Predicting the distribution of invasive crayfish (Pacifastacus leniusculus) in a Kusiro Moor marsh (Japan) using classification and regression trees

Summer distributions of the invasive signal crayfish (Pacifastacus leniusculus) were investigated in relation to physicochemistry in a Kusiro Moor marsh and its inflows and outflows in northern Japan. Maximum crayfish abundance and biomass were 1.04 individuals/ m 2 and 3.56 g dry mass (DM)/ m(2) in littoral marsh habitats, and 5.84 individuals/m(2) and 13.48 g DM/m(2) in stream habitats. Classification tree analysis was used to predict crayfish occurrence at 102 sites from all habitats (i.e. littoral marsh, pelagic marsh and stream) while regression tree analyses were used to predict crayfish abundance at littoral marsh and stream sites separately. The classification tree showed that crayfish occurrence was primarily determined by undercut bank volume regardless of habitat identity. When undercut bank volume was < 0.0054 m(3), crayfish were predicted to be absent at marsh sites, but expected to occur at stream sites where pH and water temperature exceeded 6.5 and 14.3 degrees C, respectively. The regression tree using only littoral marsh sites showed that undercut bank volume, followed by dissolved oxygen level, determined the splits of the tree. Crayfish abundance was highest when undercut bank volume was > 0.61 m(3), and moderately high when dissolved oxygen was > 9.09 mg/l and undercut bank volume was < 0.61 m(3). On the other hand, the regression tree using only stream sites showed that water temperature was the major predictor that determined the splits. We discuss the roles of physicochemical factors as limiting factors of the distribution pattern of the invasive crayfish.