The spawning migration of capelin (Mallotus villosus) in Icelandic waters

Ólafsdóttir, Anna Heiða (2012) The spawning migration of capelin (Mallotus villosus) in Icelandic waters. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Capelin (Mallotus villosus) is one of the most important commercial fish stocks in Icelandic waters and a major forage species as important prey for predatory fish, sea birds and marine mammals. Capelin have adapted to the sub-arctic environment by migrating north (67-72°N) to feed during summer in deep cold waters (>500m; 1-3°C) before migrating south (63-65°N) in winter to spawn in warmer shallow waters (<100m; 5-7°C) on the south and west coasts of Iceland. To examine mechanisms regulating location and timing of the spawning migration, acoustic recordings were analyzed from cape lin winter acoustic assessment surveys from thirteen cohorts spawning in 1992-3, 1995, and 1998 to 2007. The southward capelin spawning migration of all studied cohorts (from 63-67°N) utilized a consistent route within southward flowing cold sub-arctic waters (l- 3°C) off the east coast of Iceland. Migration was not passive but active. The route followed the outer shelf edge but was beyond it (>200m bottom depth). Further south (<65°N), after the front with warmer Atlantic waters (>4.5°C) was encountered, the migration route veered inshore (<200m bottom depth) towards coastal spawning grounds. The annual spawning migration moved southward across latitude 67°N between December 23 to January 20 in all years. Both an increased spawning biomass and colder summer feeding temperatures resulted in earlier migration. The migration was not continuous but staged, with a staging area located offshore in the transition zone between the offshore and the inshore phase of the migration (63.8-65.8°N). Capelin amassed in the staging area until roe content reached 12- 14%. Several mechanisms are involved in the migration. The movement southward appears to be innate, but the route tracks an offshore thermal corridor until reaching the Atlantic Ocean front. Capelin then stage in the warmer waters until gonad maturity reaches a roe content of 12- 14% before moving inshore towards the spawning grounds. The dependence on temperature suggests that warming temperatures north of Iceland (to >4.5°C) could result in capelin spawning grounds shifting north as occurred in the 1930's. The extended migration route over deep waters seaward of the shelf edge and the act of staging may have evolved to minimize the period of spatial overlap with cod (and hence predation) that occurs on the shelf.

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