Biodiversity of freshwater fish and aquatic macrophytes in Japanese lakes: A broad assessment

Abstract

There is an urgent need to measure trends in biodiversity. This has not been quantified in Japanese lakes because the distribution data for freshwater species is sporadic and has not been digitized. No nationwide monitoring has been conducted since the mid-1990s. In this study, we developed a research network with regional environmental research organizations, including prefectural research institutes and museums, and assessed the biodiversity in lakes. We reviewed the literature on the distributions of strictly freshwater fish and aquatic macrophytes in 19 lakes. We also newly surveyed the presence/absence of fish in seven lakes and macrophytes in 12 lakes. Overall, the richness of native species of fish and macrophytes had declined from pre-1999 to post-2000. On average, 25% of the fish and 48% of the macrophytes species had disappeared. Many exotic fish and macrophyte species were found to have invaded these lakes, even those with high native species richness. Furthermore, the introduction of fish species native to Japan into drainages where they did not occur historically was observed in many lakes, and the numbers of translocated and exotic fish species were similar. The status of biodiversity varied greatly among the five indices we used, highlighting the need to incorporate multiple indices in biodiversity assessments. Finally, we discuss the potential and constraints of our assessment for broad-scale freshwater biodiversity monitoring.