Ranging patterns of bottlenose dolphins living in oceanic waters: implications for population structure

Very little is known about the ecology of common bottlenose dolphins (Tursiops truncatus) living in oceanic waters. This study investigated the ranging and residence patterns of bottlenose dolphins occurring in the Azores (Portugal), the most isolated archipelago in the North Atlantic. Data were collected during standardized boat-based surveys conducted over a 6-year period in an area of approximately 5,400 km² (main study area). To investigate the extent of movements of individual animals, non-systematic surveys were also conducted outside this area. Only 44 individuals out of 966 identified were frequently sighted within and between years. The remaining individuals were either temporary migrants from within or outside the archipelago, or transients. Resident dolphins showed strong geographic fidelity to the area. Long-distance movements (of almost 300 km), consistent with foraging or exploratory trips, were observed among non-resident dolphins. Home range size was estimated for 31 individuals sighted ≥10 times. Range areas of these dolphins varied in size and location, but considerable overlap was observed in the areas used, suggesting the absence of habitat partitioning between resident and non-resident dolphins. Estimates of home range size of bottlenose dolphins in the Azores were found to be considerably larger than those previously reported for this species. It is hypothesized that dolphins living in the Azores carry out extensive movements and have large home ranges in response to the lower density and patchy distribution of prey compared to other areas. The extensive ranging behaviour and the lack of territoriality provide an opportunity for interbreeding between dolphins associated with different islands, thus preventing genetic differentiation within the population of the Azores.     

Association patterns and kinship in female Indo-Pacific bottlenose dolphins (Tursiops aduncus) of southeastern Australia

Kinship has been shown to be an important correlate of group membership and associations among many female mammals. In this study, we investigate association patterns in female Indo-Pacific bottlenose dolphins (Tursiops aduncus) inhabiting an embayment in southeastern Australia. We combine the behavioral data with microsatellite DNA and mitochondrial DNA data to test the hypotheses that genetic relatedness and maternal kinship correlate with associations and social clusters. Mean association between females was not significantly different from a random mean, but the standard deviation was significantly higher than a random standard deviation, indicating the presence of nonrandom associates in the dataset. A neighbor-joining tree, based on the distance of associations between females, identified four main social clusters in the area. Mean genetic relatedness between pairs of frequent female associates was significantly higher than that between pairs of infrequent associates. There was also a significant correlation between mtDNA haplotype sharing and the degree of female association. However, the mean genetic relatedness of female pairs within and between social clusters and the proportion of female pairs with the same and different mtDNA haplotypes within and between clusters were not significantly different. This study demonstrates that kinship correlates with associations among female bottlenose dolphins, but that kinship relations are not necessarily a prerequisite for membership in social clusters. We hypothesize that different forces acting on female bottlenose dolphin sociality appear to promote the formation of flexible groups which include both kin and nonkin.     

The emergence of unshared consensus decisions in bottlenose dolphins

Unshared consensus decision-making processes, in which one or a small number of individuals make the decision for the rest of a group, are rarely documented. However, this mechanism can be beneficial for all group members when one individual has greater knowledge about the benefits of the decision than other group members. Such decisions are reached during certain activity shifts within the population of bottlenose dolphins residing in Doubtful Sound, New Zealand. Behavioral signals are performed by one individual and seem to precipitate shifts in the behavior of the entire group: males perform side flops and initiate traveling bouts while females perform upside-down lobtails and terminate traveling bouts. However, these signals are not observed at all activity shifts. We find that, while side flops were performed by males that have greater knowledge than other male group members, this was not the case for females performing upside-down lobtails. The reason for this could have been that a generally high knowledge about the optimal timing of travel terminations rendered it less important which individual female made the decision. This contribution is part of the special issue “Social Networks: new perspectives” (Guest Editors: J. Krause, D. Lusseau and R. James).     

Investigating stock structure and trophic relationships among island-associated dolphins in the oceanic waters of the North Atlantic using fatty acid and stable isotope analyses

