Impact of introduced trout on Nesameletus mayfly populations measured using electrofishing
McIntosh, Angus R.
published: Jul 1, 2002
ArtNo. ESP141015403010, Price: 29.00 €
Despite the widespread introduction of salmonid fishes for sports fisheries their effects on recipient invertebrate communities are not well understood. This lack of knowledge is associated with the methodological difficulties involved in assessing the effects of predatory fish on stream invertebrate populations. A sampling technique, involving the use of backpack electrofishing equipment, was used to compare the abundance of Nesameletus mayflies in 15 streams varying in the presence of introduced brown trout (Salmo trutta) and native Canterbury galaxias (Galaxias vulgaris). Nesameletus were induced to drift into a downstream net by applying electric current to 2 m2 of stream bed using a small (12 cm diameter) anode. Density estimates for Nesameletus from electrofishing were on average five times higher than those obtained from Surber sampling. Fishless streams and streams with only Canterbury galaxias contained a biomass and density of Nesameletus that were two to three orders of magnitude higher than streams with only brown trout. When Canterbury galaxias and trout were both present, however, densities of Nesameletus were intermediate to those found in trout versus troutless streams. This could be because 'trout only' streams contained larger trout which were more effective predators. Streams with trout were wider, but were otherwise not consistently different from those without trout so comparisons were unlikely to be confounded by physical conditions. The average size of nymphs in fishless streams was significantly smaller because a new generation of nymphs had hatched in fishless streams, but was absent from streams containing fish. The effects of the introduced fish on Nesameletus populations indicate that troutless streams are important habitats for maintaining regional Nesameletus spp. populations and that further range expansions of brown trout could decrease populations of vulnerable invertebrates.