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At least 433 records · Page 24Linked to original sources
USGS2006

Causes of mortality in California sea otters during periods of population growth and decline

Elevated mortality appears to be the main reason for both sluggish growth and periods of decline in the threatened California sea otter population . We assessed causes of mortality from salvage records of 3,105 beach-cast carcasses recovered from 1968 through 1999, contrasting two periods of growth with two periods of decline . Overall, an estimated 40%-60% of the deaths were not recovered and 70% of the recovered carcasses died from unknown causes . Nonetheless, several common patterns were evident in the salvage records during the periods of population decline . These included greater percentages of (1) prime age animals (3-10 yr), (2) carcasses killed by great white shark attacks, (3) carcasses recovered in spring and summer, and (4) carcasses for which the cause of death was unknown. Neither sex composition nor the proportion of carcasses dying of infectious disease varied consistently between periods of population increase and decline . The population decline from 1976 to 1984 was likely due to incidental mortality in a set-net fishery, and the decline from 1995 to 1999 may be related to a developing live-fish fishery. Long-term trends unrelated to periods of growth and decline included a decrease in per capita pup production and mass/length ratios of adult carcasses over the 31-yr study. The generally high proportion of deaths from infectious disease suggests that this factor has contributed to the chronically sluggish growth rate of the California sea otter population .

California↗
USGS2006

Immobilization of free-ranging male pacific walruses ( Odobenus rosmarus divergens ) with carfentanil citrate and naltrexone hydrochloride

The major challenges in immobilization of free-ranging walruses ( Odobenus rosmarus divergens ) are to produce a deep level of anesthesia very quickly (to avoid darted animals from entering the water and drowning), and to find a drug or drug combination that requires only a small volume to be delivered by dart, is safe, reversible, and that provides an adequate period of immobilization to permit attachment of instruments, phlebotomy, and measuring. Tiletamine-zolazepam is recommended for immobilization of pinnipeds, with inhalant anesthesia recommended for more extensive procedures requiring better analgesia (Gales 1989). Drugs that have been used on free-ranging walruses include ketamine (Hagenbeck et al . 1975), phencyclidine combined with acepromazine (DeMaster et al . 1981), etorphine (Born and Knutsen 1990, Hills 1992, Griffiths et al . 1993), tiletamine-zolazepam (Stirling and Sjare 1988, Griffiths et al . 1993), medetomidine and ketamine (Lydersen et al . 1992), and carfentanil (Hills 1992, Lanthier et al . 1999). Carfentanil but not etorphine is presently licensed and available in the United States. Forty-eight adult male walruses were immobilized with carfentanil citrate in the summers of 1995-1997 at Maggy Beach (58°57’N, 161°76’W), a land haul-out located at Cape Peirce within the Togiak National Wildlife Refuge in southwest Alaska. The number of animals present during immobilizations ranged from three to several thousand. Criteria for choosing individual walruses included good body condition, the presence of two tusks of sufficient diameter for the attachment of radio transmitters, and presence of the animal at the edge of the herd. In addition, we chose animals that were resting quietly and which had not recently hauled out (as judged by skin color). Walruses were darted from ranges of approximately 10-15 m using a Cap-Chur rifle (Palmer Chemical and Equipment Co., Douglasville, Georgia, GA 30133). Carfentanil citrate (Wildlife Pharmaceuticals, Fort Collins, CO 80524) was administered by dart with a 10-cm needle to the lumbar region. Most animals received a dose of 2.7 or 3.0 mg of carfentanil. One animal received a second dose after the first syringe apparently burst on impact. Induction times were measured as being the time at which the animal collapsed, failed to respond to external stimuli, or as the time when the darter stood up and moved towards the animal.

