Nearshore Marine Ecosystem Research

With the exception of 13 small remnant populations, sea otters were extirpated from their historic range in the north Pacific Ocean during the 18th and 19th centuries as a result of the commercial harvest for their fur. During most of the 20th century, through protection and reintroduction, sea otter populations generally increased in abundance and distribution such that most of their range in Alaska, with the exception of southeast Alaska, was occupied by 2000. 

Although population abundance data are incomplete, there is evidence of increasing, stable and declining sea otter populations in different areas within their range. The factors that ultimately regulate sea otter population abundance are not completely understood, but can include predation, human harvest, food limitation, disease and catastrophic events such as oil spill. 

There is good evidence that the recent declines in sea otters in SW Alaska are related to killer whale predation and the Exxon Valdez oil spill reduced the size of the western Prince William Sound population in 1989. Human harvest of sea otters can adversely affect sea otter abundance, evidenced by the commercial fur trade leading to near extirpation. Because sea otters occupy relatively small home ranges and do not migrate, sustainable harvest requires management at appropriate spatial scales. Recently, harvest of sea otters for subsistence have been increasing, although effects of the harvest at current levels on population trend are unknown. Because sea otter populations occur over vast and remote areas and may display divergent trends in abundance over relatively small spatial scales, determining population status and trends can be challenging.

Methods to assess sea otter population status and trends are important to evaluate the recovery of populations and the potential effects of human perturbations (e.g., harvest, contaminants, and habitat modifications) on populations. This information is important to resource managers in identifying potential conflicts, identifying mechanisms of change, and improving the ability to detect and respond to change from human induced sources.

Objectives of our sea otter population assessment studies include: 

1) Develop and test methods to identify the degree of population structuring among north Pacific sea otter populations

2) Develop and test techniques to accurately and precisely estimate the status of sea otter populations

3) Develop and test methods to identify cause(s) of change in the status and numeric trends of sea otter populations

4) Develop and test methods to determine the role of density dependent processes in affecting change in sea otter populations

5) Evaluate the effects of population reductions and translocations on sea otter genetic variability.

Sea otters were severely impacted by the 1989 Exxon Valdez oil spill. Estimates of acute spill related mortality range from about 1,000 to 5,500 in the first months after the spill. Scientists with the Alaska Science Center were among the first responders to the 1989 spill and continue work today to document the process of recovery form this spill and to better understand the effects future contamination events on sea otters and the nearshore ecosystems they occupy.

One of the factors limiting our ability to clearly understand and document the spill effects was a lack of accurate estimates of sea otter abundance. This was true for nearly all species in the Gulf of Alaska and remains an impediment in assessing injury from such catastrophes across most landscapes today. Initial research efforts following the spill focused on damage assessment, including developing methods to accurately estimate the abundance of affected populations and studies of reproduction and survival.

Large scale ecosystem level studies of nearshore species and habitats most affected by the spill completed in 1999, found evidence of long-term spill effects among nearshore species dependent on a nearshore food web where benthic invertebrates transfer primary production to upper level consumers such as sea otters and sea ducks. Biochemical and gene techniques suggested that lingering oil may have contributed to a protracted recovery period for nearshore species. Subsequently, surveys of beaches where oil was deposited nearly a decade earlier found unanticipated volumes of oil sequestered in nearly 20 acres of widely distributed soft sediment intertidal beaches in Prince William Sound.

Our most recent surveys of sea otter abundance indicate significant progress toward recovery, when we consider the entire spill affected area in the Sound. By 2009 our estimate of sea otter abundance in the western Sound was nearly 2,000 animals more than our first post spill estimate in 1993 of about 2,000 individuals. However, when we look only at those areas that were most severely affected by the spill, where sea otter mortality approached 90% and where much of the lingering oil has been located, evidence of recovery remains incomplete. Our most recent research, based on the diving behavior of sea otters in the intertidal and published oil encounter rates, indicates that all sea otters in those heavily oiled areas are likely to encounter Exxon Valdez oil at least annually and some as often as weekly. Long term continuation of studies investigating mortality from the annual collections of beach cast sea otter carcasses implicates elevated mortality as the factor most likely contributing to delayed recovery and suggests that chronic mortality after the spill may meet or exceed the acute mortality experienced after the spill.

The Alaska Science Center, and in preceding Department of Interior agencies, has been engaged in monitoring various sea otter populations for more than 50 years, since Karl Kenyon’s seminal work in the Aleutian Islands. As sea otter populations have recovered from the fur trade and translocations contributed to expanding populations, the task of sea otter monitoring has become increasingly difficult simply because of the vast and remote nature of sea otter habitat. Moreover, it has become increasingly evident that monitoring of single species, while perhaps necessary for management purposes, often provides little insight as to why changes in abundance occur over time. As a result, we have been engaged in the development, design and testing of monitoring protocols for nearshore habitats and species, including sea otters, that might best be described as “ecosystem” or “food web” based monitoring.

The nearshore is considered an important component of the Gulf of Alaska ecosystem, including the region affected by the Exxon Valdez oil spill, because it provides:

• A variety of unique habitats for resident organisms (e.g. sea otters, harbor seals, shorebirds, seabirds, nearshore fishes, kelps, seagrasses, clams, mussels, and sea stars).
• Nursery grounds for marine animals from other habitats (e.g. crabs, salmon, herring, and seabirds).
• Feeding grounds for important consumers, including killer whales, harbor seals, sea otters, sea lions, sea ducks, shore birds and many fish and shellfish.
• A source of animals important to commercial and subsistence harvests (e.g. marine mammals, fishes, crabs, mussels, clams, chitons, and octopus).
• An important site of recreational activities including fishing, boating, camping, and nature viewing.
• A source of primary production for export to adjacent habitats (primarily by kelps, other seaweeds, and eelgrass), as well as a recipient for primary (phytoplankton) and secondary production (zooplankton) transported from offshore systems.
• An important triple interface between air, land and sea that provides linkages for transfer of water, nutrients, and species between watersheds and offshore habitats.

The underlying assumption in our monitoring design is that change will occur, and that careful consideration of what to monitor, may eventually provide insight as to why observed change occurred. In the nearshore ecosystem we work in, primary productivity is provided by at least two independent sources, the micro-algae, or phytoplankton, that occurs near the sea surface and may be transported inshore via currents. The second, and sometimes major contributor to total primary production is through the kelps and sea grasses that are conspicuous features of the nearshore zone. These combined sources of carbon fuel a diverse community of invertebrates, such as mussels, clams, snails, crustaceans, and urchins, that ultimately transfer their energy to various higher trophic level invertebrates and vertebrates, such as fishes, birds (shore birds, sea ducks and others) and mammals (primarily sea otters). Through careful selection of species and processes (growth, survival and diet) we expect to gain a better understanding of the interaction between various trophic levels that will allow us to potentially assign cause to some of the change we expect to see over time.