Bullhead Removal for Trout Conservation

Introduction

Image Details

The West Fork San Luis Rey River (WF San Luis Rey), located just north of San Diego, California on Palomar Mountain, is home to the southernmost population of native Coastal Rainbow Trout (Oncorhynchus mykiss irideus). Coastal Rainbow Trout exhibit high life history diversity as they can spend their life cycles entirely in freshwater systems like lakes and streams, or migrate to sea as juveniles and return to freshwater for spawning. This process is known as anadromy, and these sea-run forms are known as Steelhead. Often, freshwater residents are found upstream due to isolation from a natural barrier or dam structure structure
Something temporarily or permanently constructed, built, or placed; and constructed of natural or manufactured parts including, but not limited to, a building, shed, cabin, porch, bridge, walkway, stair steps, sign, landing, platform, dock, rack, fence, telecommunication device, antennae, fish cleaning table, satellite dish/mount, or well head.

Learn more about structure , but resident Rainbow Trout can also co-occur with Steelhead. In California, Coastal Rainbow Trout greater than 16 inches in length are classified as Steelhead for management reasons, and are a highly desirable fish for anglers. Additionally, the WF San Luis Rey drainage supports western pond turtles (Actinemys marmorata), arroyo toad (Anaxyrus californicus), and the southernmost population of native Arroyo Chub (Gila orcuttii) in the lower reaches.

The WF San Luis Rey is a high gradient system that flows from high, fishless meadows on Palomar Mountain to low flow, boulder channels further downstream. Palomar Mountain receives more rain than anywhere else in San Diego County, averaging around 24 inches of rain per year. As such, the WF San Luis Rey can have large flows over short durations from rain storms. A natural waterfall separates the upper section from the rest of the WF San Luis Rey, and also blocks Coastal Rainbow Trout passage to the upstream meadows. Below this is a large pool with year-round water nearly 12 feet deep that is the furthest upstream point for Coastal Rainbow Trout. The waterfall stops flowing in early-mid summer, although a small crack supplies water to support perennial pools downstream. The middle section goes mostly dry during late summer, containing both intermittent (hyporheic sections) as well as spaced pools. Pools are bedrock-boulder dominated with large cobbles, sand, undercut banks, and some woody debris that provides cover for aquatic residents.

In 2007, non-native Black Bullhead (Ameiurus melas) were found in sections of the WF San Luis Rey downstream of the natural waterfall, causing concern by managers for potential impacts on Coastal Rainbow Trout and other native aquatic species. Bullhead were mostly concentrated in slow moving waters, with about 85% estimated in perennial waters. Catfishes in general are often difficult to eradicate in introduced systems, and early efforts to remove bullhead were not successful. However, biologists from the California Department of Fish and Wildlife re-focused their efforts on bullhead removal from 2016-2018, and achieved complete bullhead eradication within this stream. 

Key Issues Addressed

Black Bullhead, native to the central United States, have high invasion potential as they can tolerate a wide range of environmental conditions including high temperatures, high salinity, and low dissolved oxygen levels. They prefer slow-moving waters like ponds, small lakes, and backwaters in streams and rivers with warm, turbid conditions and soft bottoms. The West Fork San Luis Rey River contains a unique assemblage of native fishes and amphibians that are susceptible to negative impacts from non-native Black Bullhead including direct predation, competition for resources and food, and habitat alteration. In California, bullhead introductions leading to established populations have occurred in waters such as farm ponds and stock tanks, reservoirs, and altered lower sections of rivers. Bullhead can alter habitat by increasing turbidity, which in turn, impairs visual feeding ability of predators like Coastal Rainbow Trout. Bullhead hide under debris and along the bottom during the day, and are most active when feeding at night.

