Optimal foraging

In the mid 60´ties American ecologists Eric Pianka and Robert MacArthur developed the theory of optimal foraging. 

In its most plain form the theory states that the predator would hunt in such ways that would optimize their intake of energy per unit time. One could also say that: the predator would hunt food that gave them more calories than they ultimately would use doing so.

To this theory many other ecological concepts are connected such as: competition adaptation and foraging behavior, but plain fly-fishing know-how is also closely linked to these ecological models and in the next columns we will try to highlight some interesting information.

The search images

Visual predators like trout and salmon may form what is termed as search image of certain prey. The search images will help them detect one type of pray faster than other types; this can be particular helpful if the prey is camouflaged or otherwise well hidden in the environment. The trout and salmon will more readily form these search images if there is lots of a particular kind of food at the river or in the sea. We probably use flies that contain elements that fall within different search images: It could be the search image of a prawn, little fish, nymph or adult insect. No doubt some rivers will have fish that react more eagerly to one type of fly than another – One could say that the fish had a particular strong search image of a particular prey. An example of fish with such strong search image could be salmon of some Scottish and Irish rivers where prawn flies like Alleys and Foxford shrimps work far better than traditional patterns.

Super stimulus

A certain stimulus will draw a specific response from fish – It could be that tiny fly passing quickly by the salmon – or a little shimmering imitation of a scud lifting from the bottom of the river. These flies will resemble something the salmon or trout knows as food – and often size and presentation is key elements. On some occasions all details must be in place in order to get the final response. This was very evident when I ones fished the famed River Test in Hampshire during a period of hatching Ephemera Danica; (a species of mayflies) The sky was blackened by flying insects – and trout where only inclined to rise to very correct imitations – some where so specific in their diet that they would only feed on the egg-filled females that had a tiny variation in colour – and a slightly thicker abdomen.

Sometimes anglers can exaggerate details or size of the flies in such ways that they create a supernormal stimulus effect. In ecological terms also known as; Super stimulus. We know these flies from winter steelhead fishing and the big or hot-red flies that some times can draw a salmon in late autumn.

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