The Rio Dulce/Lake Izabal system is located in Guatemala. This fluvial system contains a least 53 fish species, 5 of which are endemic also supports stocks of commercially important migratory fishes. Littoral areas in the lake and channels contain macrophyte beds that constitute important fish habitat, but the extent to which aquatic vegetation influences fish species distributions and abundance has not been investigated. We hypothesized that fish assemblage structure is significantly associated with aquatic vegetation biomass, distance from the coast, and wet/dry seasonality. Fishes were surveyed and environmental data were collected from 37 locations along the longitudinal fluvial gradient from 2009 to 2011. At each location, macrophyte beds were enclosed with block nets (100 m2), and fishes were collected using rotenone. From three randomly selected 0.25 m2 quadrants within each blocknetted area, subsamples of macrophytes were taken, taxa identified, and biomass measured. Sediment depth was measured with a pole, and water quality parameters were recorded with a multiparameter probe/meter. Fish assemblage structure was analyzed in relation to two environmental variables that revealed greatest variation (sediment depth, mean vegetation biomass) as well as season (wet, dry), distance to the coast and other spatial variables obtained using Partial Redundancy Analysis and partition the variation on the fish assemblage in spatial and environmental.
Results/Conclusions
Surveys yield a total of 13,099 specimens representing 33 species. The atherinid Atherinella meeki (19%) and cichlids Cryptoheros spilurus (15%) and Trichromis salvini (14%) were the most abundant species during the dry season. The cichlids Thorichthys aureus (29%) and Cryptoheros spilurus (25%) and characid Hyphessobrycon compressus (11%) were most abundant during the rainy season. Among variables included in the analysis, macrophyte biomass had the strongest significant relationship with fish assemblage structure. According to variation partitioning analysis, environmental variables explained 4% of variation in fish assemblage structure and spatial variables explained 5% of the variation. Fish assemblage structure was significantly different between dry and rainy seasons, with species such as Parachromis sp., Poecilia mexicana and A. meeki common in dry season samples, and T. aureus common during the rainy season. Findings support the idea that aquatic vegetation and seasonality affect fish species distributions and local assemblage structure, but somewhat surprisingly, relative position within the longitudinal fluvial gradient had much less influence. Future research could examine other factors that may affect fish assemblages, such as species interactions and fishing pressure.