The interpretation of the spatial and temporal patterns of variation in organochlorine
concentrations in marine mammal populations is complex because of the lack of wide-scale,
long-term surveys. Therefore the results from several surveys must be combined and
this causes undesired heterogeneity due to differences in the sampling and analytical
techniques used and in the biological characteristics of the individuals sampled.
Moreover, information is not homogeneously distributed in either space or in time.
Most research is concentrated in western Europe, northern America and certain areas
of Asia, while it is extremely limited or non-existent in Africa and most regions
of the southern hemisphere. Marine mammals from the temperate fringe of the northern
hemisphere, particularly fish-eating species which inhabit the mid-latitudes of Europe
and North America, show the greatest organochlorine loads; noteworthy are the extremely
high levels found in the Mediterranean Sea and certain locations on the western coasts
of the United States. Concentrations in the tropical and equatorial fringe of the
northern hemisphere and throughout the southern hemisphere are low or extremely low.
The polar regions of both hemispheres showed the lowest concentrations of DDTs and
PCBs, although levels of HCHs, chlordanes and HCB were moderate to high in the cold
waters of the North Pacific. During recent decades, concentrations have tended to
decrease in the regions where pollution was initially high but they have increased
in regions located far from the pollution source as a consequence of atmospheric transport
and redistribution. It is expected that the Arctic and, to a lesser extent, the Antarctic,
will become major sinks for organochlorines in the future; this process may already
be significant for some compounds such as HCB and HCHs. Effort should be devoted to
both assessment of organochlorine trends in the now highly polluted populations of
the temperate fringe of the northern hemisphere and to the implementation of long-term
monitoring of marine mammal populations inhabiting polar regions.