Basin-scale planning is needed to minimize impacts in mega-diverse rivers

The hydrological connectivity of freshwater ecosystems in the Amazon basin makes them highly sensitive to a broad range of anthropogenic activities occurring in aquatic and terrestrial systems at local and distant locations. Amazon freshwater ecosystems are suffering escalating impacts caused by expansions in deforestation, pollution, construction of dams and waterways, and overharvesting of animal and plant species. The natural functions of these ecosystems are changing, and their capacity to provide historically important goods and services is declining. Existing management policies-including national water resources legislation, community-based natural resource management schemes, and the protected area network that now epitomizes the Amazon conservation paradigm-cannot adequately curb most impacts. Such management strategies are intended to conserve terrestrial ecosystems, have design and implementation deficiencies, or fail to account for the hydrologic connectivity of freshwater ecosystems. There is an urgent need to shift the Amazon conservation paradigm, broadening its current forest-centric focus to encompass the freshwater ecosystems that are vital components of the basin. This is possible by developing a river catchment-based conservation framework for the whole basin that protects both aquatic and terrestrial ecosystems.

Using the most comprehensive fish occurrence database, we evaluated the importance of ecological and historical drivers in diversity patterns of subdrainage basins across the Amazon system. Linear models reveal the influence of climatic conditions, habitat size and sub-basin isolation on species diversity. Unexpectedly, the species richness model also highlighted a negative upriver-downriver gradient, contrary to predictions of increasing richness at more downriver locations along fluvial gradients. This reverse gradient may be linked to the history of the Amazon drainage network, which, after isolation as western and eastern basins throughout the Miocene, only began flowing eastward 1–9 million years (Ma) ago. Our results suggest that the main center of fish diversity was located westward, with fish dispersal progressing eastward after the basins were united and the Amazon River assumed its modern course toward the Atlantic. This dispersal process seems not yet achieved, suggesting a recent formation of the current Amazon system.

Habitat degradation leads to biodiversity loss and concomitant changes in ecosystem processes. Tropical river floodplains are highly threatened by land cover changes and support high biodiversity and important ecosystem services, but the extent to which changes in floodplain land cover affect fish biodiversity remains unknown. We combined fish and environmental data collected in situ and satellite‐mapped landscape features to evaluate how fish species with different ecological strategies and assemblage structures respond to deforestation in floodplains of the Amazon River. We surveyed 462 floodplain habitats distributed along a gradient of land cover, from largely forested to severely deforested. Rather than analyse only taxonomic metrics, we employed an integrative approach that simultaneously considers different aspects of fish biodiversity (i.e. β diversity and taxonomic and functional assemblage structure) to facilitate mechanistic interpretations of the influence of land cover. Spatial patterns of fish biodiversity in the Amazon River floodplain were strongly associated with forest cover as well as local environmental conditions linked to landscape gradients. Several species and functional groups defined by life‐history, feeding, swimming/microhabitat‐use strategies were positively associated with forest cover. Other species, including some that would usually be considered habitat generalists and species directly dependent on autochthonous resources (e.g. planktivores), were most common in areas dominated by herbaceous vegetation or open water habitats associated with the opposite extreme of the forest cover gradient. β diversity and the degree of uniqueness of species combinations within habitats were also positively associated with forest cover. Synthesis and applications. Our results demonstrating that spatial patterns of fish biodiversity are associated with forest cover, indicate that deforestation of floodplains of the Amazon River results in spatial homogenization of fish assemblages and reduced functional diversity at both local and regional scales. Floodplains world‐wide have undergone major land cover changes, with forest loss projected to increase during the next decades. Conserving fish diversity in these ecosystems requires protecting mosaics of both aquatic habitats and floodplain vegetation, with sufficient forest cover being critically important.

