VENDACE, COREGONUS ALBULA (L.) FISHERIES IN MAJOR LAKES OF THE IŃSKO LANDSCAPE PARK

Przemysław Czerniejewski¹, Wawrzyniec Wawrzyniak¹, Katarzyna Stepanowska^2
1 Department of Fisheries Management in Open Waters, Agricultural University of Szczecin, Poland
² Department of Marine Biological Resources, Agricultural University of Szczecin, Poland

ABSTRACT

Vendace landings were analysed against the background of catches of other commercially important fish species in the lakes Ińsko and Woświn. The detailed morphometry of the lakes as well as basic information on the chemistry of their water are provided. The total fish yields in both lakes were found to be much lower than the overall mean Polish yield; the total yields in the Ińsko and the Woświn amounted to 15.88 and 18.26 kg·ha^-1, respectively. Vendace contributed 34.12 and as little as 7.72% to the total yield in the Ińsko and the Woświn, respectively. Most probably, the difference is a result of a poorer water quality in the Woświn the anoxic layer in which begins at the depth of 10 m in summer. According to the vendace catch-based classification system of [3], Lake Ińsko belongs to “good” lakes, while Woświn should be classified as “poor”.

Key words: Lake Ińsko, Lake Woświn, vendace management.

INTRODUCTION

In its present form, the Ińsko Lakeland was shaped about 10 thousand years ago, during the Baltic glaciation. The most common type of lake in the Ińsko Lakeland is a trough; trough lakes are distinct in having an elongated shape, steep shores, and a considerable depth. Trough lakes occur most frequently in series, aligned along the glacial trough. The Ińsko Lakeland’s largest lake is Lake Woświn situated in the northern part of the Ińsko Landscape Park’s buffer zone. The Woświn is a typical trough lake, occupying a broad glacial trough within the bottom moraine. The same glacial trough houses a number of smaller lakes (i.a., the Dłusko) as well as Lake Ińsko, the second largest lake of the Lakeland. The Ińsko, situated south-west of the Woświn, is distinct in its high water quality; it is the deepest lake in the area. The two lakes have for years been used for tourism and recreation as well as for fisheries.

MATERIAL AND METHODS

The analysis of the Ińsko and Woświn fisheries presented below is based on data drawn from internal reports kept by fishing communities exploiting the two lakes; the data cover the periods of 1950–2001 and 1952–2001 in the Ińsko and the Woświn, respectively. In addition to fish yields, the lake morphometries are described, based on the respective bathymetric charts as well as on publications by Filipiak and Raczyński [5] and Jańczak [9]. The basic environmental information on the lakes is provided, based on results of measurements and analyses reported by the Provincial Inspectorate of Environmental Protection (PIEP) in Szczecin.

RESULTS

Lake Ińsko
Lake Ińsko is the largest lake in the Ińsko Lakeland situated in the catchment area of the Kanał Iński, Ina, and Odra. The lake covers 486.6 ha; in its north-eastern part, it features a 22.3 ha island. As a post-glacial lake formed at the intersection of several glacial troughs, the Ińsko is subdivided into a few basins, the subdivision reflected in the shoreline development index (WL = 3.19) and elongation index (2.6). Those indices suggest the presence of calm spawning and nursery grounds in the littoral. Two elongated basins and three smaller coves extend westwards. The eastern basin (called the Odnoga Linowska and covering 101.8 ha) has been separated by a causeway from the remaining part of the lake, the water exchange proceeding via a culvert in the causeway. The wide, central part of the lake features pools separated by sills. The variable depth of the lake as well as constrictions separating the basins hamper water circulation and mixing. The maximum depth of the Ińsko is 41.7 m and, coupled with the substantial surface area, results in a fairly low relative depth (0.0185). The bottom of the Ińsko is highly variable; the basin is concave, as evidenced by the low depth index (0.30). The littoral of the central part of the lake is relatively narrow, with steep slopes. However, due to numerous shallow coves in the western part, the shallow zone of less than 5 m depth is quite extensive (29.4% of the lake area). The Ińsko tributaries are small streams only, because the lake lies close to the watershed separating the River Ina catchment from that of the Rega. The Ińsko is a typical dimictic and mesotrophic lake. The temperature and oxygen profiles are shown in Fig. 1 (based on unpublished PIEP data).

