In a rocky intertidal tidepool habitat, factors like temperature, salinity, oxygen, and wave energy fluctuate tidally, daily, seasonally, and interannually. Unlike sessile organisms, motile organisms can adjust their locations in response to environmental fluctuations; therefore, they may not fit into general intertidal ecological models based on sessile emergent communities. Fish populations were sampled at several sites in San Diego, California, from 1996 to 2000 to test effects of temporal environmental variability on habitat use, assemblage structure, and population dynamics of the major tidepool fishes of San Diego: Clinocottus analis (woolly sculpin), Girella nigricans (opaleye), Gobiesox rhessodon (California clingfish), Hypsoblennius gilberti (rockpool blenny), Gibbonsia elegans (spotted kelpfish), and Paraclinus integripinnis (reef finspot). These fishes were found to spatially and temporally partition tidepool habitat among species and ontogenetically within species.

Habitat use and assemblage structure were variable on the three time scales addressed in the study (diel, seasonal, and El Nino Southern Oscillation). Middle intertidal fishes used high and low intertidal pools differently depending on time of day. Fishes used higher tidepools during the fall, when sea level is seasonally highest. Recruitment and growth were seasonal and were the main influence on temporal variability in population growth rate of the assemblage dominant, C. analis.

Changes in population dynamics and assemblage structure were observed during the 1997-98 El Nino event. C. analis population size declined due to changes in early life-history (larval and juvenile) processes, including a drop in recruitment. P. integripinnis population size increased during the El Nino event. The combination of these two changes led to higher species evenness during El Nino. One species, G. nigricans, experienced a morphological shift (increases in dorsal spot number) asociated with El Nino. Because the study spanned only one El Nino event, generalization to other ENSO events is not possible. However, as ENSO predictability increases, future studies will be able to test the generality of these results.

Rocky intertidal fishes of San Diego do not conform to the density-dependent space-limitation model proposed for many rocky intertidal taxa that are sessile and emergent. Rather, community and population regulation appear forced by pre-recruitment processes, like many tropical reef fishes. Future studies targeting the larval stage will enable further identification of specific larval vital rates important in controlling population dynamics.