Common Bottlenose Dolphins (Tursiops truncatus) are mostly at risk due to increasing anthropogenic activities. Contrary to several literary sources claiming that there is only one Mediterranean subpopulation, there are actually several subpopulations in this marine basin. The Mediterranean subpopulations are vulnerable and decreasing by the IUCN red list of threatened species, and further suspected to decline. In the subpopulation in the Adriatic sea alone, the number of dolphins has already declined by more than fifty percent in the last five decades, meaning that we need to be cautious in stating its realistic status (for more information, see: FEB 2014 Guidelines for Using the IUCN Red List Categories and Criteria). The high reduction of the dolphin number was primarily the result of planned killings which went on from the second half of the nineteenth century until the 1960’s, when dolphins were perceived as pests. In addition to the killings that still occur within the global population, further decline of bottlenose dolphins is now substituted with other growing anthropogenic pressures, some of which we observe directly while collecting data from our research area.
Some of the main threats to dolphins in the study area (and globally) are:
Loss of habitat quality, its degradation and environmental contamination.
Dumping waste, including industrial, sewage, and nuclear, was legal until the 1970’s. Regulations are in place now, but illegal dumping still occurs. In addition, run-off that soaks up toxins from fertilizers and pesticides gets washed into rivers that eventually lead to the ocean. All of this is exacerbated by maritime transportation and atmospheric pollution. Not only does chemical input affect marine animals and human health, it causes bacteria and plants to deplete oxygen and create an environment that kills healthy marine life and destroys ecosystems, a process called eutrophication. This process has affected the northern Adriatic more so than anywhere else in the Mediterranean since the 1970’s. The effects of it are serious– the Adriatic has seen rises in the level of surface temperature and salinity, and decreases in water transparency and oxygen levels (Bearzi et al. 2004).
All of the chemicals, toxins, and imbalances in the seas and oceans have serious side effects on the health and reproduction of bottlenose dolphins. The toxins are absorbed or eaten by organisms and gradually make their way up the food chain, even affecting humans. High levels of DDT, PCP’s and heavy metals have been found in dolphins from the Mediterranean (Shoham-Frider et al. 2009). In the Croatian waters of the Adriatic, dolphins were found to have higher concentrations of cadmium, arsenic, lead, and mercury in their livers, kidneys, and muscles (Bilandžić et al. 2012) The accumulation of these contaminants has serious effects on testosterone and the reproductive ability of males (Wells and Scott, 1999), and Cockcroft et al. (1989) determined that sadly, first-born calves received 80% of their mother’s body burden of contaminant residues, perhaps leading to increased neonatal mortality. The US Navy marine mammal program maintains bottlenose dolphins in open netted enclosures in the San Diego Bay and conducts tissue analysis on mothers and newborn calves from their breeding program. They found that the DDT levels in calves that died within six months was three times higher than in calves that survived. PCB levels were two and a half times higher in the mothers of calves that didn’t survive versus mothers of calves who did (Reddy et al. 2001).
Some of the toxins in the water have very long half-lives- meaning they can exist in sediments and the environment for decades. Tributyltin is a toxic compound that was used for many decades in paint on the bottom of marine vessels to prevent biofouling (the build up of algae, microorganisms, or animals such as barnacles). It leaches into marine environments and once in the tissues of animals can cause immunosuppression and in some instances, hearing loss for marine mammals. Focardi et al. (2000) determined concentrations of tributyltin (TBT), plus its degradation products monobutyltin (MBT) and dibutyltin (DBT), in livers and kidneys of dolphins stranded along the western Italian and Greek coasts. As with DDT, BTs were found to be transferred from mother to fetus.
Although our sources list contaminants in the tissues of only a handful of populations, this environmental contamination issue is one that affects all populations, worldwide.