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326F. A. SCHROEDER & J. P. CASTELLOEstuary Reduction and Limnic InfluenceThe reduction in the estuary area shouldresult in an increase in inter-and intra-specificcompetition among brackish water species, for bothspace and food. If we start with the premise that thenumber of individuals should not decline (at least atfirst instance), the population density in shallowareas with conditions of suitable salinity andvegetation will be higher. This means thatembayment areas, with seagrass meadows, whichare conducive to the proliferation of fish andcrustaceans (Seeliger et al. 1998), should bear agreater density of consumers.It is thus likely that the available food, whichpreviously supported a lower density of individualsin areas with brackisk waters, will be competed formore severely. This situation can create stress in thepopulations, increasing mortality of the estuarinedependentspecies. On the other hand, grey-mullet,which lives and growths in the fresh water area, willprobably be affected during recruitment time.While studying the effects of ‘El Niño’ onthe estuary, Garcia & Vieira (2001) noticed that thephenomenon caused a local reduction in salinity dueto high rainfall in the lagoon drainage basin. He alsofound that freshwater species, particularly Parapimelodusnigribarbis, Astyanax eigenmaniorum andOligosarcus jenynsii, had subsequently becomemore frequent in the estuary.These species will also compete for spaceand food with the estuarine-dependent species, andmay, especially in the case of the carnivore O.jenynsii, represent one more predator of larvae andjuveniles in the region.The influence of ‘limnification’ can causechanges in the dynamic movements of estuarinedependentspecies. ‘El Niño’ events reduce therecruitment of juvenile grey-mullets, negativelyaffecting the next growing season (Vieira et al.2008) and the reproductive migration (“mulletrace”).However, the variation in salinity should notaffect the biology of the species, since thefluctuation of salinity in the estuary is common,ranging from marine to limnic conditions (Seeligeret al. 1998). These changes are, however,ephemeral, while the changes that will occur due toclimate change are expected to be of a more constantnature.Thus, with the reduction of the estuary sizeand the influence of ‘limnification’, increasedmortality of the estuarine-dependent species andchanges in migration and population dynamics areexpected, reducing the available population forfishing.Figure 6. Model of a "virgin" cohort. The curve Lrepresents the weight of individual species. Curve Brepresents the biomass curve, and N is the total number ofindividuals.Figure 7. Model of an exploited cohort. Curve Lrepresents the size of individual species, B the biomass,Be the biomass with exploitation, N the number ofindividuals and Ne the number of individuals withexploitation.Increased temperatureTemperature is determinant to metabolism infish, in a manner that the second is proportional tothe first and that affects the rate of growth at all lifestages. Because the Patos Lagoon is shallow(average depth of 5 m), variations in air temperatureare quickly transmitted to the water column.Krummenauer et al. (2006) pointed out tothe low water temperature of the estuary in theautumn-winter period as a limiting factor for the cultivationof pink shrimp throughout the year. Therefore,a higher temperature could extend the growingseason for shrimp and increase their growth rate.Okamoto et al. (2006) discussed theadvantages of higher temperatures on the developmentof juvenile grey-mullet. At a higher temperature,the feed-to-weight gain ratio increases (i.e., thesame amount of food generates a greater amount ofPan-American Journal of Aquatic Sciences (2010), 5(2): 320-330