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 compared to South Twin Lake. However, the amount of habitat preferred by Rainbow Trout (based on peer-reviewed literature dissolved oxygen and temperature values) did not increase as a result of oxygenation. Using gill nets, archival tags, ultrasonic telemetry, and hydroacoustics, North Twin trout were documented to have shown a positive response to oxygenation by increasing depth distribution, but remained concentrated around the thermocline. To date, no significant increases in growth or condition of Rainbow or Brook Trout have been identified over a single summer, and high angling mortality and movement between North and South Twin Lakes precludes long-term survival comparisons.

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. 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 lakes' pelagic area primarily consumed Daphnia. Negligible changes in Largemouth Bass diets have occurred. From a stable isotope fish diet analysis completed in 2016, trout diets were similar between the North and South lakes. Similar to the gut diet analysis, stable isotope analysis also found a diet overlap between invasive Golden Shiner and trout species that may be limiting trout growth and survival.

During the 2017 contract period, we will continue to monitor nutrient, phytoplankton, and zooplankton within North Twin Lake and South Twin Lake to assess longer-term trends in the lake's response to oxygenation. Additionally, we will continue to analyze changes in trout growth, condition, harvest, and catch rate differences related to oxygenation. Finally, because we are interested in the long-term effects of oxygenation on sediment oxygen demand and potential for reversing eutrophication, we will be monitoring oxygen depletion rates in both Twin Lakes for comparison. 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.