Nereis Project

nereis project

The sea otter (Enhydra lutris) is the only member of the genus Enhydra. It is the largest member of the family Mustelidae which includes approximately 70 species; the sea otter is also the smallest marine mammal. The sea otter is limited in distribution to nearshore waters of the northern Pacific Ocean. Sea otters once ranged from northern Japan to the Alaskan peninsula and along the west coast of North America to Baja California in Mexico. Until the 1700s, sea otters were abundant throughout the waters of the North Pacific and for centuries native groups, such as the Aleuts, hunted them. During this time, the worldwide sea otter population probably numbered 150,000 to 300,000 animals. By the mid-1700s, Russian hunters had coerced the Aleuts to exploit sea otters for the fur trade, and the once abundant sea otter population plummeted. The otters that remained were chased down by English, French, Japanese and American traders. By the 1900s, the sea otter was nearly extinct with only 1,000 to 2,000 left. Only 13 remnant sea otter colonies existed from Russia to Mexico when the International Fur Seal Treaty, which banned the hunting of sea otters and fur seals, was established in 1911 (Kenyon 1975, 1981; Johnson 1982; Riedman and Estes 1990).

Based on morphological differences in color, body and skull sizes, three sub-species of sea otters have been proposed historically: Enhydra lutris lutris from the western Alaska archipelago, Enhydra lutris kenyoni from Prince William Sound, Alaska, and Enhydra lutris nereis, the California or southern sea otter (Anderson et al. 1996). Cronin et al. (1996) proposed the classification of otters from the Kuril Islands as Enhydra lutris gracilis but this nomenclature has not been officially recognized.

The status of sea otter populations is currently an issue of concern at the international level. Except for the California sea otter population, which has never increased more than 5.5% annually, all other populations have increased up to 20% per year from the time of their re-establishment in the late 1960s through the late 1980s (Estes 1990). Since then, there have been new and unexplained declines throughout the majority of the southwest Alaska stock which is now proposed as “threatened” under the Endangered Species Act (USFWS 2001; Burn et al. 2003; Doroff et al. 2003). As of summer 2003, the number of sea otters in the Aleutian archipelago has declined to about five-percent of its estimated pre-decline abundance and the rates of decline in this region appear to be increasing at an alarming rate (Maldini et al. 2004).

Once ranging from Baja California, Mexico all the way to Oregon and the southeast Alaska border, the southern sea otter is now confined to waters along the central California coast from Point Aňo Nuevo, Santa Cruz County south to Purisima Point, Santa Barbara County (Wilson et al. 1991). A small experimental population has been re-introduced at San Nicolas Island, one of the Channel Islands in Ventura County with mixed success (Jameson et al. 1992).

By the 1930s, the California sea otter population consisted only of a small group of 50 to 300 sea otters restricted to the Big Sur coast of central California. Under the protection of the International Fur Seal Treaty, this small population began a slow and steady climb from nearly extinct to a fairly stable population. From the mid 1970s to the mid 1980s, the southern sea otter population began to decline once again. In 1977, the southern sea otter was listed as “threatened” under the Endangered Species Act.  About 1,000 sea otters died over a 10 year period due to entrapment in gill nets. When gill net legislation was passed in the late 1980s requiring gill nets to move farther off shore, the sea otter population began to grow again until the mid-1990s (Siniff and Ralls 1988; Estes 1990; Estes et al. 1995; Ralls et al. 1996).

From 1995 until the present, southern sea otter numbers have declined in five out of the last six years. Some of the possible causes include entrapment in fisheries gear, disease, food limitations, and habitat loss and degradation due to contaminants (Wendell et al. 1986, Estes 2003, Miller et al. 2005). Sea otter protection is a source of ongoing conflict between the California shellfish (abalone, sea urchin, crab, lobster) industries and conservation groups (Estes and VanBlaricom 1985). While fishermen view the shellfish eating sea otter as a threat to their livelihood, conservation groups and scientists see the sea otter as a keystone species because their activity is central to the nature of their ecosystem. By consuming large numbers of sea urchins, sea otters have been shown to be instrumental in maintaining the health and persistence of kelp forest habitats in Alaska waters (Estes et al. 1978; Estes et al. 1982). Without the balancing presence of sea otters, sea urchins end up overgrazing kelp forests and creating underwater deserts known as “urchin barrens.” A sea otter’s effect on the ecosystem is disproportionate to how many sea otters there are. Very few sea otters can have a large effect.

