Biology of Marine Mammals (MSCI/BIOL.375)

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Estes, J. E., M. T. Tinker, T. M. Williams, D. F. Doak. 1998. Killer Whale Predation on Sea Otters Linking Oceanic and Nearshore Ecosystems. Science 282:473-476.

Presented by Stephen Carleton and Chris Flanary

Study Site

Aleutian Islands: Adak, Amchitka, Little Kiska, and Kagalaska Islands.

Northeastern Pacific Ocean

Background Information

Three Trophic Levels

Producers: Kelp (Laminaria)

Consumers: Sea Urchins

Predators: Sea Otters (Enhydra lutris)

Sea Otter Decline: There were larges areas of sea otter decline. In the 1990’s there was a 25% per year loss. There was an order of magnitude decrease of sea otters by 1997.

Probable Causes

Sea otter decline is evident, but what seems to be causing the loss of this keystone species?

Reduced Fertility

Redistribution

Increased Mortality

Reduced Fertility

Radio Tagging Studies: Radio tagging at Adak and Amchitka islands show birth rates and pup survival rates that are similar to stable populations.

Pup Survival Rates

Excluded: Due to this data, this theory was excluded from the study as a cause of sea otter decline.

Redistribution

Synchronous: There were synchronous declines of sea otters over a wide spread area in these islands.

Wide Spread

No Concurrent Increase: There were no population build-ups on other islands to account for the loss of population on some islands.

Radio Tagging: Radio tagged sea otters of Amchitka and Adak had no identification of redistribution during the decline of population.

Excluded: Due to this data, the theory of redistribution was excluded from the study.

Increased Mortality

Disease: There were very few beached cast carcasses of sea otters or physical signs of disease.

Contaminants/Toxins: There were some elevated levels of toxins, but they were restricted to a few small areas.

Starvation: There was a steady increase of urchin biomass, so the sea otters main food source was not depleted.

Predation: This was believed to account for 78% of the reduction in population.

Orcinus orca Introduction

Four Level Trophic System

Producers: Kelp (Laminaria)

Consumers: Sea Urchins

Predators: Sea Otters (Enhyra lutris)

Apex Predators: Killer Whale (Orcinus orca)

Coastal/Oceanic Ecosystem Interaction

The four level trophic chart. The sea otters were decreasing, the sea urchin biomass was increasing and the kelp biomass was decreasing. This data was from Adak Island.

Oceanic Impacts on Sea Otter Abundance

The sea otter counts in Clam Lagoon and Kuluk Bay. This chart shows a significant decrease in sea otter population in Kuluk Bay, which was exposed to the open ocean. The Clam Lagoon was an enclosed environment and the populations were at a steady state.

Predation Progression

Reduced or Altered Fish Stocks: This could be due to anthropogenic effects.

Increases in Ocean Temperature

Depletion of Baleen Whales

Decreases in Piscivorous Marine Birds

Declining Pinniped Populations

Orca Diet Expansion: An orca needs 1825 otters per year if feeding exclusively and it would only take 3.7 orcas to count for the decrease in the sea otter population. Marine mammals (on average) must have a higher intake of energy than many other animals (Berta, 99). Orcas are known for their varied diets (as a species) and are suspected to change diets with availability of local food supply (Baird, 94). Individual pods in close geographic proximity will exhibit dietary variety. Orcas in two overlapping populations in the Northwest Pacific exhibit different foraging behaviors, different vocalizations, different social group sizes and prey preference (Waller, 96).

They are active predators. Opportunistic feeders, their diet varies from one region to another. In the Antarctic, killer whales eat about 67% fishes, 27% marine mammals, and 6% squids. Killer whales in the Bering Sea (near Alaska), eat about 65% fishes, 20% squids, and 15% marine mammals. Food preferences of resident and transient pods in the North Pacific differ. They eat a variety of fish, including salmon, cod, flatfish, hake, herring, and smelt. They also eat other marine mammals and seabirds (www.seaworld.com). Killer whales prey on mysticete and odontocete whales, seals, sea lions, walruses, and occasionally sea otters and penguins. Killer whales tend to select habitats with suitable prey sources and will seek out new ones when initial sources are depleted (Castro, 97).

Repercussions of Sea Otter Decline

Urchin Biomass Increase: The biomass of sea urchins had an increase of 8 fold in a 10-year period. There was a kelp density decrease of 12 fold. The decrease percentages went from 1.1% in 1991 to 47.5 % in 1997.

Kelp Density Decrease

Further Studies

Closed Environments: Controlled environments where there is a four-level ecosystem.

More Expansive Tagging: More expansive tagging of orcas and sea otter to help find feeding patterns and locations.

Gut Content

Investigation of other Predators: Other predators could be causing some of this decrease such as sharks, and poachers (http://whales.magna.com).

Bibliography

Baird, R.W. 1994. Foraging behaviour and ecology of transient

killer whales (Orcinus orca). Ph.D. Thesis, Simon Fraser

University, Burnaby, B.C. 157pp.

Berta, A., J. Sumich. Marine Mammals: Evolutionary Biology. San Diego: AP, 1999.

Castro, P., M. Huber. Marine Biology. Boston: McGraw Hill, 1997.

Waller, G, ed. SeaLife: A Complete Guide to the Marine Environment. Washington DC: Smithsonian Inst. P, 1996.

http://whales.magna.com.au/DISCOVER/ORCA/orcag.html

http://www.seaworld.org/killer_whale/dietkw.html

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