Fish are by far the most important vertebrates
inhabiting fresh waters. In Britain,
they are more widely distributed and more abundant than other aquatic vertebrates
and their value both in terms of human food and sporting interest runs into millions
of pounds annually. Relatively few fresh waters in Britain are fishless though many
isolated lakes must owe their present fish stocks to man-made introductions in the
past. Fish are not found in Britain in lakes above 850 m in altitude and although
attempts have been made to introduce fish to lakes above this level the stocks
soon died out. Waters that frequently dry out in summer months are also fishless, as
no British species is truly resistant to desiccation, although certain cyprinids, such
as the tench
Tinea tinea, may survive for short periods
in wet mud. Very acidic peat pools are
also generally without fish, as pH values of less than about 4.5 are directly toxic to
most fish species. Fish are now absent from the most grossly polluted stretches of
rivers running through industrialised Britain, and from a small number of lakes
where ancient mine workings cause unusually high concentrations of toxic heavy
metals in the water.
The habitat preferences of the British
freshwater fish are somewhat obscured by
the uneven geographical distribution of several species. With the exception of
those which can tolerate sea water and can enter the sea, the powers of natural
dispersal of freshwater fish are very limited. Eels Anguilla anguilla, salmon Salmo
salar, brown trout Salmo trutta (and its sea-going race the sea trout), flounder
Platichthys flesus, three-spined stickleback Gasterosteus aculeatus, ten-spined
stickleback Pungitius pungitius, marine lamprey Petromyzon marinus, and river
lamprey Lampetra fluviatilis, frequently enter the sea and have become distributed
in much of the British Isles in suitable waters. Apart from these, few fish have
spread further north than the central lowlands of Scotland after their recolonisation
of south-east England in the last interglacial, while Britain was still connected to the
Continent. The two most successful colonists are the minnow Phoxinus phoxinus,
which has been largely spread by man as fish bait, and the perch Perca fluviatilis,
which lays long sticky ribbons of eggs which attach to wildfowl. Until man recently
assisted their dispersion by canal-building and deliberate introductions, some
species (e.g. barbel Barbus barbus, bleak Alburnus alburnus, ruff or pope
Gymnocephalus cernua, white (or silver) bream Blicca bjoerkna and burbot Lota
lota) were more or less restricted to those British rivers which flowed into the Rhine
when the landbridge between Britain and the Continent was intact. Gudgeon Gobio
gobio, dace Leuciscus leucisais, chub Squalius cephalus, rudd Scardimus erythro-
plithalmus, tench Tinea tinea and bream Abramis brama, have extended as far
north as the Scottish Border. Some species have crossed the barrier of the
Southern Uplands of Scotland and have colonised lowland Scotland and some
have just penetrated the Highlands. This group comprises grayling Thymallus
thymallus, roach Rutilus rutilus, stone loach Noemacheilus barbatulus, and bullhead
Coitus gobio. The pike f^sox Indus extends north as far as Ross but its natural
distribution is obscured by its introduction as a food fish in the past. Other species
such as the Coregonus spp., C. lavaretus and C. albula, and charr Salvelinus
alpinus, are glacial relicts and are now restricted to generally deep waters in the
north and west.
OLIGOTROPHIC LAKES
In Britain such waters generally have
a fish fauna dominated by salmonids. In those
lakes containing fish, brown trout are invariably present and, where the outflow
gives uninterrupted access to the sea, these are joined by salmon and sea trout.
These species of salmonid require a clean gravel through which a current of water
is flowing in which to spawn. Many such lakes, particularly those that are large and
deep, contain charr, and a few in Wales, the Lake District and South Scotland
contain white fish Coregonus lavaretus or vendace C. albula. Charr can seldom
survive in competition with Coregonus spp. and few lakes in Britain contain both.
Oligotrophic lakes also contain the ubiquitous eel and three-spined stickleback as
well as one or more of the three lamprey species (Lampetra planeri, L. fluviatilis
and Petromyson marinus). Minnows are not found in Scotland much further north
than the Great Glen but are widespread in stony-shored lakes further south. Perch
and pike are also found in many oligotrophic lakes but become more dominant in
slightly richer waters. In lakes within the range of the cyprinid fishes (south of Loch
Lomond) species such as roach may be found but only become abundant in more
eutrophic waters. Stream-dwelling fish such as stone loach and bullhead may
extend out from stream mouths into oligotrophic lakes.
MESOTROPHIC LAKES
In mesotrophic lakes in England and Wales
perch, pike and various cyprinid fishes,
especially roach, may begin to replace the salmonid fishes. Thus, in Esthwaite
Water with increasing nutrient enrichment, roach and rudd have greatly increased in
numbers in recent years at the expense of the trout population. Small numbers of
roach and rudd have been caught more recently in the south basin of Winder-mere.
Esthwaite is the most evolved of the larger Lake District lakes and is the only one
which does not contain either Coregonus spp. or Salvelinus alpinus. Charr are
generally only found in oligotrophic lakes in Britain, though they were formerly found
in Loch Leven (alkalinity = 35-70 p.p.m. CaCO3), Kinross, and occur in one marl
lake, Loch Borralie (alkalinity = 140-150 p.p.m. CaCO3). The richest water in which
Coregonus is now found in Britain is the Mill Loch, Lochmaben (alkalinity = 50-60
p.p.m. CaCO3), and they have disappeared from the neighbouring shallower and
more enriched Castle Loch.
EUTROPHIC LAKES
Cyprinids are usually the most abundant
fish in eutrophic lakes in England and
Wales but, in Scotland, outside the range of this family the fish fauna may be very
similar to that of oligotrophic waters. In eutrophic waters in Scotland self-
maintaining populations of fast-growing brown trout may exist. On the other hand,
the silted conditions of the inflows in many eutrophic waters in the south of Britain
may prevent natural spawning and the maintenance of a trout fishery will depend on
regular re-stocking. In such cases rainbow trout Salmo gairdneri are often stocked
instead of the native brown trout. The murky phyto-plankton-laden waters of many
eutrophic lakes militate against visual feeders such as the salmonids and favour
fish such as the cyprinids which use their sense of smell to locate food. The latter
also require weed-beds in which to spawn and these are generally more abundant
in eutrophic lakes. Consequently, where there is natural competition between these
two groups, the cyprinids are usually favoured in eutrophic waters and the
salmonids often eliminated.
Of the few species of fish which only
occur in abundance in enclosed standing
waters, carp and tench are bottom feeders, and rudd feed at the surface. Many
other species found in standing waters also occur in the slower stretches of lowland
rivers (the so-called bream zone).
A few exotic species have been introduced
into small lakes and ponds in southern
England. These include the wels Silurus glanis, which came into Britain with the
aquarium trade and has since been spread by anglers, the pike-perch Stizostedion
lucioperca, and various American bass belonging to the Centrarchidae. The carp
was originally introduced into Britain by monks in the sixteenth century and is now
widely distributed in scattered localities throughout lowland Britain, but is abundant
only in the southern half of England. The bitterling carp Rhodeus sericeus is now
fairly widely distributed in canals and slow-flowing rivers in England, where it has
been introduced by aquarists.
