Overview
Marine mammals comprise an important apex predator group in the California Current Ecosystem with 24 whale, 11 dolphin, 2 porpoise, 6 pinniped, and one fissiped species.
A number of marine mammal populations were severely depleted by commercial or bounty hunting in the late 19th and early 20th centuries. All species were protected under the federal Marine Mammal Protection Act of 1972 and since then many populations have rebounded. Much remains to be learned about their role in the ecosystem following population depletion and recovery.
Variability in marine mammal distribution and abundance is driven by ecological interactions, anthropogenic pressures, and environmental variability and change. The CCIEA Marine Mammal Team focuses on tracking indicator species and identifying relationships among marine mammals and these drivers to inform management and the public of risks to recovery and conservation issues.
Ecological Interactions
Marine mammals rely on several trophic levels for their prey. Large baleen whales are filter feeders that extract large volumes of krill, euphausiids, and small pelagic fishes. Toothed whales and pinnipeds (seals and sea lions) feed on a variety of prey species, including coastal pelagic fishes, salmon, and cephalopods. Toothed whales also eat other marine mammals. Sea otters feed mostly nearshore on marine invertebrates such as sea urchins, bivalves, crustaceans, and crabs. They also consume cephalopods and fish.
Oceanographic and climate factors on large (e.g., El Niño Southern Oscillation) and small spatial scales (e.g., relaxation of localized upwelling) affect the abundance and distribution of marine mammal prey by altering upwelling, ocean temperatures and surface mixing patterns that influence ocean productivity. Even small changes in the marine environment can lead to changes in prey distribution or availability and can have large impacts on survival, growth and reproduction of marine mammals.
Environmental Drivers
The conceptual diagram above highlights the suite of environmental drivers that influence marine mammal prey availability and population health. As marine mammals are homeotherms, small changes in temperature have less direct effects on most marine mammal species but can have indirect effects through their prey. The basin-scale climate drivers (e.g. PDO, MEI, NPGO) provide a broad-scale index of environmental conditions, which translates into effects on the local environment. For example, a negative PDO phase indicates cool and windy conditions for the north Pacific, and in turn, increased upwelling and surface mixing, resulting in cooler sea surface temperatures and often higher productivity.
Many of the environmental effects on key prey species are outlined in the CPS section. While we are in the process of researching how climate variability influences marine mammals indirectly, much less is known on the potential future impacts of climate change. For instance, "How will warming oceans / increased upwelling affect marine mammal prey availability?" and "How will acidification affect the health of marine mammals system?" Climate change and warmer ocean waters will likely result in increased metabolic rates and prey consumption rates or redistribution and changes in habitat use following changes in their prey (See Climate Change Scenarios).
Human Activities
Many marine mammals are directly and indirectly affected by anthropogenic pressures, and in some cases they are limiting recovery. We provide a very broad overview of these pressures; a detailed assessment can be found in Carretta et al. (2013). Four direct effects from anthropogenic pressures include 1) pollution (marine debris, military training, ocean noise, and persistent contaminants ), 2) loss of coastal habitat (development, tourism), 3) fisheries bycatch and entanglements, and 4) ship strikes (shipping, military training). Pollution can be toxic chemicals mediated by their prey, plastics leading to entanglements or ingestion, or ocean noise that may have a large effect on marine mammal communication or prey detection. Loss of coastal habitat due to development is of particular concern to pinnipeds that rely on coastal haul-out sites for reproduction.
Fisheries bycatch (accidental catch) is a large problem for many marine mammals that become entangled in active fishing gear, lost gear or pot-lines. In some cases, discarded fish or the netted/hooked fish themselves can provide a food resource for marine mammals, but this can increase entanglement risk and can lead to negative interactions among fishers and marine mammals. Shipping and military operations occur regularly in California Current waters and ship-strikes are still a large source of mortality for a number of large marine mammal species. Ongoing efforts to move shipping lanes, slow down vessels in key areas, or predict ship-strike risk are underway to mitigate this source of mortality.
Indicator Status and Trends
Focal marine mammal species (four species of baleen whale, Dall's porpoise, short-beaked common dolphins, southern resident killer whales, and California sea lions) were selected to ensure representation across trophic levels, sensitivity to changes in environmental conditions, and vulnerability to human activities.
Indicators of California sea lion population abundance (survey-based estimates) and population condition (pup weight and mortality) are presented at the link above.
Links to other focal species data are a work in progress; please standby for updates.