Short-beaked common dolphins (Delphinus delphis) and Atlantic spotted dolphins (Stenella frontalis) are the two most abundant cetacean species in the oceanic waters of Madeira and the Azores. They are of similar size, occur in similar habitats and are regularly observed in mixed-species groups to forage together. Genetic analyses suggested that, within each species, dolphins ranging around both archipelagos belong to the same panmictic population. We tested the hypotheses that (1) within each species, individuals from the two archipelagos belong to a single ecological stock; (2) between species, common and spotted dolphins have distinct trophic niches; using fatty acid (FA) and stable isotope (SI) analyses. Fatty acids and stable isotopes were analysed from 86 blubber and 150 skin samples of free-ranging dolphins, respectively. Sex-related differences were not significant, except for common dolphin FA profiles. In S. frontalis, FA and SI differences between archipelagos suggested that individuals belonged to different ecological stocks, despite the existence of gene flow between the two archipelagos. In D. delphis, differences were more pronounced, but it was not possible to distinguish between stock structure and a seasonal effect, due to differential sampling periods in the Azores and Madeira. Inter-specific comparisons were restricted to the Azores where all samples were collected during summer. Differences in FA proportions, noticeably for FA of dietary origin, as well as in nitrogen SI profiles, confirmed that both species feed on distinct resources. This study emphasizes the need for an integrated approach including both genetic and biochemical analyses for stock assessment, especially in wide-ranging marine top predators.     

Molecular insight into the population structure of common and spotted dolphins inhabiting the pelagic waters of the Northeast Atlantic

Several cetacean species exhibit fine-scale population structure despite their high dispersal capacities and the apparent continuity of the marine environment. In dolphins, most studies have focused on coastal areas and continental margins, and they revealed differentiated populations within relatively small geographic areas, sometimes in conjunction with a specialisation for different habitats (ecotypes). We analysed the population genetic structure of short-beaked common dolphins (Delphinus delphis) and Atlantic spotted dolphins (Stenella frontalis) in the Azores and Madeira, the two most isolated archipelagos of the North Atlantic. The archipelago of the Azores is divided into three groups of islands and stands 900 km away from Madeira. It is not known whether individuals migrate between groups of islands and archipelagos, nor whether distinct ecotypes are present. These questions were investigated by genetic analyses of 343 biopsy samples collected on free-ranging dolphins. The analyses consisted in sequencing part of the mitochondrial hyper-variable region, screening up to 14 microsatellite loci, and molecular sexing. Results did not unravel any population structure at the scale of the study area. Lack of differentiation matches expectations for spotted dolphins, which are transient in both archipelagos, but not for common dolphins, which are present year-round in the Azores and potentially resident. Absence of genetic structure over hundreds and even thousands of kilometres implies the existence of gene flow over much larger distances than usually documented in small delphinids, which could be achieved through individual movements. This finding indicates that population structure in oceanic habitat differs from that observed in coastal habitat.     

Signature whistles of free-ranging bottlenose dolphins Tursiops truncatus: stability and mother-offspring comparisons

Summary Mother-calf whistle exchanges were recorded from temporarily captured free-ranging bottlenose dolphins from 1975 to 1989. This is part of a long-term research project studying social structure and behavior of a community of approximately 100 dolphins in waters near Sarasota, Florida. Analysis of whistle exchanges from 12 mothercalf pairs shows that signature whistles can remain stable for periods up to at least 12 years. We looked for effects of vocal learning on the development of the signature whistle by comparing whistles of calves to those of their mothers. Eight female calves produced whistles distinct from those of their mothers, while four male calves produced whistles similar to those of their mothers. Male calves appeared to produce a greater proportion of whistles other than the signature whistle (termed variants). We hypothesize that these sex differences in whistle vocalizations may reflect differences in the roles males and females play in the social structure of the community.     

Sex difference in signature whistle production of free-ranging bottlenose dolphins,Tursiops truncates

Signature whistles of 42 free-ranging bottle-nose dophin calves were compared to those of their mothers. Humans judged their similarity by inspection of spectrograms. There was a sex difference in the tendency of calves to produce whistles similar to or different from those of their mothers; most female calves produced whistles that were different from those of their mothers, whereas male calves were more likely to produce whistles that were similar to those of their mothers. Because matrilineally related females associate together and use signature whistles to establish and/or maintain contact with their calves, there may be a selective pressure for females to produce whistles that are distinct from those of their mothers. There may be fewer constraints governing whistle development in males, with the result that some males produce whistles similar to those of their mothers and others do not.     

Erratum to: Prey selection by resident common bottlenose dolphins (Tursiops truncatus) in Sarasota Bay, Florida

Prey selection was investigated in wild, resident common bottlenose dolphins, Tursiops truncatus, during the summer months in Sarasota Bay, Florida, USA. Stomach content analyses of 15 dolphins with extensive sighting histories and well-documented distributions were used to determine prey use. Prey availability was assessed by purse seine surveys. We compared the relative abundances of prey available to estimates of prey use at closely matching spatial and temporal scales. G-tests determined that dolphins in this study significantly selected for prey at the species, family, and soniferous/non-soniferous prey levels (G _adj  = 753.98–1,775.93, df = 1–21, p ≤ 0.01). While comprising only 6.3% of the total available prey, soniferous fishes accounted for 51.9% of the total prey consumed. Manly’s standardized forage ratios and 95% Bonferroni confidence intervals determined significant positive selection for soniferous prey and against non-soniferous prey (β^S = 0.9461 vs. β^NS = 0.0539). Dolphins selected against Gerridae, Clupeidae, and Sparidae (β ≤ 0.0014), as well as against all the species within those families (β ≤ 0.0190). It is likely that passive listening for soniferous prey provides an ecological or energetic advantage to cetaceans utilizing this specific foraging technique.     

