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Bioaccumulation of persistent organic pollutants in small cetaceans in european waters : transport pathways and impact on reproduction (BIOCET)


Stomach content analysis results have identified the main prey species of the four studied cetacean species (common dolphin, harbour porpoise, bottlenose dolphin and striped dolphin) and described patterns of variation in relation to factors such as season, sex and year. Harbour porpoise has a narrower diet than any of the dolphin species. Diets of all species show differences between areas. In all studied cetacean species, many of the main prey species are commercially exploited fish. Fatty acid signatures of different prey species can be distinguished. Harbour porpoise, common, bottlenose and striped dolphins have significantly different fatty acid profiles. Within cetacean species, fatty acid profiles vary between areas and with body length, season, and nutritional state. Geographical differences in the fatty acid profiles of cetaceans can be related to variations in prey fatty acid signatures. The fatty acid analysis supports the differences found in the stomach content analysis.
Prior to the project, it was common knowledge and use in cetacean biology to consider that ovarian scars were permanent features and therefore could provide an insight into female individual reproductive history. Basic demographic characteristics, such as inter-birth intervals or number of past pregnancies, were estimated from ovarian counts. As part of BIOCET WP5, it was planned to use these structures in order to assess the number of past pregnancies because this parameter is central to modelling pollutant transfer to calves through both gestation and lactation, which is a route a pollutant elimination unique to females. The examination of ovaries from common dolphins showed that once maturity is reached ovarian scar counts do not increase with age, unlike what would be expected from the literature. Instead, ovarian scar counts remain at 0 until puberty and then vary over quite an extended range irrespective of age, the overall trend being flat. There can be many ovulations per year, as shown by animals in their first post-pubertal year showing scar counts in the upper part of the species range. This additionally confirms that scars represent past ovulations and not only those ovulations followed by a gestation, as it was sometimes suggested in the literature. In contrast to this lack of relationship with age there is a strong relationship with instantaneous reproductive status. Indeed pregnant females show on average about 50% less ovarian scars than non-pregnant ones. These observations suggest that, unlike previously thought, ovarian scars heal out completely in the common dolphin. Therefore the scar count at any particular time of the life is the combination of past ovulations and healing rate. The rate of healing can be approached by the observations made on pregnant females which suggest that the half-life time of ovarian scars in shorter than the duration of pregnancy (11 months in the common dolphin). Data on harbour porpoise were less clear, mostly because of a smaller sample size of mature individuals. However, they were apparently in general agreement with the lines suggested by data of common dolphins. These findings had profound effect on the way in which pollutants, age and reproductive data could linked together in the BIOCET project. Externally these results are also of importance because they are a strong invitation for cetacean biologists to fully revise the significance of ovarian scars and their value as a tool to investigate individual reproductive history and derive meaningful demographic parameters. It can be noted that in most mammals ovarian scars do regress fully and do not accumulate simply with age.
POP levels in female common dolphins were shown to be linked to diet, area and reproductive status, although not related to age; results for harbour porpoises indicated only area effects. Cadmium concentrations appeared to be closely linked to diet. The number of scars recorded on common dolphin ovaries was correlated with mercury burdens, and to a lesser extent to POP burdens and diet. Transfers of organic and metallic pollutants via food were estimated. Quantitative fatty acid signature analysis was carried out.