Although declining oxygen concentration has been reported for the oxygen minimum zones (OMZs) of the tropical oceans and the North Pacific Ocean, consistent with model predictions of the effects of global warming, its ecological impacts are poorly understood. We report the apparent impact of declining oxygen on midwater fishes within the OMZ of the southern California Current (CC). Principal component analysis of the California Cooperative Oceanic Fisheries Investigations (CalCOFI) ichthyoplankton time series from 1951 to 2008 indicates that the dominant temporal pattern (principal component 1 [PC1]) represents the marked decline of the region's mesopelagic fishes during periods of reduced oxygen. Of the 27 taxa with loadings > 0.5 on PC1, 24 were mesopelagic. PC1 was strongly correlated with intermediate-water oxygen concentrations (r = 0.75, p < 0.05), which were about 20% lower in the past decade and the 1950s than in the period from 1970 to 1995. The abundance of mesopelagic fishes represented by PC1 was reduced, on average, by 63% between periods of high and low oxygen concentrations. We hypothesize that the underlying mechanism is the shoaling of the hypoxic boundary layer during periods of reduced oxygen, which renders the mesopelagic fauna more vulnerable to visually orienting predators. The mesopelagic fish fauna provides a vital trophodynamic link between the marine plankton and many higher predators. The decline of deepwater fish populations has profound implications for commercial fisheries, marine food webs and marine conservation: climate models predict a 20 to 40% decline in global deepwater oxygen concentrations over the coming century.KEY WORDS: Oxygen · Mesopelagic fishes · Oxygen-minimum zone · Climate change · California CurrentResale or republication not permitted without written consent of the publisher
We examined climatic effects on the geographic distribution and abundance of 34 dominant oceanic fishes in the southern California region using larval fish data collected from the 50-year long California Cooperative Oceanic Fisheries Investigations (CalCOFI) surveys. The oceanic species responses to environmental changes in their geographic distributions were not very pronounced, perhaps because they lived in the deep layer where temperature change was relatively small or because the environmental variation of the CalCOFI region is not strong enough (with an average temperature gradient of the upper 100 m around 91 km 1C À1 ). Among the 34 taxa, 16 showed a significant distributional shift (median latitude or boundaries) in relation to environmental variables, and eight species significantly shifted their geographic distribution from the 1951-1976 cold period to the 1977-1998 warm period. Interestingly, the vertically migrating taxa more often showed a significant response to environmental variables than the nonmigrating mesopelagic taxa, reflecting the more significant increase in heat content of the upper ocean (o200 m), compared with the deeper zone (300-500 m) where the mesopelagic fishes typically remain. Climate change has significant effects on the abundances of oceanic fishes. Twenty-four taxa exhibited a significant change in abundance in relation to environmental variables, and 25 taxa, including both warm and cold-water taxa, showed a significant increase in abundance from the cold to warm period. Analysis of physical data indicated that the surface-layer (20-200 m) warmed significantly and the isotherms approached shoreward from the cold to the warm period. We further show that the spatial distribution of coastal-neritic fish retreated shoreward and oceanic fish extended shoreward from the cold to warm period. Our results suggest intensified stratification of the southern California region during the warm period may create a suitable habitat for the oceanic species. Moreover, such an unfavorable condition (e.g. changes in food habitat) for coastal-neritic species might result in competitive release for the oceanic fishes to flourish.
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