Fishing and drought impacts on size spectra of fish assemblages in Amazon Rivers

Background/Question/Methods

An approach that holds promise to understanding ecosystem impacts on fish assemblages is size spectra analysis: a regression of logarithmic abundance against the logarithmic body size of fish assemblages. The approach is based on the idea that the biomass and numbers of individuals of all species in a system decreases log-linearly with body size. Differences in slopes denote differences in mortality rates and trophic transfer efficiencies, whereas differences in intercepts denote differences in productivity.  However, there is no information on the validity of size spectra analyses in tropical freshwater environments. We applied size spectra analysis to the fish assemblage exploited by fisheries in the Amazon Basin to investigate if river droughts and exploitation levels influence the size spectra of the fish assemblage. 

Results/Conclusions

Size analyses were applied to an 11-year dataset of multi-gear, multi-species fish catches in the Amazon, Madeira, and Purus rivers. A total of 299,857 length measurements of 57 fish species were recorded. For each river and year the slopes and intercepts of size spectra were correlated to annual fishing effort data and river drought events. Drought intensity was calculated as the difference between the historical average and the minimum water level observed. T-Student tests were performed to evaluate the strength of correlations between the slope of size spectra and river drought and fishing effort. The linear models adjusted well to the size data (p < 0.001), supporting the validity of these models to describe the structure assemblages. Droughts influenced the of size spectra of the Amazonian fish assemblage, as shown by inverse relationships between the slope of size spectra and drought intensity (p < 0.01). Fishing effort also influenced the size spectra of the fish assemblage in the Amazon and Purus Rivers (p < 0.05), but not in the Madeira River (p > 0.05). However, after the extreme drought of 1998 caused by an El Niño event, the slope of the size spectra increased overtime, perhaps in response to the drought event. These results suggest that droughts have stronger negative influences on Amazon fish assemblages than fishing effort. Considering that extreme droughts are becoming increasingly frequent and intense in the Amazon, and that Amazonian fisheries are exploited at moderate levels, these results indicate that such stronger and more frequent drought events will influence not only fish biomass but also the yields of fish species of different sizes.