Staged spawning migration in Icelandic capelin (Mallotus villosus): effects of temperature, stock size and maturity

Capelin (Mallotus villosus) is the largest commercial fish stock in Icelandic waters and also an important forage fish. Every winter pre-spawning capelin migrate 500-1000km from their offshore (>200m bottom depth) northern feeding areas (67-71 degrees N) to inshore (<200m bottom depth) southern spawning areas (63-65 degrees N). The major migration route is east of Iceland. The route consists of both offshore and inshore phases. The migration begins offshore as capelin skirt the shelf edge north of 65 degrees N, then abruptly veer inshore between latitudes 64 degrees and 65 degrees N. Hydro-acoustic data from 1992-2007 demonstrated that the timing of the offshore phase migration varied by as much as 1month, from 22 December to 21 January. A combination of larger spawning stock and colder feeding ground temperatures (August-December) resulted in earlier offshore migration. The timing of the inshore migration phase was not dependent on the offshore migration timing, and never began prior to the first week of February. Many cohorts arrived at latitudes 64-65 degrees N in early January but staged offshore at latitudes 63.8-65.8 degrees N until early February. The longest observed delay in the staging area was 5weeks. Timing of the inshore migration was controlled by gonad maturity, with migration beginning when roe content attained 12-14%. Staging limited the time capelin spent on the continental shelf before spawning to 3weeks. We suggest that offshore staging evolved to minimize temporal overlap with predatory gadoids, especially Atlantic cod (Gadus morhua).