Correspondence Comment on “Elevated Accumulation of Tributyltin and Its Breakdown Products in Bottlenose Dolphins (Tursiops truncatus) Found Stranded along the U.S. Atlantic and Gulf Coasts” SIR: The premise of the paper by Kannan et al. (1) concerning the accumulation of tributyltin (TBT) in bottlenose dolphins (Tursiops truncatus) found stranded along the Atlantic and Gulf coasts is that the levels of TBT (and its degradation intermediates) together with presumed elevations in other chemicals may have contributed to immunosuppression, resulting in the observed dolphin mortalities over the past several years. While their data show elevated total butyltin (BT) [Butyltins here represent the sum of mono-, di-, and tributyltins.] concentrations in some individual dolphins, their speculations linking TBT to dolphin deaths were not something they actually studied (i.e., collected empirical data). In addition, a closer look at their data interpretations suggest conclusions that appear unsubstantiated as well as inaccuracies in the data presentations. Our first concern revolves around the BT level found in one bottlenose dolphin liver sample of 11 340 ng/g (all values are expressed in wet weight). While we do not dispute the veracity of this concentration, it appears to qualify statistically as an outlier and, hence, would be unrepresentative of the overall population. This single sample is 4.5 times higher than the second highest concentration in the sample population of 17, is approximately 4 standard deviations from the mean [Calculated based on our ability to extract 15 of the 17 liver BT values from ranges shown in Tables 2 and 3 of ref 1 and to deduce the average of the other two samples.], and qualifies as an outlier using the Dixon test (2). The magnitude of this single sample’s effect on the mean is great, nearly doubling the mean BT liver concentration from about 850 ng/g without the outlier to 1400 ng/g with it. Removal of this sample from the database has a significant effect on the conclusions. If not removed, then the data point should be qualified. The issue of the 11 340 ng/g value being an outlier has implications beyond statistics. Ecological risk assessments are predicated on the protection of populations rather than individuals (3). Thus, the focus needs to be on characterization of the population, and if a single individual significantly biases that characterization, then it should not be lumped in with the main population until additional data clarify its status. Without more data indicating it is part of the population, this data point should be considered an outlier. Secondly, the authors refer to the 1987-1988 large-scale bottlenose dolphin mortality event that occurred along the U.S. Atlantic Coast (4) and the 1990 event that occurred in the Gulf of Mexico (5) and implicate TBT concentrations as a probable factor based upon their analyses of butyltin residues in tissues from dolphins not associated with these events. In response to the mass mortalities, many factorss organochlorines including PCBs and DDT (6, 7), metals (8, 9), morbillivirus (10), and brevetoxin (4)shave been suggested as being responsible. This paper (1) does not make it clear
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to the reader whether any of the dolphins sampled were associated with either of the large-scale mortality events; nor does it provide any empirical data concerning causes of historic dolphin mortalities. Most, if not all, of the adults and juveniles appear to have been collected outside these events based on the information provided in Table 3 (of ref 1). It is also not clear whether the sampled animals died from the same causes as the animals involved with the mass mortality events. The authors also stated that “In general, the mean concentration of BTs in bottlenose dolphins from the U.S. Atlantic coast was higher than those of the diseased Mediterranean Sea animals (17) and finless porpoise (Neophocaena phocaenoides) and Risso’s dolphins (Grampus griseus) from Japanese coastal waters (18)”. However, examination of the cited articles, Kannan et al. (11) and Kim et al. (12), suggests this is not the case. BT concentrations in livers of Mediterranean dolphins were 1200 and 2200 ng/g, or a mean of 1700 ng/g, while mean BT concentrations in Japanese Risso’s and finless dolphin livers were 3700 and 4800 ng/g, respectively. All these values are higher than the 1400 ng/g mean for the U.S. dolphins and much higher than the 850 ng/g mean for U.S. dolphins if the 11 340 ng/g outlier is removed. In fact, approximately 80% of the 35 Risso’s dolphin samples exhibited BT liver concentrations greater than 2500 ng/g (Figure 3 of ref 12) as compared to only 2% (n ) 1) for the 17 U.S. bottlenose dolphins (1), indicating a much higher range of values for the Japanese dolphins. In a similar vein, the authors state that “Butyltin concentrations in the livers of spotted dolphin [Stenella attenuata] and pygmy whale [Kogia breviceps] were 3-4 times lower