Influences of temperature, bathymetry and fronts on spawning migration routes of Icelandic capelin (Mallotus villosus)

Capelin (Mallotus villosus) is the largest commercial fish stock in Icelandic waters and also an important forage fish. Capelin have adapted to the sub-arctic environment by migrating north (67-72 degrees N) to feed during summer in deep cold waters (>500 m; 13 degrees C) before migrating south (63-65 degrees N) to spawn in winter in warm shallow waters on the south and west coasts of Iceland (<100 m; 57 degrees C). Hydroacoustic data on capelin spawning migrations from 1992 to 2007 revealed a consistent southward route along which capelin migrated actively (ground velocity >> current velocity) off the east coast (and a lesser used route off the west coast). North of 65 degrees N, the dominant eastern route followed the bathymetry, skirting the shelf edge (>200 m bottom depth) within a funnel of near constant temperatures (approximately 2.5 degrees C). Further south, between 65 and 64 degrees N, as temperatures warmed to 4.5 degrees C (reaching 7.9 degrees C at 63.5 degrees N), capelin abruptly moved onto the shelf and towards the coastal spawning areas. Capelin spawning migrations appear to be an innately based southward search for appropriate spawning locations, guided by bathymetry and temperature. We suggest that the extended eastern migration route minimizes exposure to cod predation and that warming conditions north of Iceland may result in a northward shift in migrations and spawning locations, as occurred in the 1920s and 1930s.