By Kim Luke and Brian Williamson
The UC Davis Arboretum is a defining feature of the campus. Students, faculty, and ducks alike all enjoy the waterway that was once a part of Putah Creek. Many organisms call the Arboretum “home”, but one of recent interest is the non-native Common Carp (Cyprinus carpio). Originally native to Eurasia, Common Carp were widely introduced to aquatic environments throughout the USA as a potential sport- and food-fish. Yet sportfishing interest for Common Carp in the USA never took full flight, and while stockings quickly ceased, the species quickly become one of the most invasive animals in the world. Abundant and easy to spot, you may have seen this Arboretum dweller with its beautifully large scales perusing the waters on campus. While It’s easy to see these invasive fish on campus, some people can’t see eye-to-eye on whether their presence is welcome or not. The following excerpts are from two student researchers at the Center for Watershed Sciences, providing two separate viewpoints on the Carp.
I transferred to UC Davis in the fall of 2018, and while my time on campus has been brief, the Arboretum has quickly become my favorite place. It’s the perfect spot to visit when I need a break from studying and homework. Aside from a place for relaxation, the Arboretum also became an outdoor classroom during my laboratory course on “Biology & Conservation of Birds”. I grew to appreciate the Arboretum even more once I learned about the surprising diversity of birds that inhabit this ecosystem. So far, I’ve been able to call the Arboretum my quiet place, my classroom, and now I also get to call it the research site of my senior thesis.
This past fall I began the Carp-Dependent Ecosystem Urgent Management (Carp-DEUM) Project. The Arboretum is a beautiful place, but the water turns an opaque pea-green color during summer. Warm temperatures combined with high nutrient concentrations, especially phosphorus, conspire to produce massive and prolonged algae blooms throughout the waterway. In addition to being unsightly, algal blooms can have negative ecological and health impacts. Harmful algal blooms, or HAB’s, induce sickness and even death for humans, animals, and pets. The blooms have caused such concern on campus that signs were created warning people not to enter the Arboretum water. HAB’s also can trigger mass mortality events, such as fish kills, further complicating socioecological systems. But what does this have to do with Carp?
Carp are ecosystem engineers and can have a dominating influence over water quality when biomass is high. Carp spend most of their time swimming along benthic habitats, and by doing so accomplish two things: 1) they kick up extra nutrients from the substrate into the water column, and 2) they uproot submerged and emergent aquatic vegetation. Previous research in Minnesota lakes reveals that plant cover in aquatic ecosystems exhibits a threshold response to Carp biomass, where past a certain biomass of Carp, the amount of vegetation changes rapidly (Fig. 2). When plant coverage is reduced, and Carp biomass high, large algal blooms are often observed. My Carp-DEUM Project will estimate Carp biomass in the Arboretum waterway and evaluate whether reducing the Carp population within the Arboretum could help to mitigate HAB’s.
Because legacy phosphorus concentrations in sediments throughout the waterway likely remain high, there is a strong possibility that should Carp be removed, aquatic plant coverage would quickly expand. These changes are predicted to be coincident with increases in water clarity overall.
I want the Arboretum to be a hospitable and safe ecosystem for humans and other species, but the invasive Carp complicates management towards these goals. The project is ambitious, and I often feel overwhelmed by all I have to manage. But more than that, I’m excited for an opportunity to conduct this research and learn all I can about Carp and their ecological importance in the Arboretum!
The Arboretum played a substantial role in my choice to attend UC Davis. The idea of having a piece of “nature” – although very obviously constructed and managed – that could be a haven from the stress of school was very alluring. When I moved to Davis the Arboretum became important to me, and I have visited often to ponder, learn, recreate, and feel at ease. I noticed the foot-long fish cruising the murky water soon after arriving in Davis. A decade ago, I marched down to the concrete-lined bank of the Arboretum waterway, rolled a piece of white bread into a ball between my palms, stuck the bread ball on a hook, and cast my line into the water. After an hour of fishing I got my first bite and became acquainted with the power and resolve of the Common Carp. Thus began one of the most intimate relationships I’ve had with another species. After hooking that first beauty, I devoted much of my spare time to the pursuit of Carp. After becoming proficient at capture with rod and reel, I resigned to taking every lunch break to simply observe their behavior. I took many friends on angling expeditions to the Arboretum, several of whom caught their first fish along those concrete banks. Those friends not only gained the opportunity to come into contact with and gain the knowledge necessary to capture Carp, but also to learn about the unique ecosystem that they entered while stalking the fish – a turbid body of water in the center of a human-dominated landscape.
Much of California’s Central Valley was historically a large flooded-wetland with murky water. Heavy alterations to Central Valley waterways and invasive animals have led to the decline of turbid water and rise of submersed vegetation in California. Carp clearly bioturbate substrates, stirring up sediment and nutrients as they go. A goal of potentially removing Carp from the Arboretum would be to create clearer water in which submersed vegetation can thrive. However, with turbid water disappearing in the state, I believe we should cherish the green, muddy, and vegetation-free waters of the Arboretum.
Blooms of algae, presumably facilitated by Carp, can lead to low dissolved oxygen events. Because of this, the only fish species found in the Arboretum are those that can tolerate very low dissolved oxygen concentrations. The suite of species inhabiting the Arboretum is quite unique. If low dissolved oxygen events no longer occurred, new fish species could establish and displace current species.
Among these unique species is the native Sacramento Blackfish (Orthodon microlepidotus). Blackfish are a filter-feeding minnow that rely on plankton as a food source, and are currently quite abundant in the Arboretum waterway. The nutrients that Carp release into the water may be critical for production of food for these fish. Blackfish are in decline throughout their native range, potentially because of decreased planktonic food resources. The facilitation of Blackfish by Carp might be treasured and taken into consideration.
The Arboretum and its waterway provide a beautiful sanctuary for students, visitors, and the Davis community. Its importance to me is tied to the Carp that inhabit it. The reduction of toxin-producing algae in the Arboretum is important, but because Carp may facilitate a valuable type of habitat and unique assemblage of fishes that is rapidly disappearing from the state, other methods of algae control might also be explored.
There is no question that harmful algal blooms are occurring in the Arboretum waterway. It is important to identify the factor or combination of factors causing algal blooms to inform best management strategies to control toxin-producing algae. Carp may cause HABs, and the current study will produce useful information on this question. If Carp are identified as a major problem, then all options for management, including fish removal, might be considered. Yet a future vision of the Arboretum and valued features must be clearly defined with input from all who are invested in the waterway.