The California sea otter population, currently at about 2,000 individuals, is well below the estimated carrying capacity (16,000 animals) for the State of California. Because of its small size, sluggish growth, and numerous perceived threats, the California sea otter population is also listed as “threatened” under the US Endangered Species Act. High mortality rates appear to be largely responsible for the sluggish growth and abundance fluctuations (Estes 2003, Hanni 2003, Kreuder et al. 2003). The causes of this mortality are not well understood, and are being studied both by examining stranded carcasses and by conducting detailed longitudinal studies of living sea otters (Kreuder et al. 2003). One poorly understood threat is toxoplasmosis, an often lethal and otherwise debilitating infection caused by Toxoplasma gondii, a micro-organism that has as intermediate host wild, feral and domestic cats. Oocysts are released in cat feces and can persist in the environment for up to two years. The main infection mechanisms is thought to be the ingestion of oocysts either directly from seawater or concentrated in filter feeding invertebrates which are the otter’s staple diet.

The Importance of Male Areas

California sea otters are concentrated in numbers around Monterey Bay where several territorial male areas are found in prime feeding habitat, especially in dense kelp forest areas. These territories are held long-term by males fit enough to maintain them and the chance for young males to supplant the extant territorial males are infrequent. Young animals not holding a territory and recently weaned pups ousted from their natal area by the adult male tenant, and probably father, generally congregate in “male areas” which are found at the periphery of the sea otter range, and are supposed to be the areas where expansion into new unoccupied territories should occur. Otherwise, these young males may eventually supplant older adults in extant territories.

Not much effort has been devoted by researchers into understanding the dynamics of male areas and their importance to the health and increase of the sea otter population in California. High mortality in the young adult population, both male and female, has been advocated as a potential threat to the longevity of the California population and, as mentioned before, disease may currently play a large role in the mortality rate.

Males in male areas tend to rest in large rafts which are dynamic social units. The social role of male rafts has not been investigated. We hypothesize that male rafts play an essential role in developing young males’ fitness potential by providing “role modeling” by older mature males, which are generally found interspersed with large number of immature males and young adults. Rafts also provide shelter and potential protection by predators using the safety in number model. In addition, we propose that the social skills acquired by male otters in the social context of male areas are as important as the skills acquired by the pup form its mother up to weaning. Rafts provide tactile stimulation to young otters that have been in constant body contact with their mother up to weaning, and are a forum for mock fighting which helps develop the skills necessary to confront territorial males when young adults are ready to claim a territory. 

Elkhorn Slough and Sea Otter Population Dynamics

An important male area in Monterey Bay is located at the center of the bay, around the coastal shallow waters between Moss Landing and the Pajaro River mouth and, principally, inside Elkhorn Slough, a seasonal estuary and the second most important wetland area along the California coast. Elkhorn Slough is part of the Monterey Bay National Marine Sanctuary and is a National Estuarine Research Reserve.

The first documented invasion of Elkhorn Slough by male sea otters occurred between 1994 and 1995 (Maldini-Feinholz 1998). Up to 4% of the California population and a more significant percent of the juvenile male sea otter population used Elkhorn Slough during the past 10 years making the slough a significant sea otter habitat along the California coast (Kieckhefer et al. 2004).

Between 1994 and 2001, the research teams associated with this study counted up to 81 otters using the slough. From a maximum mean number of 52 otters in 1998, otter counts then dropped sharply to 27 in 2001, and have remained low until recently (Hoffman 2003, Maldini unpublished data). Aerial surveys in 2002 and 2003 showed an increase in otter counts in offshore areas adjacent to Elkhorn Slough, suggesting that the otters had relocated (Kieckhefer 2004). In 2006, we documented up to 83 animals in using Elkhorn Slough for resting, although the majority of the population still foraged mainly in the harbor channel and in the nearshore sandy bottom coastline rather than in the main slough channel as in the past (Maldini unpublished data).

