Semester

Spring

Date of Graduation

2024

Document Type

Thesis

Degree Type

MS

College

Davis College of Agriculture, Natural Resources and Design

Department

Wildlife and Fisheries Resources

Committee Chair

Caroline Chaves Arantes

Committee Member

Christopher Rota

Committee Member

Daniel Fitzgerald

Abstract

In Amazonian River systems, diverse fish communities depend on the floodplains for providing feeding opportunities and critical spawning habitat during the seasonal flood pulse, which annually submerges floodplain forests. In the Amazon, large-scale deforestation and hydroelectric dam construction can have profound impacts on entire ecosystems by altering river-floodplain connectivity. This study attempts to improve understanding of Amazonian fish responses to these two stressors. The first chapter aims to understand how deforestation impacts floating meadow fish assemblages. For this study, we surveyed fish in floodplain lakes along the lower Amazon River, recording their abundance in relation to forest cover and local environmental factors. Using Threshold Indicator Taxa Analysis (TITAN2), we found that forest-sensitive species declined sharply below ~40% forest cover, while species tolerant of low forest cover increased notably below ~10%. Small-bodied, sedentary species, and species in the families Cichlidae and Characidae were linked with high forest cover, while large-bodied, migratory species, as well as species of Serrasalmidae were associated with low forest cover. These findings highlight how deforestation affects floating meadow fish communities, indicating specific thresholds of fish occurrence and abundance correlated with forest cover changes, ultimately altering taxonomic and functional community structure. The second chapter analyzes long-term taxonomic and functional impacts of dam construction on fish communities in the Madeira River, the largest tributary to the Amazon River, where two dams were built in 2012. Data on fish assemblages have been collected over 19 total years covering periods before dam construction, short-term post-construction, and long-term post-construction, using gillnet sets upstream, between reservoirs, and downstream of the dams. We modeled responses of the occurrence of fish using Hierarchical Models of Species Communities (HMSC), and found that in sections containing reservoirs, there was a short-term spike in species richness and the occurrence of almost all fishes, particularly in functional groups of fishes with increased feeding opportunities in newly created reservoirs. This was followed by long-term declines in richness and occurrence, which was especially pronounced in groups of fishes that rely on floodplain forest connectivity for feeding and reproductive opportunities. In the free-flowing downstream section, we documented steady long-term declines of most groups of fish, particularly species of Pacu, and other fish that depend on seasonal access to floodplain forest. This study adds to a growing body of literature that dams in the Amazon detrimentally impact most groups of fishes through the disruption of river-floodplain connectivity, and that this impact amplifies across long time scales. Impacts on fish communities identified in this thesis have ecological, cultural, and commercial implications. Monitoring fish community responses to increasing land-cover change is imperative for conservation of ecosystems and livelihoods.

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