Demonstrating the success of habitat improvement

At the recent River Restoration Centre annual conference, one of the sessions was dedicated to pre and post monitoring of habitat restorations. It is difficult to measure the success of a particular project unless you first know what the (insert parameter of choice: invertebrate or fish numbers, flow régime or substrate size etc) were like beforehand, and subsequently, how those parameters have altered, hopefully for the good and for the long term, afterward. Of course, if you can demonstrate such success, it is often easier to acquire support / funding for similar ventures in the future. The trouble is, what may work on one river, may not be so effective on another, and most studies only focus on whether restoration has worked on one system.

That is why we were very interested to read a study comparing 16 Estonian rivers that were monitored for the management and conservation of our beloved brown trout. The work was recently published in Conservation Biology. Several very clear messages emerge that resonate very strongly with the work of the WTT.

The first was regarding the worth of installing gravel spawning substrate into rivers. The study found that while trout readily used such gravels relatively quickly after introduction, the actual success of progeny was still limited by the wider environmental conditions of the river in question. In essence, it is possible to boost juvenile trout numbers in already productive rivers where all other conditions of water quality, habitat, food etc are favourable. However, in rivers with sub-optimal conditions such as high sediment load and poor flow, adding extra spawning habitat may not immediately increase the numbers of juveniles. While WTT occasionally promotes the installation of spawning gravels, it is of little long term benefit if it will be clogged by silt from upstream after the next spate, or there is no suitable juvenile habitat nearby for the fry to move to.

The second was based upon some genetic work of the trout populations. The authors found only a very weak relationship between the effective number of breeders and the numbers of juveniles produced. Hence, many factors influence juvenile trout abundance; it isn’t simply the case that more breeding adults equals more small trout. In some instances, quite high numbers of effective breeders were found alongside low numbers of juveniles indicating that low habitat quality or lack of suitable spawning substrate was a bottleneck to population recruitment. However, in others, it was apparent that only a few effective breeders were contributing a very large number of progeny to the population overall. Such systems will clearly be heavily affected by indiscriminate removal of even a one or two of those disproportionately important breeding fish, so catch and release is the only way to safeguard the future of the trout in those rivers.

A link to the abstract of the paper is available here