Mixed species aggregations feeding upon herring and sandlance schools in a nearshore archipelago depend on flooding tidal currents

Abstract

Tidal rips and jets are common features associated with archipelagos and complex coastlines. In habitats where rips and jets develop, energy flow to piscivorous predators is hypothesized to be strongly associated with tidal phase due to interactions between currents, plankton, and schooling planktivorous fishes (the 'tidal coupling hypothesis'). This study tests 1 component of the tidal coupling hypothesis, that the feeding activity of piscivorous predators and the availability of planktivorous fishes are both strongly associated with the same tidal phase. During 1994 to 1997, I made visual counts of actively feeding, mixed-species seabird flocks and hydroacoustic measurements of the relative abundance and distribution of schooling fishes. Median feeding activity, median backscatter m-1 transect, and median prey encounter probabilities were greatest during the tidal periods Slow flood 1 and Fast flood. Positive feeding anomalies and positive prey-encounter anomalies were significantly associated with tidal flood phases, but positive backscatter anomalies were not. The results support the tidal-coupling hypothesis, but suggest that changes in the distribution or behavior of schooling fish are as important as, if not more important than, changes in relative fish abundance when determining prey availability and predator foraging-success. The data show that tidal currents can play an important role in structuring nearshore predator-prey interactions.