The freshwater fish of Britain demonstrate, as much as any group of animals, the influence of man on the aquatic environment. Drainage of wetlands and pollution of our freshwaters are easily apparent, but other less obvious changes have also occurred. The native fish species of Britain have been widely redistributed and the introduction of exotic species is widespread. It is likely that this will continue to some extent in the name of "fishery management". To understand the extent of these anthropogenic influences the original distribution of freshwater stenohaline fishes within Britain is of interest. Roach (Rutilus rutilus L.), pike (Esox lucius L.), bream (Abramis brama L.) and perch (Perca fluviatilis L.) are widespread today and although their spread has been aided by human intervention, their adaptability has ensured their success. Other species, such as silver bream (Blicca bjoerkna L.), ruffe (Gymnocephalus cernna L.), bleak (Alburnus alburnus L.), barbel (Barbus barbus L.) and spined loach (Cobitis taenia L.) have until relatively recently been confined to rivers in Eastern England from Yorkshire to the Thames. As one moves West or North the number of species declines, with Ireland possessing no native, solely freshwater fish (Wheeler, 1974). Today many species of fish are widely distributed around Britain and written reports of their redistribution are numerous. Even species not angled have been redistributed. A Cornish naturalist, Mr. J. Couch, describes the introduction of the stone loach (Barbatula barbatula L.) to a small stream running through his village in the early 1800s (Wheeler, 1974).

The introduction of exotic fish species has also been widespread. Indeed Wheeler and Maitland (1973) recorded the introduction of nineteen species over the last century. In addition there have been more introductions since 1973 and several species have been in Britain for many centuries. Carp are a long established introduced fish. Native to the Eastern Mediterranean and Black Sea basins, carp was first described as a British fish in Dame Juliana Berner's "Boke of St. Albans" published in 1496 (Regan, 1911). However it was probably introduced by monks in the early part of the 15^th century (Wheeler, 1974). The majority of species introduced from North America and Europe fail to breed and spread probably as a result of climatic factors. However, along with carp, several other species are widespread in Britain. In particular rainbow trout (Oncorhynchus mykiss Walbaum) and zander (Stizostedion lucioperca L.). This is as a direct result of stocking and redistribution of these species by anglers.

Other than direct redistribution and introduction by man, the canal system of Britain has played a considerable part in the distribution of species seen today. It started on a large scale in 1761 with the construction of the Bridgewater Canal and continued until at its peak there were 4000 miles of navigable canals.

The impacts on the aquatic environment that redistribution and introductions have wrought are considerable. Lachner et al., (1970) and Voreen (1972) studied the impacts of fish species introduction and redistribution in North America and the Netherlands and found there to have been many detrimental effects. These range from the introduction and spread of disease, genetic dilution, competition for resources and piscivory. In Ireland introduced pike has heavily predated the native salmon (Salmo salar L.) and this seems to have occurred in England as well. Barbel introduced into the primarily salmonid rivers of the West of England are unlikely to compete for food with salmonids, however barbel have been shown to interfere with salmonid spawning redds (Wheeler, 1974). In addition other aquatic flora and fauna have been affected by changes in fish populations. For instance brown trout (Salmo trutta L.) introduced into a moorland tarn removed the majority of tadpoles, Notonectidae and Dytiscidae beetles (Macan, 1965) and the role of cyprinids in destroying macrophytes is documented by Cahn as early as 1929.

With recreational angling on the increase and with many waters stocked and managed to maintain artificial fish populations the trend of redistribution and introduction is likely to continue. Many anglers in Britain seek the widespread introduction of the American large mouth bass (Micropterus salmoides Lacepede). This species is considered an angler's fish and has been introduced to France. Wutz-Arlet (1952) studied these introductions and found it to be a direct competitor for food with trout and as a piscivore the effect of its introduction into a mixed fishery could be serious. The continued redistribution and introduction of fish species and the lack of knowledge surrounding the effects that these processes have is worrying and needs fuller and further investigation.

