El Fartet (Aphanius iberus) al Baix Ter (NE de Catalunya): Mètodes de monitoratge, ús de l'hàbitat i impacte de la gambúsia

EnglishAs part of a LIFE-Nature project, new habitats (La Pletera lagoons) were created to accom-modate the expansion of the Iberian toothcarp (Aphanius iberus) in a coastal area in the Baix Emporda (Catalonia, NE Iberian Peninsula). They were repopulated in late 2002 with indi-viduals from the natural population in the neighboring Fra Ramon lagoon. Two years later, the mosquitofish (Gambusia holbrooki) was detected in the new lagoons, but not in Fra Ramon. Various passive capture methods to monitor toothcarp populations were compared. Although small-meshed fyke nets were the most efficient method, plastic minnow traps were the only trap type that captured immature individuals, and generally the captured fish covered a wider range of sizes than those caught with fyke nets. The toothcarp positively selected positions near the lagoon bottom and microhabitats with the presence of refuge (e.g. Ruppia stands). While immature individuals selected shallow areas, adults were homogeneously dis-tributed across different depths. In the Fra Ramon lagoon, toothcarp abundance increased from April to September 2005, due to the appearance of young-of-the-year. Contrarily, in La Pletera lagoons, where mosquitofish accounted for 95% of the captures, toothcarp abundance decreased in the same period. These results suggest the existence of reproductive problems and an ecological exclusion process of this toothcarp population related to the increasing mosquitofish population catalaDins d’un projecte Life-Natura es van construir a la costa baixempordanesa nous habitats (lla-cunes de la Pletera) per a l’expansio del fartet (Aphanius iberus), els quals van ser repoblats a partir d’individus procedents de la bassa de Fra Ramon a finals del 2002. Dos anys despres es va detectar la presencia de gambusia (Gambusia holbrooki) a les noves llacunes, pero no a Fra Ramon. S’han comparat diferents metodes de captura passiva per al monitoratge de poblacions de fartet. Tot i que les nanses de malla petita van ser el metode mes eficient, nomes es van agafar individus immadurs amb ampolles de plastic, que, a mes a mes, van capturar un rang mes ampli de mides. Els fartets van seleccionar positivament posicions properes al fons dins de la columna d’aigua, aixi com microhabitats amb presencia de refugis (com ara herbassars de Ruppia). Mentre que els individus immadurs es van trobar sobretot en zones somes, els adults es van distribuir de manera homogenia a les diferents profunditats. L’abundancia de far-tet va augmentar de l’abril al setembre del 2005 a Fra Ramon, per la incorporacio de reclutes de l’any. En canvi, a les llacunes de la Pletera, on les gambusies van suposar el 95 % de les captures, es va detectar una davallada de la poblacio en aquest periode. Aquestes observacions apunten l’existencia de problemes durant el periode reproductiu del fartet i un proces de des-placament ecologic com a consequencia d’un increment de la poblacio de gambusia