Freshwater Fish By Richard Shillaker

RICHARD’S NATURE NOTES: ​​​​ April, 2020

Dr Bernd Hänfling’s ‘fish detectives’ talk to WARCS about the use of environmental DNA (eDNA) to detect the species present in lakes and other water bodies got me thinking about some lesser known British freshwater fish.

The rarest freshwater fish in the UK is the Vendace, which in England is present only at Derwent Water and Bassenthwaite Lake in the Lake District. To protect the species, after it became extinct in Scotland, individuals have been translocated from these two English lakes to three Scottish sites. The closely related Powan is also only found in a few large cold lakes in the UK. Both these species belong to the Whitefish family. The Arctic Char, a member of the related Salmon family, is another species found in only a few cold-water lakes. Following a population decline, the Arctic Char has been the subject of a breeding programme at the Environment Agency’s hatchery at Kielder Water in Northumberland. Its decline has been attributed to pollution, acidification and global warming. ​​​​ 

Rare species in lowland England include the Spined Loach, so called because of the movable two-pronged spine below each eye. This fish burrows in the soft muddy bottom of slow flowing rivers in eastern England, eg the Trent and Welland. It has a peculiar method of feeding: it uses mucus to extract fine food particles from water pumped through the mouth.

The Burbot (also called Eelpout or Lingcod) is the only freshwater member of the Cod family in Europe. It is a large, strange looking bottom-living fish with a long body and three barbels on its head (a long one on its lower jaw and a short one at each nostril). ​​​​ It used to occur in lowland rivers in eastern England but the last definite record was in 1969. The reasons for its disappearance are thought to be related to pollution and the tidying up of natural “messy” edges of rivers including adjacent pools used for spawning. There are plans to reintroduce the fish to rivers in Norfolk; eDNA will be used to check that the species has indeed died out.

Another interesting bottom-living fish is the Miller’s Thumb or Bullhead. As a boy I used to catch this small fish with a large head and tapering body in stony-bottomed streams in Hertfordshire. It is also a member of a mainly marine family (the Sculpins or Cottidae). Although poorly studied, it is known that the male excavates a nest and then attracts a female by making knocking sounds. After eggs have been laid, the male looks after the brood by fanning water over the eggs and driving off potential predators. Unfortunately, they suffer from competition for food and

shelter, and from predation, by the non-native North American Signal Crayfish. The Miller’s Thumb is now included in national and international conservation measures.

A small, silvery fish that I particularly like is the European Bitterling. It is a non-native species that has become established in a number of ponds, small lakes and canals in the UK. I used to have a few in my garden pond but local supplies have dried up as you now need a licence to keep them. In the breeding season, males become more colourful and females develop a long ovipositor tube. A pair will establish a territory around a freshwater mussel. After courtship the female lays eggs inside the mussel by inserting her ovipositor into the bivalve’s inhalant siphon. The eggs are fertilised by the male releasing sperm into the mussel’s inhalent current. An interesting fact about this species is that the hormone-dependent elongation of the female’s ovipositor was investigated in the 1930s as a potential test for human pregnancy (The Bitterling Test).

Although the Bitterling is now essentially a banned fish in the UK, I have not found any reports of it having had negative effects on our native aquatic biodiversity. About a dozen non-native freshwater fish have established populations in the UK. Well known examples include the Goldfish, Orfe and Common Carp. The last species, a native of Eastern Europe and Asia, was introduced into the UK many centuries ago. Monks in the Middle Ages reared Carp as a food source, including presumably in the fish ponds at Meaux Abbey near Beverley.

One non-native invasive species that has caused particular concern is the Topmouth Gudgeon. The upward pointed mouth allows the fish to feed at the water surface, unlike our native Gudgeon which is a bottom feeder. The Topmouth Gudgeon is a small fish (4-8 cm long) which was first imported into the UK from Eastern Asia in the 1980s for the ornamental market. Although small, they are a threat to native fish because they eat their eggs and larvae, outcompete them for food and carry a parasite. They can rapidly increase in numbers as they are able to spawn up to four times a year. The Environment Agency has tried to eradicate this species from the UK and by May last year had reduced its occurrence to just 3 or 4 known sites.

Finally, I should mention the Grayling, an attractive shoaling fish with a large brightly coloured dorsal fin. It occurs in swiftly flowing clean rivers, including the chalk-stream upper reaches of the River Hull. Individual fish can be identified from the number and pattern of black dots on the skin. I have never seen this so called ‘Lady of the Stream’ but would love to do so. ​​​​