The long term monitoring of sea otter use of Elkhorn Slough as a foraging area revealed patterns of sea otter invasions of the main channel, with animals penetrating up to five miles inland to the limit of the distribution of clam beds in the estuary, and subsequent retreat from the main channel as food resources were depleted (Kieckhefer et al. 2004). In soft-bottomed habitats like the Elkhorn Slough area, sea otters prefer clams, innkeeper worms and other burrowing invertebrates that live in mud (Kvitek and Oliver 1986; Anderson and Kvitek 1987; Jolly 1997, Maldini et al. 2010 {click here for paper}). It is still unclear whether prey may have decreased due to over-foraging by otters, or by severe erosion of the slough banks due to tidal flow, which changed the substrate from mud to bedrock. Some researchers theorize that the unusually high number of otter mortalities in 2003 may have been related to the slough otters’ diet shift to offshore crabs (predominantly Dungeness). In addition, high levels of domoic acid (a fatal neurotoxin produced by oceanic diatoms) recorded during this time period may have played a significant role in sea otter mortality in 2004 (Kieckhefer, unpublished data).

Human-Related Disturbance and Habituation

In the last decade there has been an approximately 12% growth in total population in Monterey Bay (U.S. Census Bureau, 2005 American Community Survey). In addition, infrastructure development and increase in eco-tourism and recreational activities have increased access to many of natural areas in Monterey Bay.

Tourism activity in Elkhorn Slough has increased dramatically from 1990 to today with Kayak Outfitters increasing their business activities and tours becoming more popular especially on the week-ends. Boat traffic is also high in Moss Landing Harbor, at the mouth of the slough, and occasionally, small fishing vessels use the slough for recreational activities (Rodriguez et al. 2010 {click here for poster}).

Sea otters rest and forage right in the midst of intense human activity spending a large portion of their time budget in the middle of the boat channel and near the main launch ramp at Moss Landing Harbor. In the last couple of years, residents and our research team have documented an increase in human-sea otter interactions, with animals climbing on kayaks, coming to shore to chase people, and showing little fear of humans while in the water. Otters can inflict serious bite injuries if molested and rehabilitated animals released into the wild have been documented to attack or harass people or to beg for food.

Little is knows about how habituation will impact the animal fitness in the wild and whether these patterns of habituation will result in detrimental consequences for sea otters which will have to be removed from the area if they become a hazard to humans. Conversley, the consequence of human activities on the sea otter behavioral budget and resting patterns has been poorly studied. Moss Landing Harbor is an ideal location to monitor human-sea otter interactions and the potential consequences of human related traffic and activities on sea otter behavioral budgets, especially in relation to resting activities (Rodriguez et al. 2010 {click here for poster}).

Sea otters spend a considerable amount of time foraging to compensate for their high metabolic rate and can consume up to 25% of their body weight every day. An increase in energy expenditure by sea otters will result in increased metabolic requirements and therefore in increased feeding rates. Resting is a way to save energy. Resting in a raft may also provide additional insulation from severe weather conditions. When sea otters rest their body is raised as far out of the water as possible and their paws are kept out of the water and extended upward to keep them dry. This saves additional energy. The most cost efficient resting behavior is probable obtained by hauling out onto shore and becoming completely dry (Maldini et al. 2012 {click here for paper}).

Every time resting otters are disturbed, the animals roll out of sleep and often start moving away from the disturbance or dive if startled. This causes the animal to become wet and therefore consuming more energy to stay warm.

Our hypothesis is that, as a consequence of human activities, sea otters spend longer times in activities other than resting causing the animals to have to forage longer or more efficiently to make up for the loss in calories. In the long-run, this upsets the otter metabolic cycle and potentially affects them physiologically.