The conservation of the endangered Mediterranean common dolphin (Delphinus delphis): Current knowledge and research priorities

1. Preservation of endangered species is now recognized as a relevant component of regional and global conservation actions achieved through the maintenance of the remaining populations in a favourable conservation status. The Mediterranean common dolphin (Delphinus delphis) is listed as an Endangered subpopulation in the International Union for Conservation of Nature Red List, with declining numbers in some of the areas where it has been studied for numerous years. Prey depletion, fisheries bycatch, pollution, health risks, and climate change are among the recognized threats that the species is facing in the region.
2. The existing national, regional, and international legislation pertinent to Mediterranean countries should aid greater science–policy–management integration and support innovative research and monitoring towards effective conservation. The challenges inherent in studying the pelagic D. delphis should not preclude the use of long-term, year-round, multiplatform surveys, side by side with the latest molecular tools, which aid our understanding of its populations' genetic diversity, resistance to diseases, and resilience in a semi-enclosed fast-changing sea.
3. Addressing the multiplicity of problems confronting the species requires collaborative effort at all levels to share and merge resources, data, and expertise more efficiently. In this review, we provide an overview of the current knowledge on the Mediterranean common dolphin, the main threats, and knowledge gaps that are priorities for addressing its conservation, while providing clear recommendations for effectively safeguarding the species and its habitat at both local and regional levels.

     

The decline of short-beaked common dolphin (Delphinus delphis) in the waters off the island of Ischia (Gulf of Naples,Italy)

1. This study presents data on a local population of short-beaked common dolphin monitored in the waters off Ischia Island (Gulf of Naples, Italy) over a 16-year period (2000-2015). We examine dolphin occurrence and distribution and perform photoidentification analysis.
2. The data presented support the hypothesis that the waters around Ischia Island represent a feeding area, as well as a calving and an important nursery area for this local population, providing favourable conditions in which to give birth and raise calves.
3. The levelling-off of the photoidentification curves together with the continuous decline of the encounter rate lead us to believe that the area has been a hotspot for a local population (mainly resident) for years and that now this population is dying (has died) or is moving (has moved) to other locations.
4. Several expanding human activities at sea have the potential to impact on the common dolphin in the study area, the most significant possibly being habitat disturbance and degradation (including traffic and noise pollution) and overexploitation of food resources by the fishery.
5. The data presented in this study offer a strong argument for explicit and urgent population-specific conservation and management strategies to be developed and applied locally for common dolphins, considering that they rely on the region for important biological processes.

     

Occurrence and present status of coastal dolphins (Delphinus delphis and Tursiops truncatus) in the eastern Ionian Sea

• 1. Boat surveys aimed at studying short‐beaked common dolphins and common bottlenose dolphins in eastern Ionian Sea coastal waters were conducted between 1993 and 2003. During 835 survey days, 24 771 km of total effort was distributed within an area of 480 km², resulting in 428 common dolphin and 235 bottlenose dolphin sightings.
• 2. Individual photo‐identification was performed extensively throughout this study, making it possible to monitor the number of animals seen in the study area each year and their long‐term residency patterns.
• 3. Common dolphins declined across the study period, from 2.18 encounters/100 km in 1997 to 0.40 encounters/100 km in 2003. In contrast, there was a relatively stable presence of bottlenose dolphins, some individuals showing high levels of site fidelity and others using the area only occasionally.
• 4. The local decline of common dolphins and the low density of bottlenose dolphins appeared to reflect the general status of these cetacean species in the wider Mediterranean region, where common dolphins were classified as endangered in the IUCN Red List in 2003.
• 5. Based on the available evidence, we infer that the present unfavourable status of common dolphins in eastern Ionian Sea coastal waters is largely a consequence of prey depletion.

Copyright © 2005 John Wiley & Sons, Ltd.     

