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Abundance and distribution of marine mammals in nearshore waters of Monterey Bay, California

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Abstract

We studied the seasonal abundance and spatial distribution of marine mammals in nearshore waters (<1 km from shore) of Monterey Bay, California, during 1999 and 2000. The most abundant mammal was California sea lion, Zalophus californianus, followed by harbor porpoise, Phocoena phocoena, sea otter, Enhydra lutris, and harbor seal, Phoca vitulina. Seasonal abundance of harbor porpoise in the survey area was greatest during winter, pinnipeds were mostabundant during autumn, and sea otters were most abundant during spring and autumn. California sea lions were more abundant in 2000 than in 1999. Harbor porpoise were found in water of lesser clarity than expected by chance, and sea lions werefound more often in water of intermediateclarity. Distribution of sea otters and harbor seals were not affected by water clarity.

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... There was Circles are scaled to mean density of all animals in 1 km transect segments, ranging from 60 to 727 birds km -2 . Bathymetry is shown in 100 m contour intervals some variability in water clarity during winter, a factor that may have affected some species' distributions (Henkel 2006, Henkel & Harvey 2008. Other researchers testing for interspecific associations among seabirds have typically used indices such as Cole's coefficient of association (Cole 1949) that use presence-absence data , Ainley & Boekelheide 1990). ...
... These co-occurrence tests were run on 14 bird taxa and 2 mammal taxa; 1 additional bird species was used in the seasonal comparisons discussed below (Table 1). Because the abundance of several taxa was significantly greater in 2000 than in 1999 (Henkel 2004, Henkel & Harvey 2008, I also tested for positive and negative associations between pairs of 12 taxa within summer (June to August) 1999 and summer 2000 separately (4 surveys each year). ...
... Between the 2 marine mammal species, the smaller (harbor porpoise) also had only negative or neutral associations, potentially in response to avoidance of the larger California sea lion due to competition. Harbor porpoise and California sea lion may segregate using different ranges of water clarity (Henkel & Harvey 2008), a factor that did vary slightly throughout the otherwise homogenous study area. In a similar vein, the largest avian member of each guild (Brandt's cormorant, brown pelican, and western gull) had consistently strong positive associations, often all co-occurring together (Table 4). ...
Article
Patterns of spatial and temporal co-occurrence or avoidance among different species of marine birds and mammals can provide insights into the degree to which these top predators compete for prey. I conducted at-sea surveys in nearshore waters of Monterey Bay, California, USA, and used a randomization technique to assess co-occurrence patterns of marine birds and mammals in 1 km transect segments. As expected, strongest positive associations were among members of 3 different foraging guilds: pursuit-divers, surface-feeders, and plunge-divers. Within guilds, pursuit- divers exhibited marked avoidance of one another (negative co-occurrence), although surface- feeders often co-occurred with each other. The study was conducted during 2 yr, 1999 and 2000; during 2000, when predator abundance was greater and prey abundance may have been decreased, pursuit-divers exhibited more avoidance of one another than in 1999. These data suggest that competition reduces foraging ability for pursuit-diving species, making it more profitable for them to disperse more widely, whereas surface-feeding species (primarily gulls) benefit from flock foraging under most conditions. Larger animals tended to frequently co-occur, while the smallest member of each guild tended to avoid other species, indicating competitive exclusion of smaller predators.
... The other 2 DA-related sea lion stranding events occurred in the late summer/fall of 2000 and spring of 2007, and also coincided with moderate to high numbers of toxic Pseudo-nitzschia and associated high levels of DA in Monterey Bay (Fig. 2). Similarly, more sea lions were observed during these time periods in the Bay than in other years (Goldstein et al. 2008, Henkel & Harvey 2008. In 2000, coastal waters off the southern California region experienced weaker than normal upwelling and lower productivity, whereas production remained near normal in central and northern California (Durazo et al. 2001, Henkel & Harvey 2008. ...
... Similarly, more sea lions were observed during these time periods in the Bay than in other years (Goldstein et al. 2008, Henkel & Harvey 2008. In 2000, coastal waters off the southern California region experienced weaker than normal upwelling and lower productivity, whereas production remained near normal in central and northern California (Durazo et al. 2001, Henkel & Harvey 2008. Adult females (n = 10) stranded in larger than usual numbers in Monterey Bay. ...
