Short-beaked Common Dolphins Delphinus Delphis Research Articles

Trophic ecology of northern gannets Morus bassanus highlights the extent of isotopic niche overlap with other apex predators within the Bay of Biscay

Understanding the linkages within complex and evolving marine food webs is essential to comprehend marine ecosystem structure and dynamics. Carbon (δ13C) and nitrogen (δ15N) stable isotope signatures are recognized to be powerful descriptors of the trophic ecology and trophic relationships within marine communities. Apex predators such as seabirds can influence the structure of communities by predating in lower trophic levels. They also convey information both over a range of spatial and temporal scales due to their high mobility and longevity, respectively. For this reason, here, we studied the trophic ecology of northern gannets and the extent of niche overlap with other apex predators within the Bay of Biscay (North-East Atlantic), a key feeding area for numerous predatory species. Mixing models indicated that northern gannets fed primarily on European anchovy Engraulis encrasicolus and on a mixed group composed of European sardine Sardina pilchardus, European sprat Sprattus sprattus, juvenile hake Merluccius merluccius, Atlantic mackerel Scomber scombrus and Atlantic horse mackerel Trachurus trachurus. Within the apex predator community, the northern gannets trophic niche overlapped the most with common guillemots Uria aalge (15.58%, based on Stable Isotope Bayesian Ellipses (SIBER)) and overlapped to a lesser extent with cetacean species occupying shelf habitats (i.e., the harbour porpoise Phocoena phocoena (9.99%) and the short-beaked common dolphin Delphinus delphis (10.37%)). Overall, the overlap found was moderate indicating trophic similarities and dissimilarities between these species. Information on trophic interactions between co-existing species is necessary for holistic environmental management, particularly in areas where several megafauna species share their foraging grounds with fisheries activity.

Intraspecific variation in short-beaked common dolphin’s whistle repertoire

ABSTRACT Dolphins’ whistles are strongly diversified mainly related to physiology and behaviour. We present the whistle characteristics and intraspecific variations of short-beaked common dolphins Delphinus delphis of the Western South Atlantic Ocean (WSAO). A total of 1630 whistles from six encounters were analysed considering group size, contours and acoustical parameters. No significant correlation was found (p > 0.05) between number of whistles and the group size. Upsweep, downsweep and constant were the most frequent contours. A positive relation between the downsweep contour and group size was detected (p < 0.05). Minimum, maximum, centre, peak, initial and final frequencies, frequency range, frequency gradient, duration and inflection points were extracted. The frequencies ranged from 1.34 to 35.23 kHz and the duration varied from 0.11 to 2.16 seconds. Comparing encounters between years through Bonferroni method, minimum frequency, duration, and frequency range were significantly different. C5.0 decision tree showed that whistles of 2014 and 2015 had correct classifications (67.0% and 54.0%, respectively). The nMDS grouped the Atlantic and Pacific oceans based on inflection points and duration, further corroborated through PERMANOVA (p < 0.05). Our results suggest the influence of certain parameters related to intraspecific variation. Whistles comparison with other localities corroborates with differences in geographical scale.

Spatiotemporal models highlight influence of oceanographic conditions on common dolphin bycatch risk in the Bay of Biscay

The population of short-beaked common dolphins Delphinus delphis of the Bay of Biscay (northeast Atlantic) has been subjected to potentially dangerous levels of bycatch since the 1990s. As the phenomenon intensifies, it represents a potent threat to the population. Here, we investigated the relationship between bycatch mortality and oceanographic processes. We assumed that oceanographic processes spatiotemporally structure the availability and aggregation of prey, creating areas prone to attract both common dolphins and fish targeted by fisheries. We used 2 datasets from 2012 to 2019: oceanographic data resulting from a circulation model and mortality data inferred from strandings. The latter allows location of mortality areas and quantification of the intensity of mortality events at sea. We fitted a series of spatiotemporal hierarchical Bayesian models using integrated nested Laplace approximations (INLA). Results provided first insights on how bycatch of common dolphins in the Bay of Biscay might be related to key seasonal and dynamic oceanographic features. We showed that from a statistical predictive point of view, the monthly trend of 2019 bycatch mortality could be predicted with few oceanographic covariates. This study highlights how gaining knowledge about environmental influences on interactions between short-beaked common dolphins and fisheries could have great conservation and management value. Identified relationships with oceanographic covariates were complex, as expected given the dynamic aspects of oceanographic processes, dolphins and fisheries distributions. Further research focusing on smaller time scales is needed to elucidate proximal drivers of common dolphin bycatch in the Bay of Biscay.

