A CENTURY OF PIKEPERCH IN DENMARK

J.Dahl
Inland Fisheries Laboratory
Silkeborg, Denmark

ABSTRACT

Pikeperch (Stizostedion lucioperca (L.)) was first introduced into Denmark in 1879. Since then, an increasing number of introductions - most of them successful - have been made and the pikeperch is now well established in about 70 lakes and larger river systems in Denmark. Today, pikeperch occupies second place in the total catch of commercial species in Danish inland fisheries and is only surpassed by eel. The first introductions were made with eyed ova and fingerlings from Germany but during and just after the first world war, fingerlings were imported from Sweden. Since about 1930 pikeperchless lakes have been stocked with undersized fish from well established national populations.

Annual commercial catches of pikeperch amount to 49–90 t. Average yield in commercially-exploited lakes range between 3–9 kg/ha.

Pikeperch has become a useful addition to the Danish inland fish fauna. It is an effective predator of valueless species which compete with eel in turbid, eutrophic lakes, and it has added invaluably to previous non-profitable commercial lake fisheries. Furthermore, being originally regarded with suspicion by anglers it is today a highly praised game fish.

RESUME

Le sandre (Stizostedion lucioperca (L.)) a été introduit pour la première fois au Danemark en 1879. Les introductions, de plus en plus nombreuses au fil des ans, ont en général été couronnées de succès. Le sandre est aujourd'hui bien établie dans une soixantaine de lacs et de grands systèmes hydrographiques du Danemark. Il vient en deuxième position, juste après l'anguille, dans les captures d'espèces commerciales dans les eaux intérieures danoises. Les premières introductions ont été faites avec des oeufs embryonnés et des alevins importés d'Allemagne. Durant la première guerre mondiale et dans les années qui ont immédiatement suivi, des alevins ont été importés de Suède. Depuis 1930 environ, on introduit le sandre dans des lacs dont il est absent en utilisant des individus d'une taille inférieure à la normale que l'on prélève dans des populations bien établies dans le pays.

Les captures commerciales de sandre sont de 49–90 t par an. Le rendement moyen des lacs qui font l'objet d'une exploitation commerciale est compris entre 3 et 9 kg/ha.

Le sandre complète avec bonheur la faune des eaux intérieures danoises. C'est un prédateur efficace des poissons sans valeur qui font concurrence à l'anguille dans les lacs eutrophes aux eaux turbides. Il a rendu la pêche commerciale rentable dans plusieurs lacs. Enfin, il est aujourd'hui très apprécié des pêcheurs sportifs qui s'étaient pourtant montrés à l'origine assez réservés à son égard.

1. INTRODUCTION

Pikeperch (Stizostedion lucioperca (L.)) is widespread on the European continent, having invaded most of northern and western Europe, including Sweden and eastern Norway, during its westward migration after the last glaciation. But it never succeeded in reaching Danish inland waters. Therefore, pikeperch is regarded as an exotic species in Denmark, even if today pikeperch is present in 70 Danish lakes. These populations have been created by active introductions over the last 100 years.

Most Danish lakes are by nature good pikeperch waters. As the majority are situated in the part of the country which was covered by ice during the last glaciation, they rest on morainic deposits rich in minerals. To this must be added a constant supply of nutrients from intensely fertilized fields and sewage from urbanized areas. The typical Danish lake is therefore generally highly eutrophic, having a high primary production, a high degree of turbidity during summer and a correspondingly low visibility. The fish population is dominated by cyprinids and eel, with only a few predatory species of which the pike is the most important.

As lake eutrophication has been increasing during the past 50 years, the pike has been badly affected and is often unable to effectively fulfil its predatory functions. In the absence of an indigenous pelagic predator, like the pikeperch, this has caused an unbalanced fish population dominated by species which both from a commercial and a recreational point of view are regarded as valueless or even noxious (competitors with eel).

During the past century the efforts to introduce pikeperch into Danish lakes have primarily aimed at supplementing the indigenous fish fauna with a predatory fish to fill an empty niche in the ecological lake community and, secondly, to add to the Danish inland fishery a valuable fish species to benefit both commercial and recreational interests.

