Diet interactions of brown trout and perch at Malham Tarn

It is believed that brown trout and perch were introduced into Malham Tarn by the Cistercian monks in the 12th century. Further stocking of brown trout for recreational fishing started around 1860, continuing off and on until 1994 when the National Trust pushed for a more natural brown trout fishery. Indeed stocking ceased in 2001 and, since 2002, a strict catch and release policy for all fish has been in place.

A few years ago, Jon Payne, now working for one of the fisheries teams at the Environment Agency, looked at the life history and growth rates of brown trout in Malham Tarn and showed very rapid growth compared to other European populations. OK, it’s a limestone lake so the underlying nutrient base for productivity is good, but the elevation and location might suggest pretty harsh growing conditions for fish.

In a new project led by a Masters student, Tim Eldridge from University College London, we are trying to determine how diet might play a role in their incredible growth rate and whether choice of food and interactions with other fish, notably perch, changed with fish size. For example, we might expect that there is a shift from feeding predominantly on invertebrates and zooplankton to piscivory, or even cannibalism, once the fish attain a certain size. To help answer these questions, we are using stable isotope analysis (SIA), a natural chemical tracer of diet retained from within fish scales (removed so the fish can be returned unharmed). This technique relies on the fact that all nitrogen and carbon making up a fish derives from the food that it eats.brown trout, malham tarn, growth, diet, stable isotopes, perch

Sampling (using rod & line, electrofishing, and fyke netting; all under licence) is now complete and we have length & weight data from 700 fish (all safely returned) and with a sub-sample of scale samples from representative size classes. The ages and growth rates are being estimated from scalimetry (like counting the growth rings on tree). The most recent years of growth are now being separated (fiddly under the microscope) for SIA.

We should be able to ‘map’ the food web of Malham Tarn and plot perch and brown trout within that map to determine their trophic position (their place in the food web). We will also be able to determine whether diet switches occur as the fish get older / bigger, and hence if there are different feeding strategies within the population. We will also be able to determine how energy routes through the Malham Tarn food web and estimate the niche size of the fish species present.

Updates on this project work will follow.