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Separating environmental effects from fishing impacts on the dynamics of fish populations of the Southern California region


Disentangling environmental variability from fishing effects on the dynamics of fish populations is essential for sound fisheries management. Toward this goal, I compare exploited with unexploited species living in the same environment. Using greater than 50-year long larval fish time series collected in the California Cooperative Oceanic Fisheries Investigations (CalCOFI) from the southern California region, I consider fishing as a treatment effect in a long-term ecological experiment. I construct an "expert-knowledge classification system" to categorize southern California fish species collected during the CalCOFI surveys into communities based on the habitat affinities of adults (coastal, coastal-oceanic, and oceanic), and incorporate their life history traits, phylogeny, and status of exploitation into analyses. Fisheries exploitation occurs only in the coastal and coastal-oceanic communities. Within these communities, very few species exhibit a significant linear correlation with environmental variables, and exploited species are not more responsive to climate than unexploited species. However, the long-term variability in the abundance of exploited species is higher than that of unexploited species, after accounting for life history effects, phylogeny, and a changing environment. The increased variability of the exploited populations is likely caused by fishery-induced truncation of age-structure, which reduces the capability of populations to dampen the effects of environmental variability. Furthermore, the latitudinal distributions of exploited species are more responsive to climate changes than those of unexploited species, suggesting that fishing may reduce the resilience of fish populations facing environmental variation. The reduced resilience may be caused by fishery-induced truncation of age-structure or constriction of spatial distributions. My results indicate that fishing is likely to magnify uncertainty of fish populations and therefore, increase the probability of dramatic shifts of the populations facing environmental variations. A precautionary management approach is warranted not only because of normal uncertainties associated with estimates of stock size but because fishing magnifies population variability. Therefore, in addition to maintaining total viable biomass, management strategies should be implemented to conserve fish population structures in order to prevent fishing from increasing population variability

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