RESEARCH PAPER Trophic level scales positively with body size in fishesgeb_579 1..10 Tamara N. Romanuk*, April Hayward† and Jeffrey A. Hutchings Department of Biology, Dalhousie University, 1355 Oxford Street, Halifax, Nova Scotia, Canada B3H 4J1 ABSTRACT 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 (r 2 = 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 signifi- cant negative correlations were observed. The full model (order ¥ 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. Keywords Allometry, body mass, fish, FishBase, gape limitation, metabolic theory of ecology, scaling, size classes. *Correspondence: Tamara N. Romanuk, Department of Biology, Dalhousie University, 1355 Oxford Street, Halifax, Nova Scotia, Canada B3H 4J1. E-mail: tromanuk@gmail.com †Present address: Department of Biology, University of Florida, PO Box 118525, Gainesville, FL 32611, USA. INTRODUCTION Body size is of paramount importance in ecological dynamics (Elton, 1927; Peters, 1983; Calder, 1984; Schmidt-Nielsen, 1984; Cohen et al., 1993; Brown et al., 2004; Marquet et al., 2005; Arim et al., 2007; Carbone et al., 2007): it shapes the demand that organisms place on their environment for energy and materials (through its well known, if poorly understood, relationship with metabolic processes; Brown et al., 2004) and plays an important role in structuring both inter- and intra-specific interactions. Among the most important interactions affected by body size are consumer–resource interactions, as body size constrains the range of prey sizes a predator can consume (Cohen et al., 1993; Arim et al., 2007; Carbone et al., 2007; Hildrew et al., 2007). The existence of a body size hierarchy across trophic connections is widely accepted as a basic assumption of many food-web models (Cohen et al., 1993; Williams & Martinez, 2000; Becker- man et al., 2006; Petchey et al., 2008). Cohen et al. (1993), for example, have suggested that body size provides a mechanistic interpretation of the assumed trophic hierarchy of the cascade Global Ecology and Biogeography, (Global Ecol. Biogeogr.) (2010) © 2010 Blackwell Publishing Ltd DOI: 10.1111/j.1466-8238.2010.00579.x www.blackwellpublishing.com/geb 1