Effect of salmon carcasses on egg production of a freshwater amphipod, Jesogammarus jesoensis: a field observation

The effect of salmon carcasses on egg production of a freshwater amphipod, Jesogammarus jesoensis, was examined by comparing populations with and without a supply of carcasses in Naibetsu River in Hokkaido, northern Japan. The female body length in the population with carcasses became larger in the years when carcasses were abundant, being significantly larger than that in the population without carcasses. No evidence, however, was found that carcasses influenced the ratio of brooding females to mature females. Every year, the clutch size in the population with carcasses was larger than the clutch size without carcasses. Because clutch size showed a significant correlation with female size, the larger clutch size was considered to be a result of the larger female size. In addition, however, comparison of clutch size using analysis of cavariance revealed that clutch size in the population with carcasses was larger than that without carcasses, even for the same body size. Egg size showed no difference between populations and for different years. These results indicate that salmon carcasses influence egg production of J. jesoensis by increasing female body size and by increasing the investment in each clutch per unit of body size.