Abstract
An analysis of mercury levels was carried out on dace of the species Tribolodon hakonensis (Günther) and T. ezoe Okada et Ikeda, captured in the Ishikari River and its seven tributaries between October 1975 and June 1978.
Results were as follows:
1) Two groups were differentiated, a group with relatively low mercury levels and another with relatively high mercury levels, in samples captured together at the same locations and of about the same size.
2) The authors believe that these different mercury levels are not due to the species or the sex of the fish but due to other factors such as migration, various levels of mercury in feed, and formation of sub-populations, each of which is composed of members of one age-class, to give a few possible cases.
3) Mercury levels of dace from the Peipan River were higher than those from other rivers due to vein of mercury located at the headwaters of the river.
4) The ratio of methyl mercury to total mercury in the fish was approximately 72% on the average, and there was no marked difference in the ratios found in each river.
5) The authors recommend a relatively loose method when comparing of fish mercury levels for various rivers instead of statistically stricter methods, as there is as yet relatively little ecological information on dace populations.
6) Similar results were found among crucian carp, Carassius carassius, captured near the mouth of the Ishikari in June 1978.