... However, most of the female sea lion population would have been expected to remain near the Channel Islands nursing their pups. With lowered productivity and decreased prey abundance further south, females were likely traveling northwards in greater numbers to forage (Henkel & Harvey 2008), resulting in exposure to high levels of DA and increased numbers of related strandings. ...
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The neurotoxin domoic acid (DA) produced by the diatoms Pseudo-nitzschia has been responsible for deaths of marine mammals and birds in Monterey Bay, California, USA. In this study we examined links between DA-related strandings of the seasonally migratory California sea lion Zalophus californianus and regional occurrences of DA-producing diatoms using a decade-long time series. Results suggest a more complex pattern of stranding than anticipated, one not related simply to regional abundance of the toxin producers. Stranding patterns of sea lions exhibiting signs of acute DA toxicosis may be best explained by multiple causative factors including timing of toxic blooms with respect to the sea lion breeding cycle, adequacy of sea lion prey during the breeding season, as well as the geographic range of this pinniped outside of the breeding season. Three DA-related stranding events occurred in Monterey Bay in 1998, 2000 and 2007, when toxic DA blooms were present in the Bay and the California coast was experiencing El Nino/Southern Oscillation conditions. Foraging centers near the breeding site likely provided inadequate food for individuals, leading to northerly movement of these highly mobile predators to other geographic sites resulting in exposure to toxic DA-producing blooms. High toxic Pseudo-nitzschia and DA concentrations did not, however, result in DA-related stranding events in 2002, 2003 and 2004, when weaker than normal upwelling events were present in the Bay. Relative productivity of central versus southern California with respect to the breeding season thus appears to strongly influence the frequency of DA-related strandings in Monterey Bay, California.
... Desde estas plataformas y con observadores abordo, haciendo búsquedas desde un punto fijo de la embarcación durante las travesías, es posible observar y contabilizar individuos de tortugas en zonas costeras y oceánicas. Esta herramienta ha sido ampliamente utilizada en el estudio de mamíferos marinos (Carretta et al. 2001, Calambokidis & Barlow 2004, Jackson et al. 2004, Henkel & Harvey 2008, y se ha probado de manera exitosa para el estudio de aves (Camphuysen et al. 2004, Ronconi & Burger 2009) y tortugas marinas (Eguchi et al. 2007). ...
... A pesar de las particularidades que la biología de las tortugas marinas implica, el uso de esta estrategia es plenamente reconocido y validado por especialistas, y está ampliamente documentado. Además de ser una alternativa versátil que puede adaptarse a diferentes condiciones de la embarcación, siempre que tenga buena estabilidad (Henkel & Harvey 2008). ...
... To avoid measurement errors due to bubbles or foam, measurements were delayed at the spot until the views were clear (Baptist and Leopold 2010). In order to obtain the mean transparency for each 1-ha plot on a specific day, measurements were taken at three haphazardly chosen spots which were at least 30 m apart (Henkel 2003). Similar to the water transparency measurements, surface water samples were collected using a 500-mL plastic bottle from the same three spots, and averaged to obtain the daily mean values for each 1-ha plot. ...
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Background: Water clarity may negatively influence rate of plunge diving and prey capture success of piscivorous plunge-diving birds, and therefore has implications for their conservation in polluted urban wetlands. We studied the relationship between water clarity and the abundance and prey capture success of Pied Kingfishers (Ceryle rudis) and Common Terns (Sterna hirundo) in two polluted coastal waters of south-east Ghana—the Weija Lake and Densu Delta Ramsar Site. Methods: On each wetland, data on abundance and prey capture success of plunge-divers were collected in four spatio-temporal quadrats of 100 m × 100 m and analysed with concurrent measurements of water quality parameters using GLM regression with Pearson’s correlation coefficients. Results: Mean prey capture success of Pied Kingfishers (34.7 ± 13.1%) and Common Terns (35.3 ± 11.0%) were similar but the two species responded differently to water clarity. The abundance of Common Terns was significantly higher in less transparent/more turbid water while that of Pied Kingfishers showed no significant relationship with turbidity and transparency. In contrast, the prey capture success of Common Terns was neither related to transparency nor turbidity, as opposed to that of Pied Kingfishers which was significantly higher in more turbid/less transparent waters. Correlations between capture success and bird abundance, as well as capture attempts were insignificant, suggesting that increased fish abundance associated with cloudy water may not necessarily promote higher abundance and capture success of foraging birds. Thus, when foraging in less transparent water, capture success may depend more on predator avoidance by fish prey than lower prey detectability of foraging birds. Conclusion: Within a gradient of 15–51 cm transparency studied, lower water clarity did not constrain prey capture success of Common Terns and Pied Kingfishers. Further studies on the foraging ecology of plunge-divers in coastal Ghana are however required to make firm conclusions on the relationship between water clarity and foraging birds and fish prey abundances, as well as capture success.
