Research Technical Report
A PDF version of this report is available here:
Croll D.A., and B.R. Tershy (2002)
In: W.F. Perrin, B. Wursig, J.G.M. Thewissen (editors), Encyclopedia of Marine Mammals. Academic Press, San Diego, California. p. 428-431.
I. Filter Feeding and the Marine Environment
A fundamental necessity for any organism is acquiring sufficient food for maintenance, growth, and reproduction. This search for food likely drove the return of mammals to the ocean where they were able to exploit highly productive coastal waters. With their return to the sea, marine mammals evolved a number of foraging techniques. Filter feeding, found in the mysticete whales and three species of pinnipeds (crabeater seals leopard seals, and Antarctic fur seals) is the most unique of these adaptations for feeding, and is not found in any terrestrial mammals.
Filter feeding allows these marine mammals to exploit extremely abundant, but small schooling fish and crustaceans by taking many individual prey items in a single feeding event. This adaptation arose in response to the unique patterns of productivity and prey availability in marine ecosystems.
Low standing biomass and high turnover of small-sized primary producers that respond rapidly to nutrient availability characterize marine food webs. Due to spatial differences in the physical dynamics of marine ecosystems, productivity tends to be more patchy and ephemeral than in terrestrial systems. Consequently, marine grazers (e.g. schooling crustaceans and fish) often occur in extremely high densities near these patches of high primary production. Most marine mammals are primary carnivores and feed on these dense, patchily distributed aggregations of schooling prey. The spatial and temporal patchiness of this prey means that marine mammals must often travel long distances to locate prey, and the larger body size of marine mammals likely plays an important role.
Initially, thermoregulatory requirements selected for larger body sizes as mammals returned to the ocean. However, once dependent upon marine prey, large body size also provided a buffer for the patchy and ephemeral distribution of marine prey. Thus, larger individuals could endure longer periods and travel longer distances between periodic feeding events on patchy prey. While adaptive for exploiting patchy prey resources, a consequence of larger body size is a higher average daily prey requirement. For marine mammals that feed upon patchy and ephemeral resources, this requires individuals to take in large quantities of prey during the short periods of time it is available.
Filter feeding is a foraging strategy that allows individuals to capture and process large quantities of prey in single mouth full, thus allowing them to acquire energy at high rates when small prey are aggregated. Indeed, for mysticetes, large body size is probably a prerequisite for attaining a sufficiently large surface area for filter feeding. Thus, the interaction of availability of prey resources, high concentrations of prey in schools, and selection for large body size likely led to the evolution of filter feeding. Ultimately, large body size and filter feeding allowed some marine mammals to exploit the extremely high densities of schooling prey that develop at high latitudes during the spring and summer, but fast during the winter when these resources disappear. Large body size provided an energy store for wintering and long distance migration without feeding.
Due to this dependency on patchy but extremely productive food resources, it is not surprising that filter feeding whales are believed to have first evolved and radiated in the southern hemisphere during the Oligocene at the initiation of the Antarctic Circumpolar Current (ACC). It is generally agreed that the initiation of the ACC led tocooling of the southern oceans, increased nutrient availability and thus increased productivity. This increased productivity provided a rich resource of zooplankton that could be effectively exploited through filter feeding.
Present-day filter feeding marine mammals concentrate their foraging in polar regions and highly productive coastal upwelling regions. The southern ocean is still the most important foraging area for filter-feeding marine mammals. Prior to their exploitation by humans, the highest densities of mysticetes occurred in highly productive southern waters. Crabeater seals, Antarctic fur seals and leopard seals are found primarily in the southern oceans where seasonally dense aggregations of krill develop.