Rheophilic fish in oxbow lakes of the Warta River – the effect of environmental conditions on habitat selection

In order to preserve species diversity in a river ecosystem, particularly in the case of lowland rivers, areas with standing water are required as they increase the variability of environmental conditions. These areas called backwaters include flood polders and particularly oxbow lakes. Oxbow lakes are typical elements in meandering watercourses and are considered to be key components of floodplains in natural rivers (Hooke 1995; Van Geest et al. 2003). Many authors are of an opinion that oxbow lakes may have a considerable and varied effect on the diversity and viability of fish assemblages in rivers (Ward and Stanford 1995; Winemiller et al. 2000). Floodplains preserved in the natural conditions are of great importance to fish, since many species require different habitats in the course of their life cycle (Kurmayer et al. 1996; Buijse 2002; Starmach 2003). Open oxbow lakes are particularly important to ichthyofauna, i.e. those having a permanent connection with the main river channel. A permanent connection with the river has a positive effect on hydrological conditions of reservoirs and adjacent areas (e.g. water exchange), while it also promotes the use of these water bodies by ichthyofauna as spawning grounds, shelter for fry, as well as feeding and wintering grounds (Hohausowa 2000; Golski et al. 2005; Osorio et al. 2011). Both fish species characteristic of standing waters as well as rheophilic fish (preferring fast flowing waters), i.e. obligatory river species, occur in open oxbow lakes. This may indicate that in certain periods, these fish find better living conditions in lentic rather than lotic habitats. Among the large number of papers describing river ecosystems, there are only single publications or chapters related to backwaters (Penczak et al. 2000a; Winemiller et. al. 2000; Wiśniewolski et al. 2009). A few Polish scientists for years study oxbow lakes: Penczak (Penczak et al. 2000a,b; 2003a; 2005), Glińska-Lewczuk (Glińska-Lewczuk et al. 2005; 2015), Obolewski (Obolewski 2005; Obolewski et al. 2015).

In view of the fragmentary knowledge of the subject, the authors of this study decided to present the importance of these habitats of considerable conservation value for the entire river system, including rheophilic ichthyofauna, based on six oxbow lakes located in the middle course of the Warta River. The aim of this study was to determine which rheophilic species live in open oxbow lakes, whether their occurrence is permanent and whether environmental conditions connected with the type of water body and season of the year influence the habitat selection and fish behavior.

