Last week we visited the Corrie Burn Weir again to complete a formal assessment of its “porosity” to migratory fish, or in other words can the fish can up and over it, or back downstream for that matter.
The assessment method used was the “SNIFFER” barrier porosity see here for details. The method involves inspection of the barrier and identification of potential routes for fish passage. A range of measurements are then taken at each potential route including depths, current speed, vertical height, pool depth below, width, notch size etc. There are a range of tables for each species/type of fish and for upstream and downstream passage, each component allocated a score. At this weir we were interested in the passage of salmon, trout, eels and lamprey, both adults and juveniles, upstream and downstream movement.
The Corrie Burn was a relatively straightforward barrier: a single route for upstream passage for salmonids, effectively a vertical drop requiring a jump. the vertical height was over 1.4m so even for adult salmon this was difficult and as there was next to no take of pool, either in width or depth it was classed as impassable for upstream migrating salmonids. It was probably okay for downstream migrating smolts, there was sufficient depth on the crest (must be minimum 3cm, ideally 8cm+) and most of the water was landing in a shallow pool. At elevated flows, when smolts would be expected to migrate the situation will improve.
The barrier was also classified as okay for elver passage, there was good cover of damp mossy growth on the weir face on the right hand bank. Elvers require a climbing substrate of this nature to allow then to wriggle up and round the sides of barriers.
I knew the status of the weir before this formal assessment but at least we are now able to attach a formal score to it; essential if we are to attract public funding to help with its removal. On completion the results of the formal assessment were passed on to RAFTS who are coordinating barriers on a national basis for SEPA. The Corrie Burn weir is located on a small burn, it will be interesting to see how it fares when set within a national context.