Trout and suckers of Williams Lake: Understanding how they interact and vary over time

By: Luke Anderson, Fisheries Technician, and Brett Kelly, Regional Fisheries Biologist

Welcome back to the next installment of our Williams Lake blog series. If you haven’t already, check out the earlier parts in the series: Part 1 that describes how we used a picket weir to monitor rainbow trout spawning in the inlet, and Part 2, which details how we partnered with local volunteers to conduct an egg-take to boost production of young rainbow trout. This blog will cover the two netting surveys conducted in 2023.

In 2018, Fish and Game confirmed the presence of bridgelip suckers in Williams Lake after receiving several reports from the public. While they primarily feed on algae growing on rocks, they will also eat aquatic insects, potentially competing with rainbow trout. Due to concern from anglers, Fish and Game fisheries staff then initiated a monitoring plan of the Williams Lake fishery in 2021. 

One portion of the plan involves using a sampling gear called a fyke net to assess bridgelip suckers abundance. Fyke nets use a long lead and two net wings to guide fish through a series of funnels that end in a collection chamber where the fish become trapped. Fyke nets were set for multiple days at the beginning of summer 2021 and 2023 at nearshore locations close to suspected spawning areas of bridgelip suckers.

Fyke net catch rates of bridgelip suckers were noticeably lower in 2023, with less than half that of 2021. Additionally, the sizes of fish were very different between the two surveys. The average length of bridgelip suckers in 2023 was more than 2.5 inches shorter (6.6 inches) than the average length in 2021 at 9.1 inches. The increase in the proportion of juvenile suckers found in 2023 is likely due to the water temperature and timing of the surveys. 

In 2021, very hot and dry weather conditions increased water temperatures in Williams Lake earlier in the season. Even though the 2021 survey took place a couple weeks earlier in the summer than 2023, the water temperature was still about 6 degrees higher in 2021. 

While the difference may not appear to be too large, small changes in water temperature can influence fish behavior and movement, such as triggering spawning behavior. Bridgelip suckers generally spawn in shallow water during the spring when water temperatures reach 46-55 degrees Fahrenheit. So, the differences in water temperature combined with the survey timing likely influenced the number of adult bridgelip suckers found. Specifically, it is likely that netting in 2021 coincided with adult bridgelip suckers gathering in the shallow areas of the lake to spawn where the fyke nets were set. 

In addition to fyke nets, gill nets were used in the fall to examine the overall composition of fish species throughout the lake. During the 2023 gill net survey, we saw an increase in catch of bridgelip sucker and rainbow trout compared to 2021. 

The order of most common species remained the same in 2021 and 2023: bridgelip sucker followed by rainbow trout, then bull trout.  While we observed an increase in all species in the 2023 gill net survey, the increase in suckers was mostly driven by one net that encountered a large school of suckers. This net accounted for 45% of the total sucker catch, increasing the overall proportion. Suckers captured in the gill net survey also spanned a wider size range compared to those captured in the fyke nets, which were primarily juveniles.

As mentioned above, we also saw an increase in rainbow trout catch in our 2023 gill net surveys, though the average length and size range remained steady from 2021 to 2023 (Figure 1). 

In 2023, over half of the sampled rainbow trout were greater than 13 inches. In addition to the increase in catch rates, the average relative weight of rainbow trout also increased from 2021 to 2023. Relative weight is a measure of the condition or health of a fish, given its length and weight. The increase in relative weight means that the rainbow trout are slightly heavier on average than in 2021. 

In both years, the relative weight values we observed indicate that rainbow trout are in good condition, and likely indicates that the competition for food with suckers is minimal.

While these results are optimistic, continued surveys are planned for the next several years to further understand how the sucker and trout populations interact and change over time. Ultimately, this information will help guide any future management decisions. We plan on continuing this blog series, so stay tuned for more results. 

In the meantime, if you catch a fish with an orange T-bar tag, please be sure to report the tag number at tag.idaho.gov or the phone number listed on the tag.