Research Themes -
Role of Marine Mammals in Marine Ecosystems

Understanding the impact that marine mammals have on fisheries is a complex issue. On one hand information on overall consumption by marine mammals is needed. This is determined using information on the abundance of marine mammals, their energy requirements, temporal and spatial changes in diet composition and marine mammal distribution. To assess impact, information is also needed on prey abundance, and the fraction of mortality in the prey population that can be attributed to marine mammal predators. Finally, information on factors affecting prey choice are needed to predict how marine mammals will respond to changing prey abundances. These factors could include abundance of the prey or local availability, energy return or benefit compared to the energy expenditure required to capture and handle the prey, nutritional needs and specialization by the predator on particular prey.

Marine mammals play a varied role in marine ecosystems, which in most cases is poorly understood. Marine mammals may act as top level predators feeding on other marine mammals (walrus, killer whale), on fish (killer whale, harp seal, hooded seal) or they may feed at much lower levels on benthic (sea otter, walrus, grey whale), and pelagic (harp seal) invertebrates. In some cases marine mammals play a very clear role in structuring marine ecosystems. For example sea otter predation on sea urchins reduces grazing pressure, which allows the proliferation of kelp bed forests. This leads to the development of new ecosystems and an increase in marine fauna density and diversity. As top level predators, marine mammals are also accused of having a negative impact on commercial fisheries through damage to fishing gear such as nets and traps, transmission of parasites, that must be removed before fish can be sold, and finally consumption of fish limiting their availability to fishermen.

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