Clear variation in environmental conditions in open oxbow lakes in the middle Warta River is confirmed by the results of research on oxbow lakes of this river in the area of the Jeziorsko dam reservoir (Penczak et al. 2000a; 2003a), the Pilica River (Penczak et al. 2000b, 2005), the Łyna River (Glińska-Lewczuk 2005) and the Słupia River (Obolewski 2005). Differences concern oxbow lakes as well as individual seasons of the year. The major differentiating factors, directly affecting the behavior of fish, are water temperature and dissolved oxygen content. Temperature has a very strong effect on the behavior of river ichthyofauna (Opuszyński 1983; Pliszka 1964; Nunn 2003). In late autumn and winter, due to the lack of flow and thermal stratification, water at the bottom of oxbow lakes may be warmer than in the river, even by several degrees. This lead to the development of favorable overwintering conditions for fish, including rheophilic species. Typical wintering grounds for river species are primarily Madałowe and Trzykolne Młyny oxbow lakes, in contrast to the flow-through Święte oxbow lake where limited numbers of ide and burbot were reported in autumn. In the summer, water temperature in oxbow lakes on the one hand reduces the dissolved oxygen content, while on the other hand, it creates optimal conditions for eurytopic species, e.g. the bleak (Alburnus alburnus). The bleak is followed into the oxbow lake by asp. This is confirmed by the example of Madałowe where asp spectacularly hunted for bleak in the summer and reached the maximum biomass in the catches. These predators used oxbow lakes only as feeding grounds and returned to the river immediately after feeding. Large asps, i.e. above 0.5 kg, were fished almost exclusively during their spectacular hunt for bleak. Apart from Święte, periodical deficits of dissolved oxygen were observed in each of the analyzed water bodies, with the most adverse oxygen conditions being recorded at the bottom of Tarnowa and Tuchoń oxbow lakes. Based on the considerable decrease in the dissolved oxygen content with increasing depth, it may be concluded that large areas of the bottom in those water bodies were deoxygenated in critical periods and thus unavailable to the fish. If there is no permanent connection with the main river channel, oxbow lakes may act a deadly trap for rheophilic fish (having higher environmental requirements) in the summer. The research conducted on oxbow lakes of the Morava River confirm that oxygen is an important parameter differentiating both habitats (lentic vs. lotic), thus affecting their species structure and the fish population size. Migrations of fish from oxbow lakes to the river are accelerated in the summer due to the reduction in the dissolved oxygen content or high temperature (Hohausowa 2000, Hohausowa et al. 2003). When considering all the oxbow lakes together over the entire study period, the percentage of rheophilic species in the total population appears to be low. However, a comprehensive picture was provided based only on the fluctuations in the dominance index for individual species in three seasons of the year in each oxbow lake separately and the comparison of the fish species structure in the river and in a given oxbow lake. It appears that the ichthyofauna is strongly dominated by indifferent species also in the main channel, while the contribution of individual rheophilic species in the water bodies ranges from 0 to 100%, depending on the conditions. The problem of a decline in rheophilic species and the increasing dominance of species with reduced environmental requirements is observed not only in the Warta River, but also in other rivers throughout Europe and for this reason, intensive measures need to be implemented to support the declining species (Grows et al. 1998; Kruk 2004). In their research on the ichthyofauna of the Warta River upstream of Jeziorsko and of the Pilica River, Penczak et al. (2000b; 2005) reported that oxbow lakes of both catchments are visited by obligatory river species, yet rheophilic species become less common and some of them are no longer found in the main channel and therefore, they are very rarely observed in oxbow lakes. Quantitative indices for these oxbow lakes greatly fluctuated, which was confirmed by the high values of the coefficients of variation, exceeding 100%. The high variability of natural lotic assemblages was also presented by Grossman et al. (1990), Grows et al. (1998) and Golski et al. (2010). Fluctuations in open oxbow lakes are directly related to the behavior of fish. Based on the observations of changes in the population size over a period of several years, as well as those of the fish biomass and the dominance structure of rheophilic species in the investigated oxbow lakes, it may be concluded that these fish use both habitats with varied intensity throughout the season. This conclusion is consistent with the results of other studies (Hohausowa 2000; Wolter, Bischoff 2001; Hohausowa et al. 2003; Fladung et al. 2003). With regard to the middle Vistula River, Wiśniewolski et al. (2009) reported that a permanent connection of lotic and lentic elements in the river system supports an increase in their overall diversity and the number of species sensitive to habitat transformation.