Marine Mammal Science↗
USGS2006

Foraging depths of sea otters and implications to coastal marine communities

We visually observed 1,251 dives, of 14 sea otters instrumented with TDRs in southeast Alaska, and used attribute values from observed dives to classify 180,848 recorded dives as foraging (0.64), or traveling (0.36). Foraging dives were significantly deeper, with longer durations, bottom times, and postdive surface intervals, and greater descent and ascent rates, compared to traveling dives. Most foraging occurred in depths between 2 and 30 m (0.84), although 0.16 of all foraging was between 30 and 100 m. Nine animals, including all five males, demonstrated bimodal patterns in foraging depths, with peaks between 5 and 15 m and 30 and 60 m, whereas five of nine females foraged at an average depth of 10 m. Mean shallow foraging depth was 8 m, and mean deep foraging depth was 44 m. Maximum foraging depths averaged 61 m (54 and 82 for females and males, respectively) and ranged from 35 to 100 m. Female sea otters dove to depths ≤20 m on 0.85 of their foraging dives while male sea otters dove to depths ≥45 m on 0.50 of their foraging dives. Less than 0.02 of all foraging dives were >55 m, suggesting that effects of sea otter foraging on nearshore marine communities should diminish at greater depths. However, recolonization of vacant habitat by high densities of adult male sea otters may result in initial reductions of some prey species at depths >55 m.

Marine Mammal Science↗
USGS2006

A stage-based model of manatee population dynamics

A stage-structured population model for the Florida manatee (Trichechus manatus latirostris) was developed that explicitly incorporates uncertainty in parameter estimates. The growth rates calculated with this model reflect the status of the regional populations over the most recent 10-yr period. The Northwest and Upper St. Johns River regions have growth rates (8) of 1.037 (95% interval, 1.016?1.056) and 1.062 (1.037?1.081), respectively. The Southwest region has a growth rate of 0.989 (0.946?1.024), suggesting this population has been declining at about 1.1% per year. The estimated growth rate in the Atlantic region is 1.010 (0.988?1.029), but there is some uncertainty about whether adult survival rates have been constant over the last 10 yr; using the mean survival rates from the most recent 5-yr period, the estimated growth rate in this region is 0.970 (0.938?0.998). Elasticity analysis indicates that the most effective management actions should seek to increase adult survival rates. Decomposition of the uncertainty in the growth rates indicates that uncertainty about population status can best be reduced through increased monitoring of adult survival rate.

Marine Mammal Science↗
USGS2006

Capture-recapture analysis for estimating manatee reproductive rates

Modeling the life history of the endangered Florida manatee (Trichechus manatus latirostris) is an important step toward understanding its population dynamics and predicting its response to management actions. We developed a multi-state mark-resighting model for data collected under Pollock's robust design. This model estimates breeding probability conditional on a female's breeding state in the previous year; assumes sighting probability depends on breeding state; and corrects for misclassification of a cow with first-year calf, by estimating conditional sighting probability for the calf. The model is also appropriate for estimating survival and unconditional breeding probabilities when the study area is closed to temporary emigration across years. We applied this model to photo-identification data for the Northwest and Atlantic Coast populations of manatees, for years 1982?2000. With rare exceptions, manatees do not reproduce in two consecutive years. For those without a first-year calf in the previous year, the best-fitting model included constant probabilities of producing a calf for the Northwest (0.43, SE = 0.057) and Atlantic (0.38, SE = 0.045) populations. The approach we present to adjust for misclassification of breeding state could be applicable to a large number of marine mammal populations.

Marine Mammal Science↗
USGS2005

Observer variability in pinniped counts: Ground-based enumeration of walruses at haul-out sites

Pinnipeds are often monitored by counting individuals at haul-out sites, but the often large numbers of densely packed individuals at these sites are difficult to enumerate accurately. Errors in enumeration can induce bias and reduce precision in estimates of population size and trend. We used data from paired observers monitoring walrus haul-outs in Bristol Bay, Alaska, to quantify observer variability and assess its relative importance. The probability of a pair of observers making identical counts was 50 individuals. Mean count differences ranged up to 25% for the largest counts, depending on beach and observers. In at least some cases, there was a clear tendency for counts of one observer to be consistently greater than counts of the other observer in a pair, indicating that counts of at least one of the observers were biased. These results suggest that efforts to improve accuracy of counts will be worthwhile. However, we also found that variation among observers was relatively small compared to variation among visits to a beach so that efforts to account for other sources of variation will be more important.

