Assessment of inland lake coldwater fish habitat in Isle Royale and Voyageurs national parks through analysis of oxygen and temperature data

LeDuc, Jaime; Maki, Ryan; Damstra, Rick; Glase, Jay; Myers, Jared; Romanski, Mark

doi:10.36967/2294181

Voyageurs National Park (VOYA) and Isle Royale National Park (ISRO) are the only U.S. national parks to protect a series of boreal lakes on the Canadian (Precambrian) Shield. Lakes within both parks are home to coldwater fish, including Lake Trout (Salvelinus namaycush) and Cisco (Coregonus artedi). Predictions for future climate scenarios suggest that coldwater fish habitat could be threatened by elevated temperatures, prolonged periods of stratification, deepening of the thermocline, and diminished oxygen concentrations in the hypolimnion. The persistent threat of warming in VOYA and ISRO suggests there is a need for a robust coldwater fish-habitat monitoring program in support of coldwater fish conservation. The objective of this study was to assess combined oxygen and temperature (oxythermal) conditions and the availability of habitat suitable to sustain populations of coldwater fish in Little Trout, Mukooda, and Locator lakes in VOYA, and Richie Lake in ISRO. Continuous water temperature data and both discrete and continuous dissolved oxygen data from each study lake were used to determine the following for Lake Trout and Cisco for each study year: 1) the minimum volume of optimal habitat available, and the percent of the total lake volume this represents; 2) the minimum volume of non-lethal habitat available, and the percent of the total lake volume this represents; 3) the duration of exposure to sub-optimal habitat; and 4) the duration of exposure to lethal habitat. A secondary objective was to consider the merits of using continuous oxygen monitoring to improve assessments of available coldwater fish habitat and oxythermal stress. Oxythermal habitat conditions varied greatly among the four study lakes, ranging from fully suitable in two lakes (Little Trout and Mukooda), in which optimal habitat was available throughout the monitoring period each year, to uninhabitable in one lake (Richie Lake). The band of non-lethal habitat in Locator Lake in early fall appeared to be small and continued increase in atmospheric temperature could exert even greater pressure on the coldwater fish population of this lake. Considering projected climate warming, the variability in oxythermal habitat conditions among lakes and years, and the high conservation value of native coldwater fish populations, continuous oxygen and temperature monitoring throughout the water column is warranted in lakes with populations of coldwater fish in VOYA and ISRO. Adding continuous oxygen monitoring to the existing continuous temperature monitoring will ensure that low oxygen conditions are recorded, including during the late summer and fall period. Late summer and fall are when a constriction of available coldwater fish habitat is apparent but is outside of the time frame of the discrete oxygen measurements made annually during routine water quality monitoring. Monitoring of oxythermal habitat will provide resource managers with essential information to manage for sustained populations of native coldwater fish.

Assessment of inland lake coldwater fish habitat in Isle Royale and Voyageurs national parks through analysis of oxygen and temperature data

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