Keywords

1 Introduction

The northeastern region of Poland conjures up images of forests and waters, and it is home to the largest complex of lakes in Poland. There are approximately 2,700 lakes of a surface area greater than 1 ha covering an area of 1,450 km2, which is nearly 30% of the number of lakes and 45% of lake surface area in Poland [1, 2]. This is where the largest (over 100 km2) and deepest (over 100 m) lakes are located. This region was also the center of lake fisheries in the southern Baltic Sea basin for centuries.

Studies of riverine fish fauna include most river systems [3, 4]. In comparison to that of rivers, the state of knowledge about the fish fauna of lakes is much worse. Fish fauna studies of lakes are rendered difficult because, in comparison to rivers, they have a significantly wider range of habitats and surface areas are usually large. The species composition of fish inhabiting lakes is most frequently determined during studies focusing on the biology of commercial species [5] or during monitoring catches [6], or it is based on commercial catches and information obtained from professional fishermen [7, 8]. Studies of fish in Polish lakes more frequently concentrate on describing biological or ecological traits [9, 10], than on assessing the entire fish assemblage qualitatively or quantitatively [11]. There are relatively few studies that describe the fish assemblages of individual lakes and even fewer studies that describe the fishes of a few or a dozen or more lakes in a single area [12, 13]. Consequently, relatively little is known about the composition of fish assemblages in lakes. Knowledge of the composition of fish assemblages in many aquatic basins is based exclusively on knowledge obtained from the fisheries that are associated with either commercial or recreational catches.

2 Historical Data Regarding Fishes

Lakes located in the region of northeastern Poland have been the subject of more or less detailed studies regarding fishes and fisheries management [7, 14,15,16]. Nevertheless, no publication has addressed in detail the species composition or quantitative structure of assemblages in the lakes in this area. Historical publications contain summary information on the occurrence of some species in the Masurian region [14, 17,18,19], while more detailed studies analyze fisheries management [15, 16]. Historical materials and fisheries documentation permit making generalizations about the fish species composition of the area. In comparison with other regions of Poland, Masuria remains one of the least investigated areas with regard to fish assemblage species composition and quantitative structure.

The most exhaustive study of fish is the combined results of research work that focuses on the fish from lakes in the vicinity of Węgorzewo [5]. A team from the Inland Fisheries Institute conducted research focusing on the productivity of the lakes and the impact of environmental factors on fish growth. Catches of fish using nets of various mesh sizes and electro-fishing also permitted fairly precise determinations of the species composition and quantitative structure of more than a dozen lakes [5, 20].

3 Species Richness

The occurrence of at least 43 species belonging to 15 families was noted in the lakes of northeastern Poland (Table 1). The most species are from the cyprinid family, while the other families are represented by single species. Species that prefer riverine habitats occur in the littoral zone of some lakes. Among these, the rarest species is Barbus barbus, which was noted in Lake Śniardwy in the mid-twentieth century [23], but it has not been confirmed in this area for many years. The lake form of the species Salmo trutta currently occurs in lakes Hańcza and Szurpiły, while formerly it also occurred in lakes Wigry, Białe Wigierskie, Sajno, and Sajenek [24]. The lakes of northeastern Poland are the primary distribution area of Osmerus eperlanus in Polish inland waters. Generally, species that prefer cold water and require well-oxygenated water are noted with the least frequency in the lakes of this area.

Table 1 List of fish species confirmed in the Masurian Landscape Park
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The occurrence of some species is associated with fisheries management and particularly with deliberate or accidental stocking. Asian carp species (Ctenopharyngodon idella and Hypophthalmichthys spp.) do not reproduce in the inland waters of Poland [25], which is why their occurrence depends on stocking. Fortunately, the trend of stocking lakes with Coregonus peled has stopped, and the further occurrence or even hybridization with indigenous species of the genus Coregonus appears to be quite improbable [26]. Umbra krameri was introduced along with Tinca tinca stocking material from Hungary [27], but it did not establish local populations.

