Marine pollution as a morbidity factor in dolphins: evidence from
heavy metal concentrations in tissues and bones of modern
dolphins found along the Mediterranean coast of Israel and in
bones of dolphin's from ancient times.
Mia Roditi-Elasar
This thesis deals with a phenomenon known since ancient times: single
whales or dolphins stranding on the beach and dying there. Aristotle, Pliny, Aelian
and Oppian, Greek and Roman authors and philosophers described dolphins that
stranded on the beach and ended their lives because of a unique bond between pod
members.
A new facet, a modern one, was added to this known phenomenon: large
numbers of cetaceans of various species, beaching within a short time span. The
alleged causes in these cases are numerous and mostly anthropogenic: failure of the
immune system to combat viruses accelerated by sea pollution, toxic emission by
algae blooming and underwater acoustic experiments which confuse the cetacean
sonar and orientation abilities.
With worldwide industrialization, humans have exposed marine organisms to
many waste products, particularly those of heavy metals.
Typically, high
concentrations of heavy metals are found in tissues of stranded cetaceans, and are
known to cause stress and damage to the immune system, which exposes the
organism to diseases. Marine mammals lack a heavy metal elimination system and,
as top predators, concentrate metals which accumulate with age, weight, and length.
Yet, studies directly relating morbidity to pollutant concentrations or comparisons of
pollutants concentrations in sick and in healthy animals, are scarce.
The prevalent stranding phenomenon in Israel is that of single animals. From
June 1993 to August 1999, 88 whales and dolphins were found dead on the beach
or entangled in fishing nets
This thesis focused on the connection between heavy metals, as a common
pollutant, and pathologic findings from dead dolphins, where the main assumptions
were:
1. Dolphins by-caught in nets can be used as a healthy control group to the
experimental group of stranded dolphins, assumed to be ill.
2. Heavy metals, as common pollutants, may expose dolphins to diseases.
3. Heavy metals concentrations in the marine food web of the Mediterranean
Sea increased along the years due to industrial development.
Data collected from all beached animals included gender, length, weight and
fat layer thickness as a rough index for nutritional status. Teeth from mid upper and
lower jaws were extracted and sent for age determination in Dr. V. Cockroft’s
laboratory in South Africa and in Dr. C. Lockyer’s laboratory in Denmark. Twenty two
out of 88 animals were found fresh enough for a postmortem autopsy. Skin, fat,
muscle, liver, kidney, brain, and stomach content samples were taken for heavy
metals analysis in the Israeli Oceanographic & Limnological Institute in Haifa. Cold
vapor, graphite furnace and flame atomic absorption spectrophotometry methods
were used for heavy metal analysis. Bacteriological and histo-pathological samples
and smears and parasites samples were analyzed in the Israeli Veterinarian Institute
in Beit-Dagan.
In order to search for change trends in heavy metal concentration in cetacean
tissues over time, an analysis of heavy metals concentration was preformed on
bottlenose dolphin’s vertebrae from 3 different periods: a vertebra found in an
archeological site in Ashkelon from the Fatimid period (1000 years ago), a vertebra
found in the Tel-Aviv University Collection from 1960, and two vertebrae from
dolphins collected in the recent years.
Main findings
In 10 out of 22 dolphins autopsied, the final cause of death was apparently
determined. The main histo-pathological findings were located in the lungs: edema,
focal inflammation, emphysema and sometimes even acute pneumonia. In some
cases these findings could be connected to high concentrations of heavy metals. In
addition, different bacillus species, plastic bags blocking the stomach and various
parasitic infestations were found.
Kidney and liver cadmium concentrations in striped dolphins (Stenella
coeruleoalba) were higher than those found in common bottlenose dolphins
(Tursiops truncatus). Mean mercury concentrations found in livers of striped dolphins
in Israel, were lower than those found in the same species from the Mediterranean
coast of France, but higher than in dolphins from the Atlantic coast of France. Mean
liver Mercury concentrations of the bottlenose dolphins found in Israel, on the other
hand, were lower than those found in the Atlantic coast of Florida.
Evidence for stranding of single dolphins is common in literature from ancient
times. Yet, there is no mentioning of multiple strandings or epizootics as we know
today from all over the world, including the Mediterranean.
Surprisingly, of the vertebrae analyzed, the highest concentrations of Pb, Mn,
Fe, Zn and Cu were found in the one from Ashkelon. Cadmium and Hg
concentrations were similar in vertebrae from all the periods. No significant
difference was found between heavy metals concentrations in modern bones and the
bone from collection of Tel Aviv University. Since the bone from Ashkelon is the only
one located from an archeological site, it is impossible by these results to draw clear
conclusions that relate heavy metals concentrations in dolphins to the growing
amount of industrial effluents directed to the sea.
Conclusions
The primary assumption that net-entangled dolphins that subsequently
drowned could serve as a control group of "healthy" animals was proven false for the
following reasons:
1. In the progress of the research, it became apparent that part of the beached
animals could well be healthy specimens that were caught in the net, cast to sea
and drifted to shore.
2. Pathological findings indicating poor health were also found in netted dolphins
and may even have contributed to their entanglement.
A possible connection between tissue heavy metal levels and pathological findings
could be demonstrated in several dolphins. However, strict statistical testing could
not be performed because of the low number of animals from some species, lack of
similar tissues in all the dolphins, lack of teeth and mostly, inconclusive pathological
reports. In the future, reports that readily allow categorical morbidity scoring would be
mandatory
The attempt to show a difference in heavy metals concentrations along the years by
sampling dolphin bones from different periods was unsuccessful because of the
small number of samples and the period when the bone laid in the ground and may
have absorbed high concentrations of heavy metals from the soil.
This finding
questions the validity of measuring metal concentrations in bones from archeological
sites, unless they were preserved in special conditions.