Marine Mammal Science↗
USGS2006

Abundance of ringed seals ( Pusa hispida ) in the fjords of Spitsbergen, Svalbard, during the peak molting period

Ringed seal ( Pusa hispida ) abundance in Spitsbergen, Svalbard, was estimated during the peak molting period via aerial, digital photographic surveys. A total of 9,145 images, covering 41.7%–100% of the total fast-ice cover (1,496 km 2 ) of 18 different fjords and bays, were inspected for the presence of ringed seals. A total of 1,708 seals were counted, and when accounting for ice areas that were not covered by images, a total of 3,254 (95% CI: 3,071–3,449) ringed seals were estimated to be hauled out during the surveys. Extensive behavioral data from radio-tagged ringed seals (collected in a companion study) from one of the highest density fjords during the molting period were used to create a model that predicts the proportion of seals hauled out on any given date, time of day, and under various meteorological conditions. Applying this model to the count data from each fjord, we estimated that a total of 7,585 (95% CI: 6,332–9,085) ringed seals were present in the surveyed area during the peak molting period. Data on interannual variability in ringed seal abundance suggested higher numbers of seals in Van Keulenfjorden in 2002 compared to 2003, while other fjords with very stable ice cover showed no statistical differences. Poor ice conditions in general in 2002 probably resulted in seals from a wide area coming to Van Keulenfjorden (a large fjord with stable ice in 2002). The total estimated number of ringed seals present in the study area at the time of the survey must be regarded as a population index, or at least a minimum estimate for the area, because it does not account for individuals leaving and arriving, which might account for a considerable number of animals. The same situation is likely the case for many other studies reporting aerial census data for ringed seals. To achieve accurate estimates of population sizes from aerial surveys, more extensive knowledge of ringed seal behavior will be required.

Marine Mammal Science↗
USGS2007

Killer whales and marine mammal trends in the North Pacific - A re-examination of evidence for sequential megafauna collapse and the prey-switching hypothesis

Springer et al. (2003) contend that sequential declines occurred in North Pacific populations of harbor and fur seals, Steller sea lions, and sea otters. They hypothesize that these were due to increased predation by killer whales, when industrial whaling's removal of large whales as a supposed primary food source precipitated a prey switch. Using a regional approach, we reexamined whale catch data, killer whale predation observations, and the current biomass and trends of potential prey, and found little support for the prey-switching hypothesis. Large whale biomass in the Bering Sea did not decline as much as suggested by Springer et al., and much of the reduction occurred 50-100 yr ago, well before the declines of pinnipeds and sea otters began; thus, the need to switch prey starting in the 1970s is doubtful. With the sole exception that the sea otter decline followed the decline of pinnipeds, the reported declines were not in fact sequential. Given this, it is unlikely that a sequential megafaunal collapse from whales to sea otters occurred. The spatial and temporal patterns of pinniped and sea otter population trends are more complex than Springer et al. suggest, and are often inconsistent with their hypothesis. Populations remained stable or increased in many areas, despite extensive historical whaling and high killer whale abundance. Furthermore, observed killer whale predation has largely involved pinnipeds and small cetaceans; there is little evidence that large whales were ever a major prey item in high latitudes. Small cetaceans (ignored by Springer et al.) were likely abundant throughout the period. Overall, we suggest that the Springer et al. hypothesis represents a misleading and simplistic view of events and trophic relationships within this complex marine ecosystem. ?? 2007 by the Society for Marine Mammalogy.

