Eel ladder

Eel ladders are designed to exploit the eels' natural climbing behavior. Juvenile eels (glass eels and elvers) migrate upstream in large numbers, and they can climb damp surfaces, especially when a thin film of water is present. The ladder typically consists of an inclined ramp, often made of plastic or metal, with a rough texture or a series of bristles or studs that provide grip. Water is pumped to the top and flows down as a thin sheet, creating a moist, slippery path that eels can ascend.

The slope is usually between 1:1 and 1:3, and the ladder may be angled or have resting pools. Some designs incorporate a 'V' or 'U' shaped channel to concentrate water flow. The entrance is placed near the downstream face of the barrier, where eels congregate. Eels use a combination of muscular undulation and suction to move upward, and they can climb several meters per hour. The ladder must be continuously wetted, and flow rates are kept low to avoid dislodging the eels.

Studies have shown that eel ladders can be highly effective, with passage efficiencies often exceeding 80% for elvers. For example, a study on the River Severn in the UK found that a bristle-type eel ladder passed over 90% of elvers that approached it. However, effectiveness varies with design, flow, and species. American eels may require different substrate textures than European eels.

Monitoring is typically done using traps or video cameras at the top of the ladder. In some installations, eels are counted and measured to assess population trends. For instance, the Elver Pass on the River Parrett in Somerset, UK, has been monitored since 1993, providing long-term data on eel recruitment. Such data are crucial for assessing the status of eel stocks, which are listed as Critically Endangered by the IUCN.

Eel ladders are not a modern invention; historical records from 18th-century Europe describe 'eel passes' made of bundles of twigs or brushwood placed against weirs. These were used to harvest eels as much as to help them migrate. In some regions, eel ladders are combined with eel traps, allowing for sustainable harvesting of migrating elvers.

Another niche application is the use of eel ladders in urban waterways, where they are integrated into architectural designs. For example, the 'Eel Ladder' at the Vistula River in Warsaw, Poland, is a public art installation that also serves as a functional fish pass. Additionally, some ladders are designed to be portable, allowing temporary installation at seasonal barriers. Research has also explored the use of eel ladders to help eels bypass hydroelectric turbines, reducing mortality.

Eel ladders are a key mitigation measure under the European Union's Eel Regulation (EC No 1100/2007), which requires member states to ensure that at least 40% of adult eel biomass can return to the sea to spawn. In the United States, the Atlantic States Marine Fisheries Commission has recommended eel passage at dams to support American eel recovery.

Despite their benefits, eel ladders are not a panacea. They do not address other threats such as pollution, habitat loss, and overfishing. Moreover, they are often designed for juvenile eels, while adult silver eels migrating downstream may require different passage solutions, such as bypass channels or turbine shutdowns. Integrated approaches that combine eel ladders with other measures are essential for effective eel conservation.

Elver

A juvenile eel, typically 5–10 cm long, that migrates upstream from the sea.

Glass eel

A transparent juvenile eel at the stage before pigmentation, usually arriving at river mouths.

Fish passage

A structure or device that allows fish to migrate past barriers such as dams.

Silver eel

The adult, sexually maturing stage of the eel that migrates downstream to spawn.