Predation risk of age-0 cod (Gadus) relative to depth and substrate in coastal waters

Among individuals of a species, mean size in fishes increases with depth in the marine environment according to Heincke's Law. Atlantic cod (Gadus mor hua L.), like many young fish, are distributed in shallow ( <10 m) coastal water through much of their range as young juveniles (age 0) and deeper wat er as they grow. We tested the hypothesis that juvenile cod occupy such sha llows to minimize risk of encounter with larger piscivorous fish whose mean size also increases with depth according to Heincke's Law. In day and nigh t seining, we found that age-0 cod were most abundant in vegetated sites re gardless of time period. In vegetated sites, age-0 cod abundance declined a t night; the reverse was true at non-vegetated sites. Piscivorous fish were generally most abundant in vegetated sites; there were no consistent diffe rences in abundance between day and night. Relative risk of predation of in dividual age-0 cod in different habitats, depths and times was determined b y tethering. Age-0 cod prey were tethered 25 cm off the bottom in water 0.7 , 1.5, 3, 6, 10 and 20 m deep; six times during the day, three times during dusk arid three times during the night in 25-min sets on eight dates in Au gust and September 1998. A total of 576 tether sets were deployed. We deplo yed tethers on two dates for each of two vegetated-eelgrass, Zostera marina L.-and two unvegetated sites. Predation on tethered prey was lower at nigh t than during day and dusk at all locations and depths. During day and dusk , predation increased with depth at all sites (<similar to>3 times higher i n deep compared to shallow). At shallow locations (less than or equal to3 m ), predation on tethered prey was generally higher in unvegetated compared to vegetated sites during the day and dusk. Predators were captured in 12% of sets. In order of frequency, these were: cunner Tautogolabrus adspersus Walbaum, Atlantic cod, and Greenland cod, G. ogac Richardson. Mean size of all predator species captured increased with depth. We showed that predatio n risk for juvenile fish increases with depth suggesting a behavioural anti predation mechanism for Heincke's Law. (C) 2001 Elsevier Science B.V. All r ights reserved.