4 Conservation Status

Most fish species noted in this region occur commonly in various types of inland waters in Poland, and they are characterized by broad tolerance to environmental factors [28]. It must also be noted that species that are classified as threatened or are under species conservation are also observed. In total, 27% of the species confirmed in this region are classified as the highest level of critically endangered according to the International Union for Conservation of Nature (Table 1). Anguilla anguilla is a critically endangered species throughout its range of occurrence, while Vimba vimba is a critically endangered species throughout Poland. In turn, Coregonus albula, C. lavaretus, O. eperlanus, Lota lota, B. barbus, Rhodeus amarus, Misgurnus fossilis, Cottus gobio, and C. poecilopus are classified as vulnerable species, and Carassius carassius, Leuciscus aspius, and Silurus glanis are classified as near threatened (Table 1). R. amarus, M. fossilis, C. gobio, C. poecilopus, Barbatula barbatula, and Cobitis taenia are under partial species conservation. Additionally, R. amarus, C. taenia, C. gobio, and M. fossilis are listed in Annex II of the Habitat Directive, and C. albula, C. lavaretus, B. barbus, and L. aspius are listed in Annex V of the same directive.

The majority of autochthonous fish species in Poland have been severely affected by years of human pressure and have reacted with the collapse of local populations [22]. The situation of fish assemblages in the lakes of northeastern Poland is similar. The confirmation of V. vimba in the food of Phalacrocorax carbo nesting on the island in Lake Warnołty certainly qualifies as a curiosity [29]. For many years V. vimba had not been registered in fisheries catches, and the specimens found probably originated from the local form of this species that occurs in Lake Roś. In Poland, V. vimba is classified as critically endangered. Of its former range of occurrence, which included most of the Vistula and Oder river tributaries, currently only a small fragment remains. Freshwater populations have formed in a few river systems, and within the region analyzed, this species occurs in lakes Roś (the Pisa–Vistula river system) and Maróz (Łyna–Pregoła river system).

5 Alien Species

A significant part of the fish fauna of the lakes of northeastern Poland are alien species, of which nine have been noted (Table 1). To date, approximately 40 alien fish species have been noted in Polish inland waters [30]. Most of these alien species occur as the result of intentional or accidental stocking. The process itself of the spread of alien fish species is very poorly documented. Cyprinus carpio and Carassius gibelio occur in lakes as a result of intentional stocking. While Asian carps have not been permitted to be released into lakes for a few years, they are commonly expanding their distribution naturally from ponds and are released into open waters along with stocking material of other cyprinid species. Most of the alien species confirmed in Polish waters do not reproduce in these latitudes [28]; however, they do have a disadvantageous impact on native fauna and flora and sometimes on the ecological and trophic statuses of waters. When the large numbers of C. carpio released into lakes feed, they release nutrients that accumulate in the sediments, and this increase the trophic status of the waters. The groups of alien fish species reproducing in the region analyzed include C. gibelio, Pseudorasbora parva, Ameiurus nebulosus, and Neogobius fluviatilis. All of these species are invasive species in Poland [25]. C. gibelio occurs in most lakes, and its spread is linked to stocking. In the mid-twentieth century, A. nebulosus occurred sporadically in the region studied [20], but in recent years, its distribution has increased [31]. This species is noted at dispersed sites throughout nearly the entire area of northeastern Poland, although the occurrence of this species was noted previously [20]. The expansion of N. fluviatilis is progressing from the Vistula River through the Narew and Pisa rivers. In recent years, N. fluviatilis has settled in the southern part of the Masurian Lakeland.

6 Fisheries Management

Fisheries in the lake region of northeastern Poland has a very long tradition. Despite ownership and structural changes, it remains the central region of lake fisheries in Poland. Approximately 70 enterprises of various types are active in the fisheries sector of this area. As a rule, these enterprises each exploit a few lakes with surface areas of a dozen or so hectares [32]. The largest fisheries enterprises exploit lakes of a combined surface area exceeding 12,000 or so hectares. Thirteen fisheries enterprises exploit over 3,000 ha, and another 4 exploit lakes with a combined surface area exceeding 1,000 ha.

