A Cold Water Killer

Before I came to the University of Hawaii, I was researching the habitat use and behavior of an active top predator, the salmon shark (Lamna ditropis) in the North Pacific Ocean. Salmon sharks have a widespread geographic distribution. Some travel large distances as they make seasonal migrations from Alaska all the way down to Baja California, probably to feed or give birth to their young, while others overwinter in the cold waters of the Gulf of Alaska. Furthermore, they are able to withstand a broad scope of temperatures, ranging from 2.5°C (36.5°F) to 23.1°C (73.5°F). Salmon sharks are able to live in such diverse and cold habitats due to their endothermic capacity, the ability to regulate their own internal body temperature (just like us!). Salmon sharks have been shown to have internal body temperatures as high as 21°C (70°F) above the surrounding water temperature! This allows salmon sharks to be highly active predators across a broad range of depth and temperatures.

DC1 DC2Salmon shark (Lamna ditropis) and its geographic range. Images courtesy of S. Anderson (left) and C. Huh (right).

 

 

Understanding the factors that influence their distribution and habitat use is central to effective management of salmon sharkpopulations, determination of essential habitats and potential interactions with other species. Habitat use can be influenced by a wide variety of things, including food availability, competition, the risk of predators, and reproductive and social behavior. As a large top predator and consumer of a wide variety of prey (e.g., salmon, pollock, herring, squid), salmon sharks are unlikely to face either predation risk or competition for their prey. Thus, salmon shark habitat use is determined mostly by prey availability. In addition, environmental conditions such as temperature and oxygen can be other contributing factors. Just like us, salmon sharks can’t be in too hot or too cold of climates, and always need enough oxygen to breath. A better understanding of salmon shark habitat will allow us to predict potential changes in their distribution associated with climate change. Furthermore, information on any habitat that overlaps with fisheries will show potential sources of bycatch (non-targeted) mortality.

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Salmon shark dorsal fin with an attached satellite tag that allows researchers to track its movements. Image courtesy of the Census of Marine Life Tagging of Pacific Predators program (TOPP).

 

 

 

Understanding how environmental factors influence distribution patterns is important for marine resource management and conservation. Salmon sharks, like all other sharks, are slow growing, take a long time to become sexual mature, and give birth to only a few pups. This makes them particularly vulnerable to overexploitation. Given the susceptibility of sharks to overfishing and the important ecological role that salmon sharks likely play in North Pacific ecosystems as one of the largest top predators in these systems, they need to be managed responsibly.

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Salmon shark leaping after a salmon. Image courtesy of S. Anderson.

 

 

 

 

 

By Danny Coffey

 

References:

Goldman, K.J., and Musick, J.A. 2008. The biology and ecology of the salmon shark, Lamna ditropis. In: Camhi, M.D., Pikitch, E.K., and Babcock, E.A. (eds.). Sharks of the open ocean: biology, fisheries and conservation. Blackwell Publishing, Oxford, pp. 95–104.

Goldman, K.J., Anderson, S.D., Latour, R.J., and Musick, J.A. 2004. Homeothermy in adult salmon sharks, Lamna ditropis. Environ. Biol. Fishes, 71(4): 403–411.

Weng, K.C., Castilho, P.C., Morrissette, J.M., Landeira-Fernandez, A.M., Holts, D.B., Schallert, R.J., Goldman, K.J., and Block, B.A. 2005. Satellite tagging and cardiac physiology reveal niche expansion in salmon sharks. Science (Washington, D.C.). 310(5745): 104–106.

Weng, K.C., Foley, D.G., Ganong, J.E., Perle, C., Shillinger, G.L., and Block, B.A. 2008. Migration of an upper trophic level predator, the salmon shark Lamna ditropis, between distant ecoregions. Mar. Ecol. Prog. Ser. 372: 253–264.

 

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