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Home Sweet Home - part two

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More from the Wild Trout Trust's Simon Johnson in this second part of his article on trout habitat.

Getting it all taped

Understanding of the habitat requirements of the key trout life stages allows an assessment to be made of the availability of good quality habitat for these life stages present within any watercourse.  Ideally, a walkover survey of the fishery should be undertaken as part of any planning for possible enhancement work.  This can be a very simple process, with one or two assessors carrying out the survey of the fishery, roughly measuring sections of good, poor and indifferent habitat present, and marking them on a map. 

It is important to note that any assessment of the availability and quality of habitat should be undertaken during the relevant season for each lifestage. Having undertaken this survey the amount of habitat present for key trout lifestages should be clear.

What does it all mean?

The $64,000 dollar question is 'how much of each habitat type do I need'.  And of course, the answer is……it all depends!

Assuming a deposition rate of 800 eggs from our standard 450g brown trout, then even a small area (say 250m2) of good quality gravel riffle could potentially hold upwards of 10,000 eggs.  Given a hatching success of only 25%, then around 2,500 young fry might be expected. 

Excellent quality fry habitat would not be expected to routinely hold more than 1 fry per m2.  Using this density figure, a total of 2,500 m2 (i.e. a 250m length of 5m wide channel) of fry habitat would be required to support this number of fry.  If this amount of fry habitat was not available ADJACENT to the spawning site (the 'it depends' bit of the equation creeping in here), then density dependent fry mortality would be very likely to occur, with the lifecycle of the river's trout population 'bottlenecked' at the fry stage.

It can be seen from the example above, that despite the abundance of scientific literature available on trout, a large element of 'muck and magic' still creeps into any assessment of habitat needs and availability.  Use the science, consult with the Environment Agency, independent fisheries consultants or the WTT's own advisors, and combine their comments with your own empirical observations and site measurements in order to come up with a workable estimate of the management requirements for developing top class habitat for brown trout in your fishery.

Turn your back on the water... and look at the real issues  

It is important that the impact of catchment process is considered as part of any habitat evaluation.  However, for most fisheries' interests, it will only be practical to look at land use on a local scale.  This is still of great value.

The most significant issues to consider are:

•    The integrity of the riparian zone.  Well-vegetated banks are of prime importance, both as cover for fish and other animals, and to control excessive erosion.  Over-grazing by agricultural stock, ploughing too close to the riverbank and development of the banks for residential or industrial purposed can all cause significant and damaging losses of vegetation

•    Excessive run-off, often laden with a cocktail of eroded soil, pesticides and nutrients, can damage riverine ecology in a pervasive and often largely un-noticed manner.  Gradual sedimentation of spawning gravels, increased growth of algae and loss of invertebrate life may be partly attributed to this process.  In some cases, the run-off is diffuse in form, gradually seeping into the river along the length of a reach.  This is best addressed by the development of wide (ideally >10m), strongly vegetated buffer strips.  Recent changes in the agri-environmental schemes available to farmers have made the financial incentives for these strips more attractive.

•    Equally pernicious are so-called point source discharges, from for instance road drains, agricultural yard drains and residential/industrial surface water outfalls.  These discharges often by-pass buffer strips, negating their benefit.  It is thus important that they are picked up and controlled as source, perhaps by the utilisation of Sustainable Drainage Systems (SuDS), which uses elements such as grass swales, detention ponds, porous hard surfacing and reed-beds to attenuate the worst impacts of damaging run-off.

This is an edited excerpt from the Wild Trout Trust's new publication ' The Wild Trout Survival Guide' Copies are available priced at £10+£2 P&P either online at www.wildtrout.org or by contacting the WTT office on 02392 570985 / PO Box 120 Waterlooville, Hampshire PO9 0WZ.

If you fish for wild trout you should be a member of WTT; your support is vital if the Trust is to continue and extend its vital conservation work.  Join the WTT and give wild trout a future!

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