Traditionally, the fisheries sector in Poland is divided into four types. The previously dominant type of commercial fisheries has been replaced by recreational fisheries, which is now the dominant type of fisheries, at least in terms of area. The second most important type of fisheries is angling, which is usually conducted in fisheries districts exploited by the Polish Anglers Association. The least common type of fisheries is specialized fisheries conducted by scientific institutions (Inland Fisheries Institute, University of Warmia and Mazury) or fisheries that specialize in fish culture. It should be underscored here that the division of the fisheries sector is often arbitrary. In the case of many lakes, fish populations are regularly exploited by anglers, while catches are also made with professional fishery, and the decided majority of lakes are stocked more or less intensely.

7 Fisheries Yield

Following World War II, almost all the lakes in northeastern Poland were exploited by state commercial lake fish farms, which were legally required to conduct rational fishery management [33]. These farms were also required to keep detailed catch statistics and to record management practices. In the early 1950s, fisheries and biological studies were initiated in over 2,000 lakes, which corresponded to most Polish lakes. In the early period, the aim was to increase catches of commercially important fish species [20]; however, as the eutrophication of lakes progressed, changes were noted in the structure of the catches, and their yield decreased [34]. Catches of commercially important fishes have also decreased in recent years (Fig. 1). In the 2004–2016 period, annual yield decreased by approximately 300–880 tons in 2016. The three most significant species in terms of biomass (Abramis brama, Esox lucius, Rutilus rutilus) comprise 50–60% of the total catches. The species groups that contributed a 5–10% share of the catches are C. albula, Perca fluviatilis, Blicca bjoerkna, T. tinca, A. anguilla, and Sander lucioperca. However, the economic ranking of these species is different, with A. anguilla accounting for about 30% of the value of professional catches followed by E. lucius – 20%, C. albula – 13%, and S. lucioperca – 10% [32, 35].

Fig. 1
figure 1

Fisheries yield (tons) of professional fisheries in lakes in northeastern Poland in 2004–2016

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Decreases in fish catches were also noted in catch efficiency. In the 1950s and 1960s, the mean commercial catch efficiency was 26 kg/ha [20]. In recent years catch efficiency has decreased by nearly 13–7.2 kg/ha (Fig. 2). Considering, however, the most important of the species caught, the trend in changes of fishing efficiency was not as unequivocal. The catch efficiency of C. albula exhibited substantial fluctuation (from 0.5 to 1.1 kg/ha), which is characteristic of this species. Decreases in catch efficiency were noted for A. anguilla, A. brama, and R. rutilus, while increases were noted for E. lucius, T. tinca, and S. lucioperca. It must be underscored that catch efficiency does not necessarily reflect fish biomass or species composition since only commercially valuable species are subjected to intense exploitation.

Fig. 2
figure 2

Changes in mean catch efficiency (kg/ha) of catches of professional fisheries in lakes in northeastern Poland in 2002–2016

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8 Conclusions

Fish inhabiting lakes form characteristic assemblages that are generally linked with lake limnological type. Similarly, in rivers the fish fauna structure depends largely on environmental conditions. The species richness and biological diversity of fish fauna is a reflection of interactions among local species groups and environmental conditions. The species composition and quantitative structure of assemblages inhabiting the lakes of northeastern Poland are subject to long-term ecological succession. The occurrence of fish in lakes is conditioned by many factors, and the natural processes that shape the structure of fish assemblages have been aggravated by anthropogenic factors [36]. Despite very clear changes caused by water pollution, habitat transformation, and fisheries management (catches, stocking), the fish fauna of the lakes of northeastern Poland is characterized by considerable species richness in comparison to other areas of the country. A large segment of the species occurring here are subject to legal conservation and are classified as threatened according to IUCN categories. The main threats to indigenous fish fauna include habitat transformation, water eutrophication, and the spread of alien fish species.