In 1950–2001, the Ińsko supported fisheries that were systematically targeting 11 com-mercially important fish species; their catches accounted for as much as 85.9% of all the fish caught during that period (Fig. 2). Catches of other species occurring in the Ińsko showed no regularity and occurred in individual years only, which was related to periodic stocking operations and the lack of breeding potential (e.g., rainbow trout, grass carp).

The mean annual fish yield in the Ińsko over the period examined amounted to 15.88 kg·ha^-1 (range: 3.72–41.1 kg·ha^-1); the mean contribution of vendace in 1950–2000 was 34.1%, although in some years the species contributed as much as 78.66%.

Figure 3 illustrates the dynamics of vendace catches in relation to the catches of all fish species from Lake Ińsko. The mean vendace yield over 1950–2001 amounted to 5.41 kg·ha^-1, but the yields varied widely from one year to the next (range: 0–19.27 kg·ha^-1). Yields exceeding 15 kg·ha^-1 were recorded only in 1964 and 1973 (19.21 and 19.27 kg·ha^-1, respectively), while yields lower than 5.5 kg·ha^-1 (a theoretical vendace yield in vendace lakes) were typical of as many as 31 years during the period in which the lake was used as a fishing ground.

The vendace fishery showed considerable month-to-month variations in the magnitude of catches. The highest catches were reported during July-August (the long-term mean catch exceeded 600 kg per month), somewhat lower mean catches being recorded in September and October (above 400 kg per month) (Fig. 4).

Lake Ińsko was stocked with 4 types of vendace stocking materials: fertilised eggs, eyed eggs, newly hatched larvae, and fry (Table 1). The latter two juvenile stages were used most frequently and with the highest efficiency, as evidenced by increased catches in the years following the stocking operations.

Lake Woświn
Lake Woświn is situated among ridges of the frontal and bottom moraines, in the western part of the Western Pomeranian Lakeland, in the River Ina catchment. The lake covers 809.1 ha and is trough-shaped, extending from north-west to south-east and broadening in the southern part (the maximum length and width of the lake are 9500 and 2000 m, respectively). Despite its elongated S-shape (elongation index of 4.7), the shoreline is well developed (WL = 2.59). The Woświn’s basin, incising into a morainic formation, features numerous deeps (the maximum depth is 28.1 m) and shallows. The depth index of 0.33 is indicative of a variable depth; the basin resembles a cone, but due to its large area, the lake’s relative depth is as small as 0.0099. The littoral zone is relatively narrow; however, due to the presence of shallow coves (areas shallower than 5 m account for 28.8% of the lake surface) there are sufficient spawning grounds and fish nurseries. Due to its situation among arable fields and proximity to villages, the Woświn is exposed to a considerable anthropogenic pressure resulting in progressing eutrophication. As evidenced by the unpublished PIEP data for the year 2000, the Woświn is a typical dimictic and eutrophic lake. Thermal stratification is distinct in summer (Fig. 5). The epilimnion is then 6 m thick, the thermocline occurring within 6–11 m. The epilimnion is well-oxygenated down to 5 m, a rapid decrease in the dissolving oxygen content occurring in the lower part of the epilimnion and in the upper part of the metalimnion. The water from the depth of 10 m down is anoxic (Fig. 5). The spring mixing involves the entire water column: the water temperature difference between the surface and the near-bottom layer was 0.3°C, the entire water column being well-oxygenated.

The records on catches of different species and on fishery operations carried out have been kept continuously since 1952 until the data for this work were collected. A total of 18 fish species were harvested during that time; however, the fishery management of the lake was based on regular, annual fisheries of 9 species only (Fig. 6).

The mean fish yield in the period analysed was 18.26 kg·ha^-1, vendace accounting for as little as 7.72%. Since the early 1990s, the total catches have been showing a distinctly decreasing trend, which is most probably related to the unfavourable oxygen conditions in the water. The variability of vendace catches in the Woświn is shown in Fig. 7 against the background of the total fish catches. The vendace contribution to the total catch was varying extensively. On one hand, the causes of the variability should be sought in a low efficiency of natural spawning and irregular stocking operations (Table 2); on the other hand, another group of causes includes the lack of catch quota which would control the fisheries and prevent over-fishing. This conclusion is indirectly confirmed by considerable reductions in the vendace yields following high catches in the preceding years. For example, the record-breaking year 1988 during which the total vendace landing was as high as 7169 kg (8.85 kg·ha^-1), was followed by a rapid reduction in vendace catches and yield to 796 kg and 0.98 kg·ha^-1, respectively. In subsequent years, despite 4 stocking events during which vendace larvae and fry were released (averages of 5261 and 61.7 ind. per ha, respectively), the yields (except those in 1990) did not exceed 1 kg·ha^-1.

