| Summary: | Knowledge of fish habitat associations in all types of lakes is important for conservation and management to ensure that land developments or other human activities do not have irreversible impacts on lake ecology. Sensitive fish species and aquatic habitats can then be defined and considered in management decisions. One purpose of this thesis was to define fish assemblages with relation to basic habitat types of a typical boreal lake. Sensitive habitats and species were inferred from these fish assemblage definitions. Another purpose was to determine whether boreal fish assemblages are resilient to minor anthropogenic impacts, such as light fishing pressure or small water control structures. Waskesiu Lake was chosen as a typical boreal lake because its fish population structure is similar to other lakes in the mid-boreal region of Saskatchewan and because its water quality and limnological characteristics are similar. Assessments of the nearshore habitat and fish community of Waskesiu Lake were conducted through field studies and literature reviews. Field studies included the examination and description of habitat features of inshore areas of the lake. Fish communities of this nearshore portion of the lake were examined during the two field seasons in summer 1994 and 1995. These assessments indicated that there are diverse, patchily distributed habitats of importance to a fish population composed of several distinct assemblages. Sandy substrate areas were the dominant habitat types within the littoral zone, comprising 69.1% of that area within the 2 m contour interval. These sandy areas supported a predominantly schooling fish assemblage. The most commonly found fish species were juvenile white suckers, spottail shiners, juvenile yellow perch, ninespine stickleback, and fathead minnows. Sandy regions of Waskesiu Lake that were protected from strong wave action often had dense aquatic macrophyte growth and also supported the greatest numbers of fish, both benthivores and piscivores. These quiet areas contained fish assemblages that associated with both emergent and submergent macrophytes. Strongest associations were found with assemblages of brook stickleback, blacknose shiner, and northern pike. These weedy areas are important for juvenile rearing of species such as northern pike and other species that rely on aquatic macrophyte cover for feeding and protection, from predation. Distinct fish assemblages were also found at rocky substrate locations, which cover about 25% of the littoral zone floor. Consequently, fish species that associated with rocky substrate were not a dominant component of the Waskesiu Lake fish community. These species included slimy sculpin, longnose dace, and juvenile burbot. None of these species was ever captured in substantial numbers during the littoral zone surveys. However, rocky nearshore zones are also important for reproduction of other fish species, such as walleye, cisco, lake whitefish, suckers, and trout-perch, as well as slimy sculpin and longnose dace. Northern pike are the dominant (by number and biomass) predator in Waskesiu Lake's littoral zone. They comprised the majority of piscivores in both 1994 and 1995. Walleye were also a sizeable component of the nearshore piscivore community in 1994 but were notably low in numbers in the littoral zone catch in 1995. They were a larger component of the open-water catch in 1995 than northern pike. Walleye may have been foraging in off- shore waters in 1995, in response to the distribution of their prey. However, lack of offshore gill net data in 1994 precludes any solid conclusions regarding distributional changes. A large variation in abundance was apparent for fish populations between years. Substantially lower numbers of most fish species were captured in 1995. However, when lower numbers of fish were captured in 1995, similar species-habitat associations were apparent. Population size variation was also apparent from the analyses of selected fish stomach contents. Yellow perch were very abundant and the dominant forage of both pike and walleye in 1994, although not the dominant small to medium body-size fish in either survey year. White sucker and spottail shiner did not comprise substantial proportions of piscivore diets in either year, although they were the most abundant potential forage fish in gear catches in both survey years. This may have been a reflection of survey timing. Ninespine sticklebacks were forage for not only walleye and northern pike in both survey years, but also for yellow perch and cisco. The small relative body size (in comparison to perch) of ninespine sticklebacks decreases their value as important forage for large fish but they would likely supply valuable energy requirements for young northern pike, walleye, and for large yellow perch. Based on a literature review, changes in fish community composition, for both the nearshore small fish species and for the off-shore large fish species, were not apparent from early studies to this study. Given the diversity of habitat in Waskesiu Lake, and the built-in resilience of most boreal fish species, little change in the fish community assemblages was expected from pre-park years to the present. However, the apparent predominance of white suckers in the 1994 and 1995 fish catches indicates that there may have been an effect on the piscivorous populations from either recreational fishing pressure or emigration loss; or perhaps a combination of the two. Consequently, there have been no dramatic changes in Waskesiu Lake fish populations over the past 70 years. Even rare taxa have not been impacted over time. Species that may have been effected by minor perturbations, such as reductions in usage of spawning streams (i.e., Kingsmere River, Mud Creek, and Waskesiu River), have likely been able to compensate for losses by spawning in the lake or by adapting in some other way. Consequently, the results of this study indicate that boreal fish assemblages in the general region of Waskesiu Lake are resilient to minor perturbations. However, lakes in the region should still be managed carefully and considered on a lake by lake basis.
|
|---|