No Project associated with this Finding

Who is living in the lake?

Fishes and signal crayfish (Pacifastacus leniusculus) are the dominant mobile consumers found in Lake Tahoe’s nearshore. Assessment of Lake Tahoe’s nearshore fish and crayfish community should be a useful metric to detect changes in community structure and measure nearshore biological health and integrity. All native forage fishes (e.g. Lahontan redside- Richardsonius egregious and speckled dace- Rhinichthys osculus) utilize Lake Tahoe’s nearshore zone for food, coverage and habitat for spawning (Beauchamp, Byron, & Wurtsbaugh, 1994; Ngai, et al., 2010). These native fishes represent an important food source for various sport fishes (e.g. lake trout- Salvelinus namaycush) in the lake (Miller, 1951). In marinas and embayment, unintentionally introduced nonnative fishes are also found. Establishment of these fishes has virtually eliminated native cyprinid population from some areas of the lake, suggesting that lake-wide establishment of these nonnative fishes can significantly impact the native biota of Lake Tahoe (Kamerath et al., 2008). Introduced into Lake Tahoe as early as 1885, signal crayfish is currently the dominant benthic species in the lake (Abrahamsson & Goldman, 1970). In other lake systems, crayfish production often exceeds the production and consumption of all other benthic invertebrates combined (Momot, 1995; Whiteledge & Rabeni, 1997). A polytrophic feeder (Lodge, Kershne, Aloi, & Covich, 1994), crayfish can also affect the flow of energy and nutrients, often having positive and negative impacts on both algal production and benthic invertebrate production and diversity (Flint & Goldman, The Effects of a Benthic Grazer on the Primary Productivity of the Littoral Zone of Lake Tahoe, 1975; Light, 2003). In addition, data collected by Kamerath et al. (2008), suggests that crayfish is a major food source for nonnative warmwater fishes in Lake Tahoe. Given its longevity and dominance (conservatively estimated at 8 million lbs, Chandra et al. unpublished) in Lake Tahoe’s benthic community, crayfish likely plays an important role in ecological function of the lake.

Signal Crayfish (Pacifastacus leniusculus) were introduced into Lake Tahoe as early as 1885, and was established by 1936 (Abrahamsson & Goldman, 1970). Crayfish are currently the dominant benthic species in the lake and are conservatively estimated at 8 million lbs (Chandra et al. unpublished). Recent investigations of crayfish ecology and subsequent increases in their population suggest this consumer is competing and preying upon benthic invertebrates at the bottom of the lake.

Fish Monitoring Efforts in Lake Tahoe

In 1991-1994 and 2008-2009, the predominant fish species caught in the nearshore minnow traps from three sampled locations (North Stateline, Sunnyside, and Meeks Pt/Sugar Pine Point) were Lahontan reside shiners and speckled dace. Tahoe sucker, another dominant species found in the 1991-1994 sampling, was not captured in the 2008-2009 sampling. Comparison of native biomass estimates between 1988-89 and 2009 show a general decline in nearshore native fish abundance and distribution. All fish species examined, except Tahoe sucker demonstrated greater reliance in pelagic food source and all fish species have reduced trophic position. This may be attributed to the onset of cultural eutrophication which would shift productivity to the pelagic/open water zone and a subsequent decrease on energetic consumption by native fishes.

Observations suggest that lake-wide establishment of warmwater nonnative fishes has not yet occurred. In 2011, a nonnative warmwater fish control program was introduced. Extensive distributions of nonnative fishes were found at both Tahoe Keys east and west basin. Total of 12,465 non-native warmwater fishes were captured and removed from sampled sites. Species removed include largemouth bass, bluegill, black crappie, brown bullhead, goldfish, smallmouth bass, and golden shiner (Notemigonus crysoleucas) (Figure 18-5).

Where are the Crayfish?

The work of Flint (1975) and that of Abrahamsson and Goldman (1970) demonstrated clearly that crayfish concentrations vary considerably according to substrate type and the degree of local eutrophication. Abrahamsson and Goldman (1970) related crayfish size and distribution to substrate type and local nutrient levels. For example, a very stony substrate off the Coast Guard Station at Lake Forest provided good cover against predation resulting in high densities, a shortage of food, and stunted crayfish. Crayfish are widely distributed around the periphery of Lake Tahoe and comprise the bulk of the benthic biomass in the littoral zone with seasonal dynamic of movement and migration across depths (Figure 18-10). Both Flint’s research, Abrahamsson and Goldman (1970), and research from UNR today (Umek and Chandra, unpublished) from 2008-2010 suggests maximum densities occur at depths from 10 to 20 meters with rapid declines at depths greater than 40 meters, even where the bottom substrate appeared suitable. Not as many crayfish occur in shallower waters (<10 meters) possibly due to stronger predation, high light intensity, which inhibits the production of attached algae, a major food source, and the infrequent wind driven currents. None-the-less, 97 percent of the adult crayfish collected were found between the shoreline and 60 meters. They suggested that the decline at depths over 40 meters arose because crayfish eggs do not hatch in the cold temperatures at such depths during summer months. Flint identified decreasing water temperatures and sunlight as the major stimuli causing the population to migrate into deeper water to about 90 meters.

How Many Crayfish are There?

Some idea of the enormous abundance of crayfish in Lake Tahoe was revealed by Abrahamsson and Goldman (1970) and Flint (1975). Using trap catch data for the 0-40 meter depth zone, the former study estimated the size of the breeding population at 55.5 million individuals with a standing crop of 2,425,000 lbs. Flint (1975) also used traps and generated a population estimate of 375,700,000 individuals over 2.3 inches for the entire littoral zone. Chandra et al. (unpublished) suggested crayfish populations may now be approaching a conservative estimate of 220 million individuals and over 8 million lbs. Crayfish catch/trap data suggest that the population fluctuates, but generally increased over time, however they have increased in the last 20 years from 10 ind/trap (1991) to 32 ind/trap (Umek, personal communication) (Figure 18-11).

Recommended Monitoring Plan

Monitoring the density and spatial distribution of crayfish and can be used to infer their impact on the benthic biological community and their role in facilitating the establishment and spread of nonnative fishes in Lake Tahoe. An assessment of lake wide crayfish density should be made on an annual basis in seasonal intervals. To establish an annual estimate of crayfish, they will be monitored seasonally four time periods throughout the year (January, May, August, and October). Sampling locations are selected to correspond with sites for the minnow surveys, with two additional sites (Crystal Shore West Marina and Tahoe City Marina). Selection criteria used for selecting these sites are similar to criteria listed for the minnow trap surveys. Selected sites encompass unaltered habitats, along with marinas. For nonmarina locations, the collection of data for this parameter can be combined with minnow trap surveys, as similar sites and sampling time are selected and the sampling gear and methods used are identical. For marina locations, because depth gradients are difficult to capture inside marinas, six trap sets should be placed inside each marina at various locations. Catch per unit effort is calculated as the number of crayfish caught in each trap divided by time of trap deployment. For each location the size structure of each population based on carapace length and the body condition of the crayfish population (length versus weight regression) is be calculated. This data can be compared to macro-invertebrate, periphyton, and fish densities collected in similar locations (see other sections).