Potential Causes of Continental Habitat Shifts of Anguillid Eels Related to Individual Growth and Intra-Specific Interactions

A new realization of the variation in continental habitat shift patterns of anguillid eels has been made possible by otolith microchemistry research.  However, it is still unclear how habitat shifts of eels occur between freshwater and saline habitats and how they affect their continental life history.  In this study, we investigated the effect of habitat use and habitat shifts on individual body growth patterns of two temperate anguillid species (European eel, Anguilla anguilla and Japanese eel, A. japonica) at locations near the center of their distribution ranges.  Statistical mixed-effects modelling analysis for Japanese eels revealed that age and temperature had a significant effect on individual growth, but there was no significant effect of habitat type.  Habitat, together with age and temperature had a significant effect on the individual growth patterns of European eels.  The annual growth rates of European eels were higher for years when habitat shifts from rivers to estuaries occurred, whereas there was no obvious trend in Japanese eels.  These results suggested that large differences in growth potential between habitats for European eels might be compensated for by the occurrence of habitat shifters.  Detailed otolith Sr:Ca analyses of different periods of the Japanese eel found that none of the eels that appeared to move into freshwater early shifted back into the saline habitats, but a large proportion of eels that arrived in freshwater later, did return to the saline habitats.  The results suggested that timing of arrival into freshwater may be an important factor determining which Japanese eels entering freshwater will remain there and which will make habitat shifts back to estuarine habitat.  Therefore, it appears that the growth potential of habitats and/or intra-specific interactions such as arrival timing to the habitat could be related to the subsequent habitat shifts of eels, which would lead to compensation for low growth or avoidance of competition.  The analyses of this study suggest the need to protect both habitat quality and connectivity for the effective stock management of anguillid eels.