LACK OF SPATIAL COHERENCE OF PREDATORS WITH PREY - A BIOENERGETIC EXPLANATION FOR ATLANTIC COD FEEDING ON CAPELIN

We tested two biologically based predictions that potentially influenc e scales of spatial association between Atlantic cod, Gadus morhua, an d prey populations of capelin, Mallotus villosus. If cod aggregate in response to concentrations of prey, then spatial association (coherenc e) between capelin and cod was predicted to peak at the scale of maxim um capelin spatial variance. If capelin-cod coherence did not match th e scale of maximum prey spatial variability, then capelin-cod coherenc e was predicted to peak at the spatial scale that maximizes net energe tic benefit to the predator. Contrary to predictions, we found no evid ence of aggregative responses of cod to capelin over resolution scales of 20 m to 10 km. This result was observed onsistently at the tempora l scare of a single transect (c. 1 h duration) and at the scale of ave raged transects (c. 2 weeks duration). Estimates of cod foraging energ etics showed that they were not constrained by physiology to aggregate relative to capelin at any scale less than 10 km. A net energetic gai n of 478 to 784 kJ would result if a 44 cm, 752 g cod consumed a ratio n of eight to 12 capelin over a period of 58 h. Energetic calculations included costs of egestion and excretion (317 to 476 kJ), maintenance (58 kJ), digestion (125 to 188 kJ), and continuous swimming during ra tion assimilation (79 kJ). During this period, a 44 cm cod could trave l over 38 km swimming at 1 B.L. s(-1). Foraging cod are virtually cert ain to encounter capelin over this distance based on the abundance of pre-spawning capelin present in coastal bays during the spawning seaso n. This study illustrates that aggregative responses of predators do n o occur at all scales and possibly occur over a very limited range of scales.