Diet and growth rates of Meganyctiphanes norvegica in autumn

Fatty acid biomarkers analysis was performed on juvenile and adult northern krill (Meganyctiphanes norvegica) to determine variability in diet between individuals and the consequences for their growth rates. Animals were sampled from Gullmarsfjorden, western Sweden during September 2004 and incubated for 4 days to measure instantaneous growth rate (IGR) before subsequent fatty acid analysis. We found positive linear relationships between IGR and certain fatty acid concentrations within adult specimens. In particular, highest growth was seen in individuals containing high concentrations of 18:2(n-6) and 18:3(n-6), indicative of filamentous algae and/or terrestrial carbon dietary sources, and 16:4(n-1), a biomarker fatty acid for diatoms. Similar relationships were not evident for juvenile krill. In an accompanying study, stomach content analyses established that adult krill had been feeding on a diverse range of red, brown and green benthic filamentous algae, which contain fatty acids of the (n-6) series. Incidence of filamentous algae in the stomachs of juvenile krill was much lower, possibly reflecting the shallower water depths inhabited by this maturity stage. Although a benthic feeding mode has previously been observed for M. norvegica, this is the first direct evidence to indicate that benthic food sources can be important in supporting the growth of this euphausiid. Furthermore, high variability in growth rates and fatty acid concentrations between individuals suggest a diversity of feeding strategies within this fjordic population.