Between 1970 and 1994 two surveys looking at anglers' preferences for fish species found that these preferences had changed. Roach was the species most preferred by anglers in 1970 (39%), but fell to 28% of anglers in 1994. Pike were the favourite fish for 29% of anglers in 1970 but in 1994 only 12% of anglers wanted to catch pike primarily. By contrast in 1994 carp was the most popular fish with 36% of anglers stating its preference (NRA, 1994; NRA, 1970). A survey of the stocking regimes at Sites of Special Scientific Interest (SSSIs) in 1998 (detailed in this thesis) found that stocking, and therefore the preferred angling species, was dominated by the introduction of carp and bream. In addition although roach were still the most frequently caught fish, as they were in 1970, bream and carp were in the top five of most frequently caught fish. It was also found that clubs at SSSIs were generally trying to increase numbers of common carp, bream and tench (Tinca tinca L.) through stocking and to reduce numbers of pike with the aid of culling programmes. It is clear that angling is a changing sport. Angling is also a popular sport and a major recreational activity in Great Britain (Photo. 1.1). Estimates of the total gross national spending on coarse angling are over £2 billion each year (NRA, 1994). In addition it is estimated that Europe spends $1 - 4 billion p.a. on fish stocking, yet codes of practice and ecological and economic implications of stocking are largely unquantified. (Cowx, 1994).

In the past stocking with pike and roach has apparently had little impact on aquatic ecosystems. However more recent trends of stocking with benthivorous fish such as carp and bream has led to concerns about the impacts these stocked species may have on the aquatic environment. As early as 1929 Cahn described how a pond, filled with macrophytes and containing various native fish species, became turbid and devoid of plants following the introduction of carp. King and Hunt (1967) found that carp were excessively destructive to submerged vegetation. Crivelli (1983) found a strong negative correlation between carp biomass and macrophyte abundance. Decline of aquatic plants consequently causes a reduction in populations of large invertebrates such as snails and dragonflies. Other fish suffer in turn as they are dependent upon the invertebrates for food, spawning sites around the macrophytes or water clarity for visual hunting and feeding. In addition herbivorous and piscivorous birds may also disappear (Giles et al., 1990; Phillips, 1992; Dennis, 1996). Conversely the removal of fish such as carp and bream from lakes has often seen submerged macrophytes return and flourish (Wright and Phillips, 1992; Van Donk et al., 1989, 1990; Ozimek et al., 1990).

Benthic-feeding fish suck up sediment and food organisms, filter out the organisms and eject the sediment, clouding the water (Lammens and Hoogenboezem, 1991). Bream and carp in particular have been shown to increase sediment re-suspension (Breukelaar et al., 1994; Meijer et al., 1990) and settlement of winnowed sediment can smother young plants (Wright and Phillips, 1992). Fish can also disturb macrophyte propagules by their direct movement. De-oxygenated sediment disturbed through fish bioturbation can release nutrients, in particular phosphorus, into the water column (Phillips et al., 1994; Graneli, 1979) and a further indirect pathway for nutrient release is via fish excretion (Andersson et al., 1988). Increased nutrient loading has been shown to increase crops of phytoplankton and epiphytic algae, both of which compete against macrophytes for light and CO₂. High O₂ and pH regimes unfavourable to macrophytes (Sand-Jensen, 1983; Simpson and Eaton, 1986; Phillips et al., 1978) can also be created by attached epiphyton. The removal by fish of invertebrate grazers that feed on this epiphyton further increases competition (Bronmark and Weisner, 1992). In addition the presence of fish in a lake invariably reduces large bodied zooplankton populations, typically resulting in increased phytoplankton populations (Duncan, 1997; Schriver et al., 1995; Irvine et al., 1989; Andersson et al., 1978) which can shade out macrophytes. This is especially true where no macrophytes are present and acting as zooplankton refugia (Timms and Moss, 1984; Lauridsen and Buenk, 1996). All these fish effects can favour the growth of algae and competitively disadvantage macrophytes, although the relative importance of each mechanism is debatable. The decline of macrophytes and subsequent dominance by phytoplankton is highly undesirable in most lakes but especially shallow SSSIs where SSSI status is often assigned due to the specific macrophytes present. Therefore whilst the stocking of waterbodies with carp and bream has become prevalent, it has been at the possible expense of the aquatic ecosystem. It is therefore imperative to understand what species, at what densities and at what sizes can be stocked such that the environment is not adversely affected but still allows angling to take place, for angling is to many individuals a traditional part of human nature.

For the most up to date results and data please look at my published papers found on my CV page.