Black Bullhead were first documented by CDFW staff in 2007. They were suspected to have washed downstream to Coastal Rainbow Trout habitats from private stock ponds in the higher elevation meadows, and had established populations in intermittent pools. Early efforts worked with the local landowners for access to stock ponds, however in recent years, project managers have not secured access to fully remove bullhead from their source in stock ponds. Access to the remainder of the WF San Luis Rey is from National Forest lands and private access through Vista Irrigation District in downstream sites. Site access is logistically challenging as gear must be carried several miles from access points to stream sites.

Project Goals

• Eradicate non-native Black Bullhead
• Use bullhead trapping data to estimate the original size of the Black Bullhead population
• Monitor and protect the southernmost native population of Coastal Rainbow Trout in North America 

Project Highlights

Bullhead Eradicated: After removing over 1,300 Black Bullhead during the first year of surveys in 2016, project members were astonished to catch no bullhead in the following surveys in 2017. No bullhead were detected through 2019.

• Field Coordination: Project managers devised a coordinated plan to sample 16 total perennial pools throughout the WF San Luis Rey during multi-day trips. In addition, project managers needed standardized effort metrics by a field crew (catch per unit effort - CPUE - per trap) to meet assumptions for statistical population modeling.  
• Bullhead Population Estimate: In 2016, 241 total traps were set, mostly in 16 perennial pools, across four trips from August 30th to October 14th to ensure water levels would be at their lowest, forcing fish into these deeper pockets of water. In total, 1,315 Black Bullhead were captured and removed over approximately three river miles. Using this collected count data, sampling effort, and statistical modeling, project managers estimated the total Black Bullhead population prior to removal efforts at 1,361 fish. CPUE for Black Bullhead rapidly decreased with each subsequent sampling trip during 2016. No bullhead were caught in subsequent sampling trips from 2017-2019. This trend was visually supported with snorkeling surveys in 2017, as project members did not see any bullhead (compared to multiple observations before setting traps in 2016). 
• Native Species Conservation: Visually, snorkeling surveys revealed more Coastal Rainbow Trout in 2017 than in previous years. However, a high intensity rainstorm in  2017 flushed out habitat occupied by Black Bullhead, likely also to the benefit of Coastal Rainbow Trout. Further sampling and analysis needs to be conducted to determine how Coastal Rainbow Trout have responded at a population-level to mechanical bullhead removal and the possible confounding contribution of increased flows on both bullhead removal and Coastal Rainbow Trout habitat enhancements
• Non-Native Aquatics Treatment Techniques: The project team utilized Promar collapsible minnow traps (60 by 30cm, with dual 14cm openings and 1cm bar mesh -- similar to tighter meshes used in crayfish traps). Collapsible Promar traps are easier to pack than other trap types, as each crew member could carry 12 traps, bait, and backpacking gear for multiple night surveys. The project team found approximately 15 pieces of tuna-flavored cat treats (Temptations brand) as the most effective form of bait. They carefully placed traps into the deepest spots of each pool to soak overnight for capturing nocturnal bullhead. Traps were removed the following morning in the order they were set to ensure standardized effort. Black Bullhead were counted and euthanized on site. 18 Coastal Rainbow Trout were captured in total and released, while non-native bullfrogs (both adults and tadpoles) also captured in traps were removed from the stream. Field crews always included three people for multi-night sampling trips. For each trip, the field crew hiked in from the upstream site (at the natural waterfall) and began setting traps at 3 p.m. while hiking downstream to deploy all traps into the evening. They set up camp midway, and continued downstream setting traps. This strategy reduced the number of upstream and downstream trips between setting and removing traps. Early snorkeling surveys did not reveal many bullhead present during the day, so field crews stuck with setting traps overnight.

Lessons Learned

Image Details

Baited minnow traps were the most effective method for capturing Black Bullhead in this system, both for their logistical advantage for packing as well as capture efficiency. Project members tested different baits, and cat treats were the most effective. Because bullhead have a strong sense of smell compared to other fishes, project managers hypothesize that cat treats, especially the tuna-flavored, released odor into the pools that drew out bullhead from their cover in complex substrate. Previous capture methods such as electrofishing, seining, and spearing were not nearly as successful as deploying the fine-meshed traps project managers selected for the 2016 surveys. Backpack electroshockers were ineffective at bullhead removal before 2016 as many of the pools are over 12 feet deep, which is too deep to effectively shock, and complex habitat provided ample opportunities for bullhead to seek cover from seines and spears. 

