Benthic enrichment by diatom-sourced lipid promotes growth and condition in juvenile Tanner crabs around Kodiak Island, Alaska

Nearshore embayments are important nurseries for juvenile southern Tanner crabs Chionoecetes bairdi, as they provide refuge from predation and elevated water temperatures promote rapid growth. Previous investigations of juvenile Tanner crabs have shown considerable variability in size of age-0 yr crabs from different shallow water embayments surrounding Kodiak, Alaska. To determine the proportion of this presumed growth variability that is due to diet quality, we sampled crabs and sediments over 2 yr at nursery sites that had previously demonstrated disparate age-0 yr crab sizes. Juvenile crabs reside at the sediment-water interface and therefore we measured sedimentary grain size and sedimentary organic matter, as well as total lipids per weight, lipid classes and fatty acid biomarkers in both crabs and sediments. Juvenile crabs from sheltered sites, as opposed to exposed sites, were characterized by larger size and by rapid growth rates and higher tissue lipid densities. Further, higher diatom and bacterial fatty acid markers characterized both sedimentary lipids and crab lipids in animals from sheltered bays compared to those from exposed sites. Controlled laboratory experiments were run to determine the relative importance of food quantity (ration) and quality (% lipid) on juvenile growth and condition. We found both diet quantity and quality significantly affected growth and lipid storage of juvenile crabs. Our results point to the importance of trophic factors in defining high quality habitat for a commercially important cold-water crab species.