Climate-index response profiling indicates larval transport is driving population fluctuations in nudibranch gastropods from the northeast Pacific Ocean

We illustrate a novel method, climate-index response profiling, for testing mechanistic hypotheses for abundance fluctuations in long-term marine data sets. (1) Autoregressive models were fitted to time-series data for multiple species using climate indices as external regressors. The resulting regression coefficients for each species we term a climate-index response profile (CIRP). (2) Mechanistic hypotheses were rejected based on the summary response profile across species obtained by meta-analysis. (3) Monte-Carlo simulations of the CIRPs for all species at all sites were performed to generate null expectations to test the significance of nonrejected hypotheses. We applied this method to historical data of 56 nudibranch species from three intertidal sites studied independently in central California during nonoverlapping time periods between 1969 and 1995, combined with recent resurveys from these same sites. Adult populations are strongly correlated with the multivariate El Niño–Southern Oscillation index and sea-surface height, increasing during El Niño and decreasing during La Niño events. Our data are consistent with recruitment limitation via larval advection as the primary driver of adult abundance variation. El Niño conditions increase recruitment by cross-shelf onshore advection of larvae from both southern and northern populations, and alongshore larval advection from southern source populations. We hypothesize that recent declines of California nudibranchs are temporary, and populations will recover as El Niño conditions return to the California current.