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