The growth rate of Black-tailed Gull chicks is negatively related to total mercury of female parents on Kabushima (Kabu Island), Japan

The mercury (Hg) contamination at high concentrations affects the physiological state and behavior of animals, however, the effects of low Hg exposure on avian breeding success and the impact on the growth of chicks that depend on feeding from their parents are unclear. We investigated the effects of maternal Hg concentrations on breeding success in a colony of Black -tailed Gulls (Larus crassirostris) breeding on Kabushima, northeastern Honshu, Japan, in 2018 and 2019. Total Hg concentrations in blood cells in females were 2.48 ug/g dry weight +/- 0.55 (1.28-3.28) in 2018 and 2.42 ug/g dry weight +/- 0.55 (1.87-3.29) in 2019. In our two-year field study, female parents did not differ in Hg concentrations between two years, but 515Ns, i.e., the difference in diet, were significantly lower in 2019 than those in 2018. The difference in 515Ns could be related to difference in fledging success between two years (90% and 40% in 2018 and 2019, respectively). However, the results of this study showed that Hg exposure was not significantly associated with clutch size, hatching success, or nest success in the two years with different diets and was not significantly associated with any reproductive parameters at 515N. Hg exposure was significantly negatively correlated with chick weight gain (g/day), while blood Hg accumulation in female Black-tailed Gulls was below the threshold for direct effects on breeding success. However, there was no noticeable abnormal behavior when feeding the chicks at their Hg exposure levels, under the current level value. Although the breeding numbers of Black-tailed Gulls are not thought to be declining at colonies around Japan at the moment, Hg contamination may threaten their numbers in the future.