Different species of fishes are found in pelagic, soft-bottom, hard-bottom, and kelp bed habitats, but some are found in more than one of these habitats. Pelagic species such as Pacific sardine, northern anchovy, and Pacific hake have undergone large-scale natural fluctuations in abundance during the past 2,000 years. Warm El Niņo water masses bring warm-water fish into southern California and restrict the recruitment of cold-water species. Transient pelagic fishes are little affected by human activities other than fishing.
In the past, soft-bottom fishes have been good indicators of pollution. Several species, but particularly Dover sole, were afflicted by fin erosion and skin tumors near the County of Los Angeles Sanitation District's Joint Water Pollution Control Plant (JWPCP) outfall in the 1970s. However, as effluent treatment improved, these diseases have almost disappeared from the population.