Systemic Erysipelothrix rhusiopathiae in seven free-ranging delphinids stranded in England and Wales.

Microbiology records for 1127 cetaceans stranded on English and Welsh beaches and examined at the Institute of Zoology between 1990 and 2019 were reviewed to identify cases of Erysipelothrix rhusiopathiae, an uncommon but potentially fatal zoonotic pathogen. Once cases were identified, prevalence was calculated, corresponding postmortem reports were reviewed, common gross and histopathological findings were identified, and antibiotic susceptibilities were determined. Overall prevalence for E. rhusiopathiae was 0.62% (7/1127; 95% CI: 0.30-1.28%). It was isolated from 3 bottlenose dolphins Tursiops truncatus, 3 harbor porpoises Phocoena phocoena, and 1 short-beaked common dolphin Delphinus delphis, with a prevalence of 21.4% (3/14; 95% CI: 7.6-47.9%), 0.39% (3/779; 95% CI: 0.13-1.13%), and 0.47% (1/212; 95% CI: 0.08-2.62%) for each species, respectively. E. rhusiopathiae resulted in septicemia in all cases from which it was isolated. Gross necropsy findings included pulmonary edema (5/7), hemorrhage (5/7) and/or congestion of various organs (4/7), and serosanguineous effusion (3/7; pericardial: 3/7, pleural: 2/6, abdominal: 2/6). Congestion (5/5), bacterial emboli (4/5), and hemorrhage (4/5) were commonly observed on histopathology, and acute renal tubular injury (2/5) and pulmonary edema (2/5) were occasionally observed. Routine bacterial cultures were vital in identifying E. rhusiopathiae, since gross lesions were often subtle and nonspecific. The liver, kidney, and brain were key organs from which E. rhusiopathiae was consistently isolated. Antibiotic resistance was uncommon and was only observed for amikacin and trimethoprim sulfonamide. Penicillins were consistently effective, along with fluoroquinolones, macrolides, clindamycin, cephalexin, and oxytetracycline.

Meningoencephalitis in a common minke whale Balaenoptera acutorostrata associated with Brucella pinnipedialis and gamma-herpesvirus infection.

Fatal marine Brucella infections with histologic lesions specific to the central nervous system (CNS), known as neurobrucellosis, have been described in 5 species of odontocete cetaceans in the UK: striped dolphins Stenella coeruleoalba, Atlantic white-sided dolphins Lagenorhynchus acutus, short-beaked common dolphins Delphinus delphis, long-finned pilot whale Globicephala melas and Sowerby's beaked whale Mesoplodon bidens. To date, these CNS lesions have only been associated with Brucella ceti ST26 and not with B. pinnipedialis, which is rarely isolated from cetaceans and, although commonly found in various seal species, has never been associated with any pathology. This paper describes the first report of neurobrucellosis in a common minke whale Balaenoptera acutorostrata which was associated with the isolation of Brucella pinnipedialis ST24 and co-infection with Balaenoptera acutorostrata gamma-herpesvirus 2. This is the first report of neurobrucellosis in any species of mysticete and the first report of Brucella pinnipedialis in association with any pathology in any species of marine mammal, which may be due to co-infection with a herpesvirus, as these are known to be associated with immunosuppression.

