Abstract
The behaviors of some partially migratory salmonids, at both the among- and within-species levels, can be established early - on during their respective lives. For those species that have a plastic early life history in these regards, a variety of biological and environmental factors can result in some juvenile fish in a population to either stay resident within their natal streams or become migratory. This study examines the role of intraspecific competition as a trigger, either to remain resident or emigrate, for YoY (young-of-the-year) non-native (introduced) brown trout Salmo trutta found in a number of populations of selected natal streams in a large catchment in South Island, New Zealand. Brown trout are known to have the genetically-mediated flexibility leading to the potential establishment of a variety of within-population alternatives for migratory life-history outcomes (e.g., stay put, move out of the immediate area, become anadromous). The role of positive feedback loops generated by parents that either stay to live out their whole lives within their natal stream, or leave either to go to other freshwater environments (other streams, lakes) or the ocean, before spawning and producing viable offspring is examined in this study.
This thesis provides strong evidence for the role of competition in initiating downstream movement of YoY brown trout populations. These are mechanisms leading to the establishment of feedback loops between the migratory or resident life-histories of adults and the density of juvenile fish while they are stream rearing. Competition amongst juveniles was assessed by estimating and comparing the demands for energy and space of a number of rearing populations of YoY fish and the capacity of study stream to provide food and space. Stream-carrying capacity was estimated using data on fish-habitat suitability, invertebrate drift, energetic demand and the trophic selectivity of YoY trout.
In the lowland reaches of my study catchment the population densities of YoY fish in their natal streams were generally high. I found the intensity of competition within this category of natal streams, amongst the high-density populations of the offspring of migratory trout, to also be high. Anadromy was the dominant life-history strategy for adult brown trout in these low-gradient, lowland streams. Along the lowland reaches, positive associations between YoY loss rate (driven by both emigration and mortality) and intra-cohort competition, were observed. The intensity of competition amongst YoY cohorts was primarily determined by temperature-mediated energy restrictions rather than territorial limitations. In contrast, in the headwater streams which are inhabited by resident trout, the abundance of YoY juvenile fish was relatively low, and there was no evidence to indicate that competition influenced the loss rate of these fish. Hence, based on the densities, competition and self-thinning of YoY brown trout, this suggests that the majority of the recruitment across the catchment was driven by the high reproductive inputs into lowland spawning grounds from adults with migratory life-history traits. However, an analysis of the recruitment sources of fish using otolith microchemistry indicated that a high proportion of adults reproducing in the coastal part of the catchment originated from the upland part of the catchment. This indicates that populations of anadromous and potamodromous brown trout are likely comprised of individuals originated from throughout the entire catchment. Most notably, the geomorphology of the landscape limits the upstream migration and spawning distribution of large fecund migratory trout due to physical barriers.
These results highlight the genetically-mediated life-history flexibility of brown trout and their ability to shift between either migratory or resident phenotypes at early life stages. This flexibility is sustained even in populations that have originated from a likely-narrow genetic pool of introduced lineages of brown trout. Considering current changes in global-climates, the findings of temperature-driven energetic restrictions on species distributions may be highly important for brown trout management and conservation in New Zealand and elsewhere. The observed flexibility in life-history traits and high straying from natal streams contributes to their ability to utilize a wide range of opportunities across catchments.