Connection to the sea and flooding mediate the abundance of a threatened cyprinodontiform and invasive mosquitofish in a Mediterranean saltmarsh

The fish assemblages of small Mediterranean coastal lagoons remain understudied, despite their importance for biodiversity and ecological functioning. We sampled the fish assemblages and water features of 21 sites across La Rovina saltmarsh (Aiguamolls de l’Empordà Natural Park, Spain) just one month after an exceptional flooding event (Storm Gloria) and again six months later during the summer drought. Fish species composition and water quality parameters exhibited pronounced spatial variation across the saltmarsh. Fishing gear type (fyke nets vs. minnow traps) exerted negligible influence on fish species richness and community composition. Consistent with previous literature, fish species composition was not well explained by water quality parameters but rather by physiographic variables, particularly connection to the sea and distance from seawater. The threatened, endemic toothcarp ( Aphanius iberus ) and the globally invasive mosquitofish ( Gambusia holbrooki ) were the most abundant species across the saltmarsh and were even more abundant in isolated lagoons. Lagoons permanently connected to the sea exhibited higher fish richness and were the only ones in which several marine species were captured. Following the severe flooding of the saltmarsh caused by Storm Gloria, multiple fish species (notably, mosquitofish, toothcarp, and mullets) colonised numerous sites. However, these species disappeared from several locations by summer, suggesting that the hyperhaline conditions of the hot, dry season act as a severe environmental filter for many species. Notably, toothcarp, mosquitofish, and mullet species even colonised lagoons that dried up (and typically desiccate) in the summer. These findings demonstrate that both toothcarp and mosquitofish colonise new sites when Mediterranean saltmarshes are flooded, and that the genetic, ecological, and conservation implications of this eventual habitat use, alongside the role of connection to the sea, remain underestimated and warrant further investigation.