WTT Blog Posts

On another sentinel species

crayfish conference habitat river restoration invasive species

The first National Crayfish Conference to be organized in five years was held at Giggleswick School, almost on my doorstep. As I have supervised three PhD, and countless MSc and BSc student projects on invasive crayfish species during my 10 years at Queen Mary University of London, it seemed sensible to attend. I was granted time to do so on the WTT’s behalf since white clawed crayfish, the species we consider indigenous, and brown trout can both be considered flagship or sentinel species; that is their abundance / population health can tell us something about the quality of the ecosystem in which they reside. There are also other parallels of course, more of which below.

The meeting was arranged over several days to allow plenty of time for networking and a choice of site visits. The location, a plum candidate for Hogwarts, and the logistics organized by PBA Applied Ecology, were first class. The attendees comprised renowned academics and a good smattering of (hopefully) the next generation of young researchers, research institutions, consultants, charities, practitioners, and stakeholders. The time was broadly split in half; the first part focusing upon white clawed crayfish (continuing) issues, and the second, particularly on the invasive species.

Playing Devil’s advocate, I got the distinct feeling that despite the 40+ years of experience accrued since signal crayfish became established in the UK, there is still too parochial a focus on the impacts of the invader upon the indigenous species in X or Y river. There was also scant evidence of habitat restoration / rehabilitation to promote existing populations of the white clawed crayfish which are clearly affected by pressures other than invasive crayfish. One could draw analogies to trout easily here, as the habitat degradation pressures are exactly the same. Indeed, one could draw similarities in native-invasive crayfish interaction to wild-stocked trout interaction. Don’t get me wrong, ark sites and breeding programmes are important in the face of the crayfish plague, but we shouldn’t forget about doing our best for the populations out there that might be clinging on in sub-optimal, headwater retreats.

Focussing more on the invaders, there was good ‘in silico’ modelling work presented that demonstrated the efficacy of trapping in combination with targeted poisoning and mechanical male sterilization. Such models are beginning to pin-point the weak spots to exploit in signal crayfish population dynamics. There was also some interesting experimental mesocosm work on signal crayfish predation of salmonid redds and fry. Under the simplified experimental conditions, crayfish were able to capture and damage a small proportion of the fry. Such research only highlights the importance of juvenile refugia habitat near to the spawning sites.

However, I think we are in grave danger of being 'signal-centric'. Whether it was a reflection of the current distribution of invasive species, perhaps in conjunction with the northerly setting of the conference, or not (and maybe I am biased by my previous research), but I feel the regulators and policy makers have to wake up to those invasive species waiting in the wings.

Scientifically sound and robust data from at least three different universities demonstrated, using different techniques or looking at different attributes / impacts, that there are ‘worse’ invaders than signals already established in the UK: virile and red swamp crayfish (and one might consider mitten crabs here too) are the clear contenders to the crown. Virile and red swamp crayfish (and a close relative of the latter, the White River crayfish) have a very limited distribution at present (as did signals back in the day and now look where we are!); they should be nipped in the bud asap as they appear more voracious, are highly fecund, can cause worse erosion and bioturbation, can outcompete signals, and oh yes, can still carry the plague around.

Despite the despair in the air around many of the presentations, especially with regard to (not) managing the ‘idiot with a bucket’ scenario, the meeting was an absolute success in getting the blue skies researchers, the applied people on the ground, and some of the regulators in the same room and talking. I, for one, hope it is not another five years until the next meeting.

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.

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