Minke whales and harp seals are the most important marine mammal predators with respect to fish consumption. Consumption estimates for minke whales (Folkow et al., 2000) and harp seals (Nilssen et al., 2000) are shown in Figure 2.6.7. These estimates are based on stock size estimates of 85 000 minke whales in the Barents Sea and Norwegian coastal waters (Schweder et al., 1997) and of 2 223 000 harp seals in the Barents Sea (ICES 1999/ACFM:7). Consumption by harp seal is calculated for situations with both a large and a small capelin stock, while consumption by minke whales is calculated for a situation with a large herring stock and a small capelin stock.
Food consumption by harp seals and minke whales combined is at about the same level as prey consumption by cod (Figure 2.6.6). Thus, predation of fish by these two marine mammal species should be considered when calculating the mortality of capelin and young herring in the Barents Sea as their impacts are potentially significant.
The diet of marine mammals varies through the season, depending in part on where they are foraging. While most of the cetaceans leave the Barents Sea in autumn, harp seals can spend the entire year within the Barents Sea. The seals that breed and moult in the White Sea in spring perform extensive migrations covering large parts of the Barents Sea during summer, autumn and winter. They can also be joined by West Ice animals during the summer in the Northern Barents Sea, where the two stocks can overlap geographically for some months. In spring, when migrating through the southern Barents Sea, harp seals feed predominantly on fish, such as herring and small cod. Through the summer, they migrate northwards, and their diet switches to polar cod and krill, and in the autumn amphipods and capelin tend to dominate.
Additionally, there is considerable interannual variation that is based on prey availability. For example, harp seal consumption estimates show very strong patterns that are affiliated with abundance of various fish stocks through time (Figure 2.6.8). Minke whales show similar levels of variation in the prey they target through time. In the period 1992-1999, the mean annual consumption of immature herring by minke whales in the southern Barents Sea varied considerably (640 t –118 000 t) (Lindstrøm et al., 2002). Most of the herring consumed belonged to the strong 1991 and 1992 year classes. But, there was a substantial reduction in the dietary importance of herring to whales after 1995, when a major part of both the 1991 and 1992 year classes migrated out of the Barents Sea and into the Norwegian Sea. This migration reduced the role of herring as a prey species for marine mammals in the Barents Sea, which was reflected by a more northern minke whale distribution in 1995 compared to earlier years (Eriksen, 2006). However, the importance of herring as prey increased in the Norwegian Sea in 1995, where minke whales seemed to track the migrating herring towards the Polar Front, thus reducing the role of shelf feeding observed in minke whales prior to 1995 (Eriksen, 2006). The dietary importance of herring to minke whales appeared to increase in a non-linear relation with herring abundance, indicating that minke whales switch to alternative prey species when herring abundance decreases below a certain level (Lindstrøm et al., 2002).
During IMR ecosystem cruises in August-September 2003-2007 information on spatial distribution of marine mammals relative to prey distributions have been collected. During this time the Barents Sea system had low densities of capelin, a major forage fish for the pelagic marine mammals in the Barents Sea under normal circumstances. While the capelin was distributed in the central Barents Sea, abundant herring and blue whiting stocks were distributed in the southern Barents Sea and an abundant polar cod stock occurred in the northern Barents Sea (Figure 2.6.9). The main baleen whale species - minke, fin and humpback whales - were predominantly observed in Arctic Water masses north of the Polar Front. Only a small proportion of the minke and fin whales observed occurred in the southern Barents Sea (Figure 2.6.9). Furthermore, the baleen whales observed in the northern parts of the Barents Sea were typically aggregated at the rim of the capelin and polar cod distributions (Figure 2.6.9), in areas with elevated densities of larger zooplankton. This implies that the baleen whales, at least in years with low capelin densities, target other prey species such as the larger zooplankton. Furthermore, this aggregated distribution suggests that i) the baleen whales avoid areas with the highest pelagic fish densities, possibly due to prey depletion in these areas, and ii) that baleen whales and pelagic fish in arctic waters are competitors and that this competition structures the baleen whales’ distributions. Being large-bodied, homoeothermic animals, the baleen whales require a high feeding rate, which may limit their distributions to areas which have forage fish stocks which have not been depleted by pelagic fish. In the southern Barents Sea, both fin and minke whales aggregated at high herring and blue whiting densities in recent years, indicating that target pelagic fish in this area. Nevertheless, the low density of baleen whales in southern BS suggests that the abundant southern pelagic fish stocks experience relatively low predation pressure from baleen whales, even when capelin abundance is low.