Rainbow Trout stocked into North and South Twin Lakes had low year-to-year carryover and stocked juveniles did not recruit well to the fishery; little was known about the Brook Trout fishery.  Several years of study identified summer habitat restriction, caused by warm surface temperatures and low dissolved oxygen in deeper cooler waters, as a likely cause of perceived decreases in trout growth, condition, and survival.  The warm surface temperatures were caused by the mixing of the surface waters with the warm summer air temperatures and solar radiation.  The low dissolved oxygen in the deeper water was the result of lake stratification, caused by thermal density differences which eliminates deep-water mixing with the oxygen rich atmosphere, and the high biological and chemical oxygen demand in the deeper portion of the lake.  Twin Lakes were thought to have maintained adequate oxygen in the deeper portions of the lakes prior to anthropogenic watershed disturbances which increased nutrient loading into the lake, thereby increasing oxygen demand.  To restore trout habitat, a diffuse line oxygenation system was installed into North Twin Lake in 2008 and has been fully operational throughout the summer months from 2009 to present with South Twin Lake used as a non-oxygenated reference.  Oxygenation promoted significant increases in North Twin Lake suitable trout habitat while South Twin habitat remained limiting.  Using gill nets, archival tags, ultrasonic telemetry, and hydroacoustics, North Twin trout were documented to have shown a positive response to oxygenation by occupying the habitat it creates.  However, to date, no significant increases in growth or condition of Rainbow or Brook Trout have been identified over a single summer.  Identifying oxygenation’s influence on trout survival has been inhibited by an unknown number of trout migrating between the two lakes via a small channel connection and/or leaving the lakes via an unscreened outlet dam.  In 2014, we will begin to identify trout emigration and immigration numbers to understand oxygenation' s influence on survival while addressing its long-term influence on growth.

Lake ecosystem changes have also been monitored to understand the entire impact of oxygenation on the fishery.  Nutrient concentration data has been collected but needs to be statistically analyzed.  Identifying oxygenation caused nutrient changes will help us understand its potential for decreasing long-term internal nutrient loading that exacerbates oxygen demand within the hypolimnion.  Water quality may become more suitable to trout by decreasing nutrient loading in mesotrophic Twin Lakes.  Changes in nutrient levels can also cause changes within the lake food web.  Moreover, phytoplankton and zooplankton data has been collected but needs to be completely analyzed to identify significant changes in their population dynamics related to oxygenation.  A study completed in 2013 analyzed fish diets by quantifying stomach contents to look at oxygenation's influence on feeding ecology.  Rainbow Trout were found to consume significantly more pelagic large-bodied zooplankton in oxygenated North Twin compared to our reference South Twin Lake where littoral focused amphipods were the primary prey.  Feeding in the pelagic compared to the littoral zone is beneficial to trout because the water temperatures are metabolically more optimal in the deep cooler water.  Brook Trout were found to have higher relative gut weights during August, a time of peak habitat constraint, in North Twin compared to South Twin Lake, possibly due to hypolimnetic oxygenation induced increases in prey access.  Golden Shiner sampled in North and South Twin Lake pelagic areas both primarily consumed Daphnia.  Negligible changes in Largemouth Bass diets have occurred.

During the 2014 contract period we will be conducting a detailed statistical comparison of our nutrient, phytoplankton, zooplankton, and fisheries data between our treatment and reference lakes.  In order to understand oxygenation's impact on lake-wide nutrient levels, we will be conducting an additional large-scale spatial survey of nutrient levels in North Twin Lake, to account for the nutrient mobilization caused by the oxygenation system induced bubble-plume, and comparing the levels to South Twin Lake and North Twin pre-oxygenation.  Comparisons of phytoplankton and zooplankton will allow us to see how nutrient and/or habitat changes are influencing the lower trophic levels.  These analyses will allow us to describe oxygenation's influence on trophic dynamics that affect the fishery.  Additionally, we will continue to analyze changes in trout growth, condition, and survival related to oxygenation.  Installing a PIT tag array to quantify emigration and immigration in Twin Lakes will allow us to conduct a relative survival comparison between oxygenated North Twin and our reference, South Twin Lake, for the first time.  Data on growth and condition will be collected for continued assessment.  Finally, trout population estimates in North and South Twin Lake will be made to identify differences in population abundance.  By conducting this work in 2014 we will be able to evaluate how oxygenation has contributed to our goal of enhancing the Twin Lakes trout fishery.