Size-dependent susceptibility of longfin inshore squid (Loligo pealeii) to attack and capture by two predators

Cephalopods are primary prey to a wide range of predators in marine food-webs, yet a basic understanding of the mechanisms controlling predation risk and demand on their populations is lacking. Feeding experiments were conducted to evaluate how relative prey size and behavior mediate the susceptibility of squid to predation. Attack and capture of longfin inshore squid (Loligo pealeii) were quantified using two predators: bluefish (Pomatomus saltatrix) a pelagic, cruising predator, and summer flounder (Paralichthys dentatus) a benthic, ambush forager. Predator selectivity, prey susceptibility, and prey profitability were estimated as a function of relative prey size from predator-prey interactions during behavioral trials. Patterns in attack rates suggested that size-selection on squid was constrained by passive processes rather than active choice for both predators. The susceptibility of squid to predation by bluefish was strongly dependent on relative prey size; however, flounder were equally efficient at capturing all sizes of squid offered. Handling times increased exponentially with relative prey size and were the primary constraint on selectivity and profitability in flounder. Prey profitability was a dome-shaped function of relative squid size in both predators. Overall, the relative size and values at which profitability was maximized were higher in bluefish indicating they were the more efficient predator of squid. Squid succumbed to greater time-dependent mortality rates than Atlantic silversides (Menidia menidia) and mummichogs (Fundulus sp.) suggesting that when equal amounts of squid and fish are available in the environment, squid will be selectively ingested by bluefish and flounder. In addition to the influence of relative prey size, predator foraging behaviors and size-dependent encounter rates were thought to be important factors affecting selection on squid in the northwest Atlantic ecosystem.