Why Do the Radionuclide Concentrations of Pacific Cod Depend on the Body Size?

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We examined year-class-related differences in radiocesium concentrations in Pacific cod (Gadus macrocephalus) and evaluated the potential factors affecting the differences after the release of large amounts of radionuclides from Fukushima Dai-ichi Nuclear Power Plant (FNPP) in March 2011. The concentration of radiocesium was highest in the 2009 and earlier year-classes (yc) (≤2009 yc), followed by the 2010 yc, and was rarely detected in the 2011 yc. Trawl surveys throughout the year revealed that a proportion of Pacific cod born in or before 2009 and 2010 were distributed in the coastal area from winter to early summer, whereas all individuals were on the upper continental slope from early summer to winter. The concentration of radiocesium decreased more rapidly in the 2010 yc than in the ≤2009 yc. The diet of cod changed ontogenetically and spatiotemporally. The organisms preyed upon on the upper continental slope by cod of all year-classes and in the coastal area by the 2010 yc contained very low concentrations of radiocesium. However, some food items ingested in the coastal area by the ≤2009 yc had relatively high radiocesium levels. These results suggest that Pacific cod primarily accumulated radiocesium during the first few months after the FNPP accident. Age- and body size-dependent differences in growth, metabolic rate, and diet, as well as seasonal migration patterns, also affected the rate of decrease in radiocesium levels, which likely led to the differences we observed between year-classes.