... Sea otters are year round residents in Elkhorn Slough and the adjacent harbor, but are most abundant between November and December [36,37], which coincides with the observed peak in their interactions with PBRs. Sea otters were observed resting, grooming, and feeding, on PBRs. ...
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Background OMEGA is an integrated aquatic system to produce biofuels, treat and recycle wastewater, capture CO2, and expand aquaculture production. This system includes floating photobioreactors (PBRs) that will cover hundreds of hectares in marine bays. To assess the interactions of marine mammals and birds with PBRs, 9 × 1.3 m flat panel and 9.5 × 0.2 m tubular PBRs were deployed in a harbor and monitored day and night from October 10, 2011 to Janurary 22, 2012 using infrared video. To observe interactions with pinnipeds, two trained sea lions (Zalophus californianus) and one trained harbor seal (Phoca vitulina richardii) were observed and directed to interact with PBRs in tanks. To determine the forces required to puncture PBR plastic and the effects of weathering, Instron measurements were made with a sea otter (Enhydra lutris) tooth and bird beaks. Results A total of 1,445 interactions of marine mammals and birds with PBRs were observed in the 2,424 hours of video recorded. The 95 marine mammal interactions, 94 by sea otters and one by a sea lion had average durations of three minutes (max 44 min) and represented about 1% of total recording time. The 1,350 bird interactions, primarily coots (Fulica americana) and gulls (Larus occidentalis and L. californicus) had average durations of six minutes (max. 170) and represented 5% of recording time. Interactive behaviors were characterized as passive (feeding, walking, resting, grooming, and social activity) or proactive (biting, pecking, investigating, and unspecified manipulating). Mammal interactions were predominantly proactive, whereas birds were passive. All interactions occurred primarily during the day. Ninety-six percent of otter interactions occurred in winter, whereas 73% of bird interactions in fall, correlating to their abundance in the harbor. Trained pinnipeds followed most commands to bite, drag, and haul-out onto PBRs, made no overt undirected interactions with the PBRs, but showed avoidance behavior to PBR tethers. Instron measurements indicated that sea-otter teeth and gull beaks can penetrate weathered plastic more easily than new plastic. Conclusions Otter and bird interactions with experimental PBRs were benign. Large-scale OMEGA systems are predicted to have both positive and negative environmental consequences.
... It was clear however, that at least these animals were quick to use the floating PBRs for a variety of purposes. Sea otters are year round residents in Elkhorn Slough and the harbor with their greatest abundance between November and December (Feinholz 1998;Henkel & Harvey 2008), which coincides with their peak interactions with the PBRs. A group of tagged otters, routinely monitored in Moss Landing harbor, occasionally haul out onto kayaks, boats, and docks (SORAC, Carl Mayer, personal communication), setting a precedent for these animals interacting with man-made objects. ...
