Chapman, M. AnnGreen, John D.Swales, SteveRosenau, Marvin Leslie2026-07-072026-07-071991https://hdl.handle.net/10289/18431The riverine biology of adfluvial-lacustrine rainbow trout (Oncorhynchus mykiss, previously Salmo gairdneri) from the Waimarino, Tokaanu, and Hinemaiaia Rivers was investigated. These three streams are important spawning and rearing habitats for rainbow trout recruiting to an economically important sport fishery in Lake Taupo, New Zealand, yet each has very different bio-physical characteristics; the Waimarino River is an unregulated stream with seasonally varying flow and temperature regimes; the Hinemaiaia River is an hydro-electric flow-controlled stream having a seasonally varying temperature regime and daily flow fluctuations; the Tokaanu River is an unregulated groundwater-fed stream with stable flows and temperatures. Nutrient concentrations and spawning and rearing habitats also vary considerably among these streams. Scales were taken from adult rainbow trout from these streams and examined; they showed both widely-spaced outer, and narrowly-spaced inner, circulus zones. While there were no among-population differences in average circulus-spacing in the outer-growth zone, there were significant among-population differences in the inner zone; these two zones are suggested to represent (common) lake and natal stream rearing, respectively. The back-calculated minimum and average lengths at migration of juveniles from stream to lake was similar for all three populations (ca. 94 mm and ca. 140 mm, respectively) but there were significant among-population differences in age at emigration. The scale patterns and stream sampling suggested that most Hinemaiaia River rainbow trout left the river within the first year of growth (64%) whilst much higher proportions of juveniles surviving to adulthood in the other two streams overwintered before emigrating in their second year (up to 70%). Scales were also taken from juvenile rainbow trout captured from the Waimarino and Tokaanu Rivers. The patterns on these scales corresponded closely with the features seen on the inner-growth zone of the adult scales. This further strengthened the assertion that the inner-growth zone on the adult scales reflects a period of juvenile natal-stream growth before the fish go to Lake Taupo. The utilization of fluvial habitat by juvenile rainbow trout, common bullies and koaro in the Waimarino River was examined using minnow trapping as a methodology. There was segregation of habitat by these three species with regards to one or more of the following variables: 1) water depth, 2) water velocity, 3) substrate size, and 4) instream cover. Except in winter when the habitat choices by fingerlings (45-93 mm fork length) and smolts (>93 mm fork length) overlapped, there was also a segregation of habitat by the various size classes of juvenile trout, including fry (<45 mm fork length). Furthermore, there were ontogenetic trends and larger juvenile trout choose habitats with faster and deeper water and having substrates with larger diameters than did the smaller size classes of fish; larger juvenile trout were also more closely associated with instream cover. Woody and herbaceous cover were generally important for all species and size classes of fish in the Waimarino River. The distributions of juvenile trout and koaro and common bullies in the Waimarino, Tokaanu and Hinemaiaia Rivers was investigated. The species composition varied considerably among seasons, study sites and rivers. More bullies were found in the downstream reaches of these streams and this appeared to be positively related to water column depths and inversely related to velocities. There was a negative relationship between the catch rates of rainbow trout juveniles and koaro. The hydro-electric dam overflow channel on the Hinemaiaia River contained exceptional numbers of koaro. I also examined the inherited variability in parr mark numbers in juvenile rainbow trout from the Tokaanu and Waimarino Rivers. Ripe male and ovulated female trout were obtained from each of the study streams and reciprocal and pure crosses were made from the pooled gametes. The resulting juveniles were reared to fingerling size in separate tanks under similar conditions. The parr marks bisecting the lateral tine on the left side of the fish were counted and there were significant differences between the two pure crosses; the reciprocal hybrid crosses were intermediate in number. This suggests that there was a genetic difference controlling parr-mark number in these fish. The parr marks of wild-caught fingerling rainbow trout from the two study populations were also counted and there were no significant differences in number between these groups but the wild caught fish had intermediate numbers of parr marks compared to the laboratory fish. This suggests that either 1) there were genetic differences controlling parr-mark number between the laboratory and the wild fish with regards to the genetic control of parr mark number in these groups, or 2) population specific environmental influences (e.g., temperature) were modifying the parr-mark number of the genetically different wild juvenile trout, during their embryonic development in their respective natal streams, to produce a similar parr-mark number in these stocks of fish. Genetic differences in agonistic behaviour were also investigated in laboratory incubated juvenile rainbow trout from the Waimarino and Tokaanu Rivers. Because of the small sample sizes involved in this experiment, the results are considered to be only preliminary. However, if the observations reflect the behaviours in the wild populations, this genetic difference is likely to be adaptive and have evolved in response to the very different conditions in these two rearing streams. Finally, embryo and alevin development and survival rates were examined for fish from the Waimarino and Tokaanu Rivers. Two pure and two reciprocal-hybrid crosses were made and the resulting groups were incubated at the same time and under identical conditions at target temperatures of either 12 or 7°C. Within temperatures, the hatching date was the same for all crosses (12°C, 27 d; 7°C, 51 d). However, there were significant among-cross differences, within temperatures, with respect to emergence date; these differences were a result of phenotypic (egg size) and genotypic variability. Alevins from smaller eggs emerged earlier than equivalent crosses from larger eggs; alevins sired by Tokaanu River males emerged earlier than equivalent crosses of Waimarino River males. Samples of eggs were also taken ovulated rainbow trout from each population and the Tokaanu rainbow trout had lower egg weights at standard female lengths. These population differences in egg-size and rates of development are thought to be adaptive and to have evolved in response to conditions encountered by the incubating fish m their respective natal streams. The Tokaanu river has an inferior spawning environment, with lower intra-redd dissolved oxygen levels and poorer quality gravel; thus, selection has produced a population of rainbow trout having small eggs, fast developing alevins in order to maximize survival under adverse incubation conditions. Survival rates from fertilization to hatch in the experimental crosses were above 90% for all groups at both 7 and 12°C; however, embryos of larger eggs had slightly, but consistently, lower survival rates than embryos from smaller eggs, at both temperatures. Likewise, survival from hatch to emergence was also above 90% for all crosses at 12°C; however, there were substantial among cross differences in alevin survival rates at 7°C and this was the result of phenotypic (egg size) and genotypic (population) effects. These differences are also thought to be adaptive and appear to reflect selection resulting from water temperature differences that these alevins would normally encounter under natural conditions in their respective natal streams.enAll items in Research Commons are provided for private study and research purposes and are protected by copyright with all rights reserved unless otherwise indicated.Thesis