Management strategies of translocated pondweed Monochoria hastata and its ecological and economic impacts

Abstract

Understanding the impacts of, and options for, controlling invasive species is crucial to their management. Wetlands are a widely invaded ecosystem, since dispersal of aquatic species is facilitated by seasonal flooding. This study evaluated the effects of the translocated pondweed Monochoria hastata on fish and rice production in two wetlands of Bangladesh over six years (2017-2022). Fish and rice production were compared between control (negligible M. hastata) and three treatments under different M. hastata management methods comprising manual-, herbicide- and mechanical-treatment. Density of M. hastata increased significantly in all treatment groups over time in both wet and dry seasons. However, M. hastata density was lower by 270% in the dry season than the wet season. For fishes, a negative relationship between M. hastata density and fish production was recorded for snakeheads and catfishes, the most saleable fishes, whereas a mixed pattern was recorded for barbs and minnows across treatments. A positive relationship occurred between the density of M. hastata and production of the most common fish, mud eel, and therefore, the overall fish production increased in all treatment groups. Compared to control plots, rice production was lower in M. hastata infested plot groups. Among the M. hastata infested plot groups, rice production in herbicide-and mechanical-treatment groups was similar but lower than the manual-treatment group. Although manual-treatment plots yielded greater rice production, the weed management cost was also higher. This study provides evidence that translocated M. hastata can be of an invasive nature and impact rice production, not only by reducing yield but also by increasing the production costs through additional management for M. hastata control. Its presence in wetlands in Bangladesh can increase overall fish production due to the overriding influence of increased mud eel yield which has little demand locally but can decrease the species of high demand (e.g. snakehead and catfish). None of the existing control measures are effective in controlling M. hastata. Further research is needed on better management approaches for both agricultural and fish production in areas invaded by M. hastata.