Standardized diet compositions and trophic levels of sharks

Sharks are marine consumers believed to occupy top positions in marine food webs. But surprisingly, trophic level estimates for these predators are al most non-existent. With the hope of helping better define the ecological ro le of sharks in marine communities, this paper presents standardized diet c ompositions and trophic revels calculated for a suite of species. Dietary c omposition for each species was derived from published quantitative studies using a weighted average index that takes into account sample size in each study. The trophic level (TL) values of the 11 food types used to characte rize the diet (obtained from published accounts) were then used to calculat e fractional trophic levers for 149 species representing eight orders and 2 3 families. Sharks as a group are tertiary consumers (TL>4), and significan t differences were found among the six orders compared, which were attribut able to differences between orectolobiforms (TL<4) and all other orders, an d between hexanchiforms and both carcharhiniforms and squatiniforms. Among four families of carcharhiniform sharks, carcharhinids (TL=4.1, n=39) had a significantly higher TL than triakids (TL=3.8, n=19) and scyliorhinids (TL =3.9, n=21), but not sphyrnids (TL=3.9, n=6). When compared to trophic leve ls for other top predators of marine communities obtained from the literatu re, mean TL for sharks was significantly higher than for seabirds (n=28), b ut not for marine mammals (n=97). Trophic level and body size were positive ly correlated (r(s)=0.33), with the fit increasing (r(s)=0.41) when the thr ee predominantly zooplanktivorous sharks were omitted, and especially when considering only carcharhinid sharks (r(s)=0.55). (C) 1999 International Co uncil for the Exploration of the Sea.