Cephalopod biomass – estimates from predation

Abstract

North Atlantic deep sea cephalopods caught in research nets (mouth opening of9 m2) and I arge commercial trawls are compared with those eaten by Sperm Whales. 1t is shown that at three latitudes the nets and whales sample cephalopods very differently. Research nets are biased towards collecting small species and young specimens of several families, principally gonatids, cranchiids, enoploteuthids and onychoteuthids, while sperm whales are biased towards catching much !arger cephalopods, particularly histioteuthids. Commercial trawls tend to bridge the gap in size of cephalopods selected by research nets and whales but where comparisons are possible they have a bias towards catching enoploteuthids. The contribution by numbers and weight of different cephalopod families to the cephalopod fraction of the diet of sperm whales throughout the world is reviewed. A detailed study of Antarctic cephalopods in the diet of large predators including cetaceans, seals and birds indicates areas where more knowledge is required to make accurate estimates of the biomass of Antarctic cephalopods consumed. The value of direct studies of food webs for quantitative consideration of particular cephalopod taxa is demonstrated and discussed.