The Essex Fish Migration Roadmap

Essex Wildlife Trust has been working in conjunction with the Thames Estuary Partnership and the Environment Agency to create The Essex Fish Migration Roadmap. This roadmap of all the rivers across the county has subsequently led to the identification of 400 obstructions to fish passage. 

Rivers are an essential part of our environment, supporting innumerable wildlife species, providing valuable habitat, connecting local communities and linking freshwater with coastal and marine ecosystems. Curving and flowing through the land, rivers are a lifeblood of our county, but they face constant challenges.

Our rivers are home to around 3,000 species, 200 of which are listed as rare, endangered or vulnerable. Over the centuries, our connected rivers have been severely fragmented and industrialised; they have become drains for our waste and chemicals, canals for our transportation and have been artificially blocked by man-made barriers such as weirs, locks, sluices, mills and flood control structures. These structures serve a vital process but too often are impassable to fish and eels, so the once uninterrupted free flow of rivers now resembles a series of linear lakes with little or no opportunity for fish to move upstream and complete their migratory journeys.

Migratory fish species move between the freshwater and marine environment as they spend different stages of their life cycle in different habitats. Barriers can cut them off from suitable spawning sites, affecting their diversity and populations while also putting them at a greater risk to pollution events. Recovery can be dangerously slow if fish cannot move back upstream to recolonise, which also impacts their predators, affecting species like kingfisher and otter.

Essex Wildlife Trust has been working in conjunction with the Thames Estuary Partnership and the Environment Agency to create The Essex Fish Migration Roadmap. This roadmap of all the rivers across the county has subsequently led to the identification of 400 obstructions to fish passage. 

'Red' river sections signify where navigation by fish and eels is closed and 'green' sections where it is open. The roadmap will allow us to focus on a 'whole system' approach and prioritise strategic actions. This is the largest ever connectivity project attempted by the Trust, with the aim of turning the map 'green' over the next decade, by removing or bypassing obstructions to fish migration and allowing wildlife the opportunity to return to their previous range. 

Utilising GIS technology, the Roadmap provides a method that focuses on a ‘whole system’, sea-to-source approach that, like a road network, looks at rivers as interconnected migratory routes. By considering rivers as 'Highways', 'A-roads' and 'B-roads', barrier locations and river network connectivity can be easily visualised on one or more rivers, or in entire river catchments. This can help reprioritise barriers and provide a blueprint for strategic steps to open up entire migratory routes. 

The preferred option to provide fish passage is to remove the obstruction particularly where it is no longer operated or maintained. This can be complicated and expensive as there are many factors to consider, such as maintaining upstream water levels and downstream flood risk.

If the structure cannot be removed, investigation into whether it can be bypassed will follow. This is often a good solution as it gives the opportunity to provide a stretch of good habitat such as a gravel bed which several fish species chose to spawn in which may not be available in the main river due to siltation caused by the impounding effects of the structure. 

If removal or bypass are impossible due to constraints, a technical fish pass would be considered. Examples include a baffle fish pass which enables fish to migrate, and tiles or brushes specifically designed for eels.