I'm equally curious about how drought affects fish, especially in small streams. For example, in some streams when the water gets to a certain low level there may be very little flow in some stretches, accompanied by fewer pools and much shallower conditions in the pools that remain. During these occasions is the fish fatality rate higher, or are the fish just more crowded in water that's deep enough to sustain them? I expect there would be some effect on bait fish predation (an increase?) but I'm not sure how all the interaction works.

Perhaps you could address this topic in a future article. Thanks again.

A. You're pretty close on your observations, and it doesn't take a drought to produce some of the conditions you describe. Many small streams exhibit these conditions as a normal part of their hydrologic cycle in late summer and early autumn.

As water levels decline, from drought or just normal low flows, surface water decreases, often to the point where the stream bed completely dries up and open water is restricted to pools. As the surface water decreases in area, fish (we'll discuss invertebrates below) must retreat to these pools or become stranded and die. In these pools, a combination of detrimental events can occur, acting in concert with each other. As the pools become isolated, and if they are not fed by subsurface flows, they become smaller and smaller as water evaporates or percolates into the stream bed. This results in two things happening: (1) the water warms up and can potentially reach lethal temperatures, killing the fish directly, and (2) as the water temperature increases, the water becomes less capable of holding dissolved oxygen, leading to oxygen levels too low to sustain fish life. As water levels in these refugia fall, the surviving fish also become more susceptible to predation from two sources: (1) crowding into a smaller and smaller volume of water increases the chance of contact between carnivorous fish and their prey, and (2) all fish become more vulnerable to terrestrial predators such as osprey, eagles, otters, raccoons, etc. Even if flowing water remains between pools, low water levels enhance the ability of predators to reach prey.

A word or two about invertebrates. They, too, cannot escape death by dessication if the stream bed dries up or the water reaches lethal temperatures. However, they have one escape route not available to most fish - some of them can follow the water level down into the stream bed (called the hyporheos, meaning "below flow") where underground flow usually remains, thus providing livable, if not ideal, conditions until surface flows return. Some fish can survive these conditions if they can retreat into mud and remain moist and cool.

As alluded to above, subsurface flows may keep pools provided with water, preventing them from drying completely; if the inflowing water is cool enough, it also ameliorates the warming up of the water.

I hope this satisfies your inquiry.