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Home Sweet Home - why trout habitat is important

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Simon Johnson of the Wild Trout Trust takes a look at trout habitat in the first of this two-part series.

Most anglers instinctively know what makes good habitat for the brown trout they fish for.  Deep pools with undercut banks, glides between banks of water crowfoot and the crease between faster flowing water and slower sections of current.  This is all true, but it ignores the fact that, although brown trout is a single species, it comprises at least three distinct lifestages, each with its own, very demanding and different habitat requirements.  Understanding of this fact is probably THE single most important factor in the successful management of habitat for trout.   

The concept of the 'Habitat Bottleneck' helps explain why achieving the requirements of all key brown trout lifestages is of such fundamental importance.   Habitat bottlenecks can occur at any lifestage of trout.  For instance, a scarcity of suitable sized gravel will limit spawning success, resulting in a low number of trout hatching and recruiting to the fishery.  The value of abundant juvenile and adult habitat in the fishery is negated by the bottleneck occurring at the spawning phase.  Similarly, an absence of shallow riffle and glide areas suitable for juvenile trout will cause a bottleneck to this life stage, with the benefits of large numbers of swim-up fry from abundant spawning gravel being largely prevented.

Although the 'Habitat Bottleneck' concept clearly explains why all lifestages of trout need to be catered for in terms of both quality and quantity of habitat, it is at the spawning and juvenile lifestages that most trout populations are limited.  

Trout spawning requirements

Brown trout are fast water spawning fish.  They can breed at any time between October and March inclusive, with peak spawning taking place on most river systems during November, December and January.   The trout need a water depth between 10 and 50 cm with well-sorted, loose gravels at bottom, close to cover provided by deeper water, undercut banks, boulders or weed beds. The deposited eggs develop within the gravel and once hatched, feed off their yolk sac which remains attached to their body for around 14-30 days, depending on the water temperature.  During this period, they actively move away from light which helps to maintain them safely within the gravel bed.  As the yolk sac becomes exhausted, they move towards the light, emerging from the gravel to rest upon the riverbed.  >From here, they begin to dart upwards into the water column in order to intercept small food items carried by the current. 

'The killing fields': Juvenile trout habitat requirements 

Brown trout are by nature solitary and territorial.  These traits are expressed at a young age, even fry are willing and able to repel neighbours from 'their' territory.  Initially, the areas held by fry are small but as time progresses, and the fry disperse, both the fish and their holdings increase in size.  Inevitably, a point is reached where fry abundance exceeds the ability of the available habitat to support them. At this stage, the weaker fry are displaced, with many of them eventually dying due to lack of food to support their energetic needs. This process creates one of the great truisms in brown trout ecology: the fry that each section of habitat will support remains the same, independent of how many fry are stocked (naturally, by emergence from redds or artificially, by man's intervention), beyond a de-minus level.  By this process, 'density dependent' mortality, in conjunction with spawning success, places an effective limit on the number of fish supported by any river system.  The classic study, carried out over many years on the Black Brow's Beck in Cumbria clearly details this annual mass mortality of young trout.                   

The consequences of this fact are far reaching.  From the point of view of habitat management, it means that, if the number of potential fry and parr territories can be increased in any section of river, then there is a potential to support an increasing number of young fish and hence, adult trout. 

Fry and parr habitat requirements

Fry require slow-moving, shallow water (10cm-40cm) and the substrate should be a rough bed of gravel/cobble/debris or fine gravel alone if rooted plants are present.  There should be little silt.  The presence of bankside cover (typically, fringing vegetation) is important, particularly if there is no cover on the riverbed.   

Parr habitat is similar, although the depth and movement of the water are broader, reflecting the larger and more robust nature of parr.  

Adult trout requirements

Adult trout are able to survive and flourish over a wide range of habitat types, provided that depth of the water is generally greater than 30cm, and the speed is quicker, with some bankside cover, particularly in streams with a width less than 10m.             

Each life stage of trout has specific requirements and an abundance of suitable habitat must be present for ALL life stages to ensure the presence of a strong population of trout.

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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