MULTI-MESH GILLNETS IN INVESTIGATIONS OF THE ICHTHYOFAUNA OF DIFFERENT LITTORAL HABITATS IN A DEEP LAKE

Andrzej Kapusta, Tomasz Czarkowski
Department of Ichthyology, Stanisław Sakowicz Inland Fisheries Institute in Olsztyn, Poland

ABSTRACT

The investigation was undertaken to analyze the species composition, abundance, and length distribution of fish assemblages inhabiting different littoral habitats in Lake Kosno. The fish were caught during the day and at night with multi-mesh gillnets in July, September, and November 2006. The fish caught belonged to three families and eleven species. Perch, Perca fluviatilis L., and roach, Rutilus rutilus (L.) were represented most abundantly. Significant variability was noted in the abundance of fish in subsequent months. In July the abundance of fish in the littoral zone was nearly four fold higher than in September and November. Neither the time of day nor the habitat type had a significant impact on the abundance of the fish assemblages, while the time of day and the littoral zone type did impact the size of the fish caught. The application of Nordic multi-mesh gillnets to the study of the ichthyofauna composition in the littoral zone of Lake Koœno permitted determining the impact the type of littoral zone and the time of day had on the species composition and length distribution of the fish assemblages.

Key words: fish assemblages, littoral zone, Nordic multi-mesh gillnets, size structure.

INTRODUCTION

The standardization of fishing methods is the basic requirement for planning and comparing the results of national and international monitoring programs [1]. In Poland, investigations of the abundance and species composition of fish inhabiting lakes have been conducted with many methods and different fishing gear. The composition and abundance of the ichthyofauna as well as the occurrence of species was often determined based on the analysis of com-mercial catches most frequently conducted with gillnets, fyke-nets, and shore seines [8,24,25]. Gillnet sets with varied mesh sizes most frequently adapted from commercial gear were deployed in experimental catches.

The comprehensive evaluation of an environment as highly diversified as a lake requires the application of widely recognized methods to estimate the composition and abundance of icht-hyofauna. At the end of the 1960s, a special multi-mesh gillnet was designed in Sweden to catch all species of freshwater fish [1]. Analyses of catches made with this gear and many adjustments in its construction led to the benthic multi-mesh gillnet that is currently recommended as the standard gear for fish catches in lakes. Data obtained with multi-mesh gillnets permits comparing changes in the structure of fish assemblages within one and among many lakes. The aim of the current study was to analyze the species composition, abundance, and length distribution of fish assemblages inhabiting varied littoral habitats in a deep lake.

MATERIAL AND METHODS

Study area
Lake Kosno (53°38’N, 20°41’E) is a ribbon lake with a surface area of 573.6 ha, a mean depth of 13.7 m, and a maximum depth of 44.5 m, which is located in northeast Poland. In 1952 the lake was classified as α-mesotrophic, but by 2001 it was eutrophic [20]. Secchi depth in the summer period ranged between 2.7 to 3.3 m. The lake is under protection as a nature reserve and as part of the NATURA 2000 network (code PLB280007 – Puszcza Napiwodzko-Ramucka). Emerged vegetation covers approximately 6% and submerged vegetation covers approximately 15% of the total surface area of the basin. Among the emerged vegetation the dominant species are Phragmites australis (Cav.) and Typha angustifolia L., while submerged vegetation is represented by Ceratophyllum demersum L., Elodea canadensis Michx., Potamogeton spp., Stratiotes aloides L., and Fontinalis antipyretica L. ex Hedw. During the study, a stand of Nitellopsis obtusa (Desv.) was discovered in the northern part of the lake.

The lake is fished commercially, and the three basic species are common bream, Abramis brama (L.), roach, Rutilus rutilus (L.), and vendace, Coregonus albula (L.). Commercial catches indicate that the following species also occur in the lake: eel, Anguilla anguilla (L.), whitefish, Coregonus lavaretus (L.), pike, Esox lucius L., pikeperch, Sander lucioperca (L.), perch, Perca fluviatilis L., tench, Tinca tinca (L.), Crucian carp, Carassius carassius (L.), Prussian carp, Carassius gibelio (Bloch), common carp, Cyprinus carpio L., silver bream, Abramis bjoerkna (L.), grass carp, Ctenopharyngodon idella (Val.), silver carp, Hypophthamichtys molitrix (Val.), and bighead carp, Hypophthamichtys (Aristichthys) nobilis (Richardson).

