The rich krill aggregation of the Saguenay - St. Lawrence Marine Park: hydroacoustic and geostatistical biomass estimates, structure, variability, and significance for whales

The euphausiid aggregation at the head of the main channel of the estuary and Gulf of St. Lawrence was surveyed using 120- and 38-kHz hydroacoustics in the summers of 1994 and 1995. A systematic sampling grid covering an area of 1319 km(2) was visited eight times. Fish echoes were separated from krill echoes using the difference in backscattering strength at the two frequencies. Global estimates were obtained from geostatistical methods for both total biomass and the fraction exceeding 5 g wet m(-3). The euphausiids were always exclusively composed of individuals of the oldest cohorts (2+) of the two species Thysanoessa raschi and Meganyctiphanes nonvegica. Total biomass varied from 8 +/- 2 to 96 +/- 8 kt and cutoff biomass from 0 to 56 +/- 6 kt. The two types of estimates were linearly related. Biomass was autocorrelated up to distances of 10-15 km. An anisotropic structure with radii of similar to 2 x 5 km, stretched along the channel axis, was discernible at a small scale. The particular size distribution of euphausiids and the large variations in the global estimates appear to be controlled by horizontal transport. The study area appears to be the richest krill aggregation site yet documented in the northwest Atlantic, with densities similar to the rich krill aggregation areas of the Antarctic. The krill aggregation is at the heart of this traditional whale feeding ground.