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Martin S.M.,Imperial College London | Martin S.M.,MRAG Ltd. | Lorenzen K.,Imperial College London | Lorenzen K.,University of Florida | And 5 more authors.
Biological Conservation | Year: 2011

Tropical floodplain wetlands and the fish communities they support are subject to great pressure from human demands for water and aquatic living resources. In densely populated agricultural areas where such pressures are greatest, floodplain wetlands may be dewatered for the dual purpose of crop irrigation and fish harvesting. Viewed as highly destructive to fish communities, the practice is widely discouraged but remains common. We investigated the impacts of dewatering on fish abundance and assemblage structure in permanent floodplain wetlands of the lower Mekong region. Draining was carried out only in wetlands where access for fishing and water withdrawal was exclusive to individual households or groups, and where fishing was restricted to draining events. Subsequently, the impacts of draining were found to be equivalent to those of intensive fishing, rather than entirely catastrophic. Many wetlands were drained and fished repeatedly in a single dry season, with catches declining by 72% on average between consecutive events. Species richness and mean length of fish also declined with consecutive dewatering events. Fish biomass was higher in drained wetlands prior to the first and second draining events than in open access, non-drained wetlands. These surprising results suggest that draining of floodplain wetlands is not as fundamentally destructive to fish populations as is often assumed. Where fishing pressure under open access conditions is high, allocation of exclusive rights to fish and dewater wetlands can aid fish conservation as long as dewatering is carried out only once. © 2010 Elsevier Ltd.

Arthur R.I.,Imperial College London | Lorenzen K.,Imperial College London | Homekingkeo P.,Livestock and Fisheries Section of Savannakhet Province | Sidavong K.,Livestock and Fisheries Section of Savannakhet Province | And 2 more authors.
Aquaculture | Year: 2010

Herbivorous or omnivorous tilapia and carp species form the backbone of tropical inland aquaculture and fisheries enhancement and have been introduced widely outside their natural ranges. Perceptions of their impact on native fish faunas vary widely, but there have been few rigorous assessments. To quantify the impact of tilapia and carp stocking on native fish communities in freshwater wetlands of the Mekong region, we conducted observational and experimental impact-control studies replicated at the wetland level, at a total of 46 sites in Lao PDR. The studies were designed as paired comparisons of wetlands where the non-native species (Nile tilapia Oreochromis niloticus, mrigal Cirrhinus cirrhosus, rohu Labeo rohita and bighead carp Hypophthalmichthys nobilis) were stocked in substantial numbers with similar wetlands where the species were absent. Stocking of these non-native species was associated with significant increases in total fish biomass, by 180% in the observational study and by 49% in the experiment. Native fish biomass was not affected by stocking of the non-native species. No significant impacts on native fish species richness, diversity indices, species composition or feeding guild composition were detected, except for moderately negative effects on Simpson diversity and equitability in the observational study. In the experiment, no effect had a point estimate exceeding - 14%, or a 95% confidence limit exceeding - 35% of the non-impacted value. Use of these non-native tilapia and carp species in fisheries enhancement in mainland SE Asia supported substantial increases in harvestable biomass while having only mild impacts on native fish communities. Escapes of the same species from pond and cage aquaculture facilities are likely to result in lower biomass in the wild and have lower impacts than enhancement stocking. We encourage building on our results through a systematic and stakeholder-participatory evaluation of the significance and acceptability of benefits and risks associated with the use of these non-native species. This evaluation should be extended to the relative benefits and risks of replacing non-native species with native ones in aquaculture and enhancement, considering that risks associated with using the current set of non-native species are increasingly well known and appear mild-to-moderate, whilst ecological and genetic risks of releasing domesticated types of native species are poorly known. © 2009 Elsevier B.V. All rights reserved.

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