Stranded cetaceans warn of high perfluoroalkyl substance pollution in the western Mediterranean Sea

Perfluoroalkyl substances (PFASs) are a class of organohalogenated compounds of environmental concern due to similar characteristics as the well-studied legacy persistent organic pollutants (POPs) that typically show environmental persistence, biomagnification and toxicity. Nevertheless, PFAS are still poorly regulated internationally and in many aspects poorly understood. Here, we studied liver and muscle concentrations in five cetacean species stranded at the southeastern coast of Spain during 2009–2018. Twelve of the fifteen targeted compounds were detected in >50% of the liver samples. Hepatic concentrations were significantly higher than those in muscle reflecting the particular toxicokinetics of these compounds. Bottlenose dolphins Tursiops truncatus showed the highest hepatic ΣPFAS (n = 5; 796.8 ± 709.0 ng g−1 ww) concentrations, followed by striped dolphin Stenella coeruleoalba (n = 29; 259.5 ± 136.2 ng g−1 ww), sperm whale Physeter macrocephalus (n = 1; 252.8 ng g−1 ww), short-beaked common dolphin Delphinus delphis (n = 2; 240.3 ± 218.6 ng g−1 ww) and Risso’s dolphin Grampus griseus (n = 1; 78.7 ng g−1 ww). These interspecies differences could be partially explained by habitat preferences, although they could generally not be related to trophic position or food chain proxied by stable N (δ15N) and C (δ13C) isotope values, respectively. PFAS profiles in all species showed a similar pattern of concentration prevalence in the order PFOS>PFOSA>PFNA≈PFFUnA>PFDA. The higher number of samples available for striped dolphin allowed for evaluating their PFAS burden and profile in relation to the stranding year, stable isotope values, and biological variables including sex and length. However, we could only find links between δ15N and PFAS burdens in muscle tissue, and between stranding year and PFAS profile composition. Despite reductions in the manufacturing industry, these compounds still appear in high concentrations compared to more than two decades ago in the Mediterranean Sea and PFOS remains the dominating compound.

Density and Abundance of Delphinus delphis in Waters South of Samos Island, Greece (Eastern Mediterranean Sea)

The Mediterranean subpopulation of short-beaked common dolphin Delphinus delphis is ranked as endangered on the IUCN (International Union for Conservation of Nature) Red List because it has sharply declined during the last decades, resulting in sparse and decreasing populations. Monitoring the conservation status of this endangered dolphin species is particularly relevant to fulfil targets under the range of several international agreements. Moreover, estimating the abundance of D. delphis is essential to verify the effectiveness of conservation action to maintain safe population levels in the Mediterranean Sea and to suggest appropriate modifications to limit potential threats. In this regard, a monitoring program of the short-beaked common dolphin in Samos Island (Greece) was carried out from 2016 to 2019, adopting a random line transect sampling method. The overall density and abundance estimates of D. delphis, obtained by applying conventional distance sampling (CDS) on sighting data, were 0.15 individuals/km2 (CV = 13.27%; 95% CI = 0.11–0.19 individuals/km2) and 51 individuals (CV = 13.27%; 95% CI = 40-66 individuals), respectively. Although, a longer time series of sighting data should be collected and a larger area should be investigated to better understand the population trend of D. delphis and its residency pattern, the results contribute to setting up a baseline reference for future assessment of its population in the Eastern Aegean Sea.

A Robust Design Capture-Recapture Analysis of Abundance, Survival and Temporary Emigration of Three Odontocete Species in the Gulf of Corinth, Greece.

While the Mediterranean Sea has been designated as a Global Biodiversity Hotspot, assessments of cetacean population abundance are lacking for large portions of the region, particularly in the southern and eastern basins. The challenges and costs of obtaining the necessary data often result in absent or poor abundance information. We applied capture-recapture models to estimate abundance, survival and temporary emigration of odontocete populations within a 2,400 km2 semi-enclosed Mediterranean bay, the Gulf of Corinth. Boat surveys were conducted in 2011–2015 to collect photo-identification data on striped dolphins Stenella coeruleoalba, short-beaked common dolphins Delphinus delphis (always found together with striped dolphins in mixed groups) and common bottlenose dolphins Tursiops truncatus, totaling 1,873 h of tracking. After grading images for quality and marking distinctiveness, 23,995 high-quality photos were included in a striped and common dolphin catalog, and 2,472 in a bottlenose dolphin catalog. The proportions of striped and common dolphins were calculated from the photographic sample and used to scale capture-recapture estimates. Best-fitting robust design capture-recapture models denoted no temporary emigration between years for striped and common dolphins, and random temporary emigration for bottlenose dolphins, suggesting different residency patterns in agreement with previous studies. Average estimated abundance over the five years was 1,331 (95% CI 1,122–1,578) striped dolphins, 22 (16–32) common dolphins, 55 (36–84) “intermediate” animals (potential striped x common dolphin hybrids) and 38 (32–46) bottlenose dolphins. Apparent survival was constant for striped, common and intermediate dolphins (0.94, 95% CI 0.92–0.96) and year-dependent for bottlenose dolphins (an average of 0.85, 95% CI 0.76–0.95). Our work underlines the importance of long-term monitoring to contribute reliable baseline information that can help assess the conservation status of wildlife populations.

