#44: Part 3, Mechanisms of change in fish populations: Lower summer flows & drought effects on growth & survival
Dan Isaak
Hi Everyone,
Similar to the last blog, this time we’ll again consider how a climate-induced hydrologic pattern may affect BIDE processes in fish populations. But instead of too much water at certain times & places, this time the problem is too little water. Specifically, the trend in recent decades toward decreasing summer flows across significant parts of the western U.S. (blog #18). These declines aren’t trivial, amounting to decreases of 20% or more in average flows across broad regions over the last 50 years & driven by varying combinations of earlier snowmelt runoff (blogs #16 and #17; which leaves less water late in the year) and subregional trends in total precipitation (increasing in some places, decreasing in others; graphic 1).
Because the amount of water in a stream channel is the most fundamental determinant of fish habitat, these declines obviously have important and direct biological consequences. Most obviously, fish need some minimum amount of flow to live, so those places in a channel network where this minimum isn’t consistently met won’t have fish. We often think of these areas as the transition points in headwaters where flows go from being intermittent to perennial (graphic 2), but as Lake points out in our first paper (hyperlinked here: http://www.southwestnrm.org.au/sites/default/files/uploads/ihub/lake-ps-2003-ecological-effects-perturbation-drought-flowing-waters.pdf), there are many in networks where natural factors may cause minimum fish flows not to occur. If summer flows trend lower in the future, it would be expected that many of these areas will expand from where they have occurred historically. And in these new areas, no water will mean no fish—it’s as simple as that (though perhaps in a few million years we get a few nice fossils if the changes caught a few fish off guard). Another important consideration is how flow reductions will interact with the infrastructure we’ve built throughout many river basins. For example, one issue that commonly occurs in mountain basins is a restriction in fish movement past some road crossings and culverts (graphic 2). As flows decline, these restrictions could occur at a larger proportion of road crossings and serve to fragment otherwise continuous habitat.
But thankfully, climate change isn’t likely to cause all (or even many) of our streams to completely dry up and blow away. So the other important question is how flow reductions affect fish within the majority of the network that still retains sufficient flows? What does 20% or 30% or 40% less flow really mean from the standpoint of a population? Probably many things, and as always, the answer is going to be somewhat context and species specific. But this next paper by Harvey and colleagues (hyperlinked here: http://www.waterrights.ca.gov/hearings/docs/caw/exhibits/sc_10.pdf) did a nice job of exploring these questions for drift-feeding rainbow trout through a manipulative field experiment. Working in a California stream, the authors blocked off consecutive sections and diverted flow out of half the sections for one summer. They then measured the amount of aquatic insect drift, fish growth, and fish survival over a 6 week period (graphic 3). Not surprisingly, less water moving through the diverted sections of stream translated to fewer insects drifting downstream. And with a skimpier buffet line, fish growth shrank accordingly. The study was not of a duration sufficient to determine whether fish survival rates would have been affected, but growth rates in fish are often strongly correlated with survival. At the very least, some hungry fish would have eventually been forced to try emigrating from the study reaches to find habitats with more food. And emigration brings with it another set of mortality risks associated with predation or the possibility of not finding productive habitats.
So next time, we’ll flip over to the temperature side of things and think more about thermal biology before highlighting a couple recent studies that looked at the combined effects of flow and temperature on fish growth & survival. For aficionados of self-thinning theory, you’ll start to see some strong parallels as there’s a logical intersection here between that body of work and stream climatology that should synergize to rapidly improve our understanding of mechanisms by which climate regulates fish population dynamics.
Until then, best regards,
Dan