Northern Large-River Benthic and Larval Fish Drift: St. Marys River, USA/Canada

Abstract

Seasonal (summer, winter), diel (day, night), water column (surface, mid-depth, bottom), proximity to shore, weather (calm, windy), and among-transect differences in benthic and larval fish drift were measured during 1985 using 355 μm-mesh nets at three transects in the St. Marys River, the connecting channel between Lake Superior and Lake Huron. Overall, there were 65 benthic ( x ¯ density = 562/1,000 m 3) and 5 larval fish ( x ¯ density = 383/1,000 m 3) taxa collected. Occurrence of significantly higher densities of larval rainbow smelt at the middle transect, Lake Nicolet, indicates that areas above are important spawning sites for this species. Because densities declined at the downstream-most transect, Point aux Frenes, we concluded that rainbow smelt must be utilizing the Lake Munuscong area as a nursery area. Larval fish densities in summer were greater in the channel than nearshore, were greater at night than during the day, and higher in the upper half of the water column than near bottom. Densities of benthos and larval fish were greater in summer than winter. Chironomidae (52–82% of total) was the winter dominant, while Hydra (60%) dominated summer drift. Drift was greater at night than during the day for benthos and fish at two of the three transects, with benthic abundances greater at the bottom than at other depth strata. In summer, benthic drift was highest at shoreline stations and lowest in the channel; in winter there were minimal onshore/offshore differences. Current appeared to be the major factor causing elevated benthic drift rates in winter, while in summer, high winds with concomitant high waves and currents were the main factors producing significantly higher drift densities. High wind conditions produced a reversal of the usually observed diel trend of higher night benthic drift rates at one of the transects, and underscores the importance of wind in modulating drift rates in large river systems such as the St. Marys River. Overall, the multiple regression factors utilized were much more predictive of the drift of benthos than of larval fish.