Production,composition and sedimentation of salp fecal pellets in oceanic waters

Rates of fecal pellet production have been recorded from seven species of oceanic salps feeding on natural diets. Expressed as g C defecated per mg salp body C per hour, the values range between 3.7 and 27.7. Carbon: nitrogen ratios of the salp fecal pellets average 11.4; the organic matter of the pellets is mainly protein and carbohydrate. Sinking velocities of the pellets are very high, ranging from 320 to 2 238 m d^-1 for pellets from three species. However, the pellets sink slower than would be predicted from extrapolation of rates for crustacean pellets, probably due to the shape of the pellets and their density. The high rates of defecation, large size and rapid sedimentation of salp fecal pellets make them likely mechanisms for rapid transport of small particulate matter from surface waters to deep water and the benthos.     

Behavioral evidence for population structure in Lasioglossum (Dialictus) zephyrum female dispersion patterns

Summary Within-aggregation nest distribution and behavioral interactions between pairs of female Lasioglossum zephyrum were investigated. Within aggregations, nests are contagiously distributed (clumped), and this distribution is not correlated with the particle size of the soil surface. Results of behavioral interactions both in the field and laboratory show that females residing near one another behave as if they are more closely related than females residing at greater distances from one another. The behaviors used are known to be correlated with the degree of relatedness between females. This suggests that females found colonies near their natal nests, producing neighborhoods of related colonies within aggregations of L. zephyrus.     

Large-scale oceanic distribution and population structure of Calanus finmarchicus, in relation to physical environment, food and predators

The investigation was carried out from 62°N to 73°N and from 14°E to 11°W in the Norwegian Sea during 19 June-12 July 1997. Regional differences in the phase of the seasonal development of the plankton community were evident, most pronounced across the Arctic front. In the Coastal and eastern Atlantic domains, post-bloom conditions prevailed, characterised by low chlorophyll a (chl a) levels and a phytoplankton assemblage dominated by coccolithophorids and small flagellates. During the study period, egg production rates of Calanus finmarchicus were low (<10 eggs female^-1 day^-1), older copepodite stages dominated, and the seasonal descent to deeper waters had started. In the Arctic domain, bloom conditions were evident by high chl a levels and a high abundance of large diatoms. Egg production rates were higher (a maximum of 29 eggs female^-1 day^-1), but the dominance of stages CI-CIII indicated that considerable spawning had already occurred prior to the spring bloom. The seasonal descent had barely started. Both invertebrate and fish predators were most abundant in the Coastal and eastern Atlantic domains, with abundance strongly decreasing north-westwards. No tight relationship between total abundance of invertebrate or fish predators and that of C. finmarchicus was apparent. However, a weak, but significant, relationship between abundance of young stages of chaetognaths and Euchaeta spp. versus young stages of C. finmarchicus was found, indicating that these invertebrate predators develop parallel to the development of the new cohort of C. finmarchicus. In early summer, C. finmarchicus had reached overwintering stages, and had started to accumulate in deeper waters in areas with the highest abundance of horizontally migratory planktivorous fish.     

Spatiotemporal variation in survival rates: implications for population dynamics of yellow-bellied marmots

Spatiotemporal variation in age-specific survival rates can profoundly influence population dynamics, but few studies of vertebrates have thoroughly investigated both spatial and temporal variability in age-specific survival rates. We used 28 years (1976-2003) of capture-mark-recapture (CMR) data from 17 locations to parameterize an age-structured Cormack-Jolly-Seber model, and investigated spatial and temporal variation in age-specific annual survival rates of yellow-bellied marmots (Marmota flaviventris). Survival rates varied both spatially and temporally, with survival of younger animals exhibiting the highest degree of variation. Juvenile survival rates varied from 0.52 +/- 0.05 to 0.78 +/- 0.10 among sites and from 0.15 +/- 0.14 to 0.89 +/- 0.06 over time. Adult survival rates varied from 0.62 +/- 0.09 to 0.80 +/- 0.03 among sites, but did not vary significantly over time. We used reverse-time CMR models to estimate the realized population growth rate (lamda), and to investigate the influence of the observed variation in age-specific survival rates on lamda. The realized growth rate of the population closely covaried with, and was significantly influenced by, spatiotemporal variation in juvenile survival rate. High variability in juvenile survival rates over space and time clearly influenced the dynamics of our study population and is also likely to be an important determinant of the spatiotemporal variation in the population dynamics of other mammals with similar life history characteristics.     

