Trends and stability of inland fishery resources in Japanese lakes: Introductions of exotic piscivores as a drive
Although many studies have focused on marine resources, fewer studies have considered inland fishery resources. Inland fishery resources are typically monitored on the basis of catch data alone or are not assessed quantitatively, despite their social, economic, and ecological importance. Because freshwater ecosystems have been severely degraded by human activities, evaluating the trends and current status of fishery resources and assessing their drivers are urgent tasks. We compiled long-term data on the annual catch, fishing effort, and power of 23 Japanese lakes, using two sets of government statistics dating back to the 1950s that provide invaluable information but were previously neglected because of the large number of missing values. Using Bayesian state-space models, we examined the trajectories of the catch-per-unit-effort (CPUE) of entire communities, considering changes in fishing effort and fishing power, and quantified both changes in CPUE over the 10-, 20-, and 30-year periods preceding 2008 and the temporal detrended stability of CPUE over the three periods.We also investigated the relationships among the CPUE change and stability, anthropogenic drivers, and lake morphometric characteristics.
Our study revealed that the CPUEs of 15 lakes of the 23 lakes declined consistently throughout the three periods. Our macroecological analyses demonstrate that the functional group richness of exotic piscivores was the most important predictor of both changes in and the stability of the CPUE among the drivers we considered. Lakeshore development partly affected the CPUE. The CPUE and its stability was also associated with lake morphometric characteristics. In particular, the CPUE of smaller lakes were more vulnerable to temporal fluctuations. The effect of overfishing was considered to be small because both fishing effort and power declined in almost all of the lakes. Thus, our findings suggest that increasing exotic piscivore species may decrease the resources and their stability, particularly in Japanese lakes where native piscivores are rare. This might lead to a substantial decline in ecosystem services. Our study highlights the importance of assessing inland fishery resources in a comprehensive manner and the need for restoration strategies to mitigate the effects of exotic piscivores.