Ornamental trade as a pathway for Australian redclaw crayfish introduction and establishment

The ornamental trade is one of the major vectors of freshwater animal species translocation worldwide. The Australian redclaw crayfish Cherax quadricarinatus was brought into Singapore through the trade and culture of aquarium specimens during the late 1980s. From 2000 to the present, redclaw crayfish have been observed in at least 3 of Singapore's 13 reservoirs, including 2 inland reservoirs in the Central Catchment Nature Reserve (CCNR). The current paper presents the first in situ population data of feral C. quadricarinatus populations originating from ornamental trade and includes updated distribution records in Singapore based on recent surveys and museum samples. We report a C. quadricarinatus range expansion in 2 additional reservoirs (1 coastal and 1 inland), and the presence of multiple size cohorts confirms the establishment of reproducing populations in 2 coastal reservoirs and 1 CCNR reservoir. This is alarming as the current distribution borders Singapore's oldest and arguably most important nature reserve (Bukit Timah Nature Reserve: BTNR) and encircles the single remaining substantial freshwater swamp forest in the country (Nee Soon). The Nee Soon swamp forest and BTNR have extremely high conservation value as both areas together harbour the richest native freshwater fauna and flora of the island. Therefore, any range expansion of C. quadricarinatus in Singapore may have severe impacts on its native freshwater fauna and demands immediate attention. As C. quadricarinatus is one of the many invasive freshwater species occurring from the aquarium trade in Singapore, we argue that trade control might prevent further establishment of ornamentals.