Spatial patterns of recruitment in a demersal fish as revealed by seabird diet

Understanding recruitment in demersal fish requires determination of larval survival and delivery to appropriate settling habitats. Sanddabs Citharichthys sp. are an abundant neritic fish of Central California, an area of persistent upwelling. ‘Upwelling shadows’ develop in the lee of coastal promontories, retain surface waters, and may promote spatial variation in fish settlement. To test this hypothesis, we studied the diet and foraging dimensions of a seabird (Cepphus columba) specializing in sanddab consumption at windward and leeward sites over a 6 yr period. We integrated the bird’s take of sanddab with information on upwelling intensity and variability and sanddab larval abundance based on net sampling. Seabird diet at both sub-colonies was variable, but dominated by Age 1 sanddabs. Sanddabs were more prevalent in the diet of guillemots at the leeward site, and diet was more variable at the windward site. Persistent upwelling led to regional increases in sanddab larval abundance which, in turn, resulted in enhanced recruitment to leeward waters, as reflected in seabird diet. Pulsed upwelling was related to apparent increased recruitment in windward waters. This study is one of the first to suggest that seabird diet can be used as an indicator of spatial variability in recruitment and settlement of demersal forage fish.