Differing lipid contents of the animals play a relevant role in determining the differences in persistent pollutant levels which exist among them, and in particular the interindividual differences observed in the same species usually depend on this factor. In addition, the distribution of fat at intraindividual level determines the disposition of contaminants in any given individual. Cod liver, crustaceans' cuticles, and, in general, tissues rich in lipids, such as the blubber of marine mammals, may contain DDT, PCB, PCDD, and PCDF concentrations many orders of magnitude higher than the muscle flesh. Intraindividual variation is responsible for the seasonal dependence of such pollutant levels, which are higher during the spring spawning period than in autumn. It has been determined that a large proportion of herring (Clupea harengus) fat reserves are used up during spawning. This influences to a large extent the organism's energy reserves and indeed total DDT levels. Concentrations of PCBs and PCDFs in this species have been found to be seasonally dependent. Contaminant levels can depend on the age and size of the individual, as the concentrations of DDT, PCBs, and PCDDs are higher in older individuals than in younger ones. This is because the older animals have been exposed to pollutants for longer than the younger ones, but it may also be due to their migration patterns. It has, in any case, been proved that POP levels in females fall with increasing age and size, because the production of eggs/ newborn and the milk production in mammals have to be considered clearance mechanisms whereas the size increase is primarily a dilution mechanism.
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