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The biofuels community has shown considerable interest in the possibility that microalgae could contribute significantly to providing a sustainable alternative to fossil fuels. Microalgae species with high growth rates and high yields of oil that can be grown on domestic wastewater using nonarable land could produce biofuel without competing with agriculture. It is difficult to envision where the cultivation facilities would be located to produce the quantity of algae needed for fuels, given that these facilities must be close to wastewater treatment plants to save energy. Researchers investigated a possible solution called Offshore Membrane Enclosures for Growing Algae for coastal cities. This system involved growing fast-growing, oil-producing freshwater algae in flexible, inexpensive clear plastic photobioreactors attached to floating docks anchored offshore in naturally or artificially protected bays. Wastewater and carbon dioxide from coastal facilities provided water, nutrients, and carbon. The surrounding seawater controlled the temperature inside the photobioreactors and killed any algae that might escape. The salt gradient between seawater and wastewater created forward osmosis to concentrate nutrients and to facilitate algae harvesting. Both the algae and forward osmosis cleaned the wastewater, removing nutrients as well as pharmaceuticals and personal care products, so-called compounds of emerging concern. This report provided the results of two years of research into the feasibility of the Offshore Membrane Enclosures for Growing Algae system in which prototype systems were studied, built, and tested in seawater tanks. A 110-liter floating system was developed and scaled up to 1,600 liters. Algae’s ability to grow on and treat wastewater was described. The impact of biofouling on photobioreactors and forward osmosis membranes floating in the marine environment was considered. Life-cycle and technoeconomic analyses provided a perspective on what must be done to make this system commercially viable. Outreach efforts have carried the concept worldwide. Keywords: biofuels, microalgae, algae, OMEGA, offshore systems, carbon sequestration, aquaculture, wastewater treatment, biofouling, life cycle analysis, technoeconomic analysis Please use the following citation for this report: Trent, Jonathon. (NASA Ames Research Center). 2012. OMEGA (Offshore Membrane Enclosures for Growing Algae) - A Feasibility Study for Wastewater to Biofuels. California Energy Commission. Publication number: CEC-500-2013-143.
... From a management perspective, our study suggests that tributaries should be monitored with greater frequency and extent since they are the point sources for the components that set OWQ. Further, we suggest that the abovementioned trends and concepts, particularly the role of channel network configuration, can also be used to understand the spatiotemporal trends of other water quality variables. The OWQ of rivers has greater significance because it affects receiving waters such as lakes, estuaries, and coastal environments, whose biota greatly depend on aquatic light availability, including coral [Fabricius, 2005], birds [Henkel, 2006], and submersed aquatic vegetation [Dennison et al., 1993]. This study has highlighted the high spatiotemporal variability of riverine OWQ, and in doing so has opened up a number of promising research avenues including the need to understand the effects of land use and climate change on OWQ as critical steps toward a broader awareness of the fundamental role of light as a driver of multiple processes in fluvial ecosystems. ...
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Optical water quality (OWQ) governs the quantity and quality of light in aquatic ecosystems, and thus spatiotemporal changes in OWQ affect many biotic and abiotic processes. Despite the fundamental role of light in rivers, studies on riverine OWQ have been limited and mostly descriptive. Here we provide a comprehensive, quantitative analysis of the controls and spatiotemporal dynamics of riverine OWQ, focusing on the inherent optical properties (IOPs), which are those that are only affected by water constituents and not by changes in the solar radiation field. First, we briefly review the constituents attenuating light in rivers. Second, we develop a new method for partitioning (light) beam attenuation into its constituent fractions. This method distinguishes between absorption and scattering by dissolved and particulate constituents, and further isolates particulates into mineral and organic components. Third, we compare base flow IOPs between four rivers with vastly different physical characteristics to illustrate intersite variability. Fourth, we analyze the spatial and temporal patterns of IOPs for the four rivers. Fifth, we quantify a longitudinal water clarity budget for one of the rivers. Finally, available data are synthesized to identify general spatial trends robust across broad geographic areas. Temporal trends in IOPs were largely dictated by storm frequency, while spatial trends were largely dictated by channel network configuration. Generally, water clarity decreased with increasing discharge primarily owing to greater scattering by particulates and secondarily to greater absorption by chromophoric dissolved organic matter. Water clarity also generally decreased longitudinally along the river owing to increased particulate inputs from tributaries; however, for pear-shaped, dendritic basins, water clarity reached a minimum at ~70% of the channel length and then increased. By illustrating the controls and spatiotemporal variability of riverine OWQ, these findings will be of interest to water resource managers and fluvial ecologists and specifically for remote-sensing of fluvial environments and river plumes in receiving waters.
