Trophic level scales positively with body size in fishes

Aim The existence of a body size hierarchy across trophic connections is widely accepted anecdotally and is a basic assumption of many food-web models. Despite a strong theoretical basis, empirical evidence has been equivocal, and in general the relationship between trophic level and body size is often found to be weak or non-existent. Location Global (aquatic). Methods Using a global dataset for fishes (http://www.fishbase.org), we explored the relationship between body size and trophic position for 8361 fishes in 57 orders. Results Across all species, trophic position was positively related to maximum length (r2 = 0.194, b = 0.065, P < 0.0001), meaning that a one-level increase in trophic level was associated with an increase in maximum length by a factor of 183. On average, fishes in orders that showed significantly positive trophic level-body size relations [mean = 51.6 cm +/- 11.8 (95% confidence interval, CI)] were 86 cm smaller than fishes in orders that showed no relation [mean = 137.1 cm +/- 50.3 (95% CI), P < 0.01]. A separate slopes model ANCOVA revealed that maximum length and trophic level were positively correlated for 47% (27 of 57) of orders, with two more orders showing marginally non-significant positive relations; no significant negative correlations were observed. The full model (order x body size) explained 37% of the variation between body size and trophic position (P < 0.0001). Main conclusions Our results support recent models which suggest that trophic level and body size should be positively correlated, and indicate that morphological constraints associated with gape limitation may play a stronger role in determining body size in smaller fishes. Differences among orders suggest that the nature of the trophic level-body size relation may be contingent, in part, on evolutionary history.