Ecology and conservation of freshwater fish: time to act for a more effective management

Abstract

Freshwater habitats may account for less than 0.01%of the Earth’s total surface but they play an importantrole in fundamental ecosystem functions and services,besides sustaining an overwhelming diversity of spe-cies (V€or €osmarty et al. 2010; Carpenter et al. 2011).Based on current assessments and projections, theremay be between 25,000 and 40,000 species of fish inthe world, including at least 14,000 freshwater spe-cies (Kottelat & Freyhof 2007), and some 300 newspecies of fish being described every year currently.Every continent has its own distinctive fish faunawhose distribution patterns are the result from physi-cal barriers disrupting past dispersal pathways, aswell as physiological and behavioral adaptations tocope with environmental and habitat-specific charac-teristics (Lucas & Baras 2001; Helfman et al. 2009).This situation imposes different evolutionary pro-cesses resulting in a high genetic differentiationresponsible for a high number of local endemism.Although freshwater fish exhibit great diversityand are responsible for several important functions,their specific roles are still poorly understood.Besides being valuable resources for food, sport andhaving high ornamental value, they are responsiblefor fundamental ecological processes and functionssuch as nutrient cycling, trophic dynamics, productiv-ity, ecosystem engineering and ecosystems connec-tion through migrations, among others. Regrettably,major threats to fish diversity are numerous and mayinteract with each other, thus further complicating thesituation (Dudgeon et al. 2006). Examples of identi-fied threats, to name a few, are overexploitation andflow modification, habitat loss and fragmentation,deleterious effects of agriculture, introduction of non-native species, climate change and pollution includ-ing eutrophication (Dudgeon et al. 2006).Overexploitation of freshwater ecosystems receivesmuch less attention than marine ecosystems althoughthe consequences could be similar if not worse (Allanet al. 2005). Illegal fishing using, for example, pesti-cides, electrofishing and dynamite are responsible forthe collapse of fish stocks and destruction or altera-tion of important freshwater habitats all over theworld.Habitat loss, degradation and fragmentation infreshwater ecosystems are a consequence of altera-tions in land use, such as conversion to agriculture,construction on margins, diversion and abstraction ofwater for human use and human intervention on theriverbed [i.e., channelisation, dredging activities orinaccessibility caused by physical barriers (e.g.,dams)]. Such situations may disrupt migration to suit-able areas for reproduction – especially for migratoryfish, or feeding. In addition, impoundments affectboth longitudinal and lateral connectivity, altering thenatural hydrologic regimes and thus affecting the fishcommunity negatively.Species have been translocated or introduced intonew systems throughout the world for different pur-poses (e.g., stocking for aquaculture and fisheries,sport fishing, release of aquarium species, environ-mental management or frivolously – out of fun, forinstance). During recent years, the number of non-native species introduced to freshwater ecosystemshas increased considerably. Non-native species mayimpact aquatic ecosystems at the individual (e.g.,altering the behaviour of native species, influencinghabitat use and foraging, hybridisation), population(e.g., changing abundance, biomass and distributionof other species), community (e.g., altering interac-tions among populations and potentially inducingtrophic cascades) and ecosystem (e.g., changing