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

Abstract

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°N) to feed during summer in deep cold waters (>500 m; 1–3°C) before migrating south (63–65°N) to spawn in winter in warm shallow waters on the south and west coasts of Iceland (<100 m; 5–7°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°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°C). Further south, between 65 and 64°N, as temperatures warmed to 4.5°C (reaching 7.9°C at 63.5°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.

Synchronous and vertically undulating swimming behaviour of Atlantic cod Gadus morhua

“…Usually, the pelagic prey of cod form aggregations that have dimensions that are orders of magnitude larger in the horizontal than vertical plane (Ressler et al 2005, Olafsdottir & Rose 2012. Therefore, vertical searching may be an important strategy in locating these prey patches.…”

Using vessel monitoring system (VMS) data to assess the impact of marine protection boundaries on blue ling fishing northwest of the British Isles

“…Interannual variability in the spatial distribution of the landings (inferred from VMS signals) suggests that additional factors may have been influencing fishing activity. For instance, previous research has indicated that changes in sea temperature, currents, subsurface upwelling and attendant food availability (associated not only with seasonal and interannual variability, but also with less predictable events such as storms and turbidity currents) can influence the migratory routes of deep sea fish and their choice of location for gonad development and spawning activity (Overholtz et al 2011;Crawford et al 2012;Jury 2012;Olafsdottir and Rose 2012). The variability is often reflected in the spatial distribution of fishing activity, as captured in Figure 4; in general, annual fluctuations in the regions of greatest intensity tend to travel back and forth along the depth contours of the continental slope.…”

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

“…Capelin ( Mallotus villosus ) are a small (12–20 cm at maturity) schooling pelagic fish with circumpolar distribution (Vilhjálmsson, ). In Icelandic waters, capelin undertake an annual spawning migration each winter that covers 500–1000 km from northern summer feeding areas in the Iceland Sea (67–71°N; Vilhjálmsson, ) to coastal spawning waters (<200 m bottom depth) south and west of Iceland (Vilhjálmsson, ; Olafsdottir and Rose, ). This migration has been researched annually since the late 1970s because of its commercial and biological importance (Vilhjálmsson, ).…”

“…Thereafter, capelin migrate inshore to spawn. Early phases of the migration appear to vary temporarily, with the leading edge of the migration reported to reach latitude 64°N at variable times during January (Vilhjálmsson, ; Olafsdottir and Rose, ). However, such temporal variability in the early phases of migration does not appear to influence the timing of the final phase of the migration onto the continental shelf, as capelin have never been located in inshore waters before February.…”

“…In capelin, gonad maturation progresses through the winter months and spawning occurs when roe content reaches 25–30%, typically from late February to early April (Vilhjálmsson, ). Offshore staging could have evolved to enable gonad maturity to reach a specific threshold before capelin cross the continental shelf, as they do each year after they encounter the warm Atlantic waters (>4.5°C) between latitudes 64° and 65°N (Olafsdottir and Rose, ). But a question arises as to why they would remain offshore when they could just as easily move inshore and await maturation.…”