Levels of diurnal predation on a school of flat-iron herring,Harengula thrissina

Synopsis The incidence of diurnal predation (0800–1600h) on schools of several hundred thousand adult flat-iron herring,Harengula thrissina, was quantified. Nine species of piscine predators perpetrated 653 attacks and 158 captures during 42.5 h of observation. Four predators: cabrilla,Mycteroperca rosacea, cornetfish,Fistularia commersonii, green jack,Caranx caballus, and black skipjack,Euthynnus lineatus, were responsible for 92% of the attacks and 96% of the successes. Among these predators attack rates (attacks × time^–1) and success rates (successes × attacks^–1) varied with respect to time of day, although all predators were active throughout the day. As a consequence, the herring school experienced fairly homogeneous attack and success rates. These rates were estimated as roughly half the crepuscular attack rates but they were sustained over a much longer period. Piscivorous predators, particularly those species frequently found in association with schooling prey, may be focusing their attacks on opportunities, not specific times of day and/or light levels. More direct information needs to be gathered on diurnal as well as crepuscular attack and success rates in order to assess the impact that piscine predators have on schooling prey.