Eradication project of the signal crayfish Pacifastacus leniusculus from a small pond in Kushiro City, Hokkaido, Japan, using mesh crayfish traps

Abstract

Twenty mesh traps were set from spring to autumn every year from 2013 to 2015 to capture invasive crayfish (Pacifastacus leniusculus) in a small (820 m²) pond in Kushiro City, Japan. In total, 850 small crayfish (< 30 mm in carapace length), 471 medium-sized crayfish (30-39 mm in carapace length), and 80 large crayfish (>= 40 mm in carapace length were captured and killed. The numbers of captured individuals decreased from 1,205 in 2013 to 162 in 2014 and 34 in 2015. Between 2016 and 2020, crayfish were captured at least eight times each year. From June to September, there were 20 traps in the pond. However very few crayfish were captured (one in 2016, two in 2017, four in 2018, and seven in 2019). In 2020, 16 attempts were made to capture crayfish, but none was captured, although eradication has not yet been confirmed. Size-frequency data in 2013 were analyzed by sex each month. Groups I-IV represented small, small-to-medium, medium-to-large, and large crayfish respectively. In July, females of all groups were captured; males fell into groups I to III. Then, groups II, III, and IV disappeared, with (young) group I crayfish remaining from September. In 2014 and 2015, a few small and medium males and females were captured. Only two large males were captured in 2014 and one in 2015. No small individuals were captured from 2018 onwards. No medium or large individuals were captured in 2020. This study implies that Pacifastacus leniusculus in isolated small bodies of water can be eradicated by long-term trapping using large numbers of mesh crayfish traps.