Once one of the most common cetaceans in the Mediterranean Sea, the short-beaked common dolphin has declined throughout the region since the 1960s and in 2003 this population was classified as Endangered in the International Union for Conservation of Nature (IUCN) Red List. Here, we document the species' precipitous decline in eastern Ionian Sea coastal waters across 13 yr. While 150 animals were present in the study area (1050 km 2) in 1996, only 15 were observed in 2007. A 12 mo assessment of fishing effort and catch, together with circumstantial evidence, suggests that the decline was caused largely by prey depletion resulting from overfishing. We analyzed the impacts of various fishing gear and estimated the degree of resource overlap between common dolphins and local fisheries. The total biomass removed annually by 308 fishing boats in the study area averaged 3571 t, while that consumed by common dolphins was 17 t. Resource overlap between common dolphins and fisheries-expressed as an average Pianka index of 0.5-differed according to fishing gear, being higher for purse seiners (0.7) and beach seiners (0.4) and lower for bottom trawlers (0.1), trammel boats (0.2) and longliners (0.0). Only about 10 active purse seiners (4% of the total active fishing fleet) were responsible for 33% of the biomass removal, and likely had the greatest impact on prey of common dolphins. This study indicates a high risk of local disappearance of common dolphins in the very near future, unless fishery management measures are implemented immediately. Purse seining should be the main management target.

. 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.

ABSTRACT1. 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 km 2 , 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.

ABSTRACT1. Dolphins are often claimed to compete with fisheries, including through removal of substantial biomass. To calculate the biomass removed by fisheries and the degree of resource overlap with dolphins in a coastal area of Greece, estimates of dolphin abundance based on photographic capture-recapture were combined with an assessment of fishing effort and catch.2. The estimated total biomass consumed annually by local dolphin populations -15 short-beaked common dolphins and 42 common bottlenose dolphins -was 15.5 and 89.8 tonnes, respectively. The total biomass removed by the local fishing fleet (307 fishing boats) was 3469.2 tonnes, i.e. about 33 times greater than that removed by dolphins.3. Dolphins removed 2.9% of the total biomass, fisheries 97.1%. Nine purse seiners (representing only 3% of the active fishing fleet) were responsible for 31.9% of biomass removal. Similarity of biomass composition between dolphins and fisheries was expressed by a Pianka index of 0.46 for common dolphins and 0.66 for bottlenose dolphins.4. Overlap differed according to fishing gear. Common dolphin overlap was higher with purse seiners (0.82), and lower with beach seiners (0.31), bottom trawlers (0.11) and trammel boats (0.06). There was virtually no overlap with longliners (0.02). Bottlenose dolphin overlap was higher with trammel boats (0.89) and bottom trawlers (0.75), and lower with longliners (0.38), purse seiners (0.24) and beach seiners (0.18). There was minimal overlap (0.12) between the two dolphin species.5. This study suggests that ecological interactions between dolphins and fisheries in this coastal area have minor effects on fisheries. Conversely, prey depletion resulting from overfishing can negatively affect dolphins. Fisheries management measures consistent with national and EU legislation are proposed to ensure sustainability and to protect marine biodiversity.

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.