Alternative ways to measure impacts of dam closure to the structure of fish communities of a neotropical river

Abstract

Neotropical freshwaters host some of the most biodiverse ecosystems in the world and are among the most threatened by habitat alterations. The high number of species and lack of basic ecological knowledge provide a major obstacle to understanding the effects of environmental change. We assessed the impact of dam closure on the fish communities of a neotropical river by applying simple descriptions of community organizations: species abundance distribution and abundance biomass comparison curves. Fish data were collected using gillnets during three distinct time periods (1 year before, 1 year after and 5 years after dam closure), at eight sites located downstream of the dam, in the reservoir, transition zone and upstream. A total of 69 343 individuals representing 260 species were recorded. Dam closure was associated with changes in the organization of fish communities at all sites. Species richness tended to increase immediately after dam closure while evenness decreased. Changes in taxonomic structure (richness and evenness) were accompanied by a change in the distribution of biomass with the proportionate contribution by smaller individuals significantly increased relative to larger individuals. Five years on, richness had fallen to below pre‐closure levels at all sites, while the comparative stability of assemblages in the transformed habitats was reflected by biomass‐abundance distribution patterns that approximated pre‐disturbance ratios. This simplistic analysis provided a sensitive and informative assessment of ecological conditions, which highlights the impact to ecosystem process and ecological networks and has particular value in regions where a lack of detailed ecological knowledge precludes the application of traditional bioassessment methods. Copyright © 2015 John Wiley & Sons, Ltd.

Temporal fish community responses to two cascade run‐of‐river dams in the Madeira River, Amazon basin

“…These results indicate that run‐of‐river dams may not induce significant changes in fish assemblages downstream, at least in the first years after damming. Thus, the weaker impact of dams closure detected in Madeira River in this study when compared to investigations carried out in other large tropical rivers (Araújo et al, ; Lima et al, ; Lima et al, ) might have resulted from the type of turbine chosen (Kaplan horizontal bulb), an aspect that should be addressed in future projects of hydroelectric power plants in the Amazon. However, the role of resistance of species‐rich assemblages to the dam effects (such as those in Madeira River) in precluding or reducing composition changes from damming is not well known in tropical freshwater systems and should be investigated.…”

“…Increases in richness, abundance (fourfold over in sites inside Santo Antônio reservoir) and biomass of shore fish assemblages in the sites that were flooded by the impoundments may have resulted from immediate changes of the upsurge (heterotrophic) phase of the reservoir, when a large amount of organic matter is available for decomposition (see Agostinho et al, ; Lima et al, ). Often, the main factor contributing to eutrophication of accumulation reservoirs is the input of an enormous amount of organic matter resulting from the drowning and death of the riparian forest (Fearnside, ), which also generates high greenhouse gas emissions (Fearnside & Pueyo, ).…”

“…Previous studies have shown that large dams usually result in decreased fish abundance and in alterations in fish composition downstream of the impoundments after some years of dam closure (Hallwass et al, ; Lima et al, ; Petrere, ). Moreover, high densities of migratory and non‐migratory rheophilic fishes have been recorded immediately downstream the dams (at dam's foot) due to the impossibility to overcome the barrier created by the dam (Agostinho et al, ; Lima et al, ; Pompeu & Martinez, ). However, we did no observe those changes for the 20‐km‐stretch downstream the two run‐of‐river dams in Madeira River, at least in the first 3 years after dams closure.…”

“…The present study confirmed the occurrence of early and significant changes in some fish assemblage attributes, as expected for the heterotrophic phase of accumulation reservoirs. Reductions in species richness and fish diversity still can be expected after the heterotrophic phase of the reservoir (Agostinho et al, ; Sá‐Oliveira et al, ), but they are usually detected a few years after dams closure in accumulation reservoirs (Lima et al, ; Lima et al, ). Compared to vertical models, horizontal bulb turbines work with slower water velocity, which results in fish experiencing a more prolonged, albeit less severe, pressure change (Pracheil et al, ).…”

“…These man‐made reservoirs modify limnological characteristics and can trigger changes in fish assemblage structure and composition, favouring the replacement of reophilic fish assemblages by a set of predominantly sedentary fish species (Agostinho, Gomes, Santos, Ortega, & Pelicice, ; Agostinho, Pelicice, & Gomes, ; Poff, Olden, Merritt, & Pepin, ). The construction of large reservoirs in large rivers causes rapid and drastic changes in the environment, promoting almost immediate responses in fish populations (Agostinho et al, ; Gehrke, Gilligan, & Barwick, ; Lima, Agostinho, Soares, & Monaghan, ; Lima et al, ; Sá‐Oliveira, Hawes, Isaac‐Nahum, & Peres, ). In addition, dams are recognized by fragmenting lotic habitats and preventing most fish migrations because the reservoirs may act as ecological traps for migratory fish species even when a transposition system (fish passage) is present (Agostinho, Agostinho, Pelicice, & Marques, ; Pelicice & Agostinho, ).…”