DNA evidence of the consumption of short-beaked common dolphin Delphinus delphis by the shortfin mako shark Isurus oxyrinchus

DNA evidence of the consumption of short-beaked common dolphin Delphinus delphis by the shortfin mako shark Isurus oxyrinchus

Mortality trends of stranded marine mammals on Cape Cod and southeastern Massachusetts, USA, 2000 to 2006

To understand the cause of death of 405 marine mammals stranded on Cape Cod and southeastern Massachusetts between 2000 and 2006, a system for coding final diagnosis was developed and categorized as (1) disease, (2) human interaction, (3) mass-stranded with no significant findings, (4) single-stranded with no significant findings, (5) rock and/or sand ingestion, (6) predatory attack, (7) failure to thrive or dependent calf or pup, or (8) other. The cause of death for 91 animals could not be determined. For the 314 animals that could be assigned a cause of death, gross and histological pathology results and ancillary testing indicated that disease was the leading cause of mortality in the region, affecting 116/314 (37%) of cases. Human interaction, including harassment, entanglement, and vessel collision, fatally affected 31/314 (10%) of all animals. Human interaction accounted for 13/29 (45%) of all determined gray seal Halichoerus grypus mortalities. Mass strandings were most likely to occur in northeastern Cape Cod Bay; 97/106 (92%) of mass stranded animals necropsied presented with no significant pathological findings. Mass strandings were the leading cause of death in 3 of the 4 small cetacean species: 46/67 (69%) of Atlantic white-sided dolphin Lagenorhynchus acutus, 15/21 (71%) of long-finned pilot whale Globicephala melas, and 33/54 (61%) of short-beaked common dolphin Delphinus delphis. These baseline data are critical for understanding marine mammal population health and mortality trends, which in turn have significant conservation and management implications. They not only afford a better retrospective analysis of strandings, but ultimately have application for improving current and future response to live animal stranding.

Importance of biological parameters in assessing the status of Delphinus delphis

Short-beaked common dolphins Delphinus delphis in the eastern North Atlantic (ENA) are subject to mortality due to entanglement in various types of fishing gear. However, for this region, there is no population-level information available on trends in abundance, (incidental) mortality rates or even the actual distributional range. Working under the assumption that only 1 population exists in ENA waters, the current study presents basic life history data and investigates whether biological information obtained from postmortem data is, in itself, useful for managing this population. Life his- tory parameters were estimated by analysing postmortem data obtained over a 16 yr period by UK, Irish, French, Galician (northwest Spain) and Portuguese stranding and bycatch observer pro- grammes. An annual pregnancy rate of 26%, a calving interval of 3.79 yr, an average age attained at sexual maturity of 8.22 yr and an average length at sexual maturity of 188 cm were determined. With respect to the findings based solely on mortality data, significance testing failed to detect differences that could be construed as evidence of the population exhibiting what might be density-dependent compensatory responses. The low annual pregnancy rate reported throughout the sampling period may suggest either that the level of anthropogenic mortality did not cause a substantial population level decline, or a prey base declining at approximately the same rate as the dolphin population. However, this approach alone does not facilitate an assessment of the current state of the D. delphis population in the ENA. Population abundance estimates, trends in abundance and knowledge of fac- tors that affect the dynamics of the population, such as annual mortality rates in fisheries, temporal variations in prey abundance and effects of contaminants on reproductive activity, are required not only to set management objectives, but also to give context to cross-sectional life history information.

