Anthropogenic threats to coastal dolphins in the Mediterranean Sea: the Balearic Islands and the Eastern Ionian Sea as case-studies = Les amenaces antropogèniques als dofins costaners al Mar Mediterrani: les Illes Balears i el Mar Jònic oriental com a casos pràctics

Abstract

The Mediterranean Sea, the largest and deepest enclosed sea on Earth, is a marine biodiversity hotspot. Its cetacean diversity is also remarkable; although the species with a regular occurrence and resident populations are eleven, a total of twenty-one species of cetaceans occur or have occurred in various degrees. Among the planet’s marine environments, the Mediterranean Sea is one of the most affected by anthropogenic activities. In such a complex scenario of multiple pressures acting simultaneously certain habitats and species face high risk. As very mobile, long-lived vertebrates, situated at the highest levels in the marine trophic webs and with a very low reproductive rate, cetaceans are among them. Human density and intensity of activities are particularly significant in the coastal zone. Pressure is thus most intense on coastal species, such as the common bottlenose dolphin Tursiops truncatus and the short-beaked common dolphin Delphinus Delphis. This thesis focuses in these two species, aims to identify the main threats affecting them and discusses the most adequate conservation measures in order to secure their survival in the increasingly fragile waters of the Mediterranean Sea. The study areas used as case-studies in the present work are the Balearic Islands (Spain) and the coastal waters of the Eastern Ionian Sea (including the Gulf of Ambracia and the Inner Ionian Sea Archipelago, in north-western Greece). These areas are remarkably diverse in terms of environmental features and threats caused by human activities, therefore offering opportunities for understanding their links with dolphin status and conservation needs.

The studies included in the present PhD thesis involved different research methods. Line-transect based aerial surveys were exclusively conducted in the Balearic Islands, while in all three study areas extensive effort was dedicated to boat-based surveys and photo-identification. A significant part of the research focused on the study of dolphin-fisheries interactions; the methods used in this part of the study differed between areas and included observers on-board of small-scale fishing boats and bottom-trawlers, direct data collection in fishing ports and landing sites on the activity of the fishing fleet and their captures, ecosystem modelling, and fishermen interviews.

As this thesis shows, anthropogenic threats faced by coastal dolphins in the Mediterranean Sea are very diverse and differ quite significantly between areas. Therefore, the strategies and management measures required to trigger some effective conservation results should be tailored on a case-by-case basis.

For the conservation of bottlenose dolphin population in the Balearic Islands, high priority should be given to produce ecosystem-based management strategies, including: (a) promotion of sustainable fisheries to ensure stable food resources to dolphins; (b) mitigation of adverse dolphin – fishery interactions, particularly eradication of dolphin kills; (c) prevention of marine pollution; and (d) regulation of tourism, with special attention to boat traffic in areas where bottlenose dolphins aggregate and show a higher site-fidelity and where, ideally, marine protected areas could be implemented.

The situation on western Greece is quite different between the two study areas. In the Inner Ionian Sea Archipelago, where short-beaked common dolphins have suffered a dramatic decline in recent times, fishery management measures are urgently needed to reduce current over-exploitation and allow for the recovery of endangered marine megafauna. Contrarily, in the neighbouring waters of the semi-enclosed Gulf of Ambracia, which host one of the highest densities of bottlenose dolphins in the Mediterranean Sea, priority should be paid to improve the rapidly deteriorating water and seafloor quality, while ensuring continued ecosystem services and biodiversity conservation. 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.

El Mar Mediterrani es veu altament afectat per activitats antropogèniques. La seva alta densitat de població i la intensitat de les activitats que en resulten són particularment importants a les zones litorals. Espècies de cetacis costaners, com el dofí mular Tursiops truncatus i el dofí comú Delphinus delphis en sufreixen les consecuències. Aquesta tesi té com a finalitat la identificació de les principals amenaces que afecten a aquestes dos espècies i proposa una sèrie de mesures de conservació per tal d'assegurar la seva supervivència en les aigües, cada vegada més fràgils, de la Mediterrània. Les àrees d'estudi utilitzades com a casos pràctics són les Illes Balears i les aigües costaneres del Mar Jònic oriental (incloent-hi el Golf d'Ambracia i l'arxipèlag interior del mar Jònic, al nord-oest de Grècia). Les amenaces antropogèniques a les que s’enfronten els dofins costaners a la Mediterrània són molt diverses i varien de forma significativa entre àrees. En conseqüència, les estratègies i les mesures de gestió necessàries per la seva conservació també ho fan.

Per tal de protegir la població dofí mular a les Illes Balears, caldrà donar prioritat a la realització d'estratègies de gestió que tinguin en compte les necessitats dels ecosistemes: (a) promoció de la pesca sostenible; (b) la mitigació d’efectes adversos resultants d’interaccions entre dofins i pesca; (c) la prevenció de la contaminació marina; i (d) la regulació del turisme, amb especial atenció al trànsit d'embarcacions a les zones on els dofins és concentren més sovint i on, idealment, s’hi podrien crear àrees marines protegides. Pel que fa a la Grècia occidental, a l'arxipèlag Interior del Mar Jònic, on la població de dofins comuns ha patit una disminució dramàtica en els últims temps, mesures adequades per la gestió de les activitats pesqueres són necessàries per reduir l'actual sobreexplotació i permetre la recuperació de megafauna marina. D’altra banda, en el Golf d'Ambracia, on trobem una de les majors densitats de dofins mulars del mar Mediterrani, la millora de la qualitat de les seves aigües i el seu fons marí és la principal prioritat per protegir la seva important biodiversitat.