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 eliminate 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.

Lake ecosystem changes have also been monitored to understand the entire impact of oxygenation on the fishery.  Data on lake nutrient levels has been monitored but needs to continue to understand oxygenation’s longer-term potential for decreasing 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 will continue to be monitored.  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.  These stomach content findings will be tested by analyzing diets using stable isotope analysis.

During the 2016 contract period. we will continue to monitor nutrient, phytoplankton, and zooplankton within North Twin Lake and South Twin Lake to develop longer-term trends and annual variability.  Additionally, we will continue to analyze changes in trout growth, condition, and survival related to oxygenation. By continuing our monitoring efforts, we will be able to further evaluate how oxygenation has contributed to our goal of enhancing the Twin Lakes trout fishery.