The following paper describes the history of pikeperch introduction in Denmark, the stocking practice and the significance of this important addition to the indigenous Danish freshwater fish fauna.

2. HISTORY OF INTRODUCTIONS

The first attempt to introduce pikeperch into Danish inland waters was made in 1879 when a group of enthusiastic fishermen transferred a few (20) fish of about 1.5 kg each from Lake Borgdorf in south Schleswig (Germany) to the Odense River in Funen. However, mainly because of the very primitive means of transport available, this first initiative failed and another attempt was not made until 20 years later, when in 1898 200 pikeperch (weighing between 125 g and 1 kg) were transferred from the Jels lakes in north Schleswig (which at that time was under German government) to a lake close to the then Danish-German border. This time the introduction succeeded, and pikeperch still occur in this lake. From that year on pikeperch introductions were made with increasing intensity.

In the years up to the first world war all stocking material was imported from Germany, both as fry, fingerlings or larger fish and as eyed ova. However, due to a lack of knowledge of proper transportation techniques and the ecological requirements of the pikeperch, many of these first attempts failed. Out of 16 introductions in nine inland waters made up to 1914 only three were successful resulting in lasting, self-reproducing populations. In two of these cases stocking was made with two-year old fish or spawners, in the third case with eyed ova (200 000–300 000 ova per year over a five-year period).

It could be said that in these years the success of a pikeperch introduction depended on sheer luck.

During and after the first world war, pikeperch was stocked into an increasing number of lakes, the main part of the stocking material now being provided by Swedish fish farms, where in these years artificial propagation of pikeperch was rapidly developing. Gradually, the majority of transfers were of one and two-year old fish and spawners from national stocks which by that time were well established. Since the middle thirties introductions have been made exclusively with two-year old or older fish from national stocks. With a few exceptions an increasing number of liberations were successfully undertaken, and since the beginning of the fifties all introductions have been successful.

Today pikeperch occur in 70 Danish fresh- and brackish-water areas. Out of these 51 have been established by direct stocking, whereas 15 other lakes have been populated by invasion from stocked populations in other lakes within the same river systems. In four additional areas, stocking has been undertaken within the last few years so it is too early to evaluate success.

Pikeperch are present to this day in two lakes in South Jutland, most probably as a result of liberations undertaken when this part of the country was under German government (1864–1920). We do not know when these liberations took place, most probably though before 1898.

Table 1 shows the different types of Danish inland waters where pikeperch has been introduced during the last 100 years, both with success and failure.

The diagrams in Figs. 1a-c summarize the history of pikeperch introductions into Danish inland waters. In Fig. la the columns represent the total number of introductions undertaken (128). The hatched part of the columns represents the first introduction made, thus showing the total number of waters areas stocked. The diagram illustrates the stocking activity throughout the period, showing three peaks, one before 1914, a second between 1915–34 and a third from about 1950 onward.

In Fig. 1b the total number of introductions is separated into those which failed and those which have proved successful. Fig. 1c again shows the total number of introductions, but here illustrating the different origin of the stocking material throughout the years.

3. STOCKING PRACTICE

During the first half of the century most pikeperch introductions were undertaken uncritically and unprofessionally, often with no regard to whether the water in question was suitable for pikeperch or whether the stocking material at hand was appropriate, hence the many unsuccessful introductions in these years (Table 2, Fig. 1b).

Since 1951, however, the Danish Inland Fisheries Act states that any new introduction of pikeperch requires permission from the Ministry of Fisheries. This means that from this year on all pikeperch introductions have been performed under control. Also the catch and sale of undersized pikeperch for stocking requires permission from the fishery authorities.

Nowadays almost all new introductions are undertaken with two-year old (20–30 cm) fish as stocking material. Normally a stocking rate of 2 kg/ha (4–5 fish per kg) is recommended. But for economical reasons lower rates are often used. It is recommended that stockings be repeated at least through a three-year period and the introduced fish be protected from fishing until natural reproduction has been verified. Experience has shown that a newly-created pikeperch population can be exploited between five and eight years after the first introduction.