Common dolphins in the Gulf of Corinth are Critically Endangered

1. Regional populations (“subpopulations”) of globally abundant species can be exposed to human impacts that threaten their viability. Given the value of cetacean subpopulations as evolutionary significant units, keystone and umbrella species, it is important to assess their conservation status separately and propose area-specific conservation measures.
2. We used a threat assessment process and applied IUCN Red List criteria to a regional population of common dolphins Delphinus delphis in the semi-enclosed Gulf of Corinth, Greece. We compiled subpopulation-specific information about abundance and trends, estimated the geographic range of the subpopulation (area of occupancy and extent of occurrence), and calculated the probability of extinction through stochastic modelling.
3. The subpopulation qualified as Endangered according to criteria A (population size reduction over three generations) and B (geographic range), and as Critically Endangered under criteria C (population size and decline) and D (very small or restricted population). The probability of extinction was estimated to be ≥50% in three generations, qualifying the subpopulation as Critically Endangered under criterion E (quantitative analysis). We concluded that the subpopulation should be classified as Critically Endangered.
4. Considering the high extinction risk faced by common dolphins in the Gulf of Corinth, we recommend that: (a) immediate action is taken to mitigate anthropogenic activities known or suspected to have a negative impact on cetaceans in the area (particularly commercial fishing); and (b) a marine protected area is established in the Gulf of Corinth as a management tool for enforcing conservation action and facilitating the recovery of common dolphins.

     

Environmental variables influencing occurrence and distribution of Delphinus delphis in the eastern Aegean Sea (eastern Mediterranean Sea)

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Present distribution of common dolphin Delphinus delphis in French Mediterranean and adjacent waters as obtained from small boat surveys

1. Common dolphin distribution in the western Mediterranean is still poorly known, with the exception of the Alboran Sea. In French waters, the species occurrence is suspected to have strongly decreased during the 20th century.
2. Small boat dedicated surveys from 1988 to 2012 were undertaken to describe common dolphin distribution in five regions of French waters and three southern regions of the western basin. A total survey effort of 38,561 km resulted in sightings of 25 common dolphin groups in the western basin.
3. Common dolphins were rarely observed off the French continental coast, more frequently around Corsica, and were quite frequent in waters off western Sardinia. Their most favoured habitat was found to be in neritic or upper slope waters.
4. The analysis of stranding records suggested that common dolphins were more abundant in French inshore waters prior to 1980.
5. A major increase of pelagic fish landings occurred from the beginning of the 1960s in the western part of French waters. Local populations of common dolphins may have declined as a consequence of prey depletion.

     

Population genetic structure of common bottlenose dolphins (Tursiops truncatus) in the Adriatic Sea and contiguous regions: implications for international conservation

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The role of historical dolphin takes and habitat degradation in shaping the present status of northern Adriatic cetaceans

• 1. Nine cetacean species have been reliably reported to occur in the shallow northern Adriatic Sea since the 17th century. However, only two species were considered regular there until the 1970s: the short‐beaked common dolphin (Delphinus delphis) and the common bottlenose dolphin (Tursiops truncatus).
• 2. Short‐beaked common dolphins have progressively disappeared from the northern Adriatic and are now rare in the region. The systematic culling campaigns and other takes that occurred between the second half of the 18th century and the 1960s, and habitat degradation in subsequent years are the most likely causes of their decline.
• 3. Today, common bottlenose dolphins are the only regular component of the northern Adriatic cetacean fauna; however, they now occur at low densities, and their fragmented groups are facing significant anthropogenic threats.
• 4. The future of northern Adriatic dolphins will depend on precautionary action to prevent further decline and on intensified research effort aimed at identifying the most effective mitigation strategies.

Copyright © 2004 John Wiley & Sons, Ltd.     

A stranding record of the short-beaked common dolphin (Delphinus delphis) in Algerian West Coast,during 2008–2012

1. Marine mammals stranding data provide valuable information on causes of mortality for cetacean populations. The aim of this study was to investigate the characteristics of the stranding events of common dolphins along the Algerian west coast, and estimate the proportion of human vs. non-human induced mortality of the population.
2. Between 2008 and 2012, out of the 52 cetacean stranding records along the Algerian west coast, common dolphins (Delphinus delphis) were recorded in 14 cases. All recorded strandings were of individual cases with no mass strandings registered during the reported period.
3. To examine the proportion of human and non-human induced mortality affecting this population, necropsies were conducted. In 10 cases, an interaction with some type of fishing gear had occurred; in three, there was no link with human activity; and in one case, it was not possible to determine the cause of death due to the condition of the carcass (severely decomposed).