Epidemiological pattern of tattoo skin disease: a potential general health indicator for cetaceans

The presence of tattoo skin disease (TSD) was examined in 1392 free-ranging and dead odontocetes comprising 17 species from the Americas, Europe, South Africa, New Zealand and Greenland. We investigated whether TSD prevalence varied with sex, age and health status. TSD was encountered in cetaceans from the Pacific and Atlantic Oceans as well as in those from the North, Mediterranean and Tasman Seas. No clear patterns related to geography and host phylogeny were detected, except that prevalence of TSD in juveniles and, in 2 species (dusky dolphin Lagenorhynchus obscurus and Burmeister's porpoise Phocoena spinipinnis), in adults was remarkably high in samples from Peru. Environmental factors and virus properties may be responsible for this finding. Sex did not significantly influence TSD prevalence except in the case of Peruvian P. spinipinnis. Generally, there was a pattern of TSD increase in juveniles compared to calves, attributed to the loss of maternal immunity. Also, in most samples, juveniles seemed to have a higher probability of suffering TSD than adults, presumably because more adults had acquired active immunity following infection. This holo-endemic pattern was inverted in poor health short-beaked common dolphins Delphinus delphis and harbour porpoises Phocoena phocoena from the British Isles, and in Chilean dolphins Cephalorhynchus eutropia from Patagonia, where adults showed a higher TSD prevalence than juveniles. Very large tattoos were seen in some adult odontocetes from the SE Pacific, NE Atlantic and Portugal's Sado Estuary, which suggest impaired immune response. The epidemiological pattern of TSD may be an indicator of cetacean population health.

Fine-scale habitat selection by white-beaked and common dolphins in the Minch (Scotland, UK): evidence for interspecific competition or coexistence?

A decline in white-beaked dolphins Lagenorhynchus albirostris had been reported in the Minch (Scotland, UK) since the 1990s, coinciding with an increasing occurrence of short-beaked common dolphins Delphinus delphis. This has led to suggestions that rising seawater temperatures are causing D. delphis to out-compete and exclude L. albirostris in this region. A total of 793 km (66.3 hours) of survey data were collected in the Minch during August 2007, to examine fine-scale habitat selection by L. albirostris and D. delphis and investigate whether their ecological overlap is sufficient to result in interspecific competition and/or habitat partitioning. Ten sightings of L. albirostris (70 animals) were recorded in a relatively small spatial area in the northern Minch. In contrast, the eleven sightings of D. delphis (1486 animals) were more widely distributed. The relative abundance per 1/4 ICES rectangle ranged from 0.41 to 0.53 animals/km for L. albirostris and 0.13 to 6.68 animals/km for D. delphis. The mean group size and group body mass were higher for D. delphis than for L. albirostris indicating D. delphis as the dominant delphinid in the Minch during August. Lagenorhynchus albirostris occurred in waters significantly deeper and further from shore than D. delphis, suggesting interspecific differences in preferred habitat. Most dolphin schools were recorded as foraging/feeding. Behaviour and seabird associations indicated that the two species differed in diet and/or foraging strategy, with L. albirostris foraging sub-surface and D. delphis exhibiting surface-feeding with associated gannets Morus bassanus. This is consistent with published information on the stomach contents of Scottish animals. The results suggest that there are subtle differences in habitat selection and diet between these two species, which may enable L. albirostris and D. delphis to coexist in the Minch. Whether these differences result from niche partitioning arising from previous/ongoing interspecific competition or are the result of genuine differences in the habitat preferences of each species, remains unclear.

