R.S.J. Linfield
Anglian Water Authority
Huntingdon, Cambs., England
ABSTRACT
The history of zander introductions within the United Kingdom is briefly reviewed and the current distribution of the species updated to the end of 1981. Fisheries survey data is presented to demonstrate that a severe decline has occurred in the quality of natural coarse fish populations in many of the rivers and drainage channels which have been colonized by the species during the past two decades. Reasons are advanced for attributing this decline primarily to predation by zander and an account is given of an exercise aimed at restoring a satisfactory predator-prey balance in an extensive network of drainage channel fisheries in eastern England. Policies are discussed for the future management of these and other recreational fisheries in the Anglian region which have acquired zander since 1963 and measures are considered for limiting the further spread of the species.
RESUME
L'auteur fait un bref historique des introductions de sandres au Royaume-Uni et indique la répartition de cette espèce à la fin de 1981. Un certain nombre d'enquêtes montrent que la qualité des populations naturelles de poissons communs a nettement diminué dans beaucoup de cours d'eau et de canaux de drainage colonisés par le sandre au cours des vingt dernières années. L'auteur explique pourquoi le prédatisme du sandre est considéré comme la cause principale de cette diminution et rend compte des efforts visant à rétablir un équilibre satisfaisant entre les prédateurs et les proies dans un vaste réseau de drainage de l'est de l'Angleterre. II s'intéresse à l'aménagement futur des pêcheries dans ce réseau ainsi que dans d'autres parties de l'Anglia où se pratique la pêche récréative et où le sandre a commencé à apparaître en 1963. II préconise aussi un certain nombre de mesures pour limiter la propagation de cette espèce.
The zander (Stizostedion lucioperca) was first introduced to the U.K. in 1878 when 24 zander averaging 0.9 kg in weight were netted from Bothkamper Lake in Schleswig-Holstein, Germany, and transferred successfully to enclosed waters at Woburn Park, Bedfordshire (Cawkwell and McAngus, 1976). Further importations occurred in 1910 (Germany to Woburn) and 1960 (Sweden to Mepal Pit, Cambridgeshire) and a number of successful stockings from Woburn to enclosed waters in southeast England also took place between the end of the second world war and 1962. Introductions to the Grand Union Canal and the River Ouzel were carried out in circa 1950 but the fish apparently perished. Thus the species remained very limited in its distribution within the U.K. for the greater part of a century and the preliminary evidence seemed to indicate that it was only in certain favourable years that the species was able to breed successfully (Maitland, 1969). However, the introduction of zander to Norfolk's Great Ouse Relief Channel in 1963 changed the situation dramatically and the species subsequently bred very successfully in the wild and rapidly spread through the adjoining rivers of East Anglia. Currently the colonization of the extensively interconnected rivers of this region of England is still only partial but the species is steadily extending its range and has been illegally introduced to other parts of the U.K. by anglers.
The historical sequence of events concerning the original importations of zander into Britain and the subsequent establishment and spread of the species has already been described in some detail by Cawkwell and McAngus (1976) and by Wheeler and Maitland (1973). The earlier accounts were updated to 1978 in respect of the Anglian region by Linfield and Rickards (1979) and to 1980 by Klee (1981). The distribution at 1 January 1982 is now summarized in Fig. 1. The main advances since 1978, based largely on fisheries surveys and reported captures by rod and line, have involved further progression upstream on the River Cam (including invasion of Reach and Burwell Lodes), completion of the colonization process within the major channels of the Middle Level Drainage System (and also many small interconnecting drains not shown in Fig. 1 for reasons of clarity) and confirmation of successful spawning of the species in the Suffolk Stour. The Stour is believed to have acquired zander as a result of a water transfer scheme (Cawkwell and McAngus, 1976; Linfield and Rickards, 1979) and the first ripe females (three fish), the largest weighing 2.04 kg (4.5 1b) were captured from the river by Anglian Water Authority fisheries staff in May 1977 during a routine fishery operation. Mature fish have subsequently been caught on many occasions and a breeding population has become fully established. In 1980 large numbers of zander fry were also taken from Abberton Reservoir, a 490-ha pumped storage reservoir which draws water from the lower Stour. The Reservoir is listed as being of international conservation importance in the Ramsar Convention.