Marine Mammal Science↗
USGS2007

Estimation of walrus populations on sea ice with infrared imagery and aerial photography

Population sizes of ice-associated pinnipeds have often been estimated with visual or photographic aerial surveys, but these methods require relatively slow speeds and low altitudes, limiting the area they can cover. Recent developments in infrared imagery and its integration with digital photography could allow substantially larger areas to be surveyed and more accurate enumeration of individuals, thereby solving major problems with previous survey methods. We conducted a trial survey in April 2003 to estimate the number of Pacific walruses (Odobenus rosmarus divergens) hauled out on sea ice around St. Lawrence Island, Alaska. The survey used high altitude infrared imagery to detect groups of walruses on strip transects. Low altitude digital photography was used to determine the number of walruses in a sample of detected groups and calibrate the infrared imagery for estimating the total number of walruses. We propose a survey design incorporating this approach with satellite radio telemetry to estimate the proportion of the population in the water and additional low-level flights to estimate the proportion of the hauled-out population in groups too small to be detected in the infrared imagery. We believe that this approach offers the potential for obtaining reliable population estimates for walruses and other ice-associated pinnipeds. ?? 2007 by the Society for Marine Mammalogy.

Marine Mammal Science↗
USGS2009

Stress-related hormones and genetic diversity in sea otters (Enhydra lutris)

Sea otters ( Enhydra lutris ) once ranged throughout the coastal regions of the north Pacific, but were extirpated throughout their range during the fur trade of the 18th and 19th centuries, leaving only small, widely scattered, remnant populations. All extant sea otter populations are believed to have experienced a population bottleneck and thus have lost genetic variation. Populations that undergo severe population reduction and associated inbreeding may suffer from a general reduction in fitness termed inbreeding depression. Inbreeding depression may result in decreased testosterone levels in males, and reduced ability to respond to stressful stimuli associated with an increase in the stress-related adrenal glucocorticoid hormones, cortisol and corticosterone. We investigated correlations of testosterone, cortisol, and corticosterone with genetic diversity in sea otters from five populations. We found a significant negative correlation between genetic diversity and both mean population-level ( r 2 = 0.27, P < 0.001) and individual-level ( r 2 = 0.54, P < 0.001) corticosterone values, as well as a negative correlation between genetic diversity and cortisol at the individual level ( r 2 = 0.17, P = 0.04). No relationship was found between genetic diversity and testosterone ( P = 0.57). The strength of the correlations, especially with corticosterone, suggests potential negative consequences for overall population health, particularly for populations with the lowest genetic diversity.

Marine Mammal Science↗
USGS2010

Stable isotopes differentiate bottlenose dolphins off west-central Florida

Distinguishing discrete population units among continuously distributed coastal small cetaceans is challenging and crucial to conservation. We evaluated the utility of stable isotopes in assessing group membership in bottlenose dolphins ( Tursiops truncatus ) off west-central Florida by analyzing carbon, nitrogen, and sulfur isotope values (δ 13 C, δ 15 N, and δ 34 S) of tooth collagen from stranded dolphins. Individuals derived from three putative general population units: Sarasota Bay (SB), nearshore Gulf of Mexico (GULF), and offshore waters (OFF). Animals of known history (SB) served to ground truth the approach against animals of unknown history from the Gulf of Mexico (GULF, OFF). Dolphin groups differed significantly for each isotope. Average δ 13 C values from SB dolphins (−10.6‰) utilizing sea grass ecosystems differed from those of GULF (−11.9‰) and OFF (−11.9‰). Average δ 15 N values of GULF (12.7‰) and OFF (13.2‰) were higher than those of SB dolphins (11.9‰), consistent with differences in prey trophic levels. δ 34 S values showed definitive differences among SB (7.1‰), GULF (11.3‰), and OFF (16.5‰) dolphins. This is the first application of isotopes to population assignment of bottlenose dolphins in the Gulf of Mexico and results suggest that isotopes may provide a powerful tool in the conservation of small cetaceans.

Florida↗
USGS2010

Behavioral response of manatees to variations in environmental sound levels

Florida manatees ( Trichechus manatus latirostris ) inhabit coastal regions because they feed on the aquatic vegetation that grows in shallow waters, which are the same areas where human activities are greatest. Noise produced from anthropogenic and natural sources has the potential to affect these animals by eliciting responses ranging from mild behavioral changes to extreme aversion. Sound levels were calculated from recordings made throughout behavioral observation periods. An information theoretic approach was used to investigate the relationship between behavior patterns and sound level. Results indicated that elevated sound levels affect manatee activity and are a function of behavioral state. The proportion of time manatees spent feeding and milling changed in response to sound level. When ambient sound levels were highest, more time was spent in the directed, goal-oriented behavior of feeding, whereas less time was spent engaged in undirected behavior such as milling. This work illustrates how shifts in activity of individual manatees may be useful parameters for identifying impacts of noise on manatees and might inform population level effects.

