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

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.;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.;Most fish species were similar in abundance in non-rice and wild rice bays during June and August. Aside from yellow perch (Perca flavescens ) in June, however, power was low (0.1 to 14%) for these species abundance comparisons. White sucker (Catostomus commersoni), small northern pike (≤300 mm) (Esox lucius), and three shiner species (Notropis spp.), on the other hand, were significantly more abundant (p = 0.043, 0.033, and 0.021, respectively) in wild rice bays during June. This finding may be related to wild rice being able to provide fish with a complex macrophyte habitat early in the open-water season. In contrast, shiners were significantly more abundant (p = 0.034) in the non-rice bays during August. The near-complete absence of shiners in wild rice bays during late summer may be due to their preference for other lake habitats. Although large northern pike (>300 mm) abundance was similar in both littoral bay habitat types during the open-water season, more individuals were caught in the wild rice bays of all three pairings during June and two out of three pairings during August. The attraction of large pike to wild rice habitats, particularly in June, may be due to the environment being resource rich and able to provide patchy vegetated cover.;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.;The sedimentation rate of one wild rice bay was measured using the sediment profiles of Pb-210 and Cs-137. Rates did not change after the introduction of wild rice. At 204 g m-2 yr-1, the rate of sedimentation was low when compared to rates measured in other Canadian waters. This suggested that surface sediments at the core collection site in the wild rice bay were susceptible to remobilization, likely into deeper areas of the lake.;The decomposition of wild rice straw observed along shorelines may become a potential site for secondary plant succession. This may then result in the loss of potential fish foraging and spawning habitat. Wild rice straw, in the form of floating mats in spring, may also impact fish habitat by blocking the sunlight required by developing native plant species and also by smothering them as the mats later sink to the bottom.;This comparative study investigated the impacts of introduced wild rice on littoral fishes and fish habitat in northwestern Manitoba. Based on the many effects that wild rice may have on fish resources, more scientific work is needed in order for these effects to be better understood. These studies should also be completed in the not-to-distant future as the lifting of a provincial moratorium on new commercial wild rice leases in Manitoba's Northwest region is dependent on their findings.