast following the 1976 North Pacific regime shift. It has been observed on several trophic levels. Zooplankton levels have dropped by 80% from the 1950-70 level
to the early 1990s, a period with an average temperature increase of only 1.4°C in the sea surface temperature. Researchers believe this decline in zooplankton may be the result of increased warming of the surface layer that reduces the nutrient enrichment of this layer by reducing upwelling from below the thermocline. Lower nutrients in the surface limits the growth of plant plankton which are the food for zooplankton. Zooplankton such as copepods and krill are the principal source of food for many small pelagic fish.

The abundance of many nearshore species of fish off southern California has declined since the temperature change of the mid-1970s. There has been a decline in the proportion of cooler water species and a decline in the abundance of 38 species common to the southern California coast. This overall decline is not limited to heavily fished species and appears correlated to the decline in zooplankton abundance throughout the area. Similarly, abundance of seabirds, which rely upon the fish as prey, have also declined. Species commonly found in warmer water became more abundant from 1976 to 1987, especially during the 1982-83 El Niño, then declined through 1992 despite continued warm temperatures.

Mean Proportion of Species
Mean Proportion of Species
Click on image to enlarge

1) Roemmich, D. and J. McGowan. 1995. Climatic Warming and the Decline of Zooplankton in the California Current. Science. 267:1324-1326.
2) Holbrook, S., R. Schmitt and J. Stephens, Jr.. 1997. Changes In An Assemblage of Temperate Reef Fishes Associated With A Climate Shift.
Ecol. Applic. 7(4):1299-1310.
3) Veit, R.R., McGowan, J.A., Ainley, D.G., Wahls, T.R., and Pyle, P. 1997. Apex Marine Predator Declines Ninety Percent In Association With
Changing Oceanic Climate. Global Change Biology. 3: 23-28.

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