Is salinity choice related to growth in juvenile eel Anguilla anguilla?
Résumé
Microchemical otolith studies show that some individuals never colonize freshwater (FW) habitats and settle in saltwater (SW) as soon as the glass eel stage. In order to check if these divergent migratory behavior could be determined by the salinity factor, glass eels (Anguilla anguilla), caught at the entry of the Gironde estuary (France), were tested for their salinity choice just after capture. The individuals having made a choice were kept unfed in brackish water for a few days before being tested a second time for their salinity choice. Thus, four different salinity choice groups were created: freshwater-freshwater (FW-FW), freshwater-saltwater (FW-SW), saltwater-freshwater (SW-FW) and saltwatersaltwater (SW-SW). Each salinity choice group was reared for 66 days half the eels in FW and half the eels in SW. They were fed daily and individually measured and weighed every 15 days. The feeding behavior was more intense in SW, regardless of salinity choice. This induced a larger average growth in SW than in FW. Because of cannibalism, a high mortality of small individuals was monitored in SW. in SW, the SW-SW choice group showed a significantly larger growth than the FW-FW choice group. It is concluded that the higher growth rates observed in estuarine and (or) marine populations may be related to the action of salinity on the feeding behavior of the eel and not to the food availability of the habitat. Moreover, differences in salinity tolerance between individuals could explain the settlement in SW habitats in a small part of the glass eel population. Nevertheless, salinity tolerance is probably not the only factor explaining the colonization of SW habitats by the eel.