Geology topics

Daniel H. Monson

Publications and source records attributed to Daniel H. Monson.

41 records · Page 3Linked to original sources

USGS2000

Long-term impacts of the Exxon Valdez oil spill on sea otters, assessed through age-dependent mortality patterns

We use age distributions of sea otters ( Enhydra lutris ) found dead on beaches of western Prince William Sound, Alaska, between 1976 and 1998 in conjunction with time-varying demographic models to test for lingering effects from the 1989 Exxon Valdez oil spill. Our results show that sea otters in this area had decreased survival rates in the years following the spill and that the effects of the spill on annual survival increased rather than dissipated for older animals. Otters born after the 1989 spill were affected less than those alive in March 1989, but do show continuing negative effects through 1998. Population-wide effects of the spill appear to have slowly dissipated through time, due largely to the loss of cohorts alive during the spill. Our results demonstrate that the difficult-to-detect long-term impacts of environmental disasters may still be highly significant and can be rigorously analyzed by using a combination of population data, modeling techniques, and statistical analyses.

Proceedings of the National Academy of Sciences of↗

USGS2000

Changes in sea urchins and kelp following a reduction in sea otter density as a result of the Exxon Valdez oil spill

Interactions between sea otters Enhydra lutris, sea urchins Strongylocentrotus droebachiensis, and kelp were investigated following the reduction in sea otter density in Prince William Sound, Alaska, after the Exxon Valdez oil spill in 1989. At northern Knight Island, a heavily oiled portion of the sound, sea otter abundance was reduced by a minimum of 50% by the oil spill, and from 1995 through 1998 remained at an estimated 66% lower than in 1973. Where sea otter densities were reduced, there were proportionally more large sea urchins. However, except in some widely scattered aggregations, both density and biomass of sea urchins were similar in an area of reduced sea otter density compared with an area where sea otters remained about 10 times more abundant. Furthermore, there was no change in kelp abundance in the area of reduced sea otter density. This is in contrast to greatly increased biomass of sea urchins and greatly reduced kelp density observed following an approximate 90% decline in sea otter abundance in the western Aleutian Islands. The variation in community response to a reduction in sea otters may be related to the magnitude of the reduction and the non-linear response by sea urchins to changes in predator abundance. The number of surviving sea otters may have been high enough to suppress sea urchin populations in Prince William Sound, but not in the Aleutians. Alternatively, differences in response may have been due to differences in the frequency or magnitude of sea urchin recruitment. Densities of small sea urchins were much higher in the Aleutian system even prior to the reduction in sea otters, suggesting a higher rate of recruitment.

Marine Ecology Progress Series↗

USGS1996

Comparative demography of sea otter populations

Population trends are poorly documented and demographic information is typically lacking for many carnivorous mammals. The sea otter ( Enhydru lutris ) has a well known history of decline and recovery, and while many other species have declined as precipitously, few have recovered so spectacularly. Generally speaking, northern populations (remnants within the range of E. l. lutris and E. l. kenyoni ) have recovered at high rates while recovery of the southern sea olter ( E. I. nereis ), which is listed as legally threatened under the Endangered Species Act (ESA), has progressed more slowly. Our purpose is to contrast trends in abundance and demographic patterns between Southern California and northern (Washington, British Columbia, Alaska, and Asia) sea otter populations. Specifically, we provide (1) a brief review of the main findings to date: (2) a summary of ongoing and planned studies; and (3) recommendations for future research. A more detailed account of these and other issues concerning the conservation and management of sea otters is provided in the U.S. Fish and Wildlife Service's Recovery Plan for the Califomia Sea Otter (U.S. Fish and Wildlife Service 1996).

Endangered Species UPDATE↗

USGS1995

Reproduction, preweaning survival, and survival of adult sea otters at Kodiak Island, Alaska

Radiotelemetry methods were used to examine the demographic characteristics of sea otters inhabiting the leading edge of an expanding population on Kodiak Island, Alaska. Fifteen male and 30 female sea otters were instrumented and followed from 1986 to 1990. Twenty-one percent of females were sexually mature (had pupped) at age 2, 57% by age 3, 88% by age 4, and 100% by age 5. Fifteen females produced 26 pups, an overall reproduction rate of 94% for mature females. The reproduction rate was 17, 45, 66, and 100% for 2-, 3-, 4-, and 5-year-olds, respectively. Eighty-five percent of observed pups survived to weaning (120 days), and the percentage of pups weaned ranged from 34% for pups of 2-year-olds to 100% for pups of 5-year-olds. At least three of four known pup losses occurred within a month of parturition. The mean pup dependency period for weaned pups was 153 days and the mean gestation period was 218 days. No synchrony in pupping activity was observed. Mean annual survival of adults was high. Estimates of survival ranged from 89 to 96% for females and 86 to 91% for males. Human harvest was the primary source of known mortality of adults. Our estimates of reproductive rates and survival of adults are at the high end of those reported for sea otters, but preweaning survival stands out as being particularly high. Abundant food resources and the availability of protected water presumably contributed to the high reproductive success observed in this recently established sea otter population.

Alaska↗

USGS1990

Procedures and rationale for marking sea otters captured and treated during the T/V Exxon Valdez oil spill

Four methods were used for marking sea otters ( Enhydra lutris ) captured and treated during the response to the T/V Exxon Valdez oil spill. Colored and numbered flipper tags were placed on each sea otter that was brought to the otter treatment centers. These tags allowed individual recognition and permitted the tracking of individuals through the treatment and holding processes. Recovery of tagged carcasses may provide a crude measure of the fates of rehabilitated otters. Seven sea otters were instrumented with radio transmitters attached to flipper tags as part of a pilot release program. The results of that study were inconclusive. Forty-five sea otters were implanted with radio transmitters as part of a study to assess the fate of rehabilitated sea otters. Specific objectives of the study include estimating survived rates and monitoring the reproductive success of the sample of rehabilitated sea otters and comparing the results with similar variables in a control population. Transponder chips were injected in the perianal region of all but seven of the instrumented sea otters, and all were tagged with red flipper tags. The advantages and disadvantages of each type of tag are discussed.

Alaska↗