Mugilids Display Distinct Trait-Mediated Patterns with a Reinvasion of Para Grass Urochloa mutica in a Tropical Estuary

Abstract

Aggressive invasions by species such as para grass Urochloa mutica (Forssk.) are pervasive throughout the tropics, but the impacts on estuarine habitats and biota are poorly understood. After weed control was reduced in a tropical estuary, U. mutica reinvaded this area composed of two contiguous habitats: a shallow pond and a deeper river. These habitats are especially important for Mugil cephalus, a culturally prized native mullet, and introduced Moolgarda engeli not targeted in the fishery. We investigated the potential impacts of U. mutica on juvenile and adult mullets by comparing the catch per unit effort (CPUE) of the two mullet species and the physiochemical measurements from the two contiguous habitats during different stages of its reinvasion. Overall, negative effects on the CPUE of M. cephalus and M. engeli were found as a result of the U. mutica reinvasion. The M. cephalus CPUE was consistently higher in the shallow pond habitat compared to the deeper river habitat. It decreased after U. mutica was established, correlative to the invasive grass canopies smothering the shallow habitat favored by juveniles. The M. engeli CPUE significantly declined during the initial periods of U. mutica reinvasion, but did not decline further after the grass became established. No differences in the M. engeli CPUE between habitats were found, indicating that M. engeli are habitat generalists. These findings are among the first in an insular tropical estuary demonstrating a negative relationship between an invasive grass and relative abundances of two confamiliar fish, while revealing distinct patterns attributed to species-specific traits. Importantly, the broad overlapping biogeographic distribution of M. cephalus and U. mutica, and the negative relationship between these species, raises widespread concern for this mullet as a globally important fisheries species.