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Benthic marine invertebrate communities were organized along a gradient of wave-induced substrate motion on the subtidal high-energy beach in Monterey Bay, California. Two general zones were distinguished from 6 to 30 m of water. A shallow zone «14 m) contained sediments that were commonly disrupted by wave activity and it was primarily occupied by small, mobile, deposit-feeding peracarid and ostracod crustaceans. Patterns ofcrustacean morphology and mobility were related to their depth zonation. Few animals lived in permanent tubes or burrows in the crustacean zone. Wave disturbance decreased with increasing water depth, while the numbers of sessile and semisessile species, commensalanimals, and suspension or selective-surface-deposit feeders increased. Thedeeper zone (>14 m) was dominated by polychaete worms living in relatively permanenttubes and burrows. A variety ofdescriptive-correlative evidence indicates that communityzonation is strongly influenced by wave-induced bottom disturbance. The evidence includes: 1) a positive correlation between water depth and the numbers of tube dwellers, burrow dwellers, and commensal animals which apparently cannot establish or maintain populations in shifting sediments; 2) other depth and thus substrate disturbance related natural history patterns; 3) a positive correlation between the strength of wave activity and the widthanddepth limits of the faunal zones (Le., when wave disturbance is more intense, the crustacean zone endsand the polychaete zone begins in deeper water); 4) a correspondence between the largest decrease in polychaete population size and the season and location ofgreatest wave activity (winter months at the shallowest station); and 5) a marked similarity between community zonation along a depth-dependent gradient of oscillatory substrate motion (gently sloping sandflats) and the zonation along a constantdepth gradient ofcreeping substrate motion (submarine canyon ridge). Other explanations are inconsistent with these biological patterns and, thus, wave disturbance is apparently the major physical process affecting community zonation.
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We report results of ecosystem studies in Monterey Bay, California, during the summer upwelling periods, 1996–99, including impacts of El Niño 1997–98 and La Niña 1999. Random-systematic line-transect surveys of marine mammals were conducted monthly from August to November 1996, and from May to November 1997–99. CTDs and zooplankton net tows were conducted opportunistically, and at 10 predetermined locations. Hydroacoustic backscatter was measured continuously while underway to estimate prevalence of zooplankton, with emphasis on euphausiids, a key trophic link between primary production and higher trophic level consumers.The occurrences of several of the California Current’s most common cetaceans varied among years. The assemblage of odontocetes became more diverse during the El Niño with a temporary influx of warm-water species. Densities of cold-temperate Dall’s porpoise, Phocoenoides dalli, were greatest before the onset of El Niño, whereas warm-temperate common dolphins, Delphinus spp., were present only during the warm-water period associated with El Niño. Rorqual densities decreased in August 1997 as euphausiid backscatter was reduced. In 1998, as euphausiid backscatter slowly increased, rorqual densities increased sharply to the greatest observed values. Euphausiid backscatter further increased in 1999, whereas rorqual densities were similar to those observed during 1998. We hypothesize that a dramatic reduction in zooplankton biomass offshore during El Niño 1997–98 led to the concentration of rorquals in the remaining productive coastal upwelling areas, including Monterey Bay. These patterns exemplify short-term responses of cetaceans to large-scale changes in oceanic conditions.
Article
Harbor porpoise (Phocoena phocoena), and most other cetaceans, spend a significant proportion of time submerged, and are undetectable from the air. Abundance estimates based on line transect sampling may be severely biased by assuming that all porpoise near the line are detected [g(0) = 1]. By tracking groups of harbor porpoise from land, we estimated the proportion of time harbor porpoise spent at the surface and the probability that aerial observers detected groups within 200 m of the transect line. Two teams on land, equipped with electronic theodolites, tracked harbor porpoise while surveys were conducted from an aircraft equipped with side-bubble windows and a belly window. During 7 days, 33 hours of observation were made in a high-density area for harbor porpoise near Orcas Island, Washington. We monitored 7 different harbor porpoise groups from 15 to 66 minutes each. The average proportion of time at or near the surface was 0.231 (SE = 0.032). From a selected sample of 164 land-based sightings of harbor porpoise groups, 50 (30.5%) were observed from the aircraft. For our aerial line transect surveys of harbor porpoise conducted by experienced observers, (g) over cap(0) = 0.292 (SE = 0.107). However, for the inexperienced observers, (g) over cap(0) was 0.079 (SE = 0.046), which demonstrates the importance of experience and training.
Article
Benthic macrofauna were quantitatively sampled seasonally at stations along three transects through a high energy surf zone. Samples were collected on a calm, an intermediate and an exposed, high energy beach. Sediment characteristics were measured and wave parameters were calculated for each station. Two gradients of physical parameters occurred. One decreased from the swash to the nearshore zone and another increased from the sheltered to the exposed beach. Biological parameters showed similar, but opposite, gradients. Multivariate analyses of the 86 benthic species recorded and the physical parameters showed that wave current was the most important factor influencing the faunal distribution. Three faunal assemblages were identified which corresponded to the physical swash, breaker and nearshore zones, respectively. Using these, a general zonation scheme for high energy surf zones is proposed.
