Fish passes

Fish Passes and Fish Pass Approval

Salmon (Salmo salar L.) and sea trout (Salmo trutta L.) are migratory fish that are very important to the rural economy in England and Wales. Their complex life-cycle involves the migration of juvenile fish from freshwater to the sea and the migration of adults from the sea to freshwater spawning grounds.

It has long been recognised that in order to sustain these migratory fish populations unrestricted access to spawning grounds must be ensured. Unfortunately, many man-made obstructions such as dams, weirs and mills, restrict this access to spawning areas.

The Salmon and Freshwater Fisheries Act 1975 provided legislative powers requiring new obstructions, or those rebuilt for more than half their width, to include a fish pass. This fish pass also had to be  evaluated by a representative of the minister responsible, before the fish pass was approved.

Approvals

The approval process had two distinct phases. First the design was evaluated before construction and if it was satisfactory provisional approval was given. After 3 to 5 years of data collection on the operation of the fish pass an application could be made for final approval. If the data showed that the fish pass was working satisfactorily then final approval was given.

In 1995 the responsibility for the approval of fish pass designs was passed to the Environment Agency from the Ministry of Agriculture Fisheries and Food. Designs for prospective fish passes are submitted via the local Environment Agency area office to a national group of fish pass specialists who advise whether to issue approval or not.

The purpose of this approval process is to ensure that suitable fish pass structures are installed as necessary. A consistent approach is taken whether the fish pass is being built by an external developer or the Environment Agency, and local officers are not allowed to influence the approval process through their presence on the fish pass panel.

In many cases significant cost savings can be made through using tried and tested designs and through the identification of design flaws at this stage of the fish pass construction process. Some submitted fish pass designs are unlikely to work satisfactorily because of incorrect depths of water or because of insufficient attention to siting of the structure.

The fish pass group provides an opportunity to avoid these costly errors and to promote the use of best practice in fish pass implementation.

Design

Fish passage problems can occur at almost any site where the water level difference between upstream and downstream of the structure is greater than about 0.5m. Typically these sites can be identified by fish leaping clear of the water in an attempt to ascend the structure. Where fish passage is adequate fish do not usually leap.

Large fish such as adult salmon can ascend structures where the water velocity may be over 5m/sec, since the maximum swimming speed and endurance of a fish normally increases with increasing length of the fish.

Conversely, small fish such as first year returning sea trout of only 0.3m in length may have difficulty in ascending jets of water at more than 3.5m/sec. Another factor which affects the ability of fish to ascend fish passes is the water temperature. Higher water temperatures increase the maximum swimming speed but decrease the endurance of the fish.

A fish pass can be designed to be technically suitable for fish to use but if the fish cannot find the pass it will not be very effective. Therefore each fish pass design must be suitable for the fish species and size available and also allow the fish to find the fish pass. Various means have been used to attract fish to a fish pass.

 The fish pass should generally be positioned at the natural point where fish congregate. The fish pass design should also incorporate sufficient water discharge from the fish pass to compete favourably with the other flows at the structure.

Other fish

Other major fish migrators include eel, lamprey and shad. In recent years the importance of allowing free movement of coarse fish species has increasingly been recognised. While there is currently no primary legislative power to make passes for thse species it is being considered in the review of the present legislation.

In the meantime we actively seek and encourage opportunities to be taken to include passes for these species.

So, if there is an obstruction in a river that is a problem for fish passage the first option to consider is to remove it. If that is not possible or a new obstruction is planned then there are various potential solutions.