Reply to ‘Clarifying the interpretation of Hamer et al. (2008) by Bilgmann et al. (2008)’

In our recent study on the population genetic structure of short-beaked common dolphins Delphinus delphis in southern Australia (Bilgmann et al., 2008) we unequivocally showed a high level of genetic differentiation between the local common dolphin population in South Australia (SA) and the one in south-eastern Tasmania. The differentiation found in short-beaked common dolphins over this relatively small geographic scale of c. 1500 km is unusual for such a wide ranging and highly mobile species (Bilgmann et al., 2008). The genetic subdivision and the very low migration rates between the populations reported in our study has important implications for the management of dolphin interactions with the purse-seine fishery in SA; these interactions have lead to serious concerns over the long-term viability of the local dolphin population in this area. Around the same time of publication of our paper, Hamer, Ward & McGarvey (2008) reported on the creation of the purse-seine fishery Code of Practice (CoP) to reduce dolphin mortalities in SA. In Bilgmann et al. (2008), we referred to the dolphin bycatch rate before the implementation of the CoP. We commend the proactive industry approach to reduce dolphin bycatch. Moreover, we commend the industry for indirectly supporting our research through logistical support provided by the South Australian Research and Development Institute. Critically, the industry undertook mitigation before the publication of our findings. Hamer et al. (2008) results suggest that the CoP is highly effective, albeit their assessment covered only a 7-month period. Although we welcome the implementation of the CoP, we counsel caution about confidence in its efficacy. CoPs are not universally effective and may lead to a false sense that the problem has been resolved (Wiley et al., 2008). To the best of our knowledge, the most successful bycatch mitigation program was based upon an extensive, ongoing 100% observer coverage, application of proven mitigation measures, annual expert review and evolving fishery and mitigation practices (Waugh et al., 2008). The observer coverage reported in Hamer et al. (2008) was 11.8% and the assessment time (7 months) was short. We emphasize the importance of a careful and independent long-term monitoring of the efficacy of the industry CoP. The CoP is at its early stages of implementation, and using the ‘worst case bycatch statistics’ in the absence of a long-term, extensive observer programs is a precautionary approach. Given the findings in our study we strongly recommend the (1) continued independent monitoring and estimation of the magnitude of fishery interactions including the long-term effectiveness of the CoP; (2) identification of the boundaries of the common dolphin population in SA and its size; (3) assessment of the distributional changes of the population over time; (4) given that dolphin distribution may be closely linked with prey distribution, the quantification of dolphin diet, prey abundance and its distribution in the area.

Summer spatial distribution of cetaceans in the Strait of Gibraltar in relation to the oceanographic context

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 353:275-288 (2008) - DOI: https://doi.org/10.3354/meps07164 Summer spatial distribution of cetaceans in the Strait of Gibraltar in relation to the oceanographic context Renaud de Stephanis1,2,*, Thomas Cornulier3,4, Philippe Verborgh1,2, Juanma Salazar Sierra1,2, Neus Pérez Gimeno2,5, Christophe Guinet3 1CIRCE, Conservation Information and Research on Cetaceans, C/Cabeza de Manzaneda 3, Algeciras-Pelayo, 11390 Cadiz, Spain 2Sociedad Española de Cetáceos C/Nalon 16, La Berzosa, Madrid, Spain 3Centre d’Études Biologiques de Chizé, CNRS UPR 1934, 79 360 Villiers en Bois, France 4School of Biological Sciences, Zoology Building, Tillydrone Avenue, University of Aberdeen, Aberdeen AB24 2TZ, UK 5Laboratorio de Ingeniería Acústica de la Universidad de Cádiz (LAV), CASEM, Río de San Pedro S/N, Puerto Real, Cádiz, Spain *Email: renaud@stephanis.org ABSTRACT: The Strait of Gibraltar, the only passage between the Mediterranean Sea and the Atlantic Ocean, and characterised by a surface inflow of Atlantic waters and a deep outflow of Mediterranean waters, is inhabited by a large number of cetacean species. The present study focuses on the occurrence and the spatial distribution of cetacean species within the strait in relation to oceanographic features. Shipboard visual surveys were conducted during the summers of 2001 to 2004, covering 4926 km. A total of 616 sightings of 7 cetacean species were made. The spatial distributions of 6 species (short-beaked common dolphins Delphinus delphis, striped dolphins Stenella coeruleoalba, bottlenose dolphins Tursiops truncatus, long-finned pilot whales Globicephala melas, sperm whales Physeter macrocephalus and killer whales Orcinus orca) were examined with respect to depth and slope. The analyses indicate that these species could be ordered into 3 groups. A first group, with a northward tendency, is composed of common and striped dolphins. Due to its at-sea location and feeding habits, this group is likely to feed on mesopelagic fishes or squids associated with the surface Atlantic waters. The second group, constituted by bottlenose dolphins, long-finned pilot whales and sperm whales, is mainly found over the deep waters of the central part of the strait. While the foraging ecology of bottlenose dolphins is still unclear, both sperm whales and pilot whales are most likely to feed on squids occurring in deep Mediterranean waters. The third group, formed by killer whales Orcinus orca, was associated with blue fin tuna Thunnus thynnus fisheries in the southwestern part of the strait. KEY WORDS: Cetacean · Strait of Gibraltar · Spatial distribution · Feeding ecology · Fisheries interaction Full text in pdf format PreviousNextCite this article as: de Stephanis R, Cornulier T, Verborgh P, Salazar Sierra J, Gimeno NP, Guinet C (2008) Summer spatial distribution of cetaceans in the Strait of Gibraltar in relation to the oceanographic context. Mar Ecol Prog Ser 353:275-288. https://doi.org/10.3354/meps07164 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 353. Online publication date: January 17, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Habitat-based spatial and temporal variability in life history characteristics of female common dolphins Delphinus delphis in the eastern tropical Pacific