Stable isotopic evidence for trophic groupings and bio-regionalization of predators and their prey in oceanic waters off eastern Australia

Muscle tissue was collected for stable isotope analysis (SIA) from the main fish predators and their fish and cephalopod prey from oceanic waters off eastern Australia between 2004 and 2006. SIA of δ¹⁵N and δ¹³C revealed that the species examined could be divided into three main trophic groups. A “top predator” group consisted mainly of large billfish (Xiphias gladius and Tetrapturus audax), yellowfin (Thunnus albacares), bigeye (T. obesus) and southern bluefin (T. maccoyii) tunas and sharks; with mako (Isurus oxyrinchus) the highest. Below this tier was a second group composed of mid-trophic level fishes including albacore tuna (Thunnus alalunga), lancet fish (Alepisaurus ferox), mahi mahi (Coryphaena hippuris) and ommastrephid squid. Underlying both groups was a grouping of small fishes including myctophids, small scombrids and nomeids as well as surface fishes including macrorhamphosids. These groupings were based largely on mean animal size which showed a positive linear relation to δ¹⁵N (r ² = 0.58). Some species showed significant ontogenetic variation in either δ¹⁵N (swordfish, lancet fish, yellowfin and albacore tuna) or δ¹³C (mako shark). We also noted a consistent latitudinal change in δ¹⁵N and δ¹³C at ~28°S for the top predator species, particularly albacore and yellowfin tuna. The differences were consistent with a change from oligotrophic Coral Sea to nutrient rich Tasman Sea waters. These differences suggest that predatory fishes may have extended residence time in distinct regions off eastern Australia.     

Understanding abundance patterns of a declining seabird: implications for monitoring.

The Kittlitz's Murrelet (Brachyramphus brevirostris) is a rare, non-colonial seabird often associated with tidewater glaciers and a recent candidate for listing under the Endangered Species Act. We estimated abundance of Kittlitz's Murrelets across space and time from at-sea surveys along the coast of Alaska (USA) and then used these data to develop spatial models to describe abundance patterns and identify environmental factors affecting abundance. Over a five-week period in the summer of 2005, we recorded 794 Kittlitz's Murrelets, 16 Marbled Murrelets (B. marmoratus), and 70 unidentified murrelets. The overall population estimate (N, mean +/- SE) during the peak period (3-9 July) was 1317 +/- 294 birds, decreasing to 68 +/- 37 by the last survey period (31 July-6 August). Density of Kittlitz's Murrelets was highest in pelagic waters of Taan Fjord (18.6 +/- 7.8 birds/km2, mean +/- SE) during 10-16 July. Spatial models identified consistent "hotspots" of Kittlitz's Murrelets, including several small areas where high densities of murrelets were found throughout the survey period. Of the explanatory variables that we evaluated, tidal current strength influenced murrelet abundance most consistently, with higher abundance associated with strong tidal currents. Simulations based on the empirically derived estimates of variation demonstrated that spatial variation strongly influenced power to detect trend, although power changed little across the threefold difference in the coefficient of variation on detection probability. We include recommendations for monitoring Kittlitz's Murrelets (or other marine species) when there is a high degree of uncertainty about factors affecting abundance, especially spatial variability.     

Complex patterns of population structure and recruitment of <Emphasis Type="Italic">Plectropomus leopardus</Emphasis> (Pisces: Epinephelidae) in the Indo-West Pacific: implications for fisheries management

Here the population genetic structure of an ecologically and economically important coral reef fish, the coral trout Plectropomus leopardus, is investigated in the context of contemporary and historical events. Coral trout were sampled from four regions (six locations) and partial mtDNA D-loop sequences identified six populations (Fst = 0.89209, P < 0.0001): Scott Reef and the Abrolhos Islands in west Australia; the Great Barrier Reef (GBR), represented by northern and southern GBR samples; New Caledonia and Taiwan, with Taiwan containing two genetic lineages. Furthermore, this study identified source and sink populations within and among regions. Specifically, the northern population in west Australia (Scott Reef) was identified, as the source for replenishment of the Abrolhos population, whilst New Caledonia was a source for recruitment to the GBR. Based on these insights from a single mtDNA marker, this study will facilitate the development of rational management plans for the conservation of P. leopardus populations and therefore mitigate the risk of population declines from anthropogenic influences.     

