Cetaceans and their interactions with coastal fisheries in the Gulf of Kavala, Eastern Mediterranean sea

Abstract

Cetacears in the Gulf of Kavala and around Thassos, in the Thracian Sea, were studied as this site has been proposed as a potential cetacean conservation area by ACCOBAMS (Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area. This area is also an important Greek fishing ground, so cetacean-fisheries associations were srtuctured to understand the extent of the reciprocal negative impacts between the two. The research investigated) cetacean abundance and distribution in the study area of the Gulf of Kavala; b) cetacean strandings in the North Aegean Sea and stomach content diet analysis; c) survey of the fisheries catch per gear used in the Thracian Sea and analyses of the extent of fisheries target species overlap with cetacean prey species found through cetacean diet analyses, and J) fishermen questionnaire study on fisheries-cetacean interaction and sightings of cetaceans during fishing activities. Regarding the first task, the presence, abundance and distribution of cetacean species in the study area were investigated between 2005 and 2013 through dedicated scientific marine surveys. Data were collected during a total boat survey effort covering 14,701 km in sea conditions <Beaufort 4, in a study area of 2000 km2 • The survey used the line-transect sampling method to estimate relative abundance, calculated using Distance software 6.0. Six cetacean species v-ere recorded in the study area, estimated at a total of 5,069 individuals (95% C.I. = 3,668 - 7,004; CV = 16.56). Abundance estimates were obtained for bottlenose dolphins (Tursiops truncatus) (2,934 individuals; 95% C.I. = 1,915 - 4,494; CV= 21.91); and common dolphins! Delphinus de/phis) (1,482 individuals; 95% C.I. = 958 -2,295; CV= 22.55). Dolphin sightings were correlated to eight environmental variables using GAM and PCA. Bottlenose dolphins sightings were found to correlate with depth (p = 0.007) and median temperature (p = 0.042). Common dolphin sightings correlated with depth (p = 0.035), temperature (p = 0.022 for median temperature and p = 0.001 for temperature gradient) and salinity (p = 0.001 for median salinity and p = 0.024 for salinity gradient). Apart from, these two species, other visiting cetacean species include: striped dolphins (Stenella coeruleoalba), harbour porpoises (Phocoena phocoena) and Risso's dolphins (Grampus griseus). The Mysticete fin whale (Balaenoptera physalus) and the pinniped Mediterranean monk seal (Monachus monachus) were rarely sighted. Cetacean strandings were recorded and allowed for stomach-content analyses performed on 26 suitable specimen samples. Results from the stomach-content analyses showed that: common dolphins (N = 8) fed mainly on species from the Clupeidae and Myctophidae families and a few cephalopods, bottlenose dolphin (N = 8) fed primarily on snake blenny (Ophidion carbatum), bogue (Boops hoops), species of the Clupeidae family and cephalopods, such as squids of the genus Loligo. Striped dolphins (N = 4) fed on small pelagic fish, such as bogue and especially on Myctophidae, such as Madeira lantern fish (Ceratoscopelus maderensis) and in a smaller proportion on cephalopods. Harbour porpoises (N = 6) fed on fish, with a high preference for species of the Gobiidae family followed by Clupeidae. This present work also contributes first data on harbour porpoises' diet in the Mediterranean Sea. Risso's dolphins (N = 1) fed exclusively on cephalopods, prevalently species from the Teuthida order, and the genus Histiotheutis. In none of the cetacean stomachs investigated, was there a dominant presence of important commercial fisheries species. Trophic levels of the cetacean species were calculated and in all cases showed values higher than 4, indicating a top predator position in the food web; their values ranged from 4.22 for harbour porpoises to 4. 70 for Risso' s dolphins. [...]