Environmental variability and population dynamics of juvenile brown trout (Salmo trutta) in an upstream and downstream reach of a small New Zealand river

Abstract

Population densities, size and condition factor of juvenile brown trout and variation in environmental factors (discharge, temperature, and food availability) were examined over 2.5 years (28 months) in an upstream and downstream reach of Silverstream, a lowland river in New Zealand. Population densities of juvenile brown trout varied between the upstream and downstream reach, with the latter showing considerably higher temporal variation in fish density than the upstream reach. Juvenile brown trout from the downstream reach were larger and had higher condition factors than those from the upstream reach over the study period. Discharge had a negative effect on fish densities in 1 year in the downstream reach, and high water temperatures were recorded in one summer, with possible negative effects on fish densities. However, the different effects of environmental variables on populations between the two reaches could not explain the higher temporal variation in fish densities in the downstream reach as spatial variation in population densities also occurred in years without high discharge events or high summer water temperatures. Variability in food availability over time was higher in the upstream reach, but differences between the two reaches were small. These results suggest that factors other than condition of the local environment are important M07036;

The influence of natural variation in discharge on juvenile brown trout population dynamics in a nursery tributary of the Motueka River, New Zealand

“…Stream salmonids typically seek interstices among coarse substrate elements, or aquatic vegetation, for cover during winter, presumably for refuge from predators and floods while their swimming performance is compromised by cold temperature (Heggenes et al 1993;Heggenes 2006). On the other hand, the Rainy trout population may be inherently migratory, independent of local environmental conditions, as has been suggested for juvenile brown trout in another New Zealand stream (Kristensen & Closs 2008). Nevertheless, it seems counter intuitive for all fish to vacate a stream (which they appear to do in the Rainy River by age 2 ') if habitat and food conditions were sufficient to support residency.…”

Effects of fine sediment addition and removal on stream invertebrates and fish: a reach‐scale experiment

“…For trout diet analysis, the stomach contents of each selected fish were removed and examined under a stereomicroscope (see above). Taxa contributing more than 4% to the total number of invertebrates ingested were considered to be important prey items (Kristensen & Closs, 2008). Invertebrate prey items were identified to genus where possible.…”

“…All food items that appeared undigested and were largely intact were separated, identified and counted. Finally, the total biomass of the invertebrates eaten by each individual fish was calculated using length-dry mass models (Kristensen & Closs, 2008) and expressed as mg dry mass per g of wet fish body mass (to standardise for fish size). To assess the importance of individual prey items, diet composition (the relative abundance of all ingested prey taxa) was calculated following Bonnett, Sagar & Docherty (1989).…”

Within-reach spatial variability of snails and molluscivory by brown trout

“…Potamopyrgus is endemic to New Zealand streams and lakes, where it commonly co-exists with the invader P. acuta (McCarter 1986;Cope & Winterbourn 2004;Kristensen & Closs 2008), which is now in most freshwater systems worldwide (Dillon et al 2002(Dillon et al , 2005. Potamopyrgus is abundant in low-gradient, pasture-dominated catchments with high nutrient and periphyton levels (Quinn & Hickey 1990;Harding et al 1997).…”

“…Clearly, both Potamopyrgus and Physella are eaten by brown trout (McCarter 1986;Sagar & Glova 1995;Kristensen & Closs 2008), introduced mainly from Europe c. 1880 and now widely abundant in New Zealand streams and lakes (McDowall 1990 (10.190.4 cm FL) and eight fish at ageÂ20 months (29.490.7 cm FL) were collected to compare snail consumption patterns between age classes. Immediately after capture, fish were placed in 12 or 16 white plastic tubs (each 80)43)22 cm high), depending on year.…”