Habitat types
Fish catches were conducted at two stations in different littoral zones (FL-phytolittoral, PL-psammolittoral). Station FL was located in a littoral zone that was fully overgrown with macrophytes (mainly Nitellopsis obtusa and Potamogeton spp.), while station PL was a sandy littoral segment with little macrophytes cover on the bottom (<5%). The selected stations were located equidistant from the shore (aproximately 70 m) and differed slightly in depth (FL: 1.5-2.5 m, PL: 1.2-2.5 m). The physicochemical conditions (water temperature, oxygen content, pH) were similar at both stations (Mann-Whitney U test, p>0.05).

Fishing methods
Fishing was conducted in the day (10:00-14:00) and at night (22:00-02:00) in July, September, and November 2006. Fish were caught in each of the designated habitats, with one of the benthic Nordic multi-mesh gillnets used in the study [7]. The daytime fishing period was designated to avoid the times when the fish are most active (i.e. at sunrise and sunset). This was done to evaluate habitat use while simultaneously eliminating the impact of the horizontal fish migration that occurs at dawn and dusk [4, 18]. The gear used was 30 m long and 1.5 m high, and was constructed of 12 panels 2.5 m long fastened to a 30 m floatline with an inwater weight of 6 g per 1 m and 33 m leadline with an inwater weight of 22 g per 1 m (Table 1). The mesh size ranged from 5 to 55 mm and the twine thickness from 0.10 to 0.23 mm. The fish caught were sorted by species and measured to the nearest mm immediately after fishing was completed.

Data analysis
The analysis of the material was based on determining the catch efficiency at the two stations. The catch per unit effort (CPUE) unit was the number of fish caught in one gillnet during four hours of deployment. The species richness and the abundance of assemblages of fish caught were compared in both habitats in different months of the year during the day and at night. The statistical analysis was preceded by data transformation (logx+1). The length distribution of the fish and the species structure of fish inhabiting the different types of habitats were compared with the Chi² test. Spatial and temporal variation in the abundance and species richness of fish assemblages was analyzed with ANOVA analysis of variance, and when the assumptions of the analysis of variance were not fulfilled, the nonparametric version of ANOVA Kruskal-Wallis was applied.

RESULTS

The fish caught belonged to three families and eleven species (Table 2), as follows: eight species belonged to the family Cyprinidae, two to the Percidae, and one to the Cobitidae. Among the species confirmed, most are batch spawners and exhibit no substrate preference. The reproductive guild connected with macrophytes was represented by four species. FL was richer in species than was PL. The abundance structure of the fish assemblages of the investigated habitats differed significantly statistically (Chi² test, p<0.0001; Fig. 1). The most abundant species in both habitats were perch and roach. In FL roach occurred more frequently than did perch, while the opposite was noted in PL.

The range of variability in species richness was not great (Fig. 2). On average, more species occurred in the catches in July than in September and November although the differences noted were not significant (ANOVA Kruskal-Wallis, H_2.12 = 2.470, p = 0.2908). However, fish abundance in the littoral zone underwent significant temporal changes (ANOVA Kruska-Wallis, H_2.12 = 6.638, p = 0.0362; Fig. 3). In July the abundance of fish assemblages was nearly four times that in September and November. Neither the time of day nor type of habitat were found to have a substantial impact on the abundance of the fish assemblages (Table 3; two- way ANOVA, p>0.05).

The time of day had a significant impact on the size of the fish caught (Fig. 4). The length distribution of fish caught at night differed substantially in comparison to that of the fish caught during the day (Chi² test, p<0.0001). The smallest (<60 mm) and largest (>220 mm) specimens were caught most abundantly at night, while medium-sized specimens occurred most abundantly in the catches made during the day. The littoral zone type also had a significant impact on the length of the fish caught (Fig. 5). The length distribution of the fish caught in PL differed significantly in comparison with that of the fish caught in FL (Chi² test, p<0.0001). In PL fish of medium size (100-140 mm) occurred, while those of the smallest and largest sizes occurred in FL.

