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SMALL STREAMS: Contribution to populations of trout and sea trout.
Report of a workshop held at Carlingford, Co. Louth, Ireland on 27 and 28 November 2012 |
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Definition of a small stream
- 5/ The workshop heard that there were a number of possible ways of defining a small stream – stream order, width, flow, depth – but it was generally agreed that the simplest, and most easily measured, was width. The average widths of first and second order streams (at 1:50,000 scale) are about 3 and 6 metres respectively, and as trout tended to be the dominant salmonid in streams of less than 6 metres it was suggested that this might be a workable upper limit. On the other hand, it was pointed outthat a good deal of information existed about most second order and larger first order streams, and these tended to be reasonably well protected, while streams of less than 1.5 metres wide were less well understood and more vulnerable. First order (<3m wide) streams constitute a very significant proportion (50-60%) of the total length of typical rivers, although a much smaller proportion (10-20%) of the wetted area. Their self-defining feature, small physical size, makes small channels susceptible to physical change and other environmental pressures. For example input or removal of large woody debris has a priori more effect in narrow channels. Moreover, the high ratio of bank length to water volume makes small streams more vulnerable to harmful impacts from land use, or pollution point sources. Similarly, bankside vegetation has a correspondingly higher influence on food input and shade in small compared with wide channels.
- 6/ The workshop concluded that no
single definition of what constituted a small stream was all inclusive.
Instead, focus was needed on the role that small streams, and in particular
very small streams, play in the ecology of a catchment.
Use of Small Streams by Salmonids
- 7/ Small streams are principally used by salmonids for spawning and as nursery areas. Salmon prefer streams with a width of more than 2.5 metres; the minimum size utilised by trout, on the other hand, appears to be 0.8 metres, with streams of only 1 metre wide used by large sea trout for spawning. Juvenile trout tend to show a preference for habitat that lies close to banks, which by definition is a higher proportion of stream area in narrower channels. Electro-fishing surveys within the Tweed catchment have shown that streams averaging 2 metres and under are dominated by trout fry, while in channels averaging 2 metres and over salmon fry predominate. In very small streams 0+ trout fry move downstream early; this is essential in ephemeral streams such as winterbournes. On the island of Gothland 0+ trout fry move into brackish water, and elsewhere they may make use of pools and lake littoral zones. Evidence from Burrishoole shows that juvenile fish, particularly in older age classes, move progressively downstream from spawning areas throughout the year. Nevertheless, quite small streams may also contain small resident trout. The workshop agreed that more research is needed on the movement of immature salmon and trout, within and out of small streams.
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Russell Poole: The contribution of small streams to overall salmonid production of a given catchment and the role of small streams in the ecology of hill lough systems
See supporting paper "A predictive model for estimating river habitat area using GIS-derived catchment and river variables" in Fisheries Management and Ecology, Volume 19, Issue 1, pages 69–77, February 2012. Abstract here
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