Regional variation in oceanographic conditions influences intertidal fish assemblage structure

I investigated the relationship between regional variation in habitat and oceanographic conditions and the population and assemblage structure of intertidal fishes (the sculpins Clinocottus globiceps, Clinocottus recalvus, and Oligocottus snyderi) throughout the spring and summer in 2001, 2002, and 2003. The assemblage structure of intertidal sculpins varied markedly with distance from an upwelling center: warmer sites in the northern Monterey Bay that were furthest from upwelling activity were dominated by C. recalvus, whereas cooler sites north of the bay closest to upwelling activity were dominated by O. snyderi and C. globiceps. Interestingly, species were spatially distributed according to temperature differences as would be predicted by the geographic distributions and temperature sensitivity of each species: C. recalvus is a more southerly species tolerant of warmer temperatures, whereas O. snyderi and C. globiceps have more northerly geographic distributions and are relatively less tolerant of warmer water conditions. Regional variation in adult and juvenile assemblage structure reflected differences in recruitment rates among species, suggesting that regional differences in assemblage structure of these intertidal fishes are influenced greatly by pre-recruitment processes. In addition, possible limited larval exchange between the two regions — due to a combination of unique larval behavior of intertidal sculpins and the persistence of an upwelling front that acts as a barrier to larval dispersal — may serve to reinforce these patterns. The spatial differences in the assemblage structure of intertidal sculpins within this region are caused by a combined influence of variation in oceanographic conditions affecting both larval and benthic phases. The life histories of most marine organisms include an obligate period of pelagic larval dispersal followed by a relatively sedentary benthic adult stage. Because of the nature of the oceanic environment, which is characterized by varying spatial and temporal scales of oceanographic conditions, larvae are often dispersed so widely as to preclude their return to their natal sites. This has led to the consideration of marine populations as ‘‘open’’ populations, where the rate of export of offspring from parental populations is high, and population replenishment is strongly influenced by the influx of young produced elsewhere (Warner and Chesson 1985; Doherty and Williams 1988; Cowen and Sponaugle 2009). Hence, variable larval recruitment can be a major determinant of adult population densities, rendering population dynamics difficult to predict. In addition, local population dynamics are most likely determined by a combination of processes that affect both levels of recruitment and the subsequent survival of juveniles and adults (Cowen and Sponaugle 2009). Therefore, predictions of the dynamics of marine populations are dependent upon our understanding of the relative importance of processes that determine the rate of delivery of young to a population as well as those that determine their subsequent mortality and emigration (Caley et al. 1996; Carr and Syms 2006). For benthic marine organisms, local population structure can be affected by several unique processes, each of which occurs at different spatial scales. These processes include variation in features at both the local and the regional scale, such as benthic habitat characteristics or