Water Quality and Fish Community Composition in the Sava Lake: The Basic Ecological Considerations - Page 03

The family Cyprinidae was represented by 11 species, Percidae by 3 species, while the remaining reported families were represented by one species each. Carp contributed 32.9% of the total biomass and was by far the most important species in that respect. The contribution of pikeperch was 10.2%, while contributions of four species (bream, bighead carp, silver carp, and black bullhead) varied between 5.8% and 6.3% in terms of total biomass. The shares of the remaining 14 species were smaller and ranged from 4.6% (Prussian carp, perch) to 0.4% (Danubian spined loach, ruffe). Three species (H. nobilis, H.molitrix, and C. idella) did not develop self-sustaining populations, while others reproduce in the lake. Besides the species composition analysis, fish were classified into habitat, feeding and reproductive functional ecological guilds (Tab. 1).

Trophic structure included 5 functional ecological guilds. The relative contribution of trophic groups were 11.5%, 16.3%, 59.6%, 3.5 % and 9% for planktivores, piscivores, omnivores, herbivores and invertivores, respectively (Fig. 3).

 

Fig03
Figure 3: Relative trophic guild contribution (% of total biomass): Pl – Planktivores; Pi – Piscivores; O – Omnivores; H – Herbivores; In – Invertivores.

 

Calculated diversity indices were 0.143, 2.451, and 0.818 for Simpson's, Shanon-Weiner, and Evenness index, respectively. The F/C ratio was estimated as 4.18. The lake is used for angling and 13 species are of fishing interest. Out of the 13 species 5, 6 and 2 are of high, medium and low fishery importance, respectively. They contributed 81.1% in relative abundance in biomass. Relative importance values were 49.2%, 20.5% and 10.4% for the group of species assigned as species of high, medium and low fishery importance, respectively (Fig. 4).

 

Fig04
Figure 4: Relative contribution of species group classified according to their fishery importance (% of total biomass).

 

The Sava Lake is a man-made lake used for leisure, recreation, and recreational fishing. According to Hegediš et al. (2013), due to fishery mismanagement, the fish community in the Sava Lake prior to 2003 was highly devastated and characterized by the dominance of C. gibelio, A. melas, and L. gibbosus, which constituted more than 50% in fish biomass. Consequently, in order to rehabilitate fish resources, the intensification strategy was adopted, which involves methods to intensify fish production including stocking with desired species and removing of unwanted ones (Cowx and Welcomme, 1998, according to Lehtonen, 1999). The rehabilitation program was conducted during the autumn 2003 – spring 2009 period. Stocking involved C. carpio, A. brama, S. erythrophthalmus, R. rutilus, B. bjoerkna, S. luciopreca, and E. lucius, while specimens of A. melas and L. gibbosus were simultaneously removed. After termination of stocking activities the contribution of stocked species amounted to 57.8% in biomass, while the contribution of the formerly dominant three species dropped to 12.7% in total biomass. Therefore, the current fish community structure is newly created and might be used as a reference for monitoring and ecological integrity assessment purposes. Wiśniewolski and Prus (2009), suggested five-year monitoring intervals for reservoirs in Europe.

Species composition and abundance, diversity and trophic relations represent the essential components that contribute to the fish community structure. Providing a complete fish species list is of vital importance towards the maintenance of the lake's biodiversity, but with the aim of benefitting from an accurate inventory, future monitoring is needed to indicate changes over time. Obtained species richness (20 species) is common for Serbian reservoirs larger than 20 ha (Marković, 2011; Kovačević et al. 2011). At present, proportions of introduced species are significant, in both species number (30%) and biomass (27.8%). However, three of these species (H. molitrix, H. nobilis and C. idella) did not establish self-sustaining populations, therefore a gradual decline of their populations and their final extirpation is expected. The date of the introduction is unknown and presently their populations are constituted of large specimens (>10kg). All three species are long-lived and can attain considerable age and size, thus not allowing any assumption for the timing of their extirpation. Moreover, according to Serbian fishery related legalisations, stocking with allochthonous species is not acceptable and population renewal of these species is impossible. Diversity indices are widely used in studies on aquatic communities (Washington, 1984). Eloronta and Eloronta (1977; according to Washington, 1984), claimed that H' is a useful parameter for describing the condition of fish fauna. Diversity indices summarize the numerical associations of organisms, including both the number of species and the relative abundance of each species.

Diversity indices are numerical representations of a population's status and they are sensitive to changes in both community and ecosystem. They provide one of the most appropriate ways to detect and evaluate the influence of stress factors on a community, thus allowing for community comparison on a temporal scale.

Four species were referred to as carnivores for estimation of F/C ratio. They were pike, pikeperch, catfish and asp, i.e. species that are piscivorous specialists in the sub-adult and adult stages. Perch was not included in this category because it becomes partially piscivorous at about 12 cm in standard length (Kottelat and Fryhof, 2007). The estimated F/C ratio of 4.18 fell within the acceptable range of 3-6 for balanced fish populations (Swingle, 1950, according to Moehl and Davis, 1993), indicating a balanced community structure. Ecological classification provided information on intracommunity structure beyond the taxonomical level. Concerning habitat preferences, fish were assigned to either a water column or a benthic species. The water column guild comprises species that rely on vision during foraging; shoaling species with a pronounced daily movement pattern; species, which are associated with marginal shelters and macrophytic vegetation, and species that are sensitive to increased turbidity. The benthic habitat guild includes species known as benthic specialists, which are associated to substratum and are sensitive to sedimentation. The feeding categories in terms of relative abundance in biomass were in the following order of importance: omnivores > piscivores > planktivores > invertivores > herbivores. The omnivores constituted more than half in biomass (59.6%) and, consequently, representing the most important elements of the community's trophic structure. However, the assumed decline and final extirpation of planktivores (H. molitrix, H. nobilis) and herbivore (C. idella) species will significantly change the observed trophic structure. Within the reproductive guild category fish were assigned into 4 functional guilds. Noble et al. (2007), stated that the lithophilic and phytophilic guild, in terms of the lake's ecological assessments, are most frequently used as a measures of the reproductive structure of a fish community. In the Sava Lake only one species (A. aspius) is lithophilic, therefore it should be considered as an indicator species for monitoring purposes. As a recreational fishery, the Sava Lake is the most important lentic water in the Belgrade area. The number of game species, their abundance and ratio of high quality species reflects the lake's value as a fishery resource.

A fish fauna description of a particulate man-made lake is a prerequisite if we are to apply proper monitoring and management to its fish resources. Furthermore, it provides an essential foundation for future research, as well as for the assessment of reservoir's ecological potential.