Icelandic Crabs in Diets of Predatory Fish

Abstract

This thesis investigated the importance of crabs (infraorder: Brachyura) as prey for predatory fish around Iceland and whether predators could be used to sample crabs, by using stomach samples. Crabs contributed a small portion of the overall fish diet which was dominated by ray-finned fish and non-crab crustaceans. Crabs were most frequent in the diets of cod (Gadus morhua), haddock (Melanogrammus aeglefinus), Atlantic wolffish (Anarichas lupus) and thorny skate (Amblyraja radiata). Crab consumption was most frequent at depths ranging between 0 to 200 deep and was limited to depths below 550 m. Seasonal and regional variation was observed, with crabs being more frequent in the southwest region and in summer while being least frequent in spring. Crab consumption was correlated with a narrower diet and the impact on stomach fullness depended on predator species. Predatory fish were shown to be able to sample most of the known crab species around Iceland, and to estimate spatiotemporal distributions of crabs. Overall, 15 different predatory fish species consumed crabs and they consumed eight crab species. Arctic lyre crabs (Hyas coarctatus) and great spider crabs (Hyas araneus) were the most frequently consumed crabs. Some common crab species were rare in predatory fish diets, such as the Green shore crab (Carcinus maenas) and the invasive Atlantic rock crab (Cancer irroratus). This thesis presents novel data on the specific roles of crabs in predatory fish diets around Iceland as well as the usage of using predatory fish to sample crabs.

Í þessari ritgerð var kannað mikilvægi krabba (infraorder: Brachyura) sem bráð ránfiska við Ísland og hvort hægt væri að nota ránfiska til að safna kröbbum, með því að nota magasýni. Krabbar lögðu til lítinn hluta af heildarfæði ránfiska sem samanstóð mest af geisluggum og krabbadýrum að utanskildum kröbbum. Krabbar voru algengastir í fæðu þorsks (Gadus morhua), ýsu (Melanogrammus aeglefinus), steinbíts (Anarichas lupus) og tindaskötu (Amblyraja radiata). Krabbaát var algengast á milli 0 og 200 m dýpi og takmarkaðist við 550 m dýpi. Árstíðar og svæðisbundin mynstur voru athuguð, krabbar reyndust algengari suðvestan megin lands og á sumrin, en voru sjaldgæfastir á vorin. Krabbaát var tengt við þrengri fæðusamsetningu og áhrif krabbaáts á magafylli var háð ránfiskategund. Ránfiskar reyndust ná að veiða flestar af þekktum krabbategundum við Ísland og metið dreifingu krabba í tíma og rúmi. Í heild neyttu 15 ólíkar ránfiskategundir krabba og átta krabbategundir voru étnar. Litli trjónukrabbi (Hyas coarctatus) og stóri trjónukrabbi (Hyas araneus) voru algengustu krabbategundirnar í fæðu ránfiska. Sumar af algengu krabbategundum voru sjaldgæfar í fæðu ránfiska, til dæmis bogkrabbinn (Carcinus maenas) og ágengi grjótkrabbinn (Cancer irroratus). Þessi ritgerð kynnir ný gögn um hlutverk krabba í fæðu ránfiska við Ísland og notkun ránfiska við að safna krabbasýnum.