Stanislaus River. Photo courtesy of FishBio.

IEP ANNUAL MEETING: Understanding Predators to Better Understand Predation

Salmon populations in the Sacramento-San Joaquin Delta have significantly decreased over the past century, with most populations considered endangered, threatened, or species of concern.  Non-native sport fishes, such as striped bass or black bass, are popular sportfish and economically important to the Delta; however, as predators, they create challenges for the recovery of salmon as predation during outmigration is a major factor contributing to the population decline.

Balancing competing interests of non-native fisheries with the need to reduce predation pressure on native species will require novel and flexible management strategies and increased information on sport fish populations.

In response to the Water Infrastructure Improvements for the Nation (WIIN) Act (2016), FIshBio worked with NOAA Fisheries and the California Department of Fish and Wildlife to develop and implement a predator study on the Stanislaus River.  The program’s overarching goal was to identify potential management strategies to mitigate Chinook salmon mortality from predation.  This research into the ecology of these predators is an important step in developing strategies to reduce predation pressure on native fishes.  At the 2023 IEP Annual Workshop, Tyler Pilger, fisheries biologist with FishBio, gave a presentation on the initial results of the study, now in its fifth and final year.

The study

The Stanislaus River is an important salmon-bearing tributary to the San Joaquin River for fall-run chinook salmon, steelhead, rainbow trout, Pacific Lamprey, and other native resident fish species.  Goodwin Dam blocks the fish from upstream habitat, so spawning and rearing occur below the dam.

The migratory corridor and the top of the study area begin around Oakdale.   The diamonds on the map in the lower portion of the river are the 39 sampling sites randomly selected in 2019.  Each site was repeatedly sampled yearly on back-to-back days from February through June.

The study objectives focused on striped bass and black bass, their diets, abundance, and when they were present in the river.  For this study, largemouth, smallmouth, and spotted bass are combined into a single black bass group.

At each location, fish were captured using electrofishing from the bank out to the midline of the channel.  The caught fish are measured, scales taken for aging, and the stomach contents emptied and sent to the lab.  The fish are tagged with PIT tags and returned to the river.

Models were used to estimate the occupancy of striped bass, and mark-recapture methods were used to estimate the abundance and survival of black bass.   By sampling on back-to-back days, models that account for imperfect detection could be used.

Sampling conditions

Each panel on the slide below shows the sampling season for 2019-2022.  The vertical axis is the flow rate (or discharge), and the horizontal axis is the temperature.  The gray bars indicate the sampling events; he noted that some were missed due to COVID or critically high water temperatures.

2019 was a wet year, so there were higher flows, depicted by the blue line, than in 2020, a dry year, or 2021 and 2022, which were critically dry.  Water temperatures also were cooler longer into the season in 2019 than in the other study years.

The circles represent the mean turbidity for the sampling event, with the lines representing the minimum and maximum.  The most turbidity across all years was in 2019, but Dr. Pilger noted that the water was still relatively clear, even during the higher flow events.


The top panel on the slide below are the stomach contents for striped bass; the bottom panel are the stomach contents for black bass.  Each group of bars represents a sample year, and the height of the bars represents the proportion of predators that consumed that particular prey item.  The prey items were grouped into four categories: chinook salmon, lamprey, all other native fish, and all other non-native fish.

Across all these years, both striped bass and black bass consumed more native fishes than non-native fishes,  particularly Chinook salmon and Pacific lamprey.  Black bass consumed a lot of sucker fry and sculpin.

Presence of predators

“As this study progressed, we started seeing some interesting patterns in our data, particularly with striped bass,” said Dr. Pilger.  “We don’t catch many striped bass, but they are there.  In our early sample events, we pick up large striped bass, and then as the season progresses, we get more and more smaller size striped bass.”

On the graph, fish greater than 300 mm in length are shown in blue; those that are smaller are shown in green.