4. YIELD

Since 1925, when the total catch of pikeperch exceeded 1 000 kg for the first time, the yield increased up to 1935, since when it has stabilized on a level of between 5 and 8 percent of the total catch in Danish inland waters (40–90 t/year). Of the commercial species, pikeperch is today second both in weight of catch as well as in value in the total inland water yield, and is only surpassed by eel.

The yield per ha varies between different lakes, mainly depending on the type of exploitation. Table 3 gives a few examples of lake yields in kg/ha in relation to lake type, area, depth and type of fishery.

The table shows that commercially-exploited, deep, freshwater lakes give the highest yield, the lowest yields being obtained in shallow, brackishwater lakes. The great variation in annual yields through the nine-year period may be due partly to variations in catch per unit effort and/or to climatic changes resulting in strong or weak year-classes.

An average yield of about 5 kg/ha should be regarded as normal for Danish pikeperch lakes.

Originally, pikeperch was introduced in order to supplement the indigenous fish fauna with a valuable commercial species. This aim has been successfully achieved. As regards the recreational value of the pikeperch it is interesting to note that for many years anglers regarded the pikeperch with great suspicion. With a few exceptions this attitude prevailed up to the fifties, and in some manuals for anglers it was even stated that the pikeperch is a fish which should not be tolerated in a good fishing water. Nowadays, this situation has completely changed and today it is a highly praised game fish, even if angling for pikeperch must be considered a fishery for specialists. The preferred fishing method is trolling, either using live or artificial bait. Having first found the right method, a few sport fishermen can be responsible for quite a considerable catch. This may be seen from the diagram in Fig. 2, which illustrates the development of the pikeperch fishery in Lake Furesö, the deepest Danish lake (36 m), situated a few kilometres north of Copenhagen, Zealand. The lake is exploited both commercially and recreationally. Since the beginning of the century, at a time when the lake was exploited only by anglers, several attempts were made to create a pikeperch population, the first attempt as early as 1903. The angling club in question tried several times to stock pikeperch-imported from Germany - in the lake, but the attempts all failed. Not until 1964, when new and more professional introductions were made, using two-year old fish as stocking material, the creation of a good pikeperch population in the lake eventually succeeded. The diagram shows the development of the pikeperch fishery in the lake since its introduction in 1964. The catches from this particular lake can be separated into a commercial portion and a recreational portion. It is seen that pikeperch entered the fishery in 1970, six years after the introduction, first appearing in the anglers' catches. In 1973 pikeperch for the first time appeared in the commercial catch, due to the fact that the commercial fisherman has protected the newly created population, partly until a reasonable catch could be obtained and partly until the time when the first young pikeperch were found, proving that pikeperch had completed its first natural reproduction in the lake. From 1973 onward, pikeperch catches have increased, until 1979, the greatest portion being in the commercial catches. But from 1980, the situation has changed so that the anglers' catches now surpass the commercial catches. It is remarkable, however, that this high catch is obtained only by two or three anglers - to the great annoyance of the commercial fisherman who condemns it as overfishing and fears that this heavy fishery will eventually be detrimental to the pikeperch population. The catches illustrated in the diagram, however, only represent about 1 kg/ha so it is considered that the population is quite able to withstand the fishery.

5. THE FUTURE

Even if in a zoogeographical sense pikeperch is still regarded as an exotic species in Denmark the creation of a great number of well established pikeperch populations throughout the past century makes it reasonable today to regard the pikeperch as a true member of the indigenous Danish freshwater fish fauna, especially as, since the middle thirties, all new introductions have been made with stocking material from well established national stocks. In view of the still increasing eutrophication of Danish lakes, which adversely affects the profitability of commercial lake fisheries, and also from the angler's point of view, the pikeperch must be regarded as a fortunate addition to the fish population in the lakes. It is, therefore, to be expected that in the years to come pikeperch introductions into lakes from which the species is still absent will continue unabated.

Table 1 Number and type of waterbody where pikeperch were introduced

Table 2 Type of stocking material used in Danish pikeperch introductions

Table 3 Yield of pikeperch (kg/ha) from six Danish lakes

Fig. 1 Danish stocking activity with pikeperch

Fig. 2 Lake Fure sø. Development of the pikeperch fishery