Introduction

Marine pollutants are attracting increased attention as environmental awareness continues to grow. Several events, such as recent major oil spills, mass mortalities of cetaceans in various worldwide locations, as well as the growing number of environmentally concerned people have created an interest in how pollutants are affecting the environment and marine mammals.

Many chemical pollutants introduced anthropogenically or naturally occurring, could possibly affect seals, dolphins, whales and other marine mammals. Petroleum hydrocarbons (oil and its derivatives) are continuously released in large volume by natural seepage from the seafloor. Some of these seeps even sustain their own chemosynthetic communities (Brooks et al., 1987). But it is the single release of large amounts of oil, such as the release of oil in Kuwait by Iraq or the Exxon Valdez spill that generates the most press coverage. Organochlorines, such as DDT and its more toxic decomposition compounds, DDD and DDE, along with polychlorinated biphenyls (PCB) are man-made contaminants of the marine environment. PCB's have been associated with reproductive failure in some pinnipeds (Reijnders, l 986). Like petroleum, heavy metals are naturally occurring substances, used extensively in industry and even in the household (i.e. cadmium, nickel and lead in batteries and mercury in thermometers). They can occur locally in unusually high and potentially harmful concentrations. Certain heavy metals, such as iron and zinc, are essential for proper metabolism, but even these elements can be toxic at high concentrations.

Chemical Pollutants and Odontocetes (Toothed Whales)

There are two major approaches to pollutant research in cetaceans: Effects of pollutants and measurement of levels of pollutants.

It is extremely difficult to address the first category because the animals are not confined to an easily studied habitat. The size and life span of cetaceans as well as regulatory restrictions prevent contained laboratory studies of these animals. This paper focuses on the second category; specifically heavy metal concentrations found in bottlenose dolphins stranded on the Texas coast. Although tissues have been collected since 1983, no tissues have been analyzed for pollutants on the animals stranded in Texas.

Dolphins are used as environmental monitors for more than just assessing possible problems in the health of their populations. They have relatively long life spans, are top predators in the food chain and can therefore be indicators of long-term accumulation of low levels of pollutants. Cetaceans are also food resources for human populations in places such as the Faroe Islands, Japan and the Arctic. High levels of elements such as cadmium (Cd) or mercury (Hg) could exceed recommended daily intake and result in human health problems. Neurological damage occurred in the area around Minamata Bay, Japan in the 1950's as a result of eating fish that had concentrated extremely large amounts of Hg.

Stranded Bottlenose Dolphins in Texas

Texas has a large population of Atlantic bottlenose dolphins (Tursiops truncatus) and an average of approximately 120 strand along the coast each year. The Texas Marine Mammal Stranding Network (TMMSN) was organized in 1980 through the Department of Veterinary Anatomy at Texas A&M University with the intent of promoting research and education on the stranded animals and to attempt rehabilitation of live strandings. A standardized protocol was established by TMMSN to facilitate collection of tissues for toxicologic studies. In addition morphometric data, teeth for aging, gonads for determination of sexual maturity and stomachs for content analysis are obtained from the stranded cetaceans. Eventually this information will be incorporated with research currently being conducted at the Texas A&M University at Galveston Marine Mammal Research Program A long-term goal of the research program is to assess environmental quality as it relates to marine mammals by monitoring and integrating aspects of the animals behavior, physiology, pathology, ecology and toxicology.

Muscle, liver, kidney, bone and blubber are collected from the stranded animals and frozen until analysis. Tissues are processed in the laboratory by trimming away the contaminated edges and digesting approximately one gram of tissue in purified nitric acid. Metal concentrations are determined by flame, graphite furnace, or cold vapor atomic absorption spectrometry. Blanks, replicate samples, and known reference materials are digested and analyzed simultaneously for quality assurance.

Common Trends of Heavy Metal Concentrations in Bottlenose Dolphins and Other Species

Preliminary analysis of Texas Atlantic bottlenose dolphins reveal trends of heavy metal concentrations similar to those reported from studies of other odontocete species in different regions.

Koeman et al. (1973) established a correlation between concentrations of Hg and selenium (Se). A general theory is that Se provides protection or counteracts the harmful effects of Hg. The Texas population of stranded Tursiops truncates appears to have a similar linear relationship in liver tissue with an R^2 value of 0.96.

Non-essential trace metals such as Cd and Hg accumulate with age, increasing by over 200 fold from fetuses and juveniles to larger adults. Itano et al. (1984) determined that mercury in striped dolphin blood is not transferred across the placenta and that the average mercury concentration in liver of fetuses is 100 fold lower than in mature female striped dolphins. A dramatic increase with age has also been observed in livers and other tissues of Texas stranded bottlenose dolphins.

The distribution of trace metals in odontocetes seems to be tissue dependent. The tissue distribution pattern in bottlenose dolphins stranded in Texas correlates with that of other odontocete species. In many cases the liver is the greatest concentrator of metals. However, the largest quantities of zinc and lead are generally found in bone, and cadmium in kidney (Fujise et al. 1988). The high concentrations of cadmium in kidney are attributed to a higher level of metallothionein in that organ. Metallothionein is a protein thought to bind to cadmium in an induced defense mechanism against possible poisoning.

Discussion

Results from the stranded animals in Texas appear to show trends of heavy metals which are similar to other species of odontocetes. A statistical study is currently underway with tissues of the stranded population of bottlenose dolphins to test differences of metal levels among three factors that may influence metal concentration: geographical region, animals' age and sex. This research will serve as a reference for future monitoring studies of odontocetes in the Gulf of Mexico.

References

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