While the distribution of the species within Anglia has been largely a natural or accidental process since the 1963 stocking, the distribution outside Anglia has resulted from a number of illegal stockings by anglers. Each illegal stocking has provided a new locus for natural expansion of the species through successful reproduction of the stocked fish and this process is now ensuing in many parts of England. For purposes of water cycle management and fisheries administration, England and Wales has been divided into the ten water authority regions illustrated in Fig. 2. A recent enquiry to each of these authorities revealed that officially recognized populations of zander were still restricted to the Anglian, Severn-Trent and Thames Water Authorities. However, in a recent book on the species, Rickards and Fickling (1979) listed 15 counties considered from angling reports to support zander, including some in three further water authority regions. Whatever the true position in these areas may be, the confirmed occurrence of populations of the species in the Midlands Canal system around Coventry, and occasional specimens from the River Severn, suggest that widespread colonization will occur throughout the greater part of England in a manner not dissimilar to that which has already occurred in a large part of Anglia. Unconfirmed reports of the species in the River Thames also give rise to fears of further extensions of the species range over a considerable area in the near future.
Between 1963 and 1980 reactions to the zander in the U.K. varied greatly from fanatical enthusiasm to absolute horror. Views on their likely fisheries impact have thus been strongly conflicting and until 1980 were largely based on angling experiences and subjective assessment. Since Linfield and Rickards (1979) attempt at reconciling the various views against a background of known biological information on the species, a programme of fisheries survey work by Anglian Water Authority fisheries scientists has provided a considerable amount of quantitative data on Anglian river fisheries which has never been previously available (Anglian Water Authority, 1981). The first point to emerge from this work was that the early seventies produced relatively poor recruitment to the cyprinid stocks in the Anglian region (Linfield, 1981) with the result that biomass levels as determined during 1978, 1979 and 1980 were generally lower than might have been expected from the traditionally high standard of the Anglian coarse fisheries. The second major point was that the stocks of cyprinids in river fisheries containing zander had generally fallen to a lower level than fisheries not containing zander and the extent of the deterioration appeared loosely related to the length of time zander had been present. The first open waters to be colonized, the Relief Channel and Ely Ouse, had crashed to very low total biomass levels of circa 2 g/m2 (Klee, 1979, 1979a). Comparison of the structure of the depressed stocks in fisheries containing zander with those not yet colonized immediately suggested a mechanism for the decline. A very strong 1975 year-class for most cyprinid species in the region, particularly for roach, was suppressed in the waters containing zander and a high level of mortality appeared to be acting on young fish in the population. A predator-prey imbalance was clearly evident from a comparison of pike plus zander biomass with cyprinid biomass and the extent of the imbalance appeared loosely correlated both with the length of time zander had been present and the extent to which cyprinid biomass had become reduced. Data to support these conclusions, and other evidence indicating the role of zander in the decline of the fisheries concerned, is given by Klee (1981). The relationship between zander, predator-prey ratios and total biomass levels for 35 Anglian fisheries is summarized in Fig. 3.
It is possible that the Lower Ouse/Middle Level fisheries are exceptionally suitable for zander in the U.K. and that the problems encountered will not occur so extensively in other waters. However, the potential for zander to bring about predator-prey imbalances, particularly during periods of poor prey recruitment, has been shown from the Lower Ouse experience to be such a serious threat that a policy of protection of open fisheries from the species has been firmly adopted throughout'the Anglian region. While colonization of the remainder of the Great Ouse and its tributaries cannot be prevented, the likelihood of the species colonizing the River Nene from King's Dyke (see Fig. 1) and hence reaching adjoining catchments, has been considerably reduced by repeatedly electro-fishing and removing all zander from the last section of the Dyke between two lock structures which themselves provide a partial barrier to emigration. Also chlorination and dechlorination of water passing through the Wixoe Pumping Station on the River Stour has so far prevented the species from traversing the second stage of the Ely Ouse to Essex Water Transfer Scheme and hence colonizing other Essex rivers. The major threat as far as extension of the species range into entirely new catchment areas is concerned appears to come from illegal transfer by anglers, a form of vandalism which has successfully spread the species into other parts of the country as described earlier in this paper. Unfortunately, very little can be done to prevent this activity.
In spite of the U.K. experiences the zander appears to be a much respected species in other European countries, causing little or no damage in sport fisheries, and pike/perch species are also considered highly desirable in North America and Canada. When considering why zander should behave so differently in the U.K., the most pertinent point appears to be that until recently British anglers have generally returned all zander caught alive to the water and thus fishing mortality has not been a significant factor affecting population levels.
In Canadian situations experience has shown that reduction of the mean age of a walleye population to a critical point through fishing mortality can result in a collapse of stocks (P.J. Colby, pers.comm.). Similarly, it is understood that in many European rod and line fisheries the problem with zander appears to be more related to maintaining adequate zander stocks for anglers to catch and remove for eating than in limiting their natural expansion.
In the U.K. pike removal by netting and electro-fishing has been tried many times to reduce predator levels in certain river fisheries but with little or no lasting impact in productive lowland waters. While there seems little hope of the same methods proving any more successful with zander the reported impact on stocks by anglers in other countries seems to offer a means of zander control in the U.K. if anglers could be persuaded to change their traditional habit of returning fish alive to the water. A policy has therefore been adopted of encouraging the culling by anglers of all zander caught and this will be given an extended trial to determine its level of success in preventing further fishery damage by the species.
