Introduced wild rice : impacts to littoral fishes and fish habitat in northwestern Manitoba boreal lakes

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Lavergne, Christian S.
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The recent introduction of northern wild rice (Zizania palustris var. palustris) to northwestern Manitoba boreal waters has developed into an environmental concern to regional fishery and fish habitat managers. These concerns are centred on the effects this emergent grass may have on littoral fish populations. Hence, the purpose of this study was to investigate the impacts of introduced wild rice on littoral fishes and fish habitat in that area of the province. This was accomplished by comparing fish communities and several fish habitat characteristics between a bay without wild rice (i.e. non-rice bay) and a bay with wild rice (i.e. wild rice bay) in each of three boreal lakes during 2003. The most noticeable difference in fish habitat characteristics between the non-rice and wild rice bays were the plant communities. Plant species richness, diversity, and evenness were estimated lower in the wild rice bays than in the paired non-rice bays during June and August. Although most of these plant community measure comparisons were not significantly different, the statistical power of these tests was relatively low (33 to 66%). Only species diversity in June and species richness in August were shown to be significantly lower (p = 0.023 and 0.015, respectively) in the wild rice bays. Lower plant species richness and diversity in the wild rice bays may have caused a concomitant decrease in fish species richness and diversity. Other comparative studies within lakes have shown that plant and fish communities of altered (i.e. disturbed) habitats were lower in species richness and diversity than those of unaltered (i.e. natural) habitats. Dissolved oxygen was also apparently different between the non-rice and wild rice bays. From mid to late winter, the wild rice bays had lower levels of oxygen than the non-rice bays in two out of three comparisons during February and in all three comparisons during April. This result was likely caused by the decomposition of wild rice straw on the lake bottom. Furthermore, these lower oxygen concentrations rarely exceeded 1.0 mg L-1, which likely resulted in harsh winter conditions for fish. Dissolved oxygen was also on average 2.3 to 2.4 mg L-1 lower in the wild rice bays as compared to the paired non-rice bays during August. This was again likely due to decomposing rice straw. Depressed oxygen in the wild rice bays during late summer may have only affected fish at night when concentrations were approaching levels unsuitable for fish. Water temperature was similar in the non-rice and wild rice bays from May to August. Although emergent wild rice stands in August were observed to be relatively closed to water circulation, large amounts of incoming solar heat may have been absorbed by the aerial leaves, thus keeping temperatures at and below the water surface relatively cool and at temperatures similar to those measured in the non-rice bays...