Article
Sixty male sea otters (Enhydra lutris) were tagged on the rear flippers with colored tags. Of these, 46 (77%) were resighted. Movements of 127 km were documented for adults and 187 km for subadults. Adults maintained breeding territories that averaged 40.3 ha (n= 10, SE = 4.0). They returned to the same territory seasonally for up to seven consecutive years. Territorial males moved from areas of high male abundance to areas of high female abundance on a seasonal basis. During the winter, 74% of adult males left breeding areas and joined concentrations of males located near the ends of the range. Thirty percent of the subadult males were observed in male groups near the extremities of the range. During the summer and fall, the density of adult males (15/1,000 ha) and adult male to independent otter (non-pup) ratio (1:5) in female areas was highest. The number of adult males in areas of female abundance was inversely related to the number of dependent pups, perhaps because when pup numbers are low (late summer and fall) the number of estrous females is high. Subadult males may remain in female areas on a year round basis until their second or third year. However, they were not generally associated with adult females.
Article
The distribution of marine birds relative to water clarity was examined in the nearshore waters of Monterey Bay, California. I conducted nine at-sea surveys in 1999 and 2000 and simultaneously recorded water clarity and the density of five taxa of marine birds. Among plunge-divers, Forster's Terns (Sterna forsteri) occurred more frequently than expected over turbid water (< 2.5 m Secchi depth) and, among pursuit-diving species, Brandt's Cormorants (Phalacrocorax penicillatus) occurred most often in the clearest water available (> 5 m Secchi depth). Turbidity in Monterey Bay may be caused by suspended organic matter, including phytoplankton, during summer and fall. Forster's Tern may prefer such areas because small fish are likely to occur near this potential food source. Brandt's Cormorants probably rely on vision to catch fish near the bottom and turbid water may reduce available light and limit visual acuity. The distribution of Brown Pelicans (Pelecanus occidentalis), Western/Clark's Grebes (Aechmophorus occidentalis/clarkii), and Marbled Murrelets (Brachyramphus marmoratus) did not appear to be influenced by turbidity levels, indicating that some seabirds use marine habitats with a wide range of water clarities.
Article
The occurrence, distribution, site fidelity, and school size of bottlenose dolphins (Tursiops truncatus) in the coastal waters of north San Diego County, California were assessed during a six-year boat-based photoidentification study. A total of 146 photographic surveys were conducted between January 1984 and December 1989. Dolphin schools were encountered on 79% of all surveys, and 2, 869 individuals were observed in 145 separate schools. Three-hundred seventy-three dolphins were individually identified. All schools were sighted within 1 km of shore, and more than two thirds of the schools were encountered in the southern half of the 32-km long study area. School size (mean = 19.8, SD = 18.40) and the number of dolphins encountered per survey (mean = 26.8, SD = 22.30) were highly variable. Low resighting rates of known individuals provided little evidence for longterm site fidelity. When our six-year photoidentification database was combined with previous data, 404 dolphins were identified in the study area from September 1981 to December 1989. Jolly-Seber population estimates during the 1984-1989 study period varied between 234 and 285. The combination of regular dolphin occurrence, low site fidelity by known individuals, and the continuous increase in the rate at which new dolphins were identified indicates that numerous different individuals were visiting the study area across and within years. The open California coastline differs in habitat structure and prey distribution from more protected study areas where bottlenose dolphins display site fidelity. These habitat differences may help to explain the observed intraspecific behavioral variability of this species.
Article
The echolocation rate and behavior of wild harbor porpoises were studied using a harbor porpoise click detector (POD) deployed close to the bottom at 40 m depth in Scottish waters, UK, April—June 2001. Echolocation variables were compared among four diel phases; morning, day, evening, and night. The echolocation encounter rate, the minimum interclick interval per train, and the proportion of echolocation click trains with a minimum interclick interval below 10 msec were all significantly higher at night than during the day. The variation in echolocation rate implies that porpoises increased their echolocation rate and/or visited the depth of the POD more often at night than during the day. Further, the changes in minimum interclick interval per train suggest that they used their echolocation for foraging or investigating objects at a close range to a higher extent, and acoustically explored the environment at greater distances at night than during the day.