The length distribution (Lt) of roach and perch, the species that occurred most frequently in the littoral zone of the lake, varied depending on the time of day (Chi² test, p<0.0001; Fig. 6) and habitat type (Chi² test, p<0.0001; Fig. 7). During the day, roach of medium size dominated while at night the shortest (Lt <80 mm) and longest (Lt>240 mm) individuals dominated. Large individuals of this species preferred FL, while the smallest were more numerous in PL (Fig. 7a). Perch of the smallest size (<60 mm) occurred most frequently at night, mainly in habitats without vegetation. However, individuals with a total body length measuring from 60 to 160 mm were caught most often during the day (Fig. 6b). Fish measuring from 60 to 80 mm and those exceeding >140 mm were noted in FL, and the remainder were in PL (Fig. 7b).

DISCUSSION

The effective management and protection of fish assemblages requires knowledge of, among other things, the species composition and the abundance and size structure of fish populations. Sound results that permit determining reliably these fish assemblage parameters can be obtained through the deployment of standardized fishing gear. The species composition and the size of the species caught depends on, among other factors, the time of fishing and the methods employed [5,11,19]. The results of the current study confirmed significant differentiation in the relative density of fish depending on the date fishing was performed. With passive fishing gear, one of the indirect factors influencing fish catchability is water temperature [22]. Levels of activity, which are particular to individual fish species and developmental stages, depend on water temperature. This is why, in addition to the obvious differences stemming from fish shifting their habitats throughout the year in response to changes in water temperature or food abundance, fish activity level could have had an influence on the results obtained.

It has been confirmed repeatedly that habitat type has a significant impact on the occurrence of fish species, their composition, and the density of the ichthyofauna [9,12,23]. Littoral assemblages exhibit temporal and spatial variability in occurrence and abundance that depend on, among other factors, the presence and species structure of macrophytes [10,17,27], the spatial differentiation of the littoral zone [14,26], as well as diel fish migrations [13]. The spatial differentiation of the littoral zone structure impacts the species richness and fish assemblage composition. Homogeneous habitats exhibit less richness in fish species variety [15]. The effect of more highly diversified habitats is greater variety in the fish assemblages that inhabit them. Mosaic littoral zones overgrown with macrophytes provide shelter from predators for juvenile fish. More species were caught in FL in the current study. Slight differentiation in the species composition and fish assemblage structure may be related to the small differences in the types of littoral zones studied. Both PL and FL, which was nearly totally overgrown with one species of aquatic vegetation, are homogeneous habitats in which the quantity and availability of hiding places as well as food resources are both limited [16].

Nordic multi-mesh gillnets have only been deployed in Poland to date in the pelagic zones of lakes [21]. The application of this gear to study the ichthyofauna composition in the littoral zone of a lake permitted determining the impact of the littoral zone type and time of day on the species composition and length distribution of the fish assemblages. The time of day and habitat type had a significant impact on the size of the fish caught. The smallest perch were caught most abundantly at night, primarily in PL, while the others were caught during the day. Fish measuring in length from 60 to 80 mm and those exceeding >140 mm were caught in FL, while the remainder were caught in PL. Medium-sized roach, the second most abundantly caught species, dominated in catches made during the day, while at night the shortest (Lt <80 mm) and longest (Lt>240 mm) individuals were caught. Large individuals of this species preferred FL, while the smallest dominated in PL. The results obtained indicate that the catchability of fish of a total length <50 mm was low. Among all the fish caught, individuals of this length comprised just 3.4%, while individuals with a total length not exceeding 100 mm comprised 41.5% of the total number of fish caught. It should be kept in mind that the evaluation of the size structure of fish populations should be based on rigorously standardized fish catches [2].

Compiling a list of species occurring in a different lake habitats requires a critical approach to the collected material. It is less likely to catch fish occurring at small densities than those occurring at larger ones. The list of species occurring in the littoral zone of Lake Krosno is presented taking this into consideration. The dominant species that are associated either obligatorily or facultatively with aquatic vegetation for reproduction indicates that the littoral zone is of substantial importance for this lake’s ichthyofauna.

CONCLUSIONS

1. Data and results from Nordic multi-mesh gillnets may be used for analyses fish assem-blages in different littoral habitat.

2. Eleven species of fish were found in the littoral zone in the Lake Kosno. The fish ass-emblage was dominanted by phytolithophils and phytophils species.

3. The spatial differentiation of the littoral zone structure has an impact on the abundance and the size structure fish assemblages.

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Accepted for print: 9.11.2007

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