Spatial and temporal variations in fish assemblage: testing the zonation concept in small reservoirs.

Area of Science:

• Ecology
• Aquatic Ecology
• Fisheries Science

Background:

• Large reservoirs typically show distinct fish assemblage patterns across vertical (littoral, pelagic, bathypelagic) and longitudinal (fluvial, transitional, lacustrine) gradients.
• Understanding these spatial gradients is crucial for effective reservoir management and conservation strategies.

Purpose of the Study:

• To investigate whether small reservoirs also exhibit spatial gradients in fish assemblage attributes and structure, similar to those observed in large reservoirs.
• To assess the influence of zone (fluvial, transitional, lacustrine) and strata (littoral, pelagic, bathypelagic) on fish community composition and attributes in a small reservoir.

Main Methods:

• Quarterly fish surveys were conducted from 2003 to 2008 in the Mourão Reservoir, Brazil, using gillnets.
• Community attributes (species richness, evenness) and composition were analyzed using three-way ANOVA and Correspondence Analysis, considering zone, strata, and year.
• Analyses accounted for temporal variations due to reservoir water level fluctuations.

Main Results:

• Species richness decreased from fluvial to lacustrine zones and was higher in littoral strata, suggesting better feeding and reproduction conditions in these areas.
• Fish evenness was low and highly variable, with no clear spatial pattern observed.
• Contrary to large reservoirs, no significant longitudinal gradient in fish assemblage was detected, indicating longitudinal similarity within the small reservoir.

Conclusions:

• Small reservoirs can exhibit vertical and horizontal fish distribution gradients, influenced by abiotic and biotic factors, similar to large reservoirs.
• The absence of a longitudinal gradient in this small reservoir suggests that scale and specific environmental conditions may alter expected spatial patterns.
• Findings contribute to understanding fish ecology in smaller aquatic systems and inform management practices.