Effects of Stocked Trout on Native Fauna of Productive Lakes

  • Author / Creator
    Hanisch, Justin R
  • Salmonids are stocked around the world to create and enhance fishing opportunities. Most research into the effects of trout on lake ecosystems has occurred in alpine and other oligotrophic systems, often naturally fishless, where negative impacts are likely. In contrast, I investigated effects of stocked Rainbow Trout (Oncorhynchus mykiss) on native fishes, invertebrates and food webs in productive lakes in the boreal foothills of Alberta and examined mechanisms that may act to limit trout impacts. I used a variety of approaches, including whole-lake comparisons, Before-After-Control-Impact (BACI) analyses, and mesocosm experiments. Unlike results from alpine lakes, I found few negative effects of trout on native species. In Chapter 2, I showed that stocked trout were dietary generalists and consumed a variety of prey taxa across multiple trophic levels, not dissimilar to the native fishes, and the structure of food webs in stocked lakes was generally unaffected by trout relative to unstocked lakes. Stable isotope analysis revealed that many native organisms primarily inhabit littoral areas and may therefore benefit—passively or actively—from the predation refuge provided by dense aquatic macrophytes. In whole-lake comparisons, trout did not affect the abundance or lengths of forage fishes, but both adult and young-of-year dace (Chrosomus spp.) were captured more frequently in vegetated habitats in stocked lakes relative to unstocked lakes (Chapter 3). BACI analyses revealed that invertebrate assemblages were likely affected by trout the first year after stocking; however, by the second year, assemblages in a focal stocked lake were similar to those in unstocked controls (Chapter 4). My mesocosm experiment demonstrated that native fishes will adjust their use of macrophyte beds in the presence of trout to avoid open water habitat (Chapter 2) and that macrophyte stands and occasionally small-bodied fishes can shape the community composition and taxon-level abundance of invertebrates (Chapter 5). Indeed, the “pre-structuring” effects of forage fishes and macrophytes may limit the negative impacts of trout in my lakes compared to lakes that lack these features. My results demonstrate that stocked trout do not always negatively affect native fauna and suggest that native forage fishes and/or dense beds of macrophytes buffer against impacts of trout. Although the precautionary principle should be applied before introducing a species into a novel ecosystem, some systems, such as these isolated productive lakes, appear to have the capacity to support introduced trout with few effects on native forage fishes and invertebrates.

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  • Degree
    Doctor of Philosophy
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    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.