The ide showed different habitat preferences, depending on the season of the year (Rakowitz, Zweimuller 2000). In the Danube backwaters, fish were found during the period of intensive feeding as well as overwintering, both in the river and oxbow lakes, mainly in the periods of higher water levels. During the spawning season, ides from the Elbe (Germany) and the Vecht (the Netherlands) were found only in the river (Winter, Fredrich 2003; Fredrich 2003). These data are confirmed by the catches made within the framework of the authors’ studies. Other available literature data also indicate that ides may spawn in oxbow lakes (Witkowski et al. 1997). Based on the studies conducted in the 1950s, Jaskowski (1962) reported that ides are found in greater numbers in sections with numerous sandbars and oxbow lakes. Penczak reported that asps in the 1960s were frequently caught in oxbow lakes of the Warta River, yet the species was found in very small numbers during the studies conducted 30 years later (Penczak 1969b; Penczak et al. 2000a). In the Elbe River, asps rarely visited natural oxbow lakes to overwinter or to feed. In washlands, the species was found rarely and only very close to the main channel, as most fish overwintered in the river. The water level had no effect on the habitat use. Based on the habitat use, the asp may be classified as a definitely rheophilic species (Fredrich 2003). The results of the presented study indicate a different behavior of this species, as asps from the middle Warta River system – in contrast to the population from the Elbe – regularly use oxbow lakes as wintering grounds and feeding grounds. According to the abiotic typology, the Elbe is a large lowland river, one of the longest in Europe and therefore different from the Warta River in many ways, including depth, flow velocity, microhabitat heterogeneity. Perhaps in the case of the Elbe River, fish may find more convenient places for feeding and overwintering. In water bodies permanently connected with a river, Penczak (1986) observed greater species richness and the presence of asps in the summer season, which largely confirms the results of the studies conducted by the authors of this study. The blue bream (not reported in the Warta by Ciepłucha et al. 2014) is found in considerable numbers in the Święconka oxbow lake. Typical habitats of the rheophilic blue bream include lower river sections and estuaries, where its population may achieve considerable density (Brylińska et al. 2000; Kompowski 1991). The first report on the occurrence of blue breams in the Warta near Poznań was published by Kaj (1956). Findings of the report were confirmed by Jaskowski (1962) who caught two specimens at Wronki and Poznań. Previously, Puszczykowo near Poznań was the farthest documented location of this species (Kaj 1956). There are no data in the Polish or foreign literature on the occurrence of blue breams in oxbow lakes. Święconka is the only oxbow lake described to date with such a large population of blue bream. Furthermore, so far this location represents the southern limit of this species’ range in the Warta. The other rheophilic species were found only in the autumn. The chub was recorded in larger numbers in Madałowe and Trzykolne Młyny oxbow lakes, two nase specimens were caught in the Madałowe oxbow lake, while one specimen of gudgeon and one specimen of vimba were caught in Święte and Trzykolne Młyny oxbow lakes. The dace was recorded only in the Tarnowa oxbow lake, located opposite to the confluence of the Prosna and the Warta River. In the Spree, chubs were found only in the main river channel where flow velocity ranged from 0.15 to 0.6 ms⁻¹ (Fredrich 2003). Large numbers of daces in the confluence section of the Prosna were reported by Penczak et al. (2003b). Data collected in the 1960s by Penczak (1969a; 1969b) showed that daces and gudgeons were not found in the oxbow lakes of the Warta River upstream of Sieradz, while single specimens of chub and burbot were caught only in the winter during the ice phenomena. Promising findings are related to the burbot and vimba caught in the last year of the study, as they were previously found in large numbers in the Warta, which was documented by Rychlicki (1933), Dziekońska (1948), Iwaszkiewicz (1969), Jaskowski (1962), Mastyński (1992), Andrzejewski and Mastyński (2005). According to Penczak et al. (2005), the chub, asp and burbot are rarely found in oxbow lakes of the upper and middle course of the Warta River.

The results of this study clearly show that open oxbow lakes may play an important role in the functioning of ichthyofauna through their permanent connection with the main riverbed. This is consistent with most available literature data on the subject.

Of the nine rheophilic species, five are found in greater density, depending on the season and the nature of the water body. Significant fluctuations in the number of species and their relative densities were observed in individual oxbow lakes.

Open oxbow lakes are used primarily as wintering and feeding grounds.

Larger oxbow lakes, connected with the main channel through a large confluence at one end only, are characterized by greater diversity and density of the rheophilic ichthyofauna. In smaller water bodies with a limited connection with the river, particularly in summer, the river species are basically not found and these oxbow lakes may act as traps for fish.

Implementation of conservation measures is required in order to maintain the ecological functions of these valuable habitats. Regulation of river banks prevents large rivers from meandering and thus new oxbow lakes are not formed. Moreover, fish poaching was observed mainly in backwaters and not in the river channel. For this reason, it is important to maintain a permanent connection with the river and to fight poaching.