05 December 2006
The decline of common dolphins around the island of Kalamos, Greece
Seeing common dolphins bowriding and surrounding our research boat from all sides was a frequent event around the island of Kalamos. When I first moved to study dolphins in western Greece, back in 1996, these magnificent marine mammals were so abundant that one could frequently spot them from the coast, or even from the patio of our field station.
Tuna and swordfish were equally abundant, and from a distance it was sometimes difficult to tell a school of foraging tuna from a group of foraging common dolphins, as both animals performed a similar behaviour when catching anchovies and sardines near the surface. The sea was full of life, and navigating those waters was an endless source of wonder and excitement for pleasure boaters and researchers alike. The situation was so special that the area, one of the few in the central Mediterranean containing key common dolphin habitat, was declared a EC Site of Community Importance. This designation was expected to result in a commitment to protect the local resources and prevent habitat degradation.
However, only a few years later common dolphins around Kalamos had become a rare sight. Tuna and swordfish also vanished. What caused such a quick decline of high-order marine predators in this portion of the eastern Ionian Sea?
Support provided by OceanCare and WDCS - the Whale and Dolphin Conservation Society, was essential to answer this question. These organizations offered financial means and much encouragement to work hard and identify the threats affecting dolphins in the area. Was it pollution? Collisions with speedboats? Intentional killings? Interactions with fisheries? Pathogens?
More than a decade of intensive research at sea and much data analyses done by personnel from the Tethys Research Institute suggest that the main cause of common dolphin decline is overfishing of their prey. Purse seine nets, in particular, seem to be responsible for the local overexploitation and depletion of epipelagic stocks of sardines, anchovies and other fish that make the daily diet of common dolphins, tuna and swordfish. Prey depletion has been so intensive and continuous that large marine predators such as common dolphins can no longer find easy prey.
To face scarcity of food, common dolphins started dispersing and roving. Their formerly large groups broke up into smaller units, which became increasingly sparse. Between 1997-2004, common dolphin encounter rates declined 25-fold, possibly as a result of reduced reproductive success and increased mortality in an area that - as far as prey availability was concerned - had turned from paradise to hell.
Problems caused by prey scarcity summed up to entanglement and mortality in fishing gear, as documented by dead dolphins found stranded or adrift and showing amputations. Today, only a few common dolphins can still be found in the area, and this brings a feeling of sadness to those who have seen them thriving until only a few years ago.
The decline of common dolphins in the area of Kalamos flashes a red light for the conservation of the Mediterranean population. Once one of the most common cetacean species in the Mediterranean, common dolphins have declined throughout the region during the last 30-40 years. Conservation problems for the species have been recognised since the 1970s, but at that time there was little information about cause-effect relationships, as few were recording information at sea about population status and threats. After the turn of the century, however, threats affecting the animals became progressively clear. These basically included incidental mortality in fishing gear (also known as “bycatch”), habitat degradation and prey depletion caused by overfishing.
In 2003 the Mediterranean population of common dolphins was classified as Endangered in the IUCN Red List of Threatened Animals. In 2004, ACCOBAMS - the UNEP's Agreement on the Conservation of Cetaceans of the Black Sea, Mediterranean Sea and contiguous Atlantic Area - presented a comprehensive 90-page Conservation Plan for Mediterranean common dolphins, providing a detailed description of actions needed to protect the animals. Finally, in 2005 the Mediterranean population of common dolphins was included in Appendix I of the Convention on the Conservation of Migratory Species (CMS) as a consequence of their threatened status.
Have all these institutional steps contributed to making life easier for Mediterranean common dolphins? So far, they apparently haven’t. Despite all the expressions of concern, recommendations, strategic planning and scientific background produced, no relevant action has been taken that may result in common dolphin recovery in the region. Sadly, the threats which are thought to be causing decline are continuing to jeopardise the survival of relict groups such as those found around Kalamos, and the Mediterranean population at large.
Scientific research, conservation action plans and declarations of intents by the concerned Governments do not seem to suffice to reverse the present trends. Much public awareness actions, and attempts to define and communicate practical solutions to local problems are also essential.
For more information:
Bearzi G., Reeves R.R., Notarbartolo di Sciara G., Politi E., Canadas A., Frantzis A., Mussi B. 2003. Ecology, status and conservation of short-beaked common dolphins (Delphinus delphis) in the Mediterranean Sea. Mammal Review 33(3):224-252.
Bearzi G., Politi E., Agazzi S., Bruno S., Costa M., Bonizzoni S. 2005. Occurrence and present status of coastal dolphins (Delphinus delphis and Tursiops truncatus) in the eastern Ionian Sea. Aquatic Conservation: Marine and Freshwater Ecosystems 15:243-257.
Bearzi G., Politi E., Agazzi S., Azzellino A. 2006. Prey depletion caused by overfishing and the decline of marine megafauna in eastern Ionian Sea coastal waters (central Mediterranean). Biological Conservation 127(4):373-382.