Ontogenetic habitat shift of Anguilla marmorata and A. japonica in the rivers of southern Japan: Implications for habitat use evaluation and conservation

1. Understanding the habitat requirements of fishes throughout their life cycle is essential for effective conservation. Despite the growing need to conserve anguillid eels, the current understanding of the ontogenetic habitat shifts during their riverine stages remains limited, particularly for the Japanese eel Anguilla japonica and the Indo-Pacific eel Anguilla marmorata. 2. Here, changes in habitat use during the riverine stages of these two eel species were investigated in the southern Japan rivers, where they coexist. Eels were classified into total length (TL) classes at 10 cm intervals, and generalized additive models were constructed using these classes as an ordinal, quasi-continuous explanatory variable. 3. The models showed that the habitat use of the two species changed with growth; both species shifted to deeper and more lentic habitats as they grew larger. Smalland mid-sized (<25 and 25-55 cm in TL, respectively) A. japonica used more lentic habitats with smaller substrates, ranging from sand/silt to cobbles, than small- and mid-sized A. marmorata, which mainly used lotic habitats with cobble and boulder substrates. However, compared with small-sized conspecifics, mid-sized A. japonica mainly used habitats with larger substrates, such as pebbles and cobbles, suggesting that the interspecific overlap in habitat use is relatively high for mid-sized individuals of both species. Large A. marmorata ( = 85 cm) mainly used habitats with smaller substrates as they grew larger. 4. The results suggest that using a fine body size classification and including these classes as a quasi-continuous explanatory variable in habitat models capture gradual changes in habitat use, improving habitat use evaluation of anguillid eels. Preserving the environmental heterogeneity of river habitats in terms of water depth, current velocity and substrate size would facilitate habitat segregation of both conspecifics and allospecifics in the same rivers, thereby promoting the conservation of anguillid eels.