Fish species in the Severn Estuary

The fish community within the Severn Estuary and further west to Bridgwater Bay is diverse and rich, with over 100 different species living and feeding in the area¹. Some may be accidental marine visitors, swimming upstream from the sea, including basking shark and sunfish. Other fish are inadvertently swept downstream from rivers, exploring the estuarine environment. Many fish within the Estuary migrate, moving in waves up and down the Estuary, depending on the season¹. In fact, the Severn Estuary has the most diverse range of fish species in the UK² and is one of the most important nursery sites in Britain³!

The different fish species in the Estuary range from the common to the extremely rare. Common species known to thrive in the Estuary today are river lamprey, twaite shad, sprat, herring and the common goby¹. Rare species not often observed in the Estuary include the anchovy, John Dory, crucian carp, silver bream and the sea lamprey¹.

Fish species may be categorised by lifecycle (see below). The most common and numerous types of species are the marine-estuarine opportunists. These fish migrate to the Estuary at certain points in their life cycle, for example, their spawning season. Other categories include marine and freshwater species which stray into the Estuary, as well as completely estuarine dwelling fish.

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Life-cycle characteristics

Fish may move up and down the Estuary with the changing tides and seasons, to chase food sources and search for nursery grounds. Some fish, despite being mostly saltwater or freshwater dwelling, can tolerate a wide range of conditions and have different life-cycle characteristics:

• Marine: typically breeding offshore and are generally intolerant of freshwater

• Marine Stragglers: abundant in marine environments but infrequently venture into the Severn Estuary

• Marine Estuarine-Opportunistic: marine species found in large numbers in the Severn Estuary either as juveniles, or as feeding adults

• Freshwater: typically occurring and breeding in fresh water, are intolerant to saltwater

• Estuarine: typically occurring and breeding in estuaries, well-adapted for brackish water

• Anadromous: migrating from the sea into fresh water to breed, e.g. salmon

• Catadromous: migrating from fresh water into the sea to breed, e.g. eel

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Fish spawning within the Estuary, is largely limited to the common goby and sand smelt, as most fish specied tend to spawn further out in the Bristol Channel, with their larvae swimming inshore to the more sheltered Estuarine waters³. There are many advantages gained by moving further up the estuary. There are fewer predators in the calmer waters, and with more food available, the Estuary is the perfect home for these larvae. During winter months, these juvenile fish migrate seawards in response to reducing temperature and changing salinity (salt concentration in seawater). Once fully grown, some species will live entirely offshore, with others such as bass and cod taking short feeding trips back into the estuary¹.

This pattern of migration can be seen in sand goby, sole, dab, pout, bass, poor cod, whiting and grey mullet. The glass eel has an especially interesting migration cycle. Glass eel spawn in the Sargasso Sea off the coast of North America and make their way across the Atlantic to Europe⁵. Once the eels reach the Severn Estuary, they migrate by selective tidal transport – the eels are carried by the high and low tides, rather than swimming independently. The high tide prompts the eel to swim upwards, and the low tide encourages the eel to swim downwards⁶. In this way, the eel zigzags into the Estuary within large shoals. 

Monitoring fish species and data collection

Sampling species of fish within the Estuary proves difficult, due to the inaccessible mudflats and large tidal range¹. Previously, traditional salmon fisheries used baskets known as ‘putchers’ to catch and monitor numbers of migrating salmon⁴. Nets were also traditionally stretched across the River Severn and were used to catch migrating lamprey and eels¹.

Since the 1980s, the cooling water intake screens at Oldbury Power Station and Hinkley Point Power Station have caused some fish to become entrained. This gives researchers and surveyors easy access to different species of fish, for collection, counting and analysis. As certain species of fish are more likely to avoid the cooling-water intake frames and thus will not be collected, there is an element of bias to the data. However, this source of data has provided detailed accounts of seasonal variation in fish numbers and species composition for many years.

The 10 most common fish species caught in the Severn Estuary are (in order): sprat, whiting, sand goby, poor cod, Dover sole, pout, sea snail, bass, flounder, and dab. These make up 90% of all the samples recorded. In particular, sprat and whiting are caught much more frequently than the next most frequently caught, sand goby. The abundance of many species increased between the 1970s and 1990s, largely due to improving water quality. In fact, the species diversity (the number of different species) recorded in the Severn Estuary is increasing by around one species every two years¹. Further improvements in sediment and water quality, combined with decreases in heavy metal contamination, is the likely cause of improving species diversity. However, despite these improvements for some species, other individual species, such as the sprat⁷ have seen decreases in their numbers. Data on the numbers of the sprat population was taken from Hinkley Power Station’s cooling water intakes, highlighting the importance of this method of data collection for monitoring fish species.

