The interaction between Anguilla anguilla (Linnaeus, 1758) and Halobatrachusdidactylus (Bloch & Schneider, 1801) and potential consequences of climatic changes

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The distribution of the European eel, Anguilla anguilla (Linnaeus, 1758), includes the Mediterranean and the Atlantic coasts of Europe and North Africa, from Scandinavia to Mauritania. The Lusitanian toadfish, Halobatrachus didactylus (Bloch & Schneider, 1801), is a benthic voracious fish with a distribution ranging from Cabo Carvoeiro (centre of Portugal) to the Gulf of Guinea. Although this subtropical fish is primarily a marine species, in the northern range of its distribution area (southwest of Portugal), due to reproductive thermal constraints in the sea, it colonizes mostly brackish water systems where it becomes dominant. Therefore, in the southwest of Portugal both species occur in sympatry in estuaries and open coastal lagoons. The aim of this work was to analyse the ecological consequences of such sympatry considering the Mira estuary as a case study. Both species showed similar habits and a high diet overlap. Moreover, the Lusitanian toadfish predates on glass eels and yellow eels, which together with its dominance in the estuary may contribute to push A. anguilla specimens to peripheral areas upstream (freshwater and upper estuary) and downstream (Zostera beds near the sea). This spatial segregation reduces habitat availability for eels which may have serious consequences on a population that has already been subjected to an extensive loss of habitat induced by human influence. This situation may become even more unfavourable to eels since subtropical species such as the Lusitanian toadfish may spread northward as a result of raising temperatures caused by climatic changes.