The Upper Delaware trout fishing community has in recent weeks been very concerned about high river temperatures, which they feel have been compounded by unnecessarily low water releases from New York City’s Delaware System reservoirs. Agitated calls for additional water releases have been made repeatedly to those who manage the river. My article in The River Reporter’s special section Fishing 2015 outlined the structure and politics of the upper Delaware system and conservationists’ attempts to get thermal stress relief for the trout. The issues around trout and river temperatures are so important that they are worth discussing in more depth; this is the first of a series doing so, to be printed on an ad hoc basis.

Let’s review the basics of trout’s vulnerability to high temperatures. Trout are cold-blooded, meaning that their metabolisms are regulated by the temperatures of their environments, and they are cold-water species, meaning that when water temperatures get above 68° F their metabolisms and feeding slows down; they stop growing. Things get worse fast as water temperatures increase, and trout exposed to temperatures of 75° F or above for extended periods of time actually die. In the technical literature, 75° F is called the “incipient lethal temperature.” In the Delaware, a somewhat artificial trout sustaining “tail-water” environment is created by cold water releases of from the bottom of the city’s dams.

There is an extensive and complex scientific literature on these issues, largely based on laboratory experiments in which trout in tanks are subjected to well-regulated temperature changes and are observed carefully. Of course, Delaware River trout are not in tanks, and temperatures vary greatly during a given day and even within a stretch of river. But the theory does explain why trout are much more common above Callicoon where the water is cooler, and why trout feeding slows down when the water gets warm later in the summer.

Unlike other streams in the region such as the Beaverkill or the Lackawaxen, in which trout are stocked from state-run hatcheries, the trout in the Delaware are a wild and self-reproducing population. They are beautiful, strong and wily, tough to catch, and treasured by trout fisherman, the majority of whom practice “catch-and-release.” Since fly-fished trout are caught with tiny hooks that are usually not swallowed, released trout can be expected to survive. But all this changes when the water temperature rises above 68° F, and the fishes’ strenuous efforts when hooked puts them into thermal stress. It is feared that trout hooked and fought under these conditions will die, even though released apparently unharmed.

So, the fishing conservation community advocates a policy of not fishing when river temperatures are above 68° F. Many fishermen carry thermometers or consult the USGS river gages. Indeed, in reflection of these facts, some sections of the Beaverkill are closed to trout fishing in July and August. Official policies closing entire rivers for trout fishing when temperatures get too high are in effect states like Montana; but here on the Delaware, this is not the case. So, voluntary self-restraint is the fallback position.

Data from the USGS gages on the River at Callicoon (tinyurl.com/qy3sace) and Lordville (tinyurl.com/p7pc7zx) show that summertime temperatures frequently get into the lethal range of 75° and above. So, what happens to the trout then? Do they actually die? Do they flee upriver to where the water is colder? Do they seek out refuge in the few localities where underground springs keep the water cooler? It is suspected that all the above happens, depending on the intensity and duration of the stress event. But while we don’t know definitively what happens to thermally stressed trout in the Delaware, all in the community agree that it “ain’t a healthy situation!” to have long episodes of high river temperatures. Following columns will discuss how bad it can get and possible remedial actions.