Diet of the chaetognaths Sagitta setosa and S-elegans in relation to prey abundance and vertical distribution

The chaetognaths Sagitta setosa and S. elegans and their potential prey organisms were collected from 6 different depth strata on 6 day and night occasions in the Gullmar fjord, west Sweden. The chaetognaths were analysed for gut contents and predation impact estimated. Number of prey per chaetognath (NPC) ranged from 0.2 to 1.0 for S. setosa and 0.4 to 0.9 for S. elegans, with significantly higher values at night. For S. setosa, small copepods dominated gut contents, with the addition of the appendicularian Oikopleura dioica and chaetognaths later in the season. For S. elegans, the great majority of prey were Calanus copepodites. Predation impact was strong on single copepod species, increasing from 0.4 to a maximum of 48 % of the individuals removed daily by S. setosa at the end of October. S. elegans consumed 5 to 18 % of the deeper dwelling Calan us community daily. Cannibalism was pronounced in S. setosa; in October, 74 to 84 % of the population could potentially be removed each day, Selective feeding was observed in S. setosa by estimating clearance rates for different prey. Pseudocalanus sp. was over-represented in the guts (clearance 120 ml ind.(-1) d(-1)) as well as S, setosa (124 ml ind.(-1) d(-1)) and Paracalanus parvus (40 ml ind.(-1) d(-1)). S. setosa was able to maintain a daily ration of 11 to 17 % of body weight despite a rapid decline in copepod prey abundance, and this was possible through cannibalism and intense predation on O. dioica.