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

Stuck between a creek and a hot place: Why our rivers are getting warmer

 When I give my ‘elevator-speech’ summary of my research—I study the effects of hot river temperatures on juvenile salmon behavior and the importance of coolwater refugia…—a common question I get is, ‘But why are river temperatures rising?’ Sometimes people are a little abashed about asking, since it seems like a simple question… but really, it’s a great question, and the answer has considerable nuance.

A recent study of 40 rivers across the U.S. found that most showed significant increases in water temperature over the past half-century.¹ A combination of factors affect stream temperatures, including air temperatures, amount of solar input, and land-use (e.g. urbanization, farming, and river management). This makes sense both intuitively and from experience—rivers in tropical climates are on average warmer than in the Arctic, and small high mountain streams are colder than large rivers near their delta (where they are both larger and less shaded, and are therefore open to a lot more solar radiation).

Air temperature is a strong predictor of water temperature, and increases in air temperatures due to global warming are causing a trend of increasing stream temperatures.^1,2 However, this is not the whole story. Urban areas create ‘heat islands’ that can increase water temperatures both through hotter ambient air temperatures, as well as heated water run-off from hot pavement. In addition, land-uses such as irrigation and dams can exacerbate the warming trend further, by storing water in slow-moving or shallower areas (such as irrigation ditches and reservoirs) where it heats up before returning to the river. For example, on the Klamath River where I do my research, the Iron Gate Dam (lowest of 6 on the river) is an old dam that releases water downriver from the top of the reservoir, water that has been sitting in the sometimes 100°F summer heat all day. As a result, summer water temperatures on the Klamath can reach ~80°F (26°C), which feels like bathwater, and is nearly lethal for coldwater-adapted fish like salmon.

There are many problems associated with rising river temperatures, and not just for salmon. Warmer water temperatures can cause increases in primary productivity and lower dissolved oxygen levels, effects that cascade up through the ecosystem, changing aquatic habitat structure and availability, invertebrate community composition, habitat suitability for many fish species, and often making the ecosystem more susceptible to invasive species. On the Klamath River, the summer hot water temperatures combined with eutrophication (excess nutrients, often from farm run-off) cause massive green-algae blooms in the reservoir by late summer, which get released downstream and turn the whole river green.

The Klamath River during an algae bloom

Halting the trend in rising water temperatures is linked to the problem of stopping increasing air temperatures (and rising atmospheric carbon dioxide), and will not be a quick fix. However, there is significant mitigation and restoration that we can do to lessen the impact of elevated water temperatures, solutions ranging from urban greening to dam removal to in-stream habitat restoration. This is where my research fits in—salmon are a coldwater fish, and on rivers like the Klamath, summer water temperatures can reach levels that are sometimes lethal. As a result, the fish seek out colder areas in the river (coolwater refugia), often created by incoming coldwater tributaries. Protecting and restoring these refugia are an important way that we can mitigate the effects of hot summer rivers temperatures on salmon.

Juvenile salmon piling into a thermal refugia on the
Klamath River (photo by Kyle Swann) 

It’s important to note that knowledge of the long-term trends in river water temperatures, as well as the data that produced the now famous graphs of rising C0₂ trends, would not be possible without long-term monitoring projects that were established years ago. These kinds of long-term programs are hard to fund and maintain, yet are essential if we want to understand how our environment is changing over time—this is an interesting problem that my lab is currently researching. Stay tuned!

1) Kauschal, S. et al. (2010). “Rising stream and river temperatures in the United States.” Frontiers in Ecology and the Environment.

2) Webb, B. et al. (2007). “Long-term changes in river temperature and the influence of climatic and hydrological factors.” Hydrological Sciences Journal.