BY ANDY WALGAMOTT, NORTHWEST SPORTSMAN MAGAZINE
Up until recently, nobody could really say for sure whether the gazillions of American shad annually running up the Columbia River were having any impact on salmon and steelhead, either directly or indirectly.
It absolutely makes sense that shad are, and yet a panel of scientists who did a deep dive into that question a few years ago ultimately threw up their collective hands and said, who the hell knows.
“Their sheer numbers suggest there should be interactions with other fishes and with the birds and mammals that prey on them. However, the limited studies available do not identify clear interactions between shad and salmon or the role of shad in major ecosystem processes,” wrote the 11-member Independent Scientific Advisory Board back in 2021.
Fast forward five years, and that may be changing.
A new research paper says that young shad along with swarms of another invasive species are key parts of a new food web in a portion of the lower Snake River that is benefiting walleye and their ongoing colonization of the system to the detriment of Chinook, lamprey and other native ocean-goers.
“We find that American shad, in combination with Siberian prawn, support introduced walleye and almost certainly magnify their annual predation on salmon smolts,” write four Washington Department of Fish and Wildlife and Oregon State University scientists in the journal Biological Invasions.
Their conclusion stems from a yearlong study of the diet of walleye – a popular game fish that arrived via unknown source to Washington’s upper Columbia River system in the 1940s, ’50s or ’60s and have since spread from there into the Snake – from Little Goose Dam downstream past the mouths of the Tucannon and Palouse Rivers.
Crews fished for walleye here four times a month and caught a total of 524 of the fish, then pumped their stomachs and took a portion of each fish’s pelvic fin for isotope analysis.
Labwork found that the walleye ate a wide variety – northern pikeminnow, suckers, smallmouth bass, crappie, crayfish, mollusks, zooplankton and seven kinds of insects – but Siberian prawns, which are a kind of freshwater shrimp and grow to about half an inch long, along with shad, salmonids and lamprey represented the top four items on the menu.
By percentage, prawns were the most frequently found item in walleye stomachs nearly year-round, ranging from 28 percent in midsummer to nearly 80 percent during the winter months. They likely arrived in the Snake via barge ballast.
Only in late fall was another fish found more frequently than prawns, and that was young shad, representing 40-plus percent of occurrences in October and November.
Salmon and steelhead smolts were found in as many as 40 percent of walleye stomachs in April, coinciding with their downstream migration, while lamprey represented 35 percent that same month.
Isotope samples, which contained the unique chemical traces of each walleye prey species, corroborated the forage analysis.
The work is an example of what’s known as the “invasional meltdown hypothesis,” which a WDFW blog on the paper today summarized as “how the introduction of one or more non-native species make it easier for other non-native species to establish and thrive, leading to a cascade of invasions and a decline in native biodiversity.” In this case, the natives include Chinook, coho, A- and B-runs, lamprey, etc.
WDFW staffers Andrew Murdoch, Matt Polacek and Rochelle Polacek, and Dr. Jonathan Armstrong at OSU’s Department of Fisheries, Wildlife, and Conservation Sciences say that their findings also “challenge current perspectives on the relationship between American shad and salmonids in the Columbia River.”
“There has long been concern that shad could have negative effects on salmon, especially through indirect effects via a shared predator. However, multiple syntheses have found no evidence implying American shad are negatively affecting salmon. This is likely because prior studies on salmon predation were designed to detect direct effects and studied predator diets in spring and summer, when juvenile salmon migrate downstream,” they posit. “By considering empirical data across the full annual cycle, we arrive at a very different conclusion regarding the effects of American shad on salmon.”
They say the “indirect effect” of prawns and shad subsidizing walleye diets until the buffet of smolts arrives “is of serious concern given that American shad now comprise > 90% of the anadromous fish returning to the Columbia River Basin (a recent run exceeded 7-million fish) and they are expanding in both abundance and geographic extent.”
How to fight back and protect ESA-listed Snake salmon and steelhead?
The researchers suggest reducing shad and prawn numbers.
“Shad are the most actionable target because they contribute heavily to walleye diets and migrating adults can be removed at dam fish ladders,” they write, quickly adding, “However, suppression of invasive fishes in the western United States has rarely produced positive outcomes.”
There’s also little to no commercial market for shad, so no incentive for someone to go in with nets. Agency budgets aren’t exactly bursting at the seams for that sort of work either. That leaves anglers drifting shad darts and Dick Nites below Bonneville Dam to try and stem the tide.
The authors do add that walleye in the Columbia and Snake aren’t well studied, so it’s unclear how many would have to be removed to move the needle on salmon and steelhead recovery.
“Developing a comprehensive monitoring program is therefore essential, both to assess management effectiveness and to detect compensatory responses by other piscine predators,” they advise.
Other interesting findings from their paper include a graph that shows walleye passage at Little Goose Dam spiking in recent years, including 1,500 last year. This blog has reported on surging walleye numbers upstream at Lower Granite Dam, leading Idaho fish managers to sound the alarm about their increasing overlap with salmon and steelhead spawning and rearing waters, as well as the formation of a joint-agency work group.
The expansion of walleye up the Snake has also occurred despite state managers lifting daily catch and size limits, the authors also note.
And they compared what’s being seen on the Snake with the Upper Columbia. There, Priest Rapids Dam is difficult for shad to ascend because of how the fish ladders were built, while the lack of locks for barges to go through has kept Siberian prawns out.
“Though more data are needed to robustly evaluate this pattern, it appears that walleye populations are not increasing in areas of the Columbia River Basin where American shad and Siberian prawn are not abundant, lending additional support to the Invasional Meltdown Hypothesis,” they write.
The authors also speculate that the removal of native northern pikeminnow via the sport reward program may benefit walleye by creating a predatory niche.
This situation with lower Snake walleye, shad and Siberian prawns is similar to the story with sturgeon further upstream, which I reported on last fall. Their numbers are declining above Lower Granite, and before he retired, a longtime Idaho fisheries biologist aired his suspicions that rising prawn, opossum shrimp and sand roller numbers were leading to a decrease in sturgeon recruitment.
The natural world is pretty complex, and when you alter it like we have, all sorts of weird things happen. Here, it looks like the surfeit of American shad and Siberian prawns are providing the fuel for walleye to expand their grip in the Snake system, at the expense of salmon and steelhead.
