Diel Pelagic Foraging by a Pandalid Shrimp (Pandalus montagui) off Resolution Island (Eastern Hudson Strait)

Abstract

The vertical distribution and diet of shrimp (Pandalus montagui) were investigated using a multistage plankton sampler (BIONESS) and were compared with those of shrimp captured on the bottom using a trawl. Massive nocturnal vertical migrations of shrimp were observed in association with intense pelagic feeding activity. The sex ratio, size frequency distribution, and diet of shrimp caught in the water column differed markedly from shrimp caught during their stay on the bottom. Pelagic migrants were mostly males (75%), extending their planktonic foray into morning hours, whereas ovigerous (12%) and nonovigerous (13%) females returned to the bottom at dawn. In the trawl, the representation of males (43%) and ovigerous females (45%) differed, but also fluctuated on a diel cycle. Stomachs of shrimp captured in the BIONESS were fuller and the contents were in better condition than found in shrimps captured in the bottom trawl. Pelagic shrimp fed opportunistically on a variety of zooplankters, using copepods as a staple food. Stomachs of shrimp from the bottom trawl contained chitinous debris, sand grains, and small amounts of benthic organisms. The occurrence of daily vertical feeding migrations results in the unavailability of a variable proportion of the total shrimp biomass to bottom trawls.