More recently, developments in a technology called ‘acoustic telemetry’, was used to monitor the twaite shad, revealing brand new insights into their behaviour⁸. 25 of the fish were tagged and released back into the Estuary. Using a network of receivers, the fish were tracked moving up and down the Estuary, facing barriers such as the Diglis and Powick Weir. Tracking the twaite shads’ movements can identify where these barriers occur. By changing and adapting these weirs, the shad can travel freely along the Estuary.

Commercial and recreational fishing

Commercial fishing within the Estuary is relatively rare, with some small catches landed each year.

Recreational fishing, however, is popular within the Severn Estuary. Near Hinkley Point and Minehead Power Stations, codling, flounder, and whiting are commonly found during winter. During summer, plaice, dogfish, and school bass make an appearance⁹.  At Brean Down, winter cod and conger eels swim around the headland. Clevedon Pier offers a unique opportunity for people to easily access deep water. A fishing ticket can be purchased which allows anglers the opportunity to fish directly from the pier. Summer months bring large species, such as thornback rays and smooth-hound can⁹.  To help manage this activity, the Environment Agency sets out guidelines for each type of fish, which is usually non-fishing seasons, catch and size limits¹⁰.

Conservation

The vast diversity of fish species in the Severn Estuary catchment area, especially migratory species, makes the Estuary an important site of conservation. Monitoring and tracking fish species in the Estuary, using techniques such as acoustic telemetry (see Monitoring fish species and data collection paragraph above) provides information to aid in conservation. Protected species include the salmon, sea trout, river lamprey and twaite shad. Recently, the Environment Agency has proposed new legislation to further protect salmon and sea trout stocks in the Severn Estuary and River Severn¹¹. These fish are now falling below their Conservation Limit, meaning their numbers need to be restored to sustainable levels. By managing fishing techniques and fishing seasons, the Environment Agency aids in the conservation of fish species. Within Wales, Natural Resources Wales operates a ‘catch and release’ policy on salmon and trout fishing.

The Estuary is both a site of Special Scientific Interest (SSSI) and Special Area of Conservation (SAC). It has also been given the international designation of a Ramsar site. For more information, see section on Nature Conservation Designations.

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References

1. Fish and macro-crustacean communities and their dynamics in the Severn Estuary (Henderson and Bird, 2010) https://www.sciencedirect.com/science/article/pii/S0025326X09005190?casa_token=0tui3VqK7S0AAAAA:-uLkW6w1VrHaKiBgPJ7IuBz3VHSmB1KDy-keg7z0hMlKub7g4wFkid7gtH2DbvTruKxxq1Zyb-8
2. Evaluation of the conservation requirements of rarer British marine fishes. Nature Conservancy Council Report No. 1228. (Potts and Swaby, 1991)
3. The biology and conservation of the fish assemblage of the Severn Estuary (cSAC). Report Number CCW/SEW/08/1. © Countryside Council for Wales (Bird, 2008)
4. Lampreys, Life without Jaws. Forrest Text, Ceredigion, UK (Hardisty, 2006)
5. UK Glass Eels: The Life Cycle webpage https://glasseel.com/the-life-cycle/
6. UK Glass Eels: A Trial to Estimate Glass Eel Exploitation in the River Severn (2020) https://ukglasseelscom.files.wordpress.com/2020/09/mark-recapture-study-report-v-1-0.pdf
7. Population regulation in a changing environment: Long-term changes in growth, condition, and survival of sprat, Sprattus sprattus L. in the Bristol Channel, UK (Henderson and Henderson, 2017) https://www-sciencedirect-com.abc.cardiff.ac.uk/science/article/pii/S1385110116301174
8. Unlocking the Severn: Shad Monitoring webpage https://www.unlockingthesevern.co.uk/about-the-shad/
9. British Sea Fishing: South West England webpage https://britishseafishing.co.uk/south-coast-and-south-west-england/
10. Rod fishing byelaws: South West webpage (Environment Agency, 2021) https://www.gov.uk/guidance/regional-rod-fishing-byelaws-south-west-region
11. Proposed new byelaws to protect salmon stocks on the Severn (Environment Agency, 2021) https://www.gov.uk/government/news/proposed-new-byelaws-to-protect-salmon-stocks-on-the-severn
12. River Severn Net Limitation Order and Byelaws 2021 (Environment Agency, 2021) https://consult.environment-agency.gov.uk/west-midlands/river-severn-net-limitation-order-and-byelaws/