Use of constrained additive and quadratic ordination in fish habitat studies: an application to introduced pumpkinseed (Lepomis gibbosus) and native brown trout (Salmo trutta) in an English stream

Using fish data from an intensive radio-tracking study over three consecutive seasons of microhabitat interaction between introduced pumpkinseed Lepomis gibbosus and native brown trout Salmo trutta in an English stream, constrained additive (CAO) and quadratic ordination (CQO) were used as a novel analytical approach to determine the degree of overlap and repartition in seasonal microhabitat preference between the two species, and to assess the shape of their preference profiles across a microhabitat gradient defined by depth, velocity, substratum, macrophytes, shelters and debris. Both species showed preference for stream pool habitats with instream cover mostly irrespective of season, indicating the existence of a microhabitat overlap. However, a certain degree of microhabitat repartition also was evident, with narrower preference profiles for introduced pumpkinseed relative to native brown trout. Overall, CAO and CQO represent a logical and useful multivariate extension to available modelling approaches in fish-habitat studies due to their ability to provide for a synoptic representation of response profiles for different species over a composite (latent) variable describing (micro) habitat characteristics.