Ar. Mcintosh et al., SIZE-RELATED IMPACTS OF INTRODUCED BROWN TROUT ON THE DISTRIBUTION OFNATIVE COMMON RIVER GALAXIAS, New Zealand Journal of Marine and Freshwater Research, 28(2), 1994, pp. 135-144
The impact of small (fork length < 100 mm) and large (greater-than-or-
equal-to 100 mm) brown trout (Salmo trutta L.) on the distribution of
the common river galaxias (Galaxias vulgaris Stokell) was investigated
in the Shag River (South Island, New Zealand). Riffle reaches were sa
mpled by electrofishing and their macrohabitat was described in terms
of substratum, flow, elevation, and vegetation characteristics. Galaxi
as densities in reaches containing large trout were lower than reaches
with only small trout or no trout and trout size was the most importa
nt variable determining galaxias density. Densities of galaxias < 80 m
m were reduced in the presence of large trout, but not galaxias > 80 m
m. Reaches with large, small, or no trout varied in terms of water dep
th, percentage cobble, and elevation. However, these differences were
unlikely to account for variations in galaxias densities because selec
tion for these particular habitat features accounted for only a small
proportion of the observed variation in galaxias density. A change in
macrohabitat use by galaxias from fast current velocities in sites wit
hout trout to slower velocities in sites with large trout may be expla
ined by competition for areas of high velocity (potentially the better
feeding areas). However, a combination of interspecific competition a
nd predation by large brown trout is likely to be responsible for the
observed reductions in the density of smaller galaxias.