than in bottlenose dolphins”. Is this a valid comparison? Both spotted dolphins were male calves, which, using this paper’s data set on age effects, should have lower concentrations. In fact, the average liver concentration for the two spotted dolphin calves (360 ng/g) is nearly the same as the average concentration for the two male bottlenose dolphin calves sampled (440 ng/g). A somewhat similar case can be argued for the pygmy sperm whale comparison, as one of these three whales is a male calf with a BT concentration (350 ng/g) that puts it between the BT concentrations for the two male bottlenose calves (138 and 712 ng/g). While one might expect off-shore animals to have lower concentrations than near-shore animals, due to differing exposures, their own data do not support this contention once age-related influences are taken into account. The authors further state that “Total butyltin concentrations in the liver, kidney, and muscle of a captive adult (242 cm; 21 yr-old) female bottlenose dolphin were 78, 19, and 13 ng/g, respectively, which were 50-100-fold lower than the concentrations found in free-ranging dolphins”. However, the data reported show that for all 17 dolphins the liver BT range is 110-11 340 ng/g, or a 1.4-145-fold difference from the captive animal value. Remove the 11 340 ng/g outlier and the range is 110-2500 ng/g, or a 1.4-32-fold difference. The maximum difference for kidney BT is 35-fold (19 vs 670 ng/g), and for muscle it is 8-fold (13 vs 110 ng/g) (see Table 1 of ref 1). In only one case, the 11 340 liver value, is there a difference that is greater than 35-fold out of 44 captive dolphin/free-ranging dolphin comparisons (17 liver comparisons, 16 kidney, and 11 muscle). Thus, 98% of the comparisons did not exhibit 50-100-fold differences. Also, it is unclear why a 21-year-old animal, which spent 8 yr in
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the Mississippi Sound (Gulf of Mexico) and then 13 years in captivity (p 298 of ref 1), could be considered “background” for Atlantic coastal cetaceans. Without knowledge of its BT concentrations at the time of capture and exposure history in captivity, little can really be said about this animal. We also question the statement “The [butyltin] concentrations increased with [dolphin] length, for both sexes, during early life stages until maturity and then tended to remain constant (Figure 4)”. We are not convinced that the curvilinear line drawn in Figure 4 (of ref 1) best fits the scattering of data points presented and suggest, given the small sample size, that a variety of linear or curvilinear lines could be drawn to these data that could either support or refute the above statement. Moreover, data in Figure 4 appear plotted incorrectly against the y axis, and there appears to be an unexplained datum (n ) 18 instead of 17). The plotted BT values in Figure 4 should range from 110 to 11 340 ng/g (see Table 1 of ref 1) but appear to range from 400 to 40 000 ng/g. Also, if the extra value in the figure is the captive dolphin (78 ng/g), it would fall below the x axis because the base of the y axis begins at 102 ng/g. The above issue is further carried over into Figure 5 (of ref 1) where the concentrations of BTs from U.S. cetacean livers appear higher than those in Japanese animals. The U.S. values shown on the figure range from 400 to 40 000 ng/g, not from 110 to 11 340 ng/g as the data reported in ref 1 indicate. On the other hand, the Japanese values in the figure range from about 30 to 10 000 ng/g. The citation under the figure caption states the Japanese cetacean data came from Kim et al. (12), a paper showing BT liver concentrations for Risso’s dolphins ranging from 550 to 6000 ng/g. The text, however, cites an additional reference, Iwata et al. (13), that presents liver BT concentrations for three finless porpoise ranging from 1120 to 10 200 ng/g. Combined, the two species reported in refs 12 and 13 show a range of 550-10 200 ng/g, yet Kannan et al.’s Figure 5 shows at least nine values in the Japanese cetaceans column less than 550 ng/g, and ranging down to about 30 ng/g. Where did these values come from? Are they the Dall’s porpoise (Phocoenoides dalli), dwarf sperm whale (Kogia simus), and ginko-toothed whale (Mesoplodon ginkgodens) mentioned in the text of ref 1 but not in the cited references? Moreover, other than the finless porpoise, the Japanese cetaceans mentioned above occur both near-shore and off-shore similar to the pygmy sperm whale and spotted dolphin (14-18) and, therefore, would be expected to have lower concentrations than the strictly coastal bottlenose dolphins sampled by Kannan et al. (1). Consequently, we question whether Kannan et al.’s statement that “Butyltin concentrations in U.S. dolphins were comparable or higher than those reported for Japan, suggesting higher exposure in U.S. samples” is valid, especially since at least five, if not six, of the 17 samples analyzed by Kannan et al. exhibited BT liver concentrations lower than the