Changes in a tidepool fish assemblage on two scales of environmental variation: Seasonal and El Nino Southern Oscillation

Abstract

Intertidal organisms are influenced by the tidal, daily, and seasonal environmental variability of their habitat. Interannual variability, although often less severe than shorter-scale variability, may also be important in structuring intertidal systems. This study compares the magnitude of changes in a rocky intertidal fish guild occurring on a seasonal scale with those occurring during an El Nino Southern Oscillation (ENSO). I examined tidepool fish assemblage structure and habitat use in two southern California sites approximately every 3 months from 1996 to 2000, a period including non-ENSO conditions, the 1997-1998 El Nino, and the 1998-1999 La Nina. During each sampling period, I censused fish abundance in 105 tidepools of differing intertidal height, depth, and surface area. Several aspects of habitat use varied seasonally for the four most common species: Clinocottus analis (woolly sculpin), Girella nigricans (opaleye), Gobiesox rhessodon (California clingfish), and Hypsoblennius gilberti (notchbrow blenny). All four species migrated vertically within the intertidal zone on a seasonal scale, corresponding to seasonal changes in sea level. The assemblage dominant, C. analis, occupied tidepools of different sizes depending on season. Although seasonality in habitat use suggests an influence of environmental variability on seasonal scales, fish habitat was generally not altered by temperature and sea level changes imposed by the El Nino. Species assemblage, however, differed among climate conditions. C. analis declined in abundance during the El Nino because of lack of recruitment but increased immediately after its conclusion. Paraclinus integripinnis (reef finspot), usually rare, was more abundant during the El Nino. Effect of the El Nino on the other four species was not detected. Assemblage changes suggest that although intertidal fishes regularly experience large tidal, daily, and seasonal environmental fluctuations, interannual changes in environmental factors, even when relatively small in magnitude, can perturb the system. Perturbations in the present system, however, did not persist beyond the end of the El Nino event as they often do in lower-latitude nearshore areas.