Could the planktonic stages of polar cod and Pacific sand lance compete for food in the warming Beaufort Sea?

Abstract

The boreal Pacific sand lance (Ammodytes hexapterus) was recently detected in southeastern Beaufort Sea (Canadian Arctic), numbering as the second most abundant ichthyoplankton species after the polar cod (Boreogadus saida) in 2011. We contrast the hatching periods, growth, prey selectivity, and feeding success of the planktonic stages of the two species. Polar cod hatched from January to mid-July and sand lance from mid-July to early September, precluding any competition among the larval stages. By weight, sand lance larvae grew 3.7 times faster than polar cod larvae. The co-occurring juveniles of both species fed primarily on copepods and to a lesser extent on bivalve larvae, shifting to larger prey with growth. The feeding success of both species appeared limited by the availability of their preferred prey. A significant diet overlap in juveniles >25 mm suggested potential competition for Pseudocalanus spp., Calanus spp., and bivalve larvae. However, sand lance strongly selected for nauplii while the more diversified diet of polar cod comprised mainly the copepodites of these species. Interspecific competition for food is unlikely at this time but is predicted to amplify with a climate-related reduction in the size of zooplankton prey and an increase in the abundance of sand lance.