An assessment of the impact of culverts on Atlantic salmon (salmo salar) migration through freshwater

Abstract

The Atlantic salmon (Salmo salar L.) is a migratory fish species that spends a number o f years in freshwater before migrating to the sea to feed and grow, and then returning to freshwater to spawn. It requires unhindered access to upstream spawning areas in order to complete its life-cycle. Culverts are structures that allow rivers to flow under roads or embankments and, if poorly designed and constructed, they may restrict the upstream passage o f Atlantic salmon. Five spawning streams on the River Moy, a productive Atlantic salmon river in Co. Mayo, were surveyed and a total of seventy culverts were inspected. The suitability o f these culverts, in terms o f unhindered salmon migration, was assessed based on specific fish passage criteria detailed in the National Roads Authority (NRA) guidelines. Further analysis o f salmon spawning, fish survey and water quality information from the River Moy was conducted in order to validate the findings from the field work. In order to assess the level o f awareness within local authorities o f fish passage issues, a culvert questionnaire was sent to senior engineers within each o f the twenty-nine Irish Local Authorities. Preliminary results indicate that poorly installed culverts have restricted the distribution o f Atlantic salmon in several parts of the River Moy catchment. Where culverts were found to breach a number o f the key fish passage criteria outlined in the NRA guidelines, there was no evidence o f any salmon being present upstream o f these culverts. Similarly, where salmon were found upstream o f culverts, the majority o f those culverts matched the NRA criteria. However, such findings can only be confirmed by baseline electrofishing surveys upstream and downstream o f the studied culverts. Such surveys would positively confirm the presence or absence o f salmon. Water quality was not found to be a factor limiting salmon distribution on the River Moy. with the majority o f spawning streams having a biological quality class o f Q4 or greater. Twenty out o f twenty-nine culverts questionnaires were returned and results indicated that the level o f fish passage awareness within the Local Authorities is low. It is apparent that the NRA guidelines need to be expanded and improved, to cover existing problem culverts, and that further field surveys are required to determine the extent o f the problem across the wider River Moy catchment.