Diel feeding patterns and daily ration estimates of three subantarctic euphausiids in the vicinity of the Prince Edward Islands (Southern Ocean)

The feeding biology of three euphausiid species, Euphausia vallentini (adults and juveniles), Euphausia longirostris , and Nematoscelis megalops , was investigated in the vicinity of the Prince Edward Archipelago during austral autumn (April/May) of 1998 and 1999, using the gut fluorescence and gut fullness techniques, to assess diel feeding patterns and estimate daily rations. The results showed clear diel feeding patterns for both E. vallentini adults and juveniles. High feeding activity occurred during dark hours in E. longirostris , while N. megalops did not show distinct diel feeding pattern. E. longirostris had the highest gut pigment levels, averaging 15.2 and 57.1 ng pigm ind −1 in 1998 and 1999, respectively. A similar pattern was observed for E. vallentini : 3.2 versus 6.5 ng pigm ind −1 in juveniles and 4.4 versus 21.9 ng pigm ind −1 in adults. Low average gut pigment levels were found for N. megalops during both years (range 2.0–4.5 ng pigm ind −1 ). Gut evacuation rates varied between 0.45 and 0.99 h −1 for E. vallentini adults, 0.47 and 1.36 h −1 for E. vallentini juveniles, 0.41 and 1.08 h −1 for N. megalops . A single measurement of 0.94 h −1 was made for E. longirostris . Ingestion rates for E. vallentini ranged from 0.24 to 1.82 μg pigm ind −1 d −1 for adults and from 0.98 to 1.92 μg pigm ind −1 d −1 for juveniles. The highest individual ingestion rates were found for E. longirostris (between 1.24 and 3.65 μg pigm ind −1 d −1 ), and the lowest for N. megalops (0.23 μg pigm ind −1 d −1 ). These ingestion rates were combined with data from the gut fullness technique to estimate the contribution of herbivory to the total daily carbon uptake.