A discussion on the results of our contaminant assay of organochlorines in blubber biopsy samples, taken from free-ranging blue whales in the St. Lawrence.
The results of our study show a significant difference between levels of PCBs in male specimens, compared to females. Total PCBs were in the ~300 - 2600 parts per billion range, which is quite close to the limited data available from other studies of blue and related whales.
It is interesting to note that for other species of marine mammals or whales from areas other than the Gulf of St. Lawrence, the total PCB levels in the blubber differ--this is because both the level of contaminantion in the immediate environment, and species influences how much PCB a whale will accumulate. Some studies on the beluga whales of the St. Lawrence show much greater levels of PCB accumulation than that which we found in blue whales.
There are several reasons for the difference:
a) Blue whales feed at a lower trophic level than do belugas. Because beluga whales feed on a higher trophic level, there is more of an oppurtunity for biomagnification. PCBs and similar contaminants preferentially accumulate in the fatty tissues of organisms. Therefore, every time you move up a level in the food chain each organism will be accumulating not just the sum of all contaminants in whatever it feeds on, but also from everything that its prey feeds on, and so on down the food-chain.
b) St. Lawrence blue whales, unlike the belugas, do not spend all of their time in the Gulf, and so they are not always taking up contaminants to the same degree as the belugas.
c) Variation in PCBs found in various organisms occurs, additionally, because different species metabolize chemicals differently. Also, different PCB congener levels are found because different compounds have a greater affinity to partition into an organism. This 'desire' for a contaminant to move out of water and into an organism is often described using a Kow value (the 'octanol-water partition coefficient'), which is the ratio of a chemical's solubility in octanol to a chemical's solubility in water.
Reports in the scientific literature show that the higher-numbered PCB congeners tend to be present in greater proportions than lower-numbered ones. The results of our study are in general agreement with this (see figure below). PCB congeners assigned higher numbers are more heavily chlorinated. These PCBs are more difficult to metabolize, and so they may remain unchanged with greater frequency than the lower chlorinated congeners.
The highly chlorinated congeners are not readily broken-down, and tend to accumulate more in an unchanged form.
Our results show a very similar pattern of PCB contamination occurring in the blubber of blue whales in the Gulf of St. Lawrence, as other reserachers have described for other marine mammals in both the St. Lawrence and the arctic regions. The graph below represents the contribution of each PCB congener, to PCBs.
Notice, above, that lower numbered PCB congeners occur proportionally less than those with higher numbers. A similar distribution of PCB congeners has been found in beluga whales (Delphinapterus leucas), in Dallís porpoise, and in many seals (pinnipeds). The very high, proportionally speaking, levels of congener numbers 52, 153, and 138, is a particular feature that has been noted by others in previous studies.
Below is a similar graph for DDT, DDD, and DDE average contribution to total DDTs. It is interesting to note that DDE levels were found to be higher in females than in males--typically, the reverse has been found to be true. It is generally thought that males will have higher DDE levels, as DDE is a metabolite of DDT. Since male's metabolism of such compounds is thought to be heightened by their elevated contaminant burden, males generally higher DDE levels are rationalized. Because, particularily, our measured DDT, DDE and DDD levels had such a high amount of variability, these findings may be somewhat suspect, and warrant further investigation.
The root of the differences between male and female contaminant burdens is in that female marine mammals transfer a large portion of their contaminants to their offspring. The females of some species of marine mammals have been found to rid themselves of as much as 98% of their organochlorine contaminant burden. This is largely because the milk of marine mammals is extremely high in fat--more than 50% in some, compared to about 8%, on average, in terrestrial mammals (see introduction).
Unlike females, males lack the ability to rid themselves of their contaminant burden at any time during their life cycle. In males, therefore, the rate of PCB intake always exceeds the rate at which they are eliminated. If females do not reproduce, their contaminant intake rate also exceeds the elimination rate, and so they accumulate contaminants just like males do. In light of this, the significant sex differences shown above can be taken to suggest active reproduction.
Finally, in conclusion, the table below lists each of the major contaminant groups we tested for, the average (and range of values) concentrations in both males and females, the sample size (ie. the number of animals in each test group), and a P value from a statistical test performed to determine if the differences between the sexes were significant (the non-parametric Mann-Whitney test)--if an NS is specified, the test showed no significant difference, where otherwise there was a significant difference found.
Üsum of trans-, and cis-chlordane, trans-, and cis- nonachlor
NS = not significant, P > 0.05
For furthur information on contaminant accumulation differences between male and female marine mammals, read:
Aguilar, A., and A. Borrell. 1988. "Age and Sex-related Changes in Organochlorine Compound Levels in Fin Whales (Balaenoptera physalus) from the Eastern North Atlantic." Marine Environmental Research, vol. 25: pp. 195-211.
Borrell, A., A. Aguilar, S. Corsolini, and S. Focardi. 1996. "Evaluation of Toxicity and Sex- related Variation of PCB Levels in Mediterranean Striped Dolphins Affected by an Epizootic." Chemosphere, vol. 32: pp. 2359-2369.
Stern, G. A., D. C. G. Muir, M. D. Segstro, R. Dietz, and M. P. Heide-Jorgensen. 1994. "PCBs and Other Organochlorine Contaminants in White Whales (Delphinapterus leucas) from West Greenland: Variations With Age and Sex." Bioscience, vol. 39: pp. 245-259.
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RAW DATA for each whale (by sample number)