The vendace catches fluctuated not only between the years, but also between individual months. The highest mean catches were obtained in Lake Woświn during June-August as well as when fishing was carried out on the spawning grounds in November (the catches exceeded then the level of 200 kg per month) (Fig. 8).

DISCUSSION

Vendace catches reported from various Polish lakes fluctuate widely, the fluctuations being probably related to environmental conditions during spawning and stocking [2], variability of water temperature during the year [4], predation pressure [7,8], and the size of the breeding stock. Sometimes the fluctuations are a result of inappropriate management of the pelagic zone (no removal of smelt, a food competitor of vendace) as well as wrong fishing time and gear. Vendace should be fished when it is in its third year of life; a due consideration should be given to the spawning period and the maximum size increment [1]. When planning exploitation of vendace resources, the period of feeding intensity and growth should be taken into account as well. The highest size increments of vendace in Swedish lakes occurred from March to August [14], while in Finnish lakes the period is shortened to May-July [10]. On the other hand, Marciak [11] demonstrated that, in Polish lakes, feeding intensity and growth of vendace were at their highest from April to October, about 70% of the annual size increment being recorded from June to July (during the summer stagnation). Considering those data, intensive vendace fishing in the Ińsko and Woświn should be delayed by a few months to obtain higher catches.

As an oxyphilous species, vendace usually stays at good oxygenation area; the optima of water temperature and dissolved oxygen content are 9–14°C and not less than 2 mg·dm^-3, respectively [6]. In Lake Woświn, the layer with such an oxygen content was about 7–8 m thick, hence most probably the substantial reduction in yields recorded as of the early 1990s. The distinct catch reduction was, similarly to other lakes, intensified in 1990–1992 by the systemic transformation of Polish fisheries [3]. The trend shown above occurred not only in the Ińsko and Woświn, but also in most Polish lakes the vendace yield of which does not usually exceed 1 kg·ha^-1. According to Wołos [16], the vendace yields exceeded 12 kg·ha^-1 only in as few as 6 Masurian lakes. Owing to the variability of vendace yields, developed a classification of vendace lakes [15]; the classification was modified and adapted to the conditions of lakes in north-western Poland by Czerniejewski and Wawrzyniak [3]. According to that classification, Lake Ińsko, with its mean annual yield of 5.41 kg·ha^-1, can be placed in the lower range of “good” lakes, while the Woświn, with its mean annual yield of 1.41 kg·ha^-1, should be classified as “poor”.

Although the total fish catches from the Ińsko and Woświn were fairly substantial (more than 15 tonnes obtained in the Woświn over 28 years of fishing; in the Ińsko, 10 tonnes were caught during 9 years), the mean annual yields were 15.88 and 18.26 kg·ha^-1, respectively. The yields in different years were lower than those reported by Nowak [12] for 600 Polish lakes (24–29 kg·ha^-1) and than the theoretical yields from vendace lakes (34.0 kg·ha^-1 [13]); they were also lower than those reported from other Western Pomeranian vendace lakes [3].

CONCLUSIONS

The lakes Ińsko and Woświn are the most important, in terms of fisheries, water bodies of the Ińsko Landscape Park and are regularly used as fishing grounds. The catches obtained in both lakes, with trawls and stationary gear, consist of 11 and 9 species, respectively. The Woświn is more eutrophic of the two lakes and shows oxygen deficiency in the metalimnion. Despite higher total fish yields in the Woświn, the contribution of vendace there is much lower compared to that in the Ińsko (7.7 and 34.1%, respectively). Although both the Ińsko and the Woświn are classified as vendace lakes, the vendace yields assign the former to “good” lakes, the latter being classified as a “poor” lake.

REFERENCES

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14. Viljanen M., 1986. Biology, propagation, exploitation and management of vendace (Coregonus albula L.) in Finland. Arch. Hydrobiol. Beih. Ergebn. Limnol. 22, 73-97.

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16. Wołos A., 1998. General characteristics of coregonid management in 132 Polish lakes. Arch. Ryb. Pol. 6 (2), 265-278.

Accepted for print: 17.03.2008

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