lowest BT liver concentration reported by either Kim et al. (12) or Iwata et al. (13). There appear to be errors in Table 2 or Table 3. The mean BTs for four adult males is listed in Table 2 as 3390 ng/g with a range of 570-11 340 ng/g. The range for the two adult females is 420-2500 ng/g. From the BT concentrations presented in Table 3 (of ref 1), we can attribute the 570, 960, 1700, and 11 340 ng/g values to the four adult males and the 420 and 2500 ng/g values to the two adult females. The mean of the adult males appears, therefore, to be 3642.5 ng/g, not 3390 ng/g. Either one of the two adult male values in Table 2 (of ref 1) appears to be in error or the 960 ng/g value in Table 3 is in error. Overall, we believe the authors’ inferences may have been based on too few samples (n ) 17), especially considering they were of differing exposure histories (e.g., age, sex, state of decomposition). Any interpretation should, at a minimum,
be qualified and uncertainties discussed. Making inferences about populations based on so few data points goes against the risk assessment paradigm that is predicated on the protection of populations. The attention accorded total butyltins is questionable because BT encompasses TBT and its degradation intermediates, monobutyltin and dibutyltin, which vary greatly in toxicity and physiochemical properties affecting metabolism and, hence, bioaccumulation and biomagnification (19). Applying these distinctions to ref 1, we note the mean TBT liver value for U.S. dolphins (100 ng/g) as being lower than the mean TBT liver value for either Risso’s dolphins (820 ng/g) or finless porpoise (700 ng/g) from Japan. Finally, Kannan et al. use language suggesting a link between the butyltin residues measured, immunosuppression, and dolphin mortalities. In suggesting this link they make the statement “... it is probable that butyltin compounds in addition to PCBs have contributed to the immune suppression in bottlenose dolphins”. Regardless of whether the statement is true or not, we believe that such an inference is not supported by their data. This study measured chemical residues of a single compound in dolphin tissues and did not contain any histopathological, toxicological, or epidemiological analysis suggesting the strong link they infer. Moreover, many causes of the dolphin mass mortalities have been postulated, including links to chemicals like non-metabolizable organochlorines (e.g., PCBs and DDT), metals, viruses, and red tide. The actual causes of dolphin mass mortalities remain elusive.
Literature Cited (1) Kannan, K.; Senthilkumar, K.; Loganathan, B. G.; Takahashi, S.; Odell, D. K.; Tanabe, S. Environ. Sci. Technol. 1997, 31, 296301. (2) Sokal, R. R.; Rohlf, F. J. Biometry; W. H. Freeman and Company: New York, 1981. (3) Gentile, J. H.; Slimak, M. W. In Ecological Indicators; McKenzie, D. H., Hyatt, D. W., McDonald, V. J., Eds.; Elsevier Applied Science: New York, 1992; pp 1385-1397. (4) Geraci, J. R. Clinical investigation of the 1987-88 mass mortality of bottlenose dolphins along the U.S. central and south Atlantic coast; Final Report to the National Marine Fisheries Service and U.S. Navy, Office of Naval Research and Marine Mammal Commission; Ontario Veterinary College, University of Guelph: Guelph, Ontario, Canada, 1989. (5) Kuehl, D. W.; Haebler, R. Arch. Environ. Contam. Toxicol. 1995, 28, 494-499. (6) Kuehl, D. W.; Haebler, R.; Potter, C. Chemosphere 1991, 22, 1071-1084. (7) Kuehl, D. W.; Haebler, R.; Potter, C. Chemosphere 1994, 28, 1245-1253. (8) Rawson, A. J.; Patton, G. W.; Hofmann, S.; Pietra, G. G.; Johns, L. Ecotoxicol. Environ. Saf. 1993, 25, 41-47. (9) Wood, C. M.; Van Vleet, E. S. Mar. Pollut. Bull. 1996, 32, 886889. (10) Lipscomb, T. P.; Schulman, F. Y.; Moffett, D.; Kennedy, S. J. Wildl. Dis. 1994, 30, 567-571. (11) Kannan, K.; Corsolini, S.; Focardi, S.; Tanabe, S.; Tatsukawa, R. Arch. Environ. Contam. Toxicol. 1996, 31, 19-23. (12) Kim, G. B.; Tanabe, S.; Iwakiri, R.; Tatsukawa, R.; Amano, M.; Miyazaki, N.; Tanaka, H. Environ. Sci. Technol. 1996, 30, 26202625. (13) Iwata, H.; Tanabe, S.; Mizuno, T.; Tatsukawa, R. Environ. Sci. Technol. 1995, 29, 2959-2962. (14) Mitchell, E. D. J. Fish. Res. Board Can. 1975, 32, 889-983. (15) Morejohn, G. V. In Behavior of Marine Mammals: Current Perspectives in Research; Winn, H. E., Olla, B. L., Eds.; Plenum Press: New York, 1979; Vol. 3, pp 45-83.
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(16) Leatherwood, S.; Reeves, R. R. In Wild Mammals of North America; Chapman, J. A., Feldhammer, G. A., Eds.; The Johns Hopkins University Press: Baltimore, MD, 1982; pp 369-414. (17) Leatherwood, S.; Caldwell, D. K.; Winn, H. E. Whales, dolphins, and porpoises of the western North Atlantic: a guide to their identification; NOAA Technical Report NMFS Circular 396; U.S. National Marine Fisheries Service: 1976; 176 pp. (18) Nagorsen, D. Mammalian Species 1985, 239, 6 pp. (19) Fent, K. Crit. Rev. Toxicol. 1996, 26, 1-117.
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