Science and policy worldwide are influenced by predictions from bioeconomic theory that fishing cannot lead fish populations to extinction because fishing effort inevitably moves away from depleted resources. Yet such predictions contradict evidence of fishing‐induced extinctions and in particular a model, called ‘fishing‐down’, that explains historical reductions in mean size of harvested species in tropical multispecies fisheries through the gradual depletion and extinction of large‐bodied species. This study analysed data on fisheries for Arapaima spp., the most historically important and overexploited fishes of the Amazon Basin, to evaluate whether they supported bioeconomic or fishing‐down predictions. The evaluation was based on census data on arapaima populations and interview data from 182 fishers with respect to fishing practices and management regulations, which were collected in 81 fishing communities covering 1040 km2 of Amazonian floodplains. Arapaima populations were found to be ‘depleted’ in 76% of the fishing communities, ‘overexploited’ in 17%, ‘well‐managed’ in 5%, and ‘unfished’ in only 2%. Population densities were zero (i.e. locally extinct) in 19% of the communities. Twenty-three per cent of the fishers in each community harvested arapaima regardless of population status. Similarly, the percentage of the catch in compliance with the size regulation did not vary with population status, but compliance with the season regulation in communities with ‘overexploited’ or ‘depleted’ populations (72%) was lower than in communities with ‘well‐managed’ or ‘unfished’ populations (97%). These results support fishing‐down predictions that fishing pressure continues to occur even when fish populations are depleted. The fishing‐down process appeared to occur because of low gear selectivity and larger body‐size of target species as well as high species value and low fishing costs. These results and available data elsewhere suggest that fishing‐induced extinctions are more common than previously thought, endangering biodiversity and ecosystem functioning. Such extinctions are probably going unnoticed because high levels of illegal fishing, geographic heterogeneity, and data scarcity make their identification difficult. Copyright © 2014 John Wiley & Sons, Ltd.

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Arantes CC, Castello L, Stewart DJ, Cetra M, Queiroz HL. Population density, growth and reproduction of arapaima in an Amazonian river‐floodplain. Ecology of Freshwater Fish 2010: 19: 455–465. © 2010 John Wiley & Sons A/S Abstract –  Compensatory density effects are key features of fish population dynamics that remain poorly understood in tropical river‐floodplains. We investigated possible compensatory growth and reproductive processes for a river‐floodplain population of Arapaima sp., an extinction‐prone fish species of South America. Body growth was studied through analysis of ring patterns on the scales, and size and age at sexual maturity was studied through analysis of female gonads. Growth and maturity were compared for unmanaged conditions with relatively low population density (in 1990s) versus managed conditions with markedly higher density (in 2005–2006); between 1999 and 2005–2006, abundance increased 7.3 fold. Results contradict theoretical expectations for slower growth and delayed reproduction at higher population density. Total lengths of arapaima at low population density were significantly shorter for age classes 1–5 compared with lengths of those age classes at high population density (ancova, P < 0.0001 for both slopes and intercepts). Total length at 50% maturity (L50) only declined about 4% with increasing density (e.g., 164 cm at low density vs. 157 cm at high density). Apparent faster growth at high density and only a slight change in size at maturity resulted in fishes spawning at an earlier age with high density conditions (age 3 vs. age 4–5). We hypothesise that these patterns reflect compliance with minimum size limits of catch during the high density (managed) situation, where there was no harvest of immature fishes. Compliance with minimum size limits, thus, may have led to faster average body growth rate and earlier reproduction, which has greatly promoted population recovery.

To promote understanding of fish population dynamics in tropical river-floodplains, we have synthesized existing information by developing a largely empirical population model for arapaima (Arapaima sp.). Arapaima are characterized by very large bodies, relatively late sexual maturity, small clutches, and large parental investment per offspring, and their populations are overexploited and even declining due to overfishing. We used unparalleled time series data on growth, reproduction, catch-atage, and size-class abundance estimates for a population that has increased several-fold and undergone drastic changes in fishing practices in the Amazon, Brazil. Model population numbers were close to observed numbers, with generally low mean absolute percentage errors for juveniles (16%), adults (30%), and catch (18%). In using the model to test ecological hypotheses and to investigate management strategies, we found the following: (1) Annual recruitment is directly and positively related to spawner abundance, and it appears to be density-compensatory following a Beverton-Holt relation (R 2 = 0.85).(2) Fishingselectivity of arapaima caused by use of harpoons and gillnets can lower yield potentials dramatically through removal of the faster-growing individuals of the population. That is in part because fewer individuals live long enough to reproduce and survivors take longer to reach reproductive age. (3) Arapaima populations can sustain annual catches of up to 25% of the number of adults in the population the previous year if minimum size (1.5 m) and closed season (December-May) limits are met. (4) When 25% of the number of adults in the population the previous year is harvested under a 1.6 m minimum size limit of catch, catches are slightly smaller but abundance of adults in the population is considerably greater than under a 1.5 m limit. These findings can be used in ongoing management initiatives, but caution is needed because of present biological and ecological uncertainty about these fishes.