Ecological Potential Assessment of Sava Lake Based on Fish Community Composition: Preliminary Results - page 2

Five metrics concern fish assemblage composition and diversity. Species richness is the only true measure of diversity. Generally, higher numbers of species are representative of favourable conditions of the aquatic ecosystems. Reservoirs generally provide a favourable environment for the establishment of alien species (Irz et al. 2004). Alien species may have negative impact on native fishes through competition or predation, thus altering the community structure. Furthermore, their populations overgrow under degraded environmental conditions. Higher diversity of piscivorous species reflects better environmental conditions. Besides they are important indicators, piscivores represent a regulating factor of the abundance of other species, in particular of small-sized tolerant species often overpopulated in reservoirs. Intolerant species are characterized by higher oxygen requirements and preference towards rheophilic conditions. Higher diversity of this group and higher species densities reflect better environmental conditions. The Shannon-Weaver index (H') is a useful parameter for describing the condition of a fish community, commonly used in fish studies. It summarizes the numerical associations of organisms including both the number of species and the relative abundance of each species. Additionally, H' provides one of the most appropriate ways to detect and evaluate influence of stress factors on a community, thus allowing for community comparison on a temporal scale.

Three metrics are related to abundance. Total relative abundance is included as a measure of the environmental carrying capacity. Higher biomass sustainable by the environment points to improved conditions.

Ecological quality is reduced if one species occupies a dominant position in the community. The presence of several subdominant species characterized by substantial contribution in biomass indicates a more balanced community structure.

Three metrics are based on feeding guilds. Contribution of piscivores in total biomass is an excellent indicator of community balance. Swingle (1950, according to Moehl and Davies, 1993) described the prey/predator relationship referring prey as forage and predators as carnivores (F/C ratio): the total weight of all forage fish divided by the total weights of all carnivorous fish. The F/C ratio ranging from 3 to 6 indicates balanced populations. Invertivores characterize a certain degree of specialisation in terms of food requirements, thus they indicate a structured system with a diverse food base available to specialist species. Omnivores are less sensitive to environmental stresses due to their ability to vary their diets. Disruption of the trophic structure due to degraded conditions favours an increase in relative abundance of opportunistic omnivores.

Both litophils and phytophils are sensitive to siltation, therefore two guilds were merged as a single metric. Siltation is one of the most important factors causing deterioration of the reservoir environment, thus influencing fish assemblages composition

Based on fish community data and on previously described selected metrics, an attempt of preliminary assessment of ecological potential of the reservoir has been made (Table 4).

 

Table 4: Scoring criteria and preliminary ecological assessment of Sava Lake reservoir: a Sava Lake assessment.
Tab04

 

Establishing metrics criteria was based on expert judgment and taking into account relevant data on structure and functioning of fish assemblages from well studied reservoirs in Serbia (Simić et al. 1994; Marković and Lenhardt, 2009; Lenhardt et al. 2009; Kovačević et al. 2011; Marković et al. 2014). Scoring criteria, according to Hickman and McDonough (1996), were applied, i.e. each metric was separated into three categories assumed to represent relative degrees of the fish community condition ranging from good to poor, where values of 5, 3, and 1 represent good, fair, and poor conditions, respectively. The sum of all the metrics represents the score of the community condition and is evaluated as follows: 12-21 very poor, 22-31 poor, 32-40 fair, 41-50 good, and 51-60 excellent (Hickman and McDonough, 1996). According to the obtained assessment score, fish assemblage in the Sava Lake can be described as a community of fair condition. Since fish assemblages reflect the overall integrity and environmental quality of aquatic ecosystems (Fausch et al. 1990, according to Launois et al. 2011) the evaluated score indicates fair ecological potential of the Sava Lake reservoir.

The proposed assessment system can be used for future monitoring of resource conditions and as a tool to control the effectiveness of managing measures. However, the presented results are of preliminary importance and should be recognized as an attempt to establish baseline conditions for the studied water body. Further research is needed especially on metrics credibility, sensitivity and reliability, as well as on the scoring criteria.

 

Acknowledgment

The reported work is supported by the Ministry of Education, Science and Technological Development (project number TR 37009), and the Serbian Academy of Sciences and Arts.