     

Linking small pelagic fish and cetacean distribution to model suitable habitat for coastal dolphin species,Delphinus delphis and Tursiops truncatus,in the Greek Seas (Eastern Mediterranean)

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Abundance,distribution and diet of the common dolphin,Delphinus delphis,in the northern Aegean Sea (Greece)

1. The abundance, distribution and diet of the short-beaked common dolphin were investigated as part of the first detailed study on cetacean populations in the North Aegean Sea. Since 2004, the area has been proposed by national and international entities as a marine protected area for common dolphins and other cetacean species owing to its high biodiversity.
2. Abundance and distribution were investigated between 2005 and 2013 through dedicated scientific marine transect surveys, covering 14,701 km, in sea conditions of Beaufort sea state 3 and below. The line-transect sampling method was used to estimate relative abundance, using Distance 6.0 software. Encounter rate for this species in the study area was estimated to be 0.24 groups/100 km (1.5 dolphins/100 km), with a mean group size of 6.88 (SE = 1.90).
3. Common dolphin sightings were analysed for correlations with eight environmental variables (distance from the coast, depth, slope, median salinity, gradient of salinity, median temperature, gradient of temperature and mean current) using generalized additive modelling. Common dolphin sightings were significantly correlated to depth, temperature and salinity.
4. Stomach-content analyses were performed on eight suitable samples from common dolphins stranded in the study area. The trophic level of the species was calculated, indicating that this species is a top predator.
5. This research provides the first estimates for abundance and distribution and the first diet analysis for this Mediterranean Endangered dolphin species in the study area based on a year-round long-term study.

     

The abundance of common bottlenose dolphins (Tursiops truncatus) in the former special marine reserve of the Cres‐Lošinj Archipelago,Croatia

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Prevalence and effects of the cymothoid isopod (Ceratothoa oestroides,Risso 1816) on cultured meagre (Argyrosomus regius,Asso 1801) in the Eastern Adriatic Sea

Meagre (Argirosomus regius) is a relatively new farmed fish species in Croatian mariculture. This study is the first attempt to determine the prevalence of cymothoid isopod parasite Ceratothoa oestroides and its effects on cultured meagre in the Mediterranean area. When analysing growth of meagre at two sites in the central part of the Eastern Adriatic Sea, the greatest difference in total fish length was recorded in March 2016, when fish with parasites were 33% smaller than fish without parasites (24.47 ± 3.29 vs. 16.28 ± 1.01 cm; p < .001). The largest difference in mass was also recorded during sampling in March, when parasitized fish was 74% lighter than the unparasitized fish (171.75 ± 72.96 vs. 44.77 ± 6.57 g; p < .001). The presence of the cymothoid isopod in the buccal cavity was observed, and caused fish deformation throughout the entire duration of the study. Also, female parasites with pulli II in marsupium were found. Based on the determined prevalence and effects on production and health status, monitoring of the interaction between C. oestroides and meagre should be considered in future management plans for cultivation of this promising species for Mediterranean aquaculture.     

Pan-steatitis in farmed northern bluefin tuna, Thunnus thynnus (L.), in the eastern Adriatic

Clinical, gross and histopathological investigations were carried out into large-scale mortalities on eastern Mediterranean bluefin tuna, Thunnus thynnus (L.), farms. Fish showed only nervous signs and darkened colour. At post-mortem the liver was bronze coloured and the pyloric area waxy in consistency. There was no evidence of any other gross pathology. Histopathology showed severe hepatic necrosis and lipidosis. Peri-pancreatic lipoid tissue was heavily infiltrated with an inflammatory round cell infiltrate. Fish on all three farms had been fed on a North African pilchard diet rather than traditional local or Baltic species. Once the diet was modified, losses ceased. A diagnosis of pan-steatitis as seen in other farmed fish species, as well as in terrestrial animals, on particular fish-based diets was made, although the actual factor within the diet which induced the inflammatory effect is not known.     

