The Nexus Between Invasive Species and Lake Clarity
During a five-year study, funded by private donors, TERC researchers were able to chronicle a doubling of clarity in Emerald Bay, coincident with the disappearance of the introduced Mysis shrimp and the return of the native cladoceran, Daphnia. This occurrence presented two intriguing questions. First, were Mysis having a large impact on clarity through their intense grazing of Daphnia? The answer to that appears to be yes. Second, is it possible to harvest Mysis to the point where Daphnia can return throughout the lake and help with the restoration of lake clarity. The answer to that is being addressed in an ongoing pilot project.
The project is being funded by the California Tahoe Conservancy and the Nevada Division of Environmental Protection. It entails the location and quantification of the Mysis shrimp in Emerald Bay using echo-sounding technology, and then removing them using a custom-designed trawling net. Prior research has shown that the Mysis only have to be reduced by 50-70% for Daphnia to return.
While this sounds simple enough, the real challenge is presented by the daily, vertical migration that the Mysis undertake. To avoid predation, they spend daylight hours on the lake bottom, and then swim toward the surface at night. Using a Biosonics Echosounder researchers detect the location of a discrete band of Mysis in Emerald Bay close to midnight. The featured image above shows TERC's researcher vessel towing a trawl net precisely the depth which needs to be maintained in order to intercept the largest concentrations of Mysis… at night!
Mysis Shrimp Introduction
California and Nevada State Official Agencies introduced one of the most significant species introductions to Lake Tahoe was that of the mysid shrimp (Mysis relicta) which feeds on algae, ditritus, and zooplankton. Mysis were repeatedly introduced into the lake over a three-year period between 1963 and 1965 with the hope that it would supplement the food supply for kokanee and lake trout to produce better sports fishing for the Tahoe angler (Linn and Frantz 1965). Introduction of this organism was successful in a few other lakes (Dodds 2002); however, it was not successful at Lake Tahoe.
The failure of Mysis to supplement food supply for game fish was due to a lack of understanding of (1) the full dietary role of the Mysis shrimp in large and deep oligotrophic lakes, and (2) shrimp behavior, specifically their nightly vertical migration which reduce their utilization by fishes. Mysid shrimp remain near the sediments in daytime and migrate to upper waters at night. In fact, mysids have been shown to migrate enormous distances (400-500 meters) in Lake Tahoe each day (Rybock 1978). Thus, this species has the potential to couple both profundal (dark, deep benthic zones in lakes that do not allow enough light to support photosynthetic organisms) and pelagic (open water) habitats (Chandra 2003).
Mysis introduction was responsible for a dramatic change in the makeup of the Lake Tahoe zooplankton food web (Richards et al. 1975; 1991, Morgan et al. 1978; Threlkeld et al. 1980; Morgan et al. 1982). By the early 1970s, a significant Mysis population had established (> 300/m2). This freshwater shrimp has been known to prey on other zooplankton; however, due to the limited food sources in Lake Tahoe, this large omnivorous shrimp increased predation on native zooplankton (Rybock 1978) and led to the near-extinction of the zooplankter Daphnia, and important prey species for Kokanee (Cooper and Goldman 1980, 1982). Ironically the introduction of Mysis did not only fail as a food source, it also significantly reduced and important natural prey species. This highlights the need for a science-based component when conducting ecosystem management.