Article
Carrying capacity (K) for the California sea otter (Enhydra lutris nereis) was estimated as a product of the density of sea otters at equilibrium within a portion of their existing range and the total area of available habitat. Equilibrium densities were determined using the number of sea otters observed during spring surveys in 1994, 1995, and 1996 in each of three habitat types where sea otters currently exist. Potential sea otter habitat was defined as from the California coastline to the 40-m isobath and classified as rocky, sandy, or mixed habitat according to the amount of kelp and rocky substrate in the area. The amount of habitat available to sea otters in California was estimated using a Geographic Information Systems (GIS) program. The estimated mean number of sea otters that could be supported by the marine environment to a depth of 40 m in California was 15,941 (95% CI 13,538–18,577). The GIS-based approach incorporated detailed bathymetric contours, produced repeatable and accurate estimates, and served as an innovative method of measuring sea otter habitat. We believe the approach described in this paper represents the best available information on how a sea otter population at equilibrium would be distributed along the California coast.
Article
This study presents a detailed seasonal comparison of the abundance and distribution of cetaceans within 100-150 nmi (185-278 km) of the California coast during 1991 and 1992. The results of a shipboard line-transect survey conducted in July-November 1991 (“summer”) were compared to those from aerial line-transect surveys conducted in March-April 1991 and February-April 1992 (“winter”). Using a confidence-interval-based bootstrap procedure, abundance estimates for six of the eleven species included in the comparison exhibited significant (= 0.05) differences between the winter and summer surveys. Pacific white-sided dolphins (Lagenorhynchus obliquidens), Risso's dolphins (Grampus griseus), common dolphins (Delphinus spp.), and northern right whale dolphins (Lissodelphis borealis) were significantly more abundant in winter. The abundance of blue whales (Balaenoptera musculuss) and gray whales (Eschrichtius robustus) reflected well-documented migratory patterns. Fin whales (B. physalus) were significantly more abundant during summer. No significant differences in seasonal abundance were identified for Dall's porpoises (Phocoenoides dalli), bottlenose dolphins (Tursiops truncatus), killer whales (Orcinus orca), sperm whales (Physeter macrocephalus), or humpback whales (Megaptera novaeangliae). Significant north/south shifts in distribution were found for Dall's porpoises, common dolphins, and Pacific white-sided dolphins, and significant inshore/offshore differences were identified for northern right whale dolphins and humpback whales.
Article
Pinnipeds forage almost exclusively underwater. Consequently, observing them is difficult and relatively little is known of how they use their senses to locate prey, avoid predators, and navigate while diving. Vision has been presumed to be of primary importance, although previous measurements of visual functioning in pinnipeds have been restricted to just a few shallow-diving species. As diving pinnipeds experience rapid changes in light levels during descent/ascent and low light levels at depth, it has not been clear whether they possess visual capabilities adequate for use while diving, particularly in the case of deep-diving species. To examine this issue, behavioral psychophysics have been used to assess and compare the dark adaptation rates and relative light sensitivities of a deep-diving pinniped (northern elephant seal, Mirounga angustirostris), two shallow-diving species (California sea lion, Zalophus californianus, and harbor seal, Phoca vitulina), and a human subject. In comparison to the human subject, both the California sea lion and the harbor seal dark-adapted relatively quickly and were more light sensitive. These findings suggest that both of these species are well suited for vision in the moderately dim shallow-water environments in which they dive to forage. In contrast, the elephant seal reached complete dark adaptation in less than half the time taken by the other pinnipeds, and it was significantly more light sensitive. Unlike the shallower-diving species, the visual abilities of the elephant seal are commensurate with the extreme conditions experienced while deep diving. Thus, we conclude that elephant seals are sufficiently adapted to rely on vision underwater, even while diving to depths in excess of 1000 meters where bioluminescence may be the sole source of ambient light.
