By Kim Luke, John Durand, Rachel McConnell, Aaron Sturtevant, Nina Suzuki, Andrew L. Rypel

This spring, the Carp-Dependent Urgent Management (Carp-DEUM) Project began its first round of sampling in the UC Davis Arboretum before the Covid-19 lockdown. The project has two planned phases; a population estimate of common carp (and other arboretum fishes) in the Arboretum and a subsequent carp exclosure experiment. We want to know if removing carp can improve water quality and reduce harmful algal blooms, HABs. Carp are widely known to bioturbate sediments where previously deposited nutrients like phosphorus are bound (see YouTube video below). Re-suspension of phosphorus by carp leads to HABs, creating an interesting link between fish and human health. At the same time, this exercise also provides an opportunity to evaluate the unique fish community and limnological conditions within the Arboretum. 

Challenges in the Arboretum 

The Arboretum is the former pathway of Putah Creek, before the creek was diverted to the south of town in 1871 as part of bringing the railroad through. The channel has been heavily re-shaped by humans, making it a challenging place to sample. Natural banks have been replaced by concrete banks with steep, landscaped sides. However, a series of weirs on the eastern end of the Arboretum allow for periodic draining of parts of the Arboretum, which in turn present opportunities for studying fish. During spring this year, when the weirs were open, we prevented fish from escaping sampling sections with a block net, drained the water to a wade-able height and used a beach seine to sample every possible fish in the three middle sections (Figures 1 and 2). Of the five available sections, the three middle were chosen for initial sampling due to their relatively straight and narrow form.

Seining Methods

Once the sections were drained and the water level lowered, we began seining. A beach seine is a long net pulled manually by two poles (or brailers) on each end. The top of the net has a line of floats and the bottom a line of weights. We pulled the net along the channel from the lower weir to the upper weir, and rolled the seine up until it was flush against the concrete bank (because there is no beach to land it on). We then lifted the net from the water, and transferred the live fish into a large aerated container of water. We seined the lowest section just once because of the quantity of detritus and other debris on the bottom. The middle section was seined four times and the upper section five times in an effort to catch most of the fish present.

What We Found

We found an abundance and diversity of fish in these sections, as well as turtles, crawfish–and even a cash register! The predominant fish species found in these sites were 1058 Fathead Minnow (Pimephales promelas), and 743 Black Bullhead (Ameiurus melas). We also seined 358 Mosquitofish (Gambusia affinis), 149 Green Sunfish (Lepomis cyanellus), and 263 native Sacramento Blackfish (Orthodon microlepidotus) (Figures 4). A few carp were found in two of the three sections, and a goldfish was found as well. We found fewer carp than anticipated, though we still suspect they are an abundant fish, as they are easily spotted in many other reaches of the Arboretum. The weir sections might make it difficult for carp to pass upstream, limiting the accessibility of carp to these sections compared to lower parts of the Arboretum where more carp are typically seen.

One particularly interesting species found in abundance was the Sacramento Blackfish. Like Common Carp, this species also belongs to the cyprinid family, but unlike carp, they’re native to the Sacramento and San Joaquin watersheds. Sacramento Blackfish prefer the warm, turbid water of off-channel floodplain habitats that once dominated the Sacramento Valley in spring and summer. As adults, they eat algae and organic matter floating in the water–widely available in the Arboretum in summer. Given its murky waters full of algae, the Arboretum is an ideal habitat for Sacramento Blackfish. While preventing HAB’s and improving water quality is our goal, we will need to consider how removal of carp could affect the habitat of a native fish with a declining population.

What’s Next?

Based upon our preliminary work, we intend to sample other sections of the Arboretum, potentially with different methods. The larger, more open areas in the lower west end of the Arboretum appear to host many more carp, which can often be seen by people above water cruising, rooting and feeding. Because it may be even more difficult to sample these sections with a seine, we will use fyke nets, which are a kind of modified fish trap made from netting to conduct mark-and-recapture experiments. Once we have a better estimate on the total carp population and biomass, we will move on to the second part of the project, installing exclosures, removing carp from exclosures, and monitoring the change in vegetation and water quality in the exclosures and control plots. Given the shutdown in response to the Covid-19 pandemic, further sampling is postponed for now. We will resume once we can do so safely. In the meantime, the western end of the Arboretum can be a safe place for exercise, fresh air and carp-viewing during this time. If you visit the Arboretum, be sure to wear a face mask, and maintain a safe social distance from other human visitors of at least 6 feet.

Kim Luke is a junior specialist at the Center for Watershed Sciences at the University of California, Davis. John Durand is a Research Scientist at the Center for Watershed Sciences. Rachel McConnell and Aaron Sturtevant are graduating from UC Davis and are researchers at the Center for Watershed Sciences community. Nina Suzuki is the Waterway Steward at the UC Davis Arboretum and Public Garden. Andrew Rypel is an associate professor of fish ecology and the Peter Moyle & California Trout Chair at UC Davis and an associate director at the Center for Watershed Sciences.

Further Reading

https://californiawaterblog.com/2020/03/08/contemplating-the-carp/

Lake Wingra carp removal – https://lter.limnology.wisc.edu/research/research_highlight/water-clarity-responses-carp-reduction-shallow-eutrophic-lake-wingra

Lake Kegonsa carp removal – https://bit.ly/2YFRPhW

The “Carp Cannon” – https://kstp.com/news/u-of-m-researchers-experiment-with-carp-cannon-to-stop-spread-of-invasive-fish/5370984/

Carp Removal Studies

Lathrop, R. C., D. S. Liebl, and K. Welke. 2013. Carp removal to increase water clarity in shallow eutrophic Lake Wingra. Lakeline 33:23-30.

Carp and Macrophytes

Miller, S., and T. Crowl. 2006. Effects of common carp (Cyprinus carpio) on macrophytes and invertebrate communities in a shallow lake. Freshwater Biology 51:85-94.

Carp Effects on Water Quality

Bajer, P. G., and P. W. Sorensen. 2015. Effects of common carp on phosphorus concentrations, water clarity, and vegetation density: a whole system experiment in a thermally stratified lake. Hydrobiologia 746:303-311.