Fishways for eels and lampreys

Introduction

Figure 1: Eel pass at a hydropower plant in France. Here, migrant glass eels are led to a ramp filled with "artificial turf" and water.

Depending on the life stage and the passage design, eel and lamprey can use the migration solutions mentioned above, apart from traditional pool-type fishways without vertical slots. Eel and lamprey can pass natural-type ramps and bypasses in the same way as they do in similar river reaches in nature. It is then important that the passages also have a natural-type layout of the banks, with vegetation, and a rough, varied substrate, and no slippery walls. Yellow eels and silver eels can swim upstream along the bottom of vertical slot fishways, if there is continuous rough bottom substrate. In pool type fishways without such slots, Lamprey and eels can still migrate if there are bottom cuts or orifices and lamprey even climb across surface notches. These should have a brush structure on at least one side, so that smaller yellow eels can crawl through. However, the youngest stages, (<8 cm, glass eel and small yellow eel) require another type of passage, since their behavior differs from older eels. Glass needle migrate near the surface and can only cause low water speeds. However, they can pass obstacles by ebbing themselves through substrates and terrain, even outside the river, as long as the bank surface is moist and rough enough, for example, on moss-clad rocks. In order to imitate such conditions, specially designed eel passes have been developed. These consist of a channel with brushes, artificial turf or other structures with continuous voids that are kept moist (Armstrong et al. 2010, Environment Agency 2011 and DWA 2014). More details for the design of eel passages can be found in (Environment Agency 2011 and 2017).

Methods, tools, and devices

During planning

A fishway for eels and lamprey is often planned as a separate construction in addition to a conventional fishway and fishways for eels are normally smaller installations. Planning of the fishway will start with mapping and surveying of the barrier, any other fishways and the river reach upstream and downstream of the barrier, including information about the hydropower scheme. In particular, the dam site should be mapped for the best location of the fishway. Surveying must also be conducted in the area of the river bank where the fishway is planned, including geological surveying. Geographic data should be handled in GIS software for further planning and analyses. The design of the fishway should be conducted with conventional hydraulic- and civil engineering calculations and drawing. All material used in a fishway must be planned to withstand physical strain from water, floods and frost. Monitoring facilities should basically be planned in the upper part of the fishway.

During implementation

If a full size fishway is planned, physical implementation of an eel and lamprey fishways requires heavy machinery suited for the river size and its surrounding terrain, as for a conventional pool-type fishway, such as excavators and lorries. Work with explosives is relevant in most cases and blasted rocks and transportation of material out from the site is common. Surplus rocks should not be disposed at site because of pollution risk. The construction phase includes construction of concrete formwork, casting of concrete and iron reinforcement work.

During operation

Lamprey and eel fishways are typically in use only parts of the year and requires that their operation is started and stopped at certain dates. Injuries on such fishways from physical wear must be monitored and repaired in order to secure regular fish migration. Maintenance work normally require hand-tools more than heavy equipment. Depending of the site, removal of sediment, branches, logs and floating debris in pools and fishway entrance is common. Monitoring systems require regular inspection, depending on product and system.

Classification table

Classification	Selection
Fish species for the measure	All
Does the measure require loss of power production	Operational (requires flow release outside turbine)
	-
	-
Recurrence of maintenance	Irregular at events
Which life-stage of fish is measure aimed at	-
	-
	-
	-
	-
	Movements of migration of fish
Which physical parameter is mitigated	Flow quantity
	Flow variations
	Substrate and hyporheic zone
	Water temperature
	Ice
	Water velocity
	Water depth
Hydropower type the measure is suitable for	Plant in dam
	Plant with bypass section
Dam height (m) the measure is suitable for	Up to 20
Section in the regulated system measure is designed for	-
	Bypass section
	-
River type implemented	Steep gradient (up to 0.4 %)
	Fairly steep with rocks, boulders (from 0.4 to 0.05 %)
	Slow flowing, lowland, sandy (less than 0.05 %)
Level of certainty in effect	Moderately certain
Technology readiness level	TRL 8: system complete and qualified