Reproductive biology of pink dentex Dentex gibbosus (Rafinesque) from the Adriatic Sea, Croatia

A 4‐year study (May 1997–December 2000) of the reproductive biology of pink dentex Dentex gibbosus (Rafinesque) from the Adriatic Sea revealed that this species is a rudimentary hermaphrodite. The smallest mature males and females captured were 38.70 cm in total length (T_L) and 39.80 cm in total length (T_L) respectively. The von Bertalanffy growth parameters estimated for the entire population were: L_∞=107.24, K=0.12 and t₀=−0.90. Fifty per cent of the population were sexually mature at 41.50 cm T_L, while 100% of the specimens were sexually mature at 57.00 cm T_L. Both monthly gonadosomatic index and macroscopically determined gonad stages strongly indicate that the pink dentex from the Adriatic Sea spawn partially in August, September and October. The mean value of absolute fecundity (F) was 1672 × 10⁶ eggs. The results of great fertility of the pink dentex, partial spawning and relatively late sexual maturation suggest that the pink dentex has a high potential for commercial culture.     

Distribution,abundance, and movements of the bottlenose dolphin (Tursiops truncatus) in the Pelagos Sanctuary MPA (north‐west Mediterranean Sea)

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Dolphins in a bottle: abundance,residency patterns and conservation of bottlenose dolphins Tursiops truncatus in the semi‐closed eutrophic Amvrakikos Gulf,Greece

• 1. Boat surveys were conducted between 2002 and 2005 to study bottlenose dolphins living in the 400 km² Amvrakikos Gulf, western Greece. During 116 survey days, 4705 km of total effort resulted in the individual photo‐identification of 106 animals, through long‐term natural markings on their dorsal fins.
• 2. Mark–recapture analyses based on the M_th model provided estimates of 82 marked individuals in 2003 (95% CI=80–91), 92 in 2004 (95% CI=86–108) and 98 in 2005 (95% CI=94–110). To include the unmarked portion of the population, the proportion of unmarked individuals was computed based on the number of photographs of marked and unmarked dorsal fins. The mean proportion of unmarked animals in the population was 0.338 (95% CI=0.288–0.389). By adding this to the estimate for marked animals in 2005, considered as the most robust, a total population estimate of 148 individuals (95% CI=132–180) was obtained.
• 3. Dolphin encounter rates in 2003–2005 did not show significant variations, and averaged 7.2 groups per 100 km or 72.5 individuals per 100 km. Encounter rates within the Gulf were about one order of magnitude greater than those found for bottlenose dolphins in nearby eastern Ionian Sea coastal waters.
• 4. Mean dolphin density in the Gulf was 0.37 animals km^?2. This relatively high density, together with high levels of site fidelity shown by most individuals, was thought to be related primarily to prey availability, particularly of epipelagic schooling fish.
• 5. The importance of the semi‐closed Amvrakikos Gulf for bottlenose dolphins and other threatened species encourages the adoption of measures aimed to conserve its valuable ecosystems and raise the naturalistic profile of the area, while promoting environment‐conscious development. Meaningful action includes restoring natural hydrology (e.g. freshwater input from rivers), curtailing pollution from various sources, responsible fisheries and aquaculture management, and control of illegal fishing. Interactions between dolphins and fisheries also deserve careful quantitative investigation.

Copyright © 2007 John Wiley & Sons, Ltd.     

国外海豚体内多氯联苯的研究进展与概况

综述了20世纪70年代以来有关海豚体内多氯联苯的研究进展和概况。论述了PCBs进入海豚的方式及消除途径；PCBs在海豚体内各组织中的分布特点与差异；PCBs在雌、雄和仔海豚体内的分布特点和转移规律；不同环境条件对海豚PCBs含量及同族物构成影响；PCBs对海豚的有害影响。通过分析表明,PCBs对海洋生态系统的影响还将持续很长时间,国内应进一步加强近岸海域哺乳类动物PCBs的长期污染效应研究,进而更深入地研究其环境行为、归宿和影响效应。