When considering what steps might be taken to restore those fisheries in which cyprinid stocks have already been damaged a number of practical difficulties immediately arise. The size of the waters involved is such that instant restoration based on restocking is not feasible and reestablishment of adequate breeding stock levels for cyprinids appears the best that can be attempted. Even where this can be carried out the right balance between predators and prey has to be established otherwise the cycle of over-predation would probably be repeated.
An overall assessment of the problem areas in Anglia indicated that the waters requiring restoration fell into two categories: (a) those where the biomass of both prey and predators had crashed to an extremely low level and (b) those where a decline appeared to be rapidly progressing but where an adequate head of prey for breeding purposes was still present and predators were still at a high level. Waters in the first category comprised the Relief Channel, which received the 1963 zander stocking, and the Ely Ouse, the next river to be colonized. In both of these it was possible to recommend restocking with cyprinids to provide the right balance and provide a prey species breeding stock. This action was pursued and 12 g/m2 of cyprinids (60 000 fish of total weight approximately 14.5 t) were stocked to the Ely Ouse during 1980/81. The Relief Channel stocking was commenced in 1981 and is continuing. Waters in the second category were predominantly to be found in the Middle Level system of drains, which is a discrete catchment (bounded by a dotted line in Fig. 1) separated from neighbouring waters by pumps at the downstream end and two lock structures with restricted operation at the upstream end. These waters have traditionally offered some of the best pike fishing in the United Kingdom as well as excellent sport for other species. With the demise of the prey species through enhanced predation pressure coupled with relatively low recruitment in the early seventies, the ration of pike alone to residual prey had become unbalanced over much of the drain network and thus any restorative action in these waters necessarily involved both pike and zander. Upon considering all these factors it was ultimately recommended that an attempt be made at restoring the fisheries in the whole Middle Level system by treating all those waters in direct communication with one another as a single fishery (Anglian Water Authority, 1980). Altogether biomass estimates had been made for fish 10 cm fork length in 30 sections (length 60–380 m each) distributed along representative waters in the system and this data was pooled to provide an approximate estimate of total stocks in the 145 km of drains comprising the whole system (278.3 ha). This procedure smoothed out the extreme imbalances identified in some areas and the following results were obtained:
| Species | Total stock (t) | g/m2 |
| Prey | 6.1 | 2.2 |
| Pike | 3.1 | 1.1 |
| Zander | 0.8 | 0.3 |
| Total | 10.0 | 3.6 |
A decision was taken to aim for a 9:1 ratio of cyprinid biomass to predator biomass (pike plus zander) as a basis for subsequent natural expansion of the cyprinid population. A series of options to achieve this within a single angling season was offered to anglers' representatives based on various combinations of predator culling by the anglers themselves and restocking with cyprinids by the Anglian Water Authority. An option leaning heavily toward predator culling was recommended because of the uncertainties involved in both the acquisition of roach and bream for restocking and their likely survival rate in their new environment.
In August 1980 it was agreed between the Authority and the angling bodies involved that 3 117 kg of pike and zander would be removed (80 percent of estimated total available) by anglers followed by the introduction of 891 kg of roach and bream by the Authority. A compromise had to be reached with the Pike Anglers' Club of Great Britain, however, whereby all pike in excess of 10 lb weight (4.54 kg) would be selectively preserved. An extensive system was set up for receiving and scrutinizing cull returns and it was agreed that the situation would be reviewed at the end of the year when additional survey data from field work in July (eight further sections of total length approximately 1.5 km) would be available and the rate of culling achieved in practice could be assessed. The review document (Anglian Water Authority, 1981a), produced in January 1981, revealed the following revised stock levels for the system at the pre-cull stage.
| Species | Total stock (t) | g/m2 |
| Prey | 6.3 | 2.3 |
| Pike | 2.7 | 1.0 |
| Zander | 0.6 | 0.2 |
| Total | 9.6 | 3.5 |
The implications of these revised figures were considered in the light of the progress of the cull at that stage and crude estimates that were attempted of likely levels of production of predator flesh within the fishery during the period of the cull. Following this assessment, during which it was evident that the zander stock level had been substantially underestimated, it was concluded that the original cull target should be retained but should be broken down into 639 kg of zander (the total weight estimated to have been present) plus 2 478 kg of pike. Zander culling would continue indefinitely beyond the target (shown to be low) whereas pike culling would be called to a halt as soon as the target figure was reached. This was achieved before the end of February and, when all outstanding returns had been collected and verified as far as possible, the final figures stood at 1 415 kg for zander (984 fish) and 2 971 kg for pike (1 447 fish).
