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Fish passage refers to the movement of fish into, out of and within waterbodies, typically river systems. Man-made structures used to control and manipulate water levels, such as weirs and sluices, more often than not prevent the free movement of fish along the length of river systems.
The structures which make up the current Lower Mole Scheme form a complete barrier to fish passage, except for eels where specially built passes have been erected. This means whilst fish may be washed downstream under flood events, they are otherwise confined to a short stretch of river and can only use the limited habitat that is available to them. It also means any fish displaced downstream during high flow events are unable to make their way back upstream.
Why is fish passage so important?
Removing barriers to fish passage is important for a number of reasons:
Different fish species require a multitude of habitats across their life cycle (i.e. fry, juveniles, adults).
Each habitat provides a different function such as spawning, feeding and refuge from predators or during flood events. Removing these barriers allows these habitats to be connected so fish can freely move between them. Some species can travel several kilometres over periods of days to make use of specific habitats.
Migratory fish species such as eels and sea trout, have an intrinsic need to migrate upstream, often to the upper reaches of a river catchment, in search of suitable habitats required to complete their life cycle.
Populations that are well connected and are able to move freely are more resilient to disturbance as they are able to move away from and subsequently recolonise affected areas. Disturbances may include pollution and poor water quality events. This improved resilience (i.e. ability to adapt to adverse events) is crucial against a back drop of climate change where extreme weather events, which can have adverse impacts of water quality, are predicted to increase.
The importance of achieving fish passage is recognised under different pieces of national legislation including The Eels Regulations (England and Wales) 2009 and the Salmon and Freshwater Fisheries Act 1975 (as amended).
Achieving fish passage
Fish passage can be achieved in a number of ways. The most obvious and first choice is the removal of a structure, which often results in significant improvements to upstream habitats associated with the removal of an impoundment see our combined 'water quality and levels and understanding impoundment page' for further information. Better habitat quality, combined with the removal of a barrier, has been shown to lead to an increase in both the density and number of fish species, all of which results in overall improvement to the status of the river.
Where a structure cannot be removed, fish passage is often achieved using a rock ramp or technical fish pass, both of which come in many shapes and sizes. A fish pass has to be designed specifically for the individual site or structure, taking into account the unique characteristics and needs of the site, such as the fish species which will need to use it. It is important to understand, whilst typically designed to pass as many fish species as possible, technical fish passes are not a substitute for structure removal.
Please see the photo gallery on the right hand side for examples of what a technical fish pass and rock ramp can look like.
Fish passage refers to the movement of fish into, out of and within waterbodies, typically river systems. Man-made structures used to control and manipulate water levels, such as weirs and sluices, more often than not prevent the free movement of fish along the length of river systems.
The structures which make up the current Lower Mole Scheme form a complete barrier to fish passage, except for eels where specially built passes have been erected. This means whilst fish may be washed downstream under flood events, they are otherwise confined to a short stretch of river and can only use the limited habitat that is available to them. It also means any fish displaced downstream during high flow events are unable to make their way back upstream.
Why is fish passage so important?
Removing barriers to fish passage is important for a number of reasons:
Different fish species require a multitude of habitats across their life cycle (i.e. fry, juveniles, adults).
Each habitat provides a different function such as spawning, feeding and refuge from predators or during flood events. Removing these barriers allows these habitats to be connected so fish can freely move between them. Some species can travel several kilometres over periods of days to make use of specific habitats.
Migratory fish species such as eels and sea trout, have an intrinsic need to migrate upstream, often to the upper reaches of a river catchment, in search of suitable habitats required to complete their life cycle.
Populations that are well connected and are able to move freely are more resilient to disturbance as they are able to move away from and subsequently recolonise affected areas. Disturbances may include pollution and poor water quality events. This improved resilience (i.e. ability to adapt to adverse events) is crucial against a back drop of climate change where extreme weather events, which can have adverse impacts of water quality, are predicted to increase.
The importance of achieving fish passage is recognised under different pieces of national legislation including The Eels Regulations (England and Wales) 2009 and the Salmon and Freshwater Fisheries Act 1975 (as amended).
Achieving fish passage
Fish passage can be achieved in a number of ways. The most obvious and first choice is the removal of a structure, which often results in significant improvements to upstream habitats associated with the removal of an impoundment see our combined 'water quality and levels and understanding impoundment page' for further information. Better habitat quality, combined with the removal of a barrier, has been shown to lead to an increase in both the density and number of fish species, all of which results in overall improvement to the status of the river.
Where a structure cannot be removed, fish passage is often achieved using a rock ramp or technical fish pass, both of which come in many shapes and sizes. A fish pass has to be designed specifically for the individual site or structure, taking into account the unique characteristics and needs of the site, such as the fish species which will need to use it. It is important to understand, whilst typically designed to pass as many fish species as possible, technical fish passes are not a substitute for structure removal.
Please see the photo gallery on the right hand side for examples of what a technical fish pass and rock ramp can look like.