Plasticity in circadian activity patterns of mesocarnivores in Southwestern Europe: implications for species coexistence

Limiting similarity theory predicts that competing species must segregate along one or more dimensions of their ecological niche in order to coexist. In predator communities, interspecific interactions are influenced by a diversity of factors; therefore, the behavioural patterns of composing species will differ due to locally adapted interactions. We deployed 32–41 camera-traps in five study areas across the Iberian Peninsula to investigate the temporal relations between mesocarnivores in SW Europe. The selection for a period of the diel cycle and plasticity in activity patterns was evaluated using the Jacobs Selection Index (JSI) and the coefficient of activity overlap (?₁). Furthermore, we investigated whether temporal shifts can facilitate coexistence by reducing activity overlap. Seven species of mesocarnivores were detected and were assigned into one of three behaviourally distinct groups: diurnal (JSI_day?≥?0.8), strictly nocturnal (JSI_night?≥?0.8) or facultative nocturnal species (0.4?≥?JSI_night?>?0.8). Most species exhibited substantial flexibility, which allowed them to locally adapt their foraging strategies (intraspecific ?₁?=?0.70–0.77). Mean Δ₁ from all interspecific pairwise comparisons was negatively correlated with the number of carnivore species with ≥10 detections (r ?0.76, p?=?0.02). Our results suggest that temporal segregation is likely to play an important role in facilitating mesocarnivore coexistence, especially with increasing community complexity, where most species’ activity peaks were asynchronous. These results contribute to understanding the dynamics and behavioural strategies of coexisting mesocarnivores, crucial for forecasting the possible outcomes of conservation or management actions.     

Species-specific grazing rates of heterotrophic dinoflagellates in oceanic waters,measured with a dual-label radioisotope technique

A dual-isotope method was developed to measure grazing rates and food preferences of individual species of heterotrophic dinoflagellates from natural populations, collected from the Slope, Gulf Stream, and Sargasso Sea and from a transect from Iceland to New England, in 1983. The isotope method measures the grazing rates of microzooplankton which cannot be separated in natural populations on the basis of size. Tritiated-thymidine and ¹⁴C-bicarbonate were used to label natural heterotrophic and autotrophic food, respectively. Nine oceanic dinoflagellate species in the genera Protoperidinium, Podolampas, and Diplopsalis fed on both heterotrophic and autotrophic food particles with clearance rates of 0.4 to 8.0 l cell^-1 h^-1, based on ³H incorporation, and 0.0 to 28.3 l cell^-1 h^-1, based on ¹⁴C incorporation. Two dinoflagellate species, Protoperidinium ovatum and Podolampas palmipes, fed only on ³H-labelled food particles. Several species of dinoflagellates fed on bacteria (<1 m) which had been prelabelled with ³H-thymidine. The clearance rates of heterotrophic dinoflagellates and ciliates were similar and within the range of tintinnid ciliate clearance rates reported in the literature. As heterotrophic dinoflagellates and ciliates can have comparable abundances in oceanic waters, we conclude that heterotrophic dinoflagellates may have an equally important impact as microheterotrophic grazers of phytoplankton and bacteria in oceanic waters.Partially supported by a grant from the National Science Foundation, OCE-81-17744     

The population structure of the harbour porpoise,Phocoena phocoena,in Danish waters and part of the North Atlantic

A study of the homogeneity between putative stocks of Phocoena phocoena in the North Sea and inner Danish waters was performed using isozyme electrophoresis. Two polymorphic systems, mannose phosphate isomerase (MPI) and phosphoglucomutase (PGM) were used in the analysis. A two-locus homogeneity test showed an effect of season and locality on the genotypic distribution. This led to a division of the total sample into season and into the two localities, the inner Danish waters (IDW) and the North Sea. The samples were then compared to the Hardy-Weinberg expectations where a deficit in heterozygotes was observed in the North Sea sample in both systems and at the Pgm locus in the total sample, indicating a mixing of sub-populations. A hierarchical contingency table analysis, also based on the genotypic distribution, showed a combined effect of season and locality and only an insignificant effect of sampling period, thus supporting the division into season and locatily. The seasonal effect is further supported by earlier observations of a seasonal migration of harbour porpoises out of the Baltic in winter. A significant difference was observed between the two supposed sub-populations, IDW-summer and North Sea-summer samples, based on the genotypic distribution. This could also be a reflection of the sample sizes. On the basis of samples obtained from Canada, West Greenland and Holland a preliminary study of the population structure on a larger scale, comprising the North East and North West Atlantic, was performed. The results so far accord well with the hitherto accepted sub-populations of P. phocoena in the North Atlantic.