Effect of bait size and sampling time on the attraction of the lysianassid amphipods Anonyx sarsi Steele & Brunel and Orchomenella pinguis (Boeck)

Abstract

Traps were used to study the effect of bait size and sampling time on the attraction of Anonyx sarsi Steele & Brunei and Orchomenella pinguis (Boeck) in Baie des Rochers, Middle Saint Lawrence Estuary. Numbers of lysianassids attracted to traps increased with increasing bait size (1–700 g). However, traps with ⩾ 10 g bait yielded significantly more first (5–9 mm in length) and second (15–28 mm) cohort Anonyx sarsi per g fresh weight of bait than traps with 1g bait, while yield of Orchomenella pinguis did not differ significantly with l g bait compared with ⩾ 10 g bait. With large bait, catchability (i.e. ratio number in traps: endobenthic abundance) of second cohort Anonyx sarsi was greater than that of Orchomenella pinguis and first cohort Anonyx sarsi. Orchomenella pinguis and second cohort Anonyx sarsi were significantly less abundant in traps set for 30 min at high tide in the day compared with the night, demonstrating that darkness is an important modifying factor of their foraging behaviour. In contrast, first cohort A. sarsi were equally represented in traps deployed in the day and at night. The swift second cohort A. sarsi responded to bait faster than other lysianassids in laboratory and field experiments. These findings point to a distinct segregation of the foraging patterns of these lysianassid species and size classes, which is attributed to differences in activity rhythms and swimming speed and behaviour. Anonyx species are apparently better adapted to compete for large carrion than Orchomenella species.