The aim of this study was to investigate population densities and dynamics of brown trout along multiple tributaries of the Taieri River, a large New Zealand river. Relationships between juvenile brown trout population dynamics and food availability, discharge and water temperature were examined in the same lowland stream. Downstream movements of juvenile brown trout were also examined. Trace element analyses of eggs aimed to identify the source of fish spawning in tributary streams. An experimental study of juvenile brown trout collected from downstream and upstream reaches compared patterns of agonistic behaviour amongst trout that were considered to be dominated by either migratory (downstream) or resident (upstream) fish. Longitudinal surveys of juvenile trout abundance were expanded across four additional streams to determine whether there were consistent patterns in population dynamics of brown trout.
Large spatial and temporal variation in population dynamics of juvenile brown trout was found in Silverstream with an upstream site having significantly less temporal variation in densities than a downstream site. No effect of food availability on population dynamics was found. Discharge and temperature affected population densities of juvenile brown trout in some years, but could not solely explain the general pattern in spatial variation in population dynamics. Downstream movement was found to be a behavioural strategy used by juvenile brown trout and appeared to contribute to variation in juvenile density.
Analysis of trace element signatures in brown trout eggs indicated that upstream spawning migration by anadromous females was restricted by the presence of a steep gorge along the middle reaches of the Taieri River. Patterns of spawning by migratory and resident fish along tributary streams appeared to vary. In Silverstream, spawning by anadromous fish dominated the lower reaches, whereas considerable overlap between anadromous and resident fish was observed in Big Stream. In Cap Burn distinct trace element signatures were observed for redds spawned along upstream and downstream reaches. However, it was not possible to confidently ascribe the source of the redds to main river migrants or Cap Burn resident fish.
Behavioural observations of juvenile brown trout collected from upstream (resident) and downstream (migratory) reaches of Silverstream revealed differences in aggression level, with migratory fish being more aggressive. No difference in growth rates was found between fish from the two areas. Furthermore, the social organisation of resident fish was based on the formation of dominance hierarchies, whereas the competitive status of migratory fish was more even.
Patterns in spatial and temporal variation in the longitudinal population dynamics and patterns of abundance of juvenile trout were repeated across the five tributaries. Populations in upstream reaches consisted of fish from multiple age classes and exhibited relatively limited variation in densities, whereas juvenile fish dominated populations along downstream reaches and densities varied greatly over an annual cycle.
Taken together, these results suggest a general pattern of spatial and temporal variation in the population dynamics of brown trout sub-populations along tributaries of a large New Zealand river. Whilst various environmental factors may contribute to some of the variation, longitudinal separation into resident and migratory populations also appeared to be important. These finding are important for the management of brown trout in both main river channels and tributaries in New Zealand.
| Identifer | oai:union.ndltd.org:ADTP/217490 |
| Date | January 2007 |
| Creators | Kristensen, Esben Astrup, n/a |
| Publisher | University of Otago. Department of Zoology |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Esben Astrup Kristensen |
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