The roach and bream to be restocked were provided during April and May 1981 and released at various points within the system. This stocking actually involved 1 188 kg of fish and a further 187 kg were added in February 1982.
The Middle Level cull has been viewed as an experiment in the management of lowland fisheries which have incurred predation problems from zander and arrangements have accordingly been made for monitoring the outcome. A consortium of sponsors, namely the Angling Foundation, the Water Space Amenity Commission, Sports Council and the Anglian Water Authority have collectively financed a three-year research project on predator/prey relationships in one of the major drainage channels within the Middle Level system with a view to assessing the effectiveness of the cull and monitoring subsequent developments within the fishery. The work is being carried out by Liverpool University and will continue until 31 December 1983. In addition, the Anglian Water Authority will repeat routine survey coverage within the system during the next few years to provide broadly based quantitative data for various parts of the drainage network.
Angling results during 1981 and survey catches by workers from the University suggest that the Middle level cull has been very successful in reducing the zander biomass to a low level. However, young zander from 1980 and 1981 spawnings are well in evidence within the fishery and young pike from these year-classes are also fairly numerous. For the pike a good cross section of age groups and size classes has also survived the cull leaving a good breeding population still active within the fishery to ensure a rapid recovery of this species. While preliminary indications suggest the management action has been successful in achieving the initial objective, good spawning and a rapid re-expansion of the cyprinid stocks is essential during the period of reduced predation pressure (probably only three to four years) if the ultimate goal of a high quality fishery for both cyprinids and pike is to be re-established. It now remains to be determined what the subsequent course of events will prove to be. If the management action is successful, and zander stocks can subsequently be kept at a low level by persistent angler culling in the future, then the species may yet come to be viewed more favourably in the U.K. than has so far been the general case. However, if the action should fail and zander should come to enjoy a further rapid expansion in numbers in the Middle Level system with subsequent damage to cyprinid stocks, then fishery managers in the U.K. will have a persistent problem of some magnitude on their hands and the popularity of zander in the U.K. will remain restricted.
The author wishes to thank Mr. A.W. Davies, Director of Scientific Services, Anglian Water Authority, for his support in the preparation and subsequent presentation of this paper to the Symposium. Any views expressed are those of the author and not necessarily of the Anglian Water Authority.
Anglian Water Authority, 1980 Regional policy for zander and proposals for the Middle Level system. Committee report. Anglian Water Auth.Comm.Rep.Huntingdon, (F.26/80):14 p.
Anglian Water Authority, 1981 List of internal reports on fisheries surveys and investigations, 1978–80 inclusive. Huntingdon, Anglian Water Authority, 4 p. (mimeo)
Anglian Water Authority, 1981a Progress with management action and research in the Middle Level system. Anglian Water Auth.Comm.Rep., Huntingdon, (F.7/81):5 p.
Cawkwell, C. and J. McAngus, 1976 Spread of the zander. Angler's Mail, March 3 issue:12–3
Klee, C., 1979 Report on fish survey of the Ely Ouse from Little Thetford to Denver. March and July 1979. Report. Huntingdon, Anglian Water Authority, 27 p.
Klee, C., 1979a Report on fish survey of the Relief Channel, October 1979. Report. Huntingdon, Anglian Water Authority, 21 p.
Klee, C., 1981 An assessment of the contribution made by zander to the decline of fisheries in the lower Great Ouse area. In Proceedings of the Second British Freshwater Fisheries Conference. Liverpool, University of Liverpool, pp. 80 9
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 | zander present |  | zander not recorded |
The colonised sections of the River Stour (bottom right corner) continue downstream to Cattawade barrage, at the head of the estuary
* Modified with permission from Linfield and Rickards, 1979.
Fig. 1 The distribution of zander in Anglian watercourses at 1 January 1982
REPRODUCED WITH PERMISSION FROM THE NATIONAL WATER COUNCIL, 1982
Fig. 2 The areas of the ten Regional Water Authorities in England and Wales
(a) Relationship between total fish biomass and the presence of zander.
 | Zander present≥ 10 years at the time of the survey. |
| Zander present 4–9 years at the time of the survey. | |
| Zander present < 4 years or not recorded at the time of the survey. |
(b) Relationship between total fish biomass and the percentage of predator biomass (pike + zander) in the population.
| Pike + zander occupy > 30% total fish biomass. |
| Pike + zander occupy 20–30% total fish biomass. | |
| Pike + zander occupy < 20% total fish biomass. |
Fig. 3 The relationship between zander, predator: prey ratios and total fish biomass (fish ≥ 10 cm fork length) in 35 Anglian river fisheries. The data was collected during 1978–80 and the fisheries are identified in Linfield (1981)