Marine Mammal Science↗
USGS2010

Results and evaluation of a survey to estimate Pacific walrus population size, 2006

In spring 2006, we conducted a collaborative U.S.-Russia survey to estimate abundance of the Pacific walrus ( Odobenus rosmarus divergens ). The Bering Sea was partitioned into survey blocks, and a systematic random sample of transects within a subset of the blocks was surveyed with airborne thermal scanners using standard strip-transect methodology. Counts of walruses in photographed groups were used to model the relation between thermal signatures and the number of walruses in groups, which was used to estimate the number of walruses in groups that were detected by the scanner but not photographed. We also modeled the probability of thermally detecting various-sized walrus groups to estimate the number of walruses in groups undetected by the scanner. We used data from radio-tagged walruses to adjust on-ice estimates to account for walruses in the water during the survey. The estimated area of available habitat averaged 668,000 km 2 and the area of surveyed blocks was 318,204 km 2 . The number of Pacific walruses within the surveyed area was estimated at 129,000 with 95% confidence limits of 55,000 to 507,000 individuals. This value can be used by managers as a minimum estimate of the total population size.

Bering Sea↗
USGS2012

Movement patterns of Antillean manatees in Chetumal Bay (Mexico) and coastal Belize: A challenge for regional conservation

Information from 15 satellite-tracked Antillean manatees (Trichechus manatus manatus) was analyzed in order to assess individual movements, home ranges, and high-use areas for conservation decisions. Manatees were captured in Chetumal Bay, Mexico, and tagged with Argos-monitored satellite transmitters. Location of the manatees and physical characteristics were assessed to describe habitat properties. Most manatees traveled to freshwater sources. The Maximum Area Size (MAS) for each manatee was determined using the observation-area method. Additional kernel densities of 95% home range and 50% Center of Activity (COA) were also calculated, with manatees having 1–3 COAs. Manatees exhibited two different movement patterns: remaining in Chetumal Bay, and long-distance (up to 240 km in 89 d). The residence time in Chetumal Bay was higher for females (89.6% of time) than for males (72.0%), but the daily travel rate (0.4–0.5 km/d) was similar for both sexes. Most of the COAs fell within Natural Protected Areas (NPA). However, manatees also travel for long distances into unprotected areas, where they face uncontrolled boat traffic, fishing activities, and habitat loss. Conservation of movement corridors may promote long-distance movements and facilitate genetic exchange.

Chetumal Bay↗
USGS2012

Retrospective analysis of bottlenose dolphin foraging: a legacy of anthropogenic ecosystem disturbance

We used stable isotope analysis to investigate the foraging ecology of coastal bottlenose dolphins (Tursiops truncatus) in relation to a series of anthropogenic disturbances. We first demonstrated that stable isotopes are a faithful indicator of habitat use by comparing muscle isotope values to behavioral foraging data from the same individuals. δ 13 C values increased, while δ 34 S and δ 15 N values decreased with the percentage of feeding observations in seagrass habitat. We then utilized stable isotope values of muscle to assess temporal variation in foraging habitat from 1991 to 2010 and collagen from tooth crown tips to assess the time period 1944 to 2007. From 1991 to 2010, δ 13 C values of muscle decreased while δ 34 S values increased indicating reduced utilization of seagrass habitat. From 1944 to 1989 δ 13 C values of the crown tip declined significantly, likely due to a reduction in the coverage of seagrass habitat and δ 15 N values significantly increased, a trend we attribute to nutrient loading from a rapidly increasing human population. Our results demonstrate the utility of using marine mammal foraging habits to retrospectively assess the extent to which anthropogenic disturbance impacts coastal food webs.

Florida↗