We can answer this by understanding a few key elements of a salmon’s world: from the competitive nature of the food chain, the salmon’s feeding habits, and the life stage of the prey.
Salmonids, which include salmon, trout, freshwater whitefishes and graylings, are predators that feed on small crustaceans, aquatic insects, and smaller fish. Like certain humans, they become more selective of their food items as they grow older. Younger fish are naturally less suspicious and have added pressure to eat their prey before a competing fish does. The older and larger the fish, the more picky they are about the presentation of their food items. So unless fishermen learn what attracts the big beasts, they may only catch young, inquisitive fish that won’t break the scale nor make for an impressive brag photo.
At certain times of the year, insects and other salmon food are in different stages of their life cycle: from larvae to nymph, then emergence, and finally adulthood. These different stages have different colour hues and different body shapes that the Salmonids lock onto, making their preference in food rather specific.
It is these various animals that salmon fisherman want to mimic, and they do this with artificial flies, which are made of feathers, thread and fur tied onto a fish hook. There are thousands of artificial fly patterns, with names like Woolly Bugger, Cockroach Deceiver and Crazy Charlie. And they look remarkably real. Just by attaching eyes to your fly dramatically increases the chances of catching a fish.
According to one fishing guide, their Stimulator fly can coax reluctant large trout to strike: “it tries to imitate an egg-laying female salmonfly caught on the water and working her wings to escape.” Any hungry trout will go for it.
There are also attractor flies that don’t mimic natural prey, but appeal to natural curiosity or territorial aggression.
Now for lighting: on bright days there are more sun rays in the water, which illuminate natural food as well as the fishing “lures” or fly. If you were to try to lure a Salmonid to a fly that looks different to all the other food in the water, it would be ignored through natural suspicion, especially by the larger, wiser fish. The fly must match the natural colouration of other insects in order to be found credible – and must therefore be bright. Silver lures give off a bright flash, especially on sunny days, which will be seen from quite a way off.
Dark days and nights on the other hand share the same principle: the fly needs to be darker or it will look too different from bugs and small fish. However in cloudy or dusk conditions fish hunt from below looking up, so it’s the silhouette of the fly that stands out more than the colour. Therefore the shape and movement of the fly are important for convincing a savvy Salmonid. Waves and currents can give the fly a realistic motion, and sometimes incorporating mobile synthetic materials into the fly can make it wriggle and squirm.
Depth also plays a strong factor: in deeper regions, there is less light at different ends of the color spectrum. Red is the first color to disappear at depth, so you’d want purple and blue flies for catching fish that lurk in the depths. Interestingly, black flies work no matter what time of day or depth.
There is an age-old fly fishing analogy that confirms the requirement for your bait to look realistic: “If you went to a steakhouse, and the waiter brought you a bright green steak that was moving oddly, would you eat it?”
