Chena River Chinook Salmon Study

As mentioned in the previous post, Mark Wipfli and Jason Neuswanger recently attended the international symposium Advances in the Population Ecology of Stream Salmonids in Luarca, Spain.  This post summarizes part of Jason’s talk, Intra-school competition and drift-feeding behavior in wild juvenile Chinook salmon (coauthors: Nick Hughes, Mark Wipfli, Lon Kelly, Amanda Rosenberger).

We begin with a video of  juvenile Chinook salmon behavior.  It’s in high-definition, so watch it full-screen for best viewing.  We show a wide variety of behaviors here because they’re all very interesting, but the most common one in the Chena is drift-feeding, in which fish hold a fairly steady position in the river, facing upstream, and dart back and forth to intercept drifting prey as it passes.  You’ll also notice that these fish are often gathered into schools or shoals, because there are plenty of predators in the Chena and there’s safety in numbers.

It’s slightly unusual to see young-of-the-year salmonids grouped up like this, because normally they inhabit smaller streams, where it’s safe to spread out and defend larger individual feeding territories.  In this presentation, we looked at whether they are still territorial in a large-river setting.  This inquiry addresses a broader ecological question:  Can any schooling/shoaling fish also be territorial at the same time?

To answer that, we’re using 3-D video measurement technology introduced in this earlier post and updated in this poster (PDF download) at Luarca.  It allows us to do things like measure the 3-D configuration of the whole group of fish, and the distances between them (the green arrows):

We can also measure the habitat.  The video below shows prominent logs in brown, the boundary of dense cover in green, the surface in blue, the bottom in beige, and velocity vectors in yellow, which were calculated by tracking naturally drifting particles as they pass by the cameras.

We set out to use this system to detect territoriality in juvenile Chinook salmon.  We we found it, we hoped to learn something about the relative quality of each territory by seeing how much prey each fish catches.  However, when we started watching the feeding fish very closely, we noticed this happening a lot: the fish captures an item and spits it right back out.

Also, they often chase items they decide not to capture:

In fact, most times these fish chase a potential prey item, they get nothing for it.  About a third of the time, they decide not to even capture it.  Of the items they do capture, they spit more than half out.  The percentage of prey capture attempts that end with ingesting actual food is much lower for Chinook salmon in the Chena than for brook trout in still water, which were the subjects of the only other studies we found on this topic.  It makes sense that fish in a flowing river should feed less efficiently than those in still water, because there’s more suspended debris in a river, and because the fish have less time to identify moving objects before capturing them.

This principle may apply to juvenile salmonids in rivers in general, in which case ours is the first clear documentation of a pattern that could be very important for mathematical models of drift-feeding fish behavior.  Failing to account for the high debris error rate would lead current models to overestimate the amount of food these fish eat and underestimate the minimum size of suitable prey.  These errors would then influence predictions of growth, habitat suitability, and abundance.

The high debris error rate also means we can’t judge the relative quality of feeding territories by counting up the foraging attempts.  Two fish could be making the same number of attempts, while one gets mostly food and the other, positioned behind it, is stuck with nothing but debris to spit out.  That’s frustrating for our efforts to understand territoriality, but we can still tell a lot about territories even from failed attempts.  First and foremost, are these fish territorial in the first place?

Here’s the raw data to answer that question.  Each colored dot represents a foraging attempt, and each color represents a different fish.  The yellow arrow shows the direction of the water’s flow from the upstream end of the school.  These data cover 20 minutes of foraging by one school of juvenile Chinook salmon in the 70mm length range on August 14th, 2009.

You can tell there’s some exclusivity to the feeding areas of each fish, but it’s hard to judge exactly how much.  We can learn more by wrapping shapes called convex hulls around almost all the foraging attempts by each fish.  We use an objective algorithm to identify a few outlier points to exclude from these shapes, and the result is a very good representation of the core area each fish was using:

Now we can tell that these fish have very well-separated territories, but those territories look very different from the typical salmonid territories, which are spread across a large area of the stream in a 2-D mosaic.  All of these territories would fit within one typical territory for many other fish species of the same size.  Despite their close proximity, our calculations show that no fish performed more than 45% of its foraging attempts within the boundaries of another fish’s territory, and many had little overlap at all:

Summary statistics for a few territories. Click to view full-size.

This analysis highlights the exciting things we can do with the new 3-D video analysis program we developed, available at http://vidsync.sourceforge.net.  The first 20 minutes of video we’ve analyzed in detail have taught us some interesting things about juvenile Chinook salmon, and probably juvenile salmonids in general.  And that’s only the beginning — we have more than 12 hours to analyze, and will film many more this summer.  Stay tuned!