Article
We investigated effects of marine climate variability on pinniped populations and assessed the initial stages of recovery following implementation of the U. S. Marine Mammal Protection Act (MMPA) based on long-term (1973-1997) population surveys at the South Farallon Islands and Point Reyes Peninsula, central California. California sea lions increased over the study period, with peak numbers observed during and after major El Niño events. The rate of increase for California sea lions appears to have decreased in recent years. Steller sea lions decreased at the South Farallon Islands and remain depleted at Point Reyes Peninsula. Harbor seal populations increased in a logistic and non-linear fashion at Point Reyes Peninsula and the South Farallon Islands, respectively. Harbor seals were more abundant at the South Farallon Islands during years of relatively high sea-surface temperature, which may be related to their inability to find sufficient prey in coastal waters under these conditions. Northern elephant seal abundance increased in a logistic fashion over the study period at both the South Farallon Islands and Point Reyes Peninsula; however, productivity at the South Farallon Islands decreased in recent years. Maximum haulout numbers for elephant seals at the South Farallon Islands increased in the 1970s, maintained an asymptote throughout the 1980s and early 1990s, but recently declined; additional studies are needed to investigate which age classes are associated with this decline. Protection afforded by the MMPA has facilitated partial to full recovery of all populations except for Steller sea lion. Oceanographic relationships do not appear to confound interpretations of population recovery and may help to explain changes in the Steller sea lion population.
Article
Sea otter, Enhydra lutris, predation had no detectable effect on abundance and size distribution of deep-burrowing bivalve prey in the Elkhorn Slough, California, USA. Up to 23 otters were present for 6 mo of the study period (March 1984 through April 1985). This is in contrast to previous studies of sea otter predation, especially on the shallow-burrowing Pismo clam Tivela stultorum, which can be found along the wave-exposed coast near the slough. The deep-burrowing clams Tresus nuttallii and Saxidomus nuttalli made up 61% of the prey taken in the slough, and are more difficult for otters to excavate than Pismo clams. The occurrence of foraging otters was highest in an area where the two bivalve prey were extremely abundant (18 individuals m–2). However, the otters did not selectively prey on the largest clams available within the study sight, but foraged preferentially in a patch of smaller individuals where bivalve burrow depth was restricted by the presence of a dense clay layer. This foraging strategy maximized the amount of prey biomass obtained per unit volume of sediment excavated. Our findings suggest that in soft-sediment habitats deep-burrowing bivalves may be more resistant to otter predation than shallower burrowers.
Article
A census of marine mammals was conducted off the coast of California (USA) in 1979–1980. The distribution of seasurface chlorophyll was determined at the same time by onboard fluorometry and by remote sensing using the Coastal Zone Color Scanner on the Nimbus-7 satellite. Comparisons of species and chlorophyll distributions indicate that marine mammals are not randomly distributed with respect to chlorophyll. Cetaceans were more abundant in the productive coastal waters than in the offshore oceanic waters of the California Current. This supports the hypothesis that the distributions of some cetacean species may be related to the mesoscale features that are manifest in the patterns of chlorophyll as revealed in the satellite imagery. It is suggested that oceanic chlorophyll may be used as a habitat descriptor for selected marine mammals, and that remote sensing will provide complementary data useful in the interpretation of observed distribution patterns of marine mammals and in the estimation of their abundance.
Article
Periods of elevated temperatures in northern Monterey Bay suggest that this is a region of increased residence time during periods of active upwelling. Nearshore increases in near-surface temperature often coincide with offshore decreases in temperature as cold, upwelled water is advected into the center of the bay—the juxtaposition of warm and cold water enhancing the thermal signature of a feature described as an ‘upwelling shadow’. We present a variety of data collected from 1988 to 1993 invoking a shallow, stratified, cyclonic circulation in northern Monterey Bay to explain the upwelling shadow as a dynamic response to upwelling north of the bay. Residence times associated with this warm recirculation are of the order of 8 days which is comparable to semi-enclosed embayments whereas Monterey Bay is an open embayment. We estimate a continuous replacement of upwelling shadow water with offshore water at a rate of 0.083 day−1, and we suggest that a circulation velocity of 0.1 m · s −1, is needed to produce the thermal signature of the feature given published values of surface heat flux in the central California coastal region. The thermal signature of this upwelling shadow persists through much of the upwelling season, surviving brief relaxations in upwelling but breaking down during prolonged relaxations of the order of a week or longer. The stratification and coherent recirculation of this feature is likely to be very important for such biological processes as productivity and the dispersal and recruitment of larvae.