Disruption of salinity regimes in Mediterranean coastal wetlands and its impact on the coexistence of an endangered and an invasive fish

Abstract

Mediterranean coastal wetlands have been extensively eliminated and degraded by human activities. Salinity regimes are one of the main abiotic factors structuring biological communities in coastal wetlands and can be disrupted outflows from irri- gated fields, especially through dilution. Here, we analyse patterns in salinity re- gimes as well as their temporal trends in coastal lagoons of the Ebro Delta, focusing on the impacts of rice farming. We then analyse the relationships between salinity regimes and the populations of two fish species, the Iberian toothcarp (Aphanius iberus) and the Eastern mosquitofish (Gambusia holbrooki). The former is a globally threatened cyprinodontid endemic to Spain and its decline has been linked to the impacts of mosquitofish. The influence of rice farming in the Ebro Delta has disrupted the salinity regimes of its coastal lagoons, most of which have winter salinity peaks and low values during summer due to irrigation drainage from inundated rice paddies. Active management of these outflows has favoured increases of summer salinity in some lagoons between years 2001 and 2009, but during the same period summer salinity values have de- creased in other lagoons. Between years 2005 and 2009 we sampled fish 55 times in the autumn, within 16 sites, capturing more than 10,000 fish. We related fish cap- tures to the seasonal variation in water salinity. Toothcarp were more abundant in sites with higher summer salinities, i.e. with little influence of drainage effluent from rice fields. Mosquitofish tended to be less abundant when winter conductivities were high, suggesting that the maximum salinity levels during winter act as a filter for the species. The agriculture-induced drop in salinity during summer seems to allow mosquitofish to occupy coastal lagoons. We discuss the interactions of rice culture with natural wetlands as well as the management implications for the conservation of the Iberian toothcarp.