Population Trends and Trophic Dynamics in Pacific OCS Ecosystems : What Can Monitoring Data Tell Us ?

Abundances of many coastal species off southern California have undergone dramatic declines over the past two decades concurrent with a regional climate shift. This shift in climatic conditions represents a natural, large-scale perturbation and provides an opportunity to understand whether and how various components of coastal ecosystems respond. We conducted time-series analyses on long-term data sets of non-exploited fish populations from the Southern California Bight to describe their temporal trends and explore the timing and magnitude of change. The species examined were classified as to trophic level, mode of reproduction, extent of geographic range, association with benthic or pelagic food webs, and habitat. In general, the magnitude of decline was similar for all species, regardless of classification. Trends were similar at all locations examined within the Bight, suggesting regional declines in abundances rather than redistribution of individuals. These patterns are consistent with the explanation that a regional decline in productivity associated with the shift to an alternate climatic regime is responsible for regional decline in fish stocks. Introduction Understanding the processes that link and/or limit component populations within aquatic communities represents a significant ecological challenge. This understanding is essential, however, for predicting how perturbations to populations at one trophic level might be propagated throughout the rest of the community (Wootton 1994; Albritton et al. 2001). Current models predicting the responses of populations to “top down” or “bottom up” effects often differ widely in their estimates of the magnitude of change expected by species at other trophic levels (Hairston et al. 1960; Menge and Sutherland 1976; Fretwell 1977, 1987; Oksanen et al. 1981; Carpenter and Kitchell 1993; Estes and Duggins 1995; Hairston and Hairston 1997; see Sinclair et al. 2000 for a review). Attempts to experimentally test predictions of these models in temperate marine systems have been hindered by the logistical difficulties inherent in conducting long-term manipulative studies encompassing multiple levels of community organization at meaningful spatial scales. Final Study Report – Schmitt and Brooks 8 Increasingly, marine ecologists are turning to observational studies of the responses of communities to environmental changes to gain insight into the mechanisms structuring marine communities (Carpenter 1990; Corten 1990; Aebischer et al. 1990; Barry et al. 1995; Holbrook and Schmitt 1996; Veit et al. 1996; Beamish et al. 2000). We use such an approach here. The behavior of the California Current system is one of the most important determinants of environmental conditions in the nearshore marine environment off Southern California (Brodeur et al. 1996). The typically high productivity of this system is attributed to coastal upwelling, which brings deeper, nutrient-rich waters to the surface near shore. This high supply rate of nutrients enhances primary productivity, which, in turn, drives the secondary productivity of nearshore benthic and pelagic food webs. Oceanographic conditions within the California Current system underwent a substantial shift in the late 1970s when there was a rapid, large, and persistent increase in seawater temperature (Miller et al. 1994; Minobe 1997; but see Mantua et al. 1997; Beamish et al. 1998). Between 1976 and 1977, recorded seawater temperatures within the Southern California Bight rose an average of 1-2C above the mean for the previous two decades (Roemmich 1992). Associated with this warming event was a decrease in upwelling intensity and productivity of nearshore surface waters (Roemmich and McGowan 1995; McGowan et al. 1996). Numerous studies have documented decreases in the abundances of various nearshore species within the California Current region since the regime shift of the late 1970s. While these declines are often cited as evidence of a tight linkage between large scale changes in oceanographic circulation patterns and the productivity of nearshore marine ecosystems, few studies have attempted to compare observed trends in the magnitude and timing of these declines among species with differing life histories, geographical ranges, or trophic status (but see Barry et al. 1995; Veit et al. 1996; Holbrook et al. 1997). Such comparisons could provide useful insight into whether and how various components of the nearshore coastal ecosystem respond to the same disturbance. We use data collected from fish impingement studies conducted at coastal electrical generating stations located throughout the Southern California Bight to describe trends over a 17 year period and establish the timing and magnitudes of change in the abundances of 37 species of nearshore fishes. These data thus provide a long-term estimate of fish abundances that is both integrated through time and spatially extensive. Specifically, we explore the extent to which responses (i.e., effect sizes and response times) were manifested similarly between pelagic and benthic ecosystems, among various habitat types (pelagic vs. reef associated vs. benthic), among different trophic levels, and among species that vary in life history attributes (e.g. reproductive mode and geographic range).