Cutthroat trout

Cutthroat trout occupy cold western North American waters, and the honest habitat statement has to start with the fact that they are not one thing.

THE SPECIES COMPRISES MANY SUBSPECIES, each adapted to a particular basin — interior forms in high desert streams and lakes, coastal forms running to the sea, Rocky Mountain forms in cold headwaters. Their habitat requirements overlap but are not identical, and a general statement will be wrong somewhere.

COLD, CLEAN, WELL-OXYGENATED WATER IS COMMON TO ALL OF THEM, with most forms restricted to headwaters and high-elevation streams and lakes in the modern range.

STREAM FORMS USE POOL AND RIFFLE HABITAT with cover — undercut banks, woody debris, boulder — and are strongly associated with structurally complex reaches.

LAKE FORMS OCCUPY COLD OLIGOTROPHIC LAKES and spawn in inlet and outlet streams, so lake populations depend on tributary access.

THE COASTAL FORM IS ANADROMOUS, moving between small coastal streams, estuaries and nearshore marine water, and it uses estuarine habitat more heavily than most sea-run salmonids.

AND THE RANGE HAS CONTRACTED UPSTREAM for the same reasons everywhere: warming, sedimentation, and competition and hybridisation with introduced rainbow and brown trout. Many remaining populations are isolated above barriers, which protects them genetically and exposes them demographically.

Cutthroat trout are spring spawners, and in their high-elevation habitat that puts reproduction into the snowmelt period.

SPAWNING IS IN SPRING AND EARLY SUMMER, triggered by rising water temperature and, in most of the range, coinciding with or following snowmelt. In high country that can mean early summer rather than spring in any calendar sense.

FLOW IS THE COMPLICATION. Spawning during or just after peak runoff means redds are cut in water that is high, cold and moving, and scouring flows can destroy them. Year-class strength therefore varies with the shape of the melt as much as with the number of spawners.

LAKE POPULATIONS RUN INTO TRIBUTARIES to spawn, so inlet and outlet stream access decides whether a lake population reproduces at all. Barriers on those streams sever it.

THE ANADROMOUS COASTAL FORM RETURNS TO SMALL COASTAL STREAMS and, unlike Pacific salmon, does not die after spawning — repeat spawning is normal.

MATURITY IS AT A FEW YEARS and varies substantially between forms and environments.

AND HYBRIDISATION IS THE CENTRAL REPRODUCTIVE THREAT. Cutthroat hybridise readily with introduced rainbow trout, producing fertile offspring, so the reproductive output is not lost but the genetic integrity of the population is. Genetically pure populations are now the conservation unit, and many survive only above barriers that keep rainbows out.

Cutthroat trout handling matters disproportionately because so many populations are small, isolated and genetically distinct, and because much of the fishing happens in exactly the conditions that make release survival worst.

THE TEMPERATURE PROBLEM IS THE CENTRAL ONE. Cutthroat occupy cold headwaters, and in late summer many of those waters approach the upper limit of what the species tolerates. A fish caught, fought and handled in water near that limit has very little physiological reserve, and mortality after release rises steeply with water temperature. Several fisheries now close or restrict angling above a stated temperature — the practice known as a hoot-owl restriction, limiting fishing to the cool early hours — and that restriction is the single most effective handling measure available.

SO THE FIRST DECISION IS WHETHER TO FISH. Carry a thermometer. If the water is warm, fish early or not at all.

THE HANDLING PROPER is standard salmonid practice done strictly: knotless rubber net, wet hands, barbless hooks, fish kept in the water, unhooked in the net, and returned without lifting where possible. Air exposure of even a few seconds measurably reduces survival in warm water.

THE FISH IS HARMLESS to a handler.

AND THE GENETIC CONTEXT IS THE REASON FOR THE CARE. Many cutthroat populations are remnant and isolated, several subspecies are of conservation concern, and hybridisation with introduced rainbow trout is a major threat. A local population may not be replaceable from anywhere. Cleaning and drying gear between waters is part of handling for the same reason.

Cutthroat trout reproduction is a conventional salmonid strategy with one unconventional and serious problem attached, and the problem is genetic rather than physical.

THEY SPAWN IN SPRING, WHICH SEPARATES THEM FROM MOST TROUT IN THIS CATALOGUE. Cutthroat cut redds in gravel in small tributaries and headwater streams as flows drop and water warms after snowmelt, rather than in autumn as brown and brook trout do. Eggs incubate through late spring and early summer.

THE TRIBUTARY REQUIREMENT IS SPECIFIC AND FRAGILE. Cutthroat depend heavily on small, cold, clean headwater streams for spawning and early rearing, and connectivity from the main river or lake to those tributaries is essential. Culverts, diversions, dewatering and low-flow barriers cut populations off from their spawning water while leaving the adults apparently fine.

AND THE SPRING TIMING IS EXACTLY THE PROBLEM, because rainbow trout also spawn in spring. Where rainbows have been introduced into cutthroat water — which is across enormous parts of the western range — the two species hybridise readily and produce fertile offspring. The result is not a mixed population but a progressive genetic swamping, and pure cutthroat populations have been lost from vast areas without anybody removing a single fish.

WHICH MAKES THE CONSERVATION UNIT A POPULATION RATHER THAN A SPECIES. Cutthroat comprise a number of subspecies with restricted and in some cases very small ranges, and the surviving genetically pure populations are frequently confined to headwater reaches above a barrier that kept the rainbows out. A waterfall is, in this context, a conservation asset.

BROWN TROUT AND BROOK TROUT COMPETE RATHER THAN HYBRIDISE, and both displace cutthroat in parts of the range through competition and predation — a second and separate introduction problem.

RECRUITMENT DEPENDS ON SPRING AND SUMMER FLOWS, and warming, reduced snowpack and low summer flows all act on the incubation and early rearing period directly.

AND MANAGEMENT IS POPULATION-SPECIFIC, with closures, catch-and-release requirements and gear restrictions varying by water and by subspecies. The competent authority is the source.