Striped bass: will eating them all solve the problem?

A few weeks ago, I had the opportunity to help out a labmate with her research on the Mokelumne River (and take a classic fish photo.) Unfortunately, I didn’t catch it angling… an electro-fishing boat caught it. For those who haven’t heard of this, it’s a boat that sends out pulses of electricity through the water to momentarily stun fish, a common way to catch them for fisheries research. Cheating? Perhaps. But the fish aren’t hurt, and it’s a whole lot easier (though less fun) than fishing all day to catch your sample size quota.

Striped bass classic shot

E-fishing boat courtesy of collaborator EBMUD:
the hanging metal cables shock the water

We were up on the Mokelumne, which joins the Sacramento-San Joaquin Delta near San Francisco, to remove all the striped bass from a pool below an irrigation dam. California’s Sacramento-San Joaquin Delta is plagued by many problems, and foremost among them is the conflict over water needs between agriculture and ESA-listed native fish populations, including salmon, steelhead, and Delta smelt. As a result, a lot of effort goes into trying to figure out what is causing high mortality in native fish populations, and what can be done to boost survival. One possible culprit: striped bass.

The last thing the juvenile salmon sees (looking down the striped bass gullet)

Striped bass are native to the Eastern U.S., and were introduced to the west coast as a game fish, a sadly familiar story. (As an interesting sidenote, Wikipedia lists many common local names for striped bass, including striper, linesider, rock... and my favorite, pimpfish). Striped bass eat juvenile salmon (see my post from last October), so one suggestion for boosting salmon populations is to decrease predation pressure by attempting a large-scale striped bass removal, through unlimited fishing allowances. But will this work?

My labmate is looking at an interesting aspect of this question. She is measuring juvenile salmon mortality as the fish move past an irrigation dam, both when striped bass are present in the pool below the dam, and when they are absent (through removal). The idea is to see if salmon mortality drops dramatically when the bass are absent, or if other predators in the system (some of them native, like the pikeminnow) fill the predatory niche, making removal ineffective… or perhaps even exacerbating the problem, since striped bass also prey on some of the other fish that eat salmon. If striped bass are removed, this would release these other predator populations from bass predation pressure as well, and could potentially cause a sharp rise in other predator populations. (For a more complete, interesting discussion of this topic, check out well-known fisheries biologist Peter Moyle’s post on the California WaterBlog).

The fieldwork for this project was quite fun, involving many slippery, spiny fish and a beautiful day in the California sun. I joined in for the bass removal day, so a group of juvenile salmon had already been released through the dam the previous day and mortality measured while striped bass were present. We were there to remove most of the striped bass, prior to another salmon release. To remove striped bass from the pool, an electro-fishing boat made four passes through the pool below the dam, shocking the water and catching a cross-section of the fish community each time. Striped bass key in on pools below dams, where salmon coming through are funneled through a small area in large numbers, providing an easy feast for waiting predators, so this pool is usually full of bass around this time of spring, when juvenile Chinook are out-migrating to the ocean.

The irrigation dam and pool

We took striped bass diet samples to get an idea of what proportion of the bass diet is made up of juvenile salmon. This is done through gastric lavage (aka making the fish barf by flushing them out with water). The only downside to this is that the spines on the dorsal fin of striped bass are surprisingly sharp, and fish are slippery and wiggly. Suffice it to say that I got some war wounds.

Barfing a fish

One striped bass' breakfast

We then returned all other fish species to the pool, but retained the striped bass and transported them to another river later that day. It will be interesting to see the results of this research. However, I doubt very much that removing all striped bass from the Sacramento-San Joaquin system will be enough to cause significant increases in salmon populations. The Delta is a highly altered system—it used to be an extensive wetland, and is now mostly agricultural fields and channelized habitat—so extensive habitat restoration will likely be needed to help restore native salmon populations. However, this is a complex question that deserves its own post. Stay tuned.

But predator removal could well be a part of the solution, and hopefully this study will help us determine whether this is true. Striped bass for dinner, anyone?

The crew (fish and human)

And most importantly... when you're doing fieldwork in the sun all day,
you have to be innovative about how to keep the chocolate snacks from melting

Who’s eating all the fish?

What are the major causes of salmon mortality? Unsurprisingly, this is one of the major questions in salmonid research. Of the thousands of eggs that an individual adult female salmon lays, on average only 1-2 adults return to successfully spawn. Salmon have a complex life history, spanning both freshwater and marine realms; there are many opportunities for them to die along the way, but it’s not always easy for researchers to parse out what’s killing them, and at which life stage.

This week, I helped out one of my lab-mates who’s studying one aspect of this complex question. She’s trying to figure out whether bird predation is a major cause of juvenile steelhead mortality in several small creeks just north of Santa Cruz, California. However, quantifying predation can be extremely difficult—the challenge is not just to show whether one animal is eating another, but also to quantify the predation rate (i.e. what percentage of the out-migrating juvenile salmon population is being eaten?). To do this, she and several researchers at the National Marine Fisheries Service lab in Santa Cruz came up with an ingenious method. A local biologist discovered a PIT tag (a small tag used to individually ID fish) on the nearby Año Nuevo Island, sparking the question: are birds eating young salmon in the creek and estuary and then depositing the tags (i.e. crapping them out) on the island?

The old foghorn keeper's house from the late 19th century

Año Nuevo Island is a beautiful state reserve just off the coast north of Santa Cruz, and provides important breeding and resting habitat for Northern Elephant Seals, California and Stellar’s Sea Lions, Rhinoceros Auklets, Brandt’s Cormorants, and Western gulls. At this time of year, its beaches and rocky terraces are teeming with wildlife—elephant seal pups, abandoned by their mothers and resting until they’re ready to start their own ocean journey, lie in adorable, fat, glassy-eyed piles. Huge droves of California sea lions blanket the beaches as well, barking noisily. And Western gulls add to the relentless cacophony; it’s a place that is at once peaceful and frantic, depending on your mood and ability to filter out the constant noise.

Sea lions and elephant seals blanketing the beach of the island

Wallowing baby elephant seals

We made a research trip out to the island yesterday to search for the PIT tags, heading across the ~1km stretch of ocean in a tiny dingy, banging the sides to scare off over-curious marine mammals. Our mission was to use a PIT tag detector to scan as much of the island as possible (marine mammals permitting)—the detector picks up the individual tag ID if it’s near a tag. Luckily, we didn’t need to actually find or retrieve the tags, since they are about the size of a grain of rice. Data on how many PIT tags are found and the detection likelihood, combined with data on total out-migrating salmon population size, will allow us to estimate avian predation rates on juvenile salmon in nearby creeks. In addition, the tags will tell us which particular individual fish were eaten, allowing us to quantify the characteristics of these fish to see if there is size-based mortality (i.e. were these fish disproportionately small or large compared to the average size of fish in the out-migrating population?)

Western gulls staking out their territory

Scanning for PIT tags

Estimating predation rates, as well as pinpointing potential predators, is an important step towards good management practices. So what are the potential predators that could be depositing these tags on the island? Possible culprits include avian predators—Western gulls and Brandt’s cormorants both use Año Nuevo Island for breeding—but also California sea lions, who also eat salmon. Determining which of these possible predators is actually depositing the tags on the island requires more (past and ongoing) research, including several studies analyzing the diet and movement patterns of Western gulls.