2023 Volume 28 Issue 2 Pages 437-452
Largemouth bass (Micropterus nigricans) is listed among the world’s 100 worst invasive alien species by the International Union for Conservation of Nature (IUCN). Because of M. nigricans’ serious negative impacts on aquatic ecosystems that it invades, major efforts have been made to control and locally eradicate it. However, there are few reports of its complete eradication from lake or river systems. Here, we discuss a comprehensive program of M. nigricans eradication in Kanabara Dam Lake, Nagano Prefecture, Japan. Operating since 2007, the program includes the removal of spawning beds; removal of larvae and fry through scooping; and capture of immature and adult fish via net traps, cast nets, gill nets, fishing, and spear guns, and by hand. The M. nigricans population was monitored via snorkelling and environmental DNA analysis. Net traps were effective for capturing smaller fish, whereas 3-meter-long gill nets and spear guns were effective for larger fish. Yearly catches of M. nigricans reached a maximum of 1,472 in 2010, excluding young-of-the-year. The number of spawning beds reached a maximum of 131 in 2012 but decreased rapidly and substantially thereafter. In 2014 > 5,000 larvae were observed around a few spawning beds, having been missed at the time of spawning. Most of these larvae were quickly caught via hand nets from the shore or by snorkelling. Interestingly, Amur goby (orange type, Rhinogobius kurodai) increased in abundance during the eradication program and preyed upon M. nigricans eggs. We found no evidence of new M. nigricans spawning beds after 2015, and the last adult was captured in 2018. These results suggest that no breeding of largemouth bass occurred after 2018 and point to functional eradication with no further population growth in the lake. On the other hand, our environmental DNA analysis detected M. nigricans-specific DNA in all four surveys conducted between 2018 and 2022. Therefore, largemouth bass may still persist in or around Kanabara Dam Lake, although the estimated DNA concentrations were very low and we found no sign of increase after 2018. This suggests that, despite the apparent success of eradication efforts, the risk of reintroduction remains and ongoing monitoring is needed.