They analyzed the pattern across all years through an occupancy modeling framework.  On the graph below, the circles represent striped bass larger than 300 millimeters, and the squares represent smaller fish.  The x-axis is the date; the y-axis is the estimated proportion of the 39 sampling sites occupied by striped bass.

“At the beginning of our sampling events in each season, we saw a higher proportion of sites occupied by large striped bass than small striped bass,” said Dr. Pilger.  “But then as the season progressed, both sizes classes occupied greater proportions of the river.”

Black bass

For black bass, Dr. Pilger said they used a robust design mark-recapture program that allowed them to estimate abundance within each season by assuming that the population is closed to births, deaths, immigration, or emigration, and to estimate survival from year to year, assuming that the population is open.

The slide on the lower left shows the abundance of black bass.  The confidence intervals were wide in the first year because they didn’t have as many tagged fish, and there weren’t many recaptures.  But as the years progressed, there were more recaptures, and the confidence intervals tightened.  Bass abundance jumped in 2021.

As for survival, they ran a series of models looking at different covariates for survival, such as flow conditions, time of year, and others.  The model that most strongly supported survival was the length at initial tagging, shown in the figure on the upper right.  He noted that the survival rates only are for fish that can be tagged; the smallest size is 80 millimeters.

“What we see across the four years that we have so far is that black bass survival is associated with the fork length at tagging, and it’s quite high,” said Dr. Pilger.  “So even across the range, we’re seeing survival rates around 80% or higher.”

The researchers also measured the fish.  The slide below shows black bass ‘population pyramids’ for each year of the study.  Each bar represents a size in a 10-millimeter size gin, and the width of the bar represents the proportion of the catch in that size bin.

“In 2019, we saw a good chunk of fish that were under 150,” he said.  “We also saw a good chunk of fish between 180 and 300 millimeters – that’s where those population pyramids get wide.  So the fish under 150 survived to a pretty good clip into 2020, as well as those larger fish.”

“Then, in 2021, we saw a huge influx of fish under 150 millimeters, so this meant that there was a pretty substantial recruitment event.  We think that conditions during the summer and fall of 2020 allowed high spawning success for black bass for those fish to recruit and for us to capture them in 2021.  And again, that group of smaller striped bass, age one plus striped bass, survived to a pretty good clip into 2022 which we can see on that last plot.”

He also noted that all black bass over 90 millimeters are capable of consuming Chinook salmon fry.

Summary of results

“Both striped bass and black bass consume more native fishes than non-native fishes, which occurred in both high and low flow years,” said Dr. Pilger.  “The proportions of fish they were eating remained consistent across those different flow years.  All sizes of striped bass that are in the Stanislaus River are capable of consuming native fish.  Black bass are shifting to piscivory within their first year.  We don’t see any evidence of actual recruitment of striped bass in the river; they’re coming in to feed, not to breed.”

By May, the lower migratory corridor is estimated to be occupied 50% or more by striped bass in both low and high-flow years; they are present year-round at some low level because other monitoring programs have documented their presence.  Black Bass are abundant in this migratory corridor and widely distributed.

There was a high survival for fish greater than 80 millimeters, so he said once they get to that age, they will likely be around for a couple of years.

The current hypothesis for recruitment is that it may be linked to flow conditions in the summer or fall, but they are still working to figure that out.


What does this mean for juvenile anadromous fish?

The migratory corridor is a gauntlet of both roving and sedentary predators.  The vulnerability of chinook salmon to striped bass predation increases throughout the season, while the vulnerability of black bass predation persists throughout the year as they are there, year after year.

“What we are hypothesizing right now is that these native fish are vulnerable to predation, regardless of how high the flows are, and what we’re basing that off of the data we have from 2019 to 2022,” he said.  “Now, in 2023, we have very different conditions – we have both high flow and high turbidity.  This is our last year of sampling for this study.  So this will be an important data point to see how high flow and high turbidity influenced the vulnerability to predation.”

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