Spatial and temporal variability in mean age, breeding season and mean body length were examined for female short-beaked common dolphins Delphinus delphis in the eastern tropical Pacific (ETP). Specimens were obtained from central stock D. delphis incidentally killed in the tuna purse-seine fishery between 1973 and 1993. To examine spatial variability in life history parameters, oceanographic variables were used to characterize 2 primary habitats within the range of central stock animals: the Costa Rica Dome (CRD) and the surrounding areas of the CRD. Mean age of mature females did not vary through time or between habitats, and calving occurred throughout the year in both habitats. However, mean body length of D. delphis sampled 'on' the CRD was 10 cm shorter than those sampled 'off' the CRD during the years 1973-1974 but was not significantly different between the 2 habitats in the years 1988-1989. The difference between spatial groups in 1973-1974 is likely attributed to animals moving within the region in response to changes in their environment. That is, animal size and location is consistent with southern stock D. delphis moving into the higher quality habitat of the CRD area during the strong El Nino of 1972-1973 when preferred habitat was likely reduced in the area that the southern stock inhabits. Our evidence of animal movement suggests animals cross stock boundaries during periods of significant environmen- tal change. This result has implications for development and implementation of management plans and estimation of life history parameters for D. delphis inhabiting the ETP.

Long-term dietary segregation of common dolphins Delphinus delphis in the Bay of Biscay, determined using cadmium as an ecological tracer

Dietary studies in marine mammals are traditionally performed by stomach contents analyses, which may be insufficient to determine long-term dietary preferences of these upper level predators. Our primary objective was to test the efficiency of trace metal measurements as comple- mentary tools in dietary studies. Variations in cadmium (Cd) exposure through the diet and its effec- tive renal levels in the short-beaked common dolphin Delphinus delphis were investigated to study the long-term feeding ecology of this predator in the neritic and oceanic waters of the Bay of Biscay. Based upon previous stomach contents analyses, the main prey occurring in the diet of common dol- phins were analysed for their Cd contents. Results showed that cephalopods, and especially oceanic Cranchids and Histioteuthids, constituted a major source of Cd for common dolphins. Estimated Cd intake would therefore be 12 times higher in oceanic common dolphins (1400 ± 65 µg d -1 ) compared to neritic ones (120 ± 30 µg d -1 ). Accumulation of renal Cd concentrations with age was 5 times higher in by-caught oceanic dolphins than neritic ones (p < 0.0001). Within the neritic area, renal Cd accu- mulation rate was 2 times higher in by-caught individuals compared to stranded ones (p = 0.002). Thus, the use of Cd concentrations in by-caught dolphins proved efficient for assessing the existence of dietary segregation between neritic and oceanic common dolphins from the Bay of Biscay. However, using Cd data to make inferences about the feeding ecology of stranded animals should be considered cautiously.