Fog rolls in one drizzly May morning on the Olympic Peninsula of northwestern Washington. The precipitation is somewhere between mist and rain, decreasing visibility to about a hundred feet in the temperate rainforest. The river is high, but still clear, and there is only one other car in the boat launch parking lot. May is kind of an “off month” for fishing here because the only two major game fish species in this particular river at the time are Spring Chinook salmon (Oncorhynchus tshawytscha) and Coastal Cutthroat Trout (Oncorhynchus clarkii clarkii). “Springers,” as the Chinook are locally known, are notorious for being extremely difficult to target so few people fish for them. Coastal Cutthroat, also called harvest trout or bluebacks, are easy to target, but smaller than the other game fish in the area and protected by strict angling regulations.
As a fly fisherman, I enjoyed the peace of the late spring season. I also have a fascination with the Coastal Cutthroat. The trout species I fish for tend to feed seasonally on whatever major benthic (i.e., bottom-dwelling) invertebrate is most active at the time. Many types of flying insects lay their eggs in water, and after the eggs hatch, the juveniles go through multiple aquatic stages referred to as larvae or nymphs, and later sub-adults; these juvenile forms are enticing snacks for hungry trout. For example, West-slope Cutthroat Trout (O. c. lewisi) in the Yakima River east of the Cascades mountains will feed on emerging sub-adult March Brown Mayflies (Rhithrogena morrisoni) in late spring—and will readily consume a fisherman’s imitation of R. morrisoni at that time. However, West-slope Cutthroat are very unlikely to eat an imitation of a March Brown during any other time of year. Coastal Cutthroat seem to follow somewhat different rules about what prey to attack, at least in certain areas of their diet.
On that May morning, I decided to skate an October Caddis fly imitation to try to elicit a surface eat from the Cutthroat I knew lurked under the surface of the water. October Caddis are large moth-like insects in the genus Dicosmoecus. They live benthic lives until they become adults through complete metamorphosis. After emerging they are inch-long orange-bodied insects that blunder around near the water trying to find a mate and lay eggs to sew the next year’s generation. The kicker is that adult October Caddis, as suggested by their name, are only around between late September and early November. Within three casts, skittering my big fuzzy fly erratically across the surface with sporadic pauses, I got a very enthusiastic take from a Coastal Cutthroat. This fish had just tried to eat something that resembled nothing it had seen for six months, nor would see for another four months. This unusual behavior piqued my interest: why were Coastal Cutthroats less picky about eating food out of season compared to other trout?
There are many different subspecies of cutthroat trout, the number varying depending on whether you’re a “lumper” or a “splitter.” Lumpers combine several genetically unique strains of the Lahontan Cutthroat subspecies into one group, whereas splitters treat each strain as a separate subspecies. According to the splitter philosophy, there are 14 subspecies of cutthroat trout, two of them extinct [1]. Coastal Cutthroat Trout are the only subspecies to be fully anadromous, meaning they spawn in freshwater and migrate to salt water to feed for part of the year. Some of the other subspecies exhibit adfluvial life history patterns (spawning in streams, and migrating to freshwater lakes for part of the year), but no other subspecies actually go to the ocean. It’s possible that this unique life history translates into the flexibility of their foraging behavior.
In addition to being unique among cutthroats for their anadromy, Coastal Cutthroats are one of the few anadromous fish species that actually feed both in the ocean and after they move into freshwater. This could require them to use different behavioral tactics for each of the very different environments the trout enter—open ocean, bays, estuaries, rivers, and streams. In the salt water, trout feed on small baitfish, saltwater sculpins, crustaceans, fry of their close relatives Chum Salmon (O. keta, which move into salt water when still very small, unlike Coho, O. kisutch, and Chinook Salmon), and just about anything else they can find and catch. When Coastal Cutthroat move into the streams, they are still quite generalist in their diet. However, unlike Rainbow Trout (O. mykiss), Coastal Cutthroat Trout very seldom feed on tiny benthic invertebrates in their larval/nymph stages. Cutthroat, with relatively large mouths and specialized teeth at the base of their tongue called basibranchial teeth, are consummate predators. In the rivers, they generally attack relatively large prey that is, in almost all cases, moving. That is where the skated October Caddis comes in.
A Coastal Cutthroat’s attack of an October Caddis skittering across the water surface is best described as explosive. The fish literally erupts at the surface as it attempts to intercept the rapidly- and erratically-moving insect. It is a classic example of ram feeding, where the predator captures the prey not by sucking it into the mouth or biting it, but by engulfing it through rapid forward motion of the predator’s entire body with the mouth open wide. This is considered to be an appropriate method for capturing prey with excellent predator evasion capabilities [2]. Fish relying heavily on this prey capture technique are expected to have large mouths and excellent acceleration—both of which are conspicuous features of Coastal Cutthroats.
What determines whether a fish swims right past a fisherman’s fly or launches itself in attack? Behavioral ecologists have a term for external sensory factors that elicit a particular innate behavioral sequence: releasers [3]. It’s interesting to think about what exactly acts as the releaser of a Coastal Cutthroat’s predatory attack behavior. Trout in general seem, in my experience, to be pickier about how their food behaves than about the prey’s physical appearance. Coastal Cutthroats may have generalized this even further, perhaps as an adaptation to the diverse prey they encounter in all of the different habitats in which they feed. According to this idea, motion would be a key feature of prey items that act as effective releasers of an attack. And erratic motion—of a river sculpin or juvenile Coho Salmon darting away in a panic, or of an October Caddis skittering across the surface—appears to be most effective of all.
There are some exceptions to the trend that motion is a necessary factor in triggering a predatory response in Coastal Cutthroats. The most glaring example is that sometimes Coastal Cutthroats will feed on static mayfly subadults. Mayflies emerge from their nymphal stage into a subadult known as a “dun” and float down the river until their wings become fully operational. Many species of trout will feed on mayfly duns as they float at the surface, creating a “hatch” (fly fishers’ term for a large-scale emergence of adult/subadult aquatic insects) that typifies what many people associate with fly fishing.
For the most part, though, Coastal Cutthroats focus their attention on moving prey, so much so that they can be fooled into eating an October Caddis imitation in May. Even though I never ended up catching the aforementioned fish, the experience made me think about how the fish would spend the next few months. The trip downstream into the salt water would be followed by a couple of months of voracious foraging in the marine environment, replenishing its energy stores in preparation for a return to freshwater with the first autumn rains in September. Maybe I’ll even encounter this same fish again and coax it into another surface eat with an October Caddis fly—in October.
Lyle Hahn is a senior at Davis High School, and he spends a great deal of his time standing in western rivers attempting to get trout to display feeding behavior.
References
- Trotter, P. (2008). Cutthroat: Native Trout of the West (2nd ed.). University of California Press.
- Norton, S. F. (1995). A functional approach to ecomorphological patterns of feeding in cottid fishes. Environmental Biology of Fishes, 44, 61–78.
- Tinbergen, N. & Perdeck, A. C. (1950). On the stimulus situation releasing the begging response in the newly hatched herring gull chick (Larus argentatus argentatus Pont.). Behaviour, 3, 1–39.
Main photo: An adult Coastal Cutthroat Trout in a Pacific Northwest rainforest river; photo by John McMillan. All photos in this piece were used with the photographer’s permission.
Edited by Jessica Schaefer
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