The Influence of Forage Fish Ecology in Delimiting Marine Biological Hotspots in Eastern Canada

Investigations at regional scales consistently reveal areas where high abundances of top predators persistently overlap during certain periods of the year, or biological hotspots. As tracking, survey or by-catch data are often used to identify and describe hotspots, factors driving their formation are often speculative. Forage fish species provide essential linkages for energy transfer among trophic levels and, thus, high-density aggregations of forage fish species are generally thought to underlie the formation of hotspots. This uncertainty underscores the lack of knowledge of the basic ecology of forage fish species, which is critical to predict spatial and temporal shifts in hotspots. Capelin (Mallotus villosus) is the focal forage fish in the Northwest Atlantic on which most top predators rely for prey. We examined the distributional patterns of predators and capelin through at-sea surveys off the northeast Newfoundland coast during July-August, 2000-2010. High densities of chick-rearing seabirds and whales were persistently concentrated among years in the same 10 km² area. Combining hydroacoustics with underwater imagery and bio-physical monitoring, we investigated the mechanisms underlying hotspot formation. The hotspot was centered on four persistently used deep-water (17-40 m) spawning sites of capelin. Persistent sites were located within bathymetric depressions that allowed the retention of suitable sediment (range: 0.25-22.5 mm) for successful spawning. The timing of spawning and hotspot formation was earlier in years when temperature during capelin gonadal development (Feb-Jun) was warmer (r²=0.650, p=0.053). This hotspot represented critical foraging habitat for predators. Breeding seabirds minimized energetically costly search activities by using memory to repeatedly return to the hotspot. When the hotspot was not active during chick-rearing, breeding seabirds exhibited prey switching and lower reproductive success. When hotspot formation overlapped with the opening of the inshore fishery for Atlantic cod (Gadus morhua), high by-catch of breeding seabirds in gillnets was documented. As this hotspot was well defined in space and time, restricted fisheries activities during capelin spawning within this area is a suitable management option. This study illustrates the importance of elucidating the biology and behaviour of prey species when delimiting hotspot areas for protection as well as possible shifts in space and time among years. As energy flow through marine food webs is maximized within these areas, targeting persistent biological hotspots for protection is critical.