Project members were shocked to discover that no bullhead were caught in traps each year after their first season in 2016. Following this effort, they initially saw a spike in native Coastal Rainbow Trout in 2017, however this also coincided with a large rain event during the 2016-2017 wet season with over seven inches of rain falling over a 24-hour period. California was in a significant drought from 2012 to 2016, which in the WF San Luis Rey  may have produced favorable low flow and soft substrate conditions for Black Bullhead. High flows from this rain may have assisted project members with bullhead removal as any remaining fish were flushed out of the system. Project managers suspect that the right combination of environmental factors (e.g. long-term drought and this high intensity rain event) and successful minnow trapping enabled Black Bullhead eradication at this time. During low flow events, like during a drought, fine particulate matter settles and builds up in the stream, which creates undesirable habitat for Coastal Rainbow Trout spawning beds. Followed by a good flow year, like the 2016-2017 wet season, increased spawning activity and juvenile survival tends to favor more fish the following year. Project managers therefore hypothesize that perhaps good rain is equally if not more important for native Coastal Rainbow Trout populations than competition from Black Bullhead.

Logistically, project managers strongly recommend knowing the dynamics of the particular drainage being studied. The project team mapped out all of the perennial pools before setting traps to figure out the most efficient way of site access and how to cover the entire survey area in one trip. Because they had to set traps overnight, project members had to through-hike the stream as well as double back to access all pools. Yet, by approaching from the top and setting traps downstream, they minimized the time spent hiking. In addition, project managers stress that field crews are reliable and enjoyable, as in this case, teams spend three days together camping and hiking through remote conditions. Lastly, project members emphasize that if working in this particular drainage, even a 30% chance of forecasted rain on Palomar Mountain likely means that it will rain.

Next Steps

• Continue Annual Monitoring: Continue with yearly monitoring surveys to ensure Black Bullhead do not enter Coastal Rainbow Trout habitats by getting washed downstream during high flow events.
• Remove Bullhead at the Source: Work with a local private landowner to gain full access to his stock pond to completely remove bullhead at their source population. 
• Increase Genetic Diversity of Coastal Rainbow Trout: Ultimately, this Rainbow Trout population is relatively isolated in the WF San Luis Rey and has low genetic diversity (e.g. low heterozygosity). Project biologists would like to introduce new genes to this population to increase overall genetic health through translocations. This would entail introducing additional Coastal Rainbow Trout from the nearest healthy native population (north of Los Angeles) to WF San Luis Rey. Biologists would try to target smaller sized trout for relocation to increase their ability of making the 2 hour rugged and stressful hike with buckets (or backpacks) of translocated fish to stream sites for relocations.

Funding Partner

• California Sport Fish Restoration Act funding

Resources

• California Trout: Coastal Rainbow Trout
• California Trout: WF San Luis Rey Native Trout Protection
• UC-Davis California Fish Species: Black Bullhead
• Barabe, R. (2020). “Black Bullhead Removal from a Headwater Trout Stream in Southern California.” North American Journal of Fisheries Management. DOI: 10.1002/nafm.10470.
• Heritage and Wild Trout Program

Contact

• Russell Barabe, Fisheries Biologist, California Department of Fish and Wildlife, Heritage and Wild Trout Program, russell.barabe@wildlife.ca.gov

Case Study Lead Author

• Alex Koeberle, CART Research Specialist, University of Arizona, akoeberle@arizona.edu

Suggested Citation

Koeberle, A.,L. (2020). “Bullhead Removal for Coastal Rainbow Trout Conservation in Southern California.” CART. Retrieved from https://www.fws.gov/project/bullhead-removal-trout-conservation.