Download full issue - PanamJAS

Potential effects of climate change on fisheries in Patos Lagoon325superimposed decadal cycle. Costa et al. (in press),for a shorter time period, found a much higherincrease in rainfall rates (up to 40% in 40 years)when compared with the data from Pelotas.Toldo et al. (2006) points to an increasein sedimentation rate in Patos Lagoon in the last150 years, i.e., before and after the agriculturalcolonization of the banks of its hydrographicbasin. Soil impermeabilization (due to building citiesand roads), destruction or reduction of riparianvegetation and agriculture all decrease the rates ofwater penetration into the soil, causing morewater (and therefore more sediment) to be carried toPatos Lagoon. This process, along with increasedrainfall in the drainage areas of the rivers basins,may explain the increased outflow of contributingrivers.Figure 5. Conceptual model of the evolution of a cohort.This scenario may be relieved by the highrates of evaporation in the Patos Lagoon, estimatedto be approximately 600 m 3 /s (Hirata & Möller2006). As evaporation is directly proportional totemperature, increased heat in the region shouldincrease evaporation rates, reducing the amount ofwater available.Assuming that freshwater input will increase(in a linear fashion or in a decadal cycle) in thesystem despite the evaporation rates, more freshwater will be carried to the estuary over the next 30years resulting in ‘limnification’, therefore,decreasing the size of the estuary (Costa et al. inpress).The ‘limnification’ process increases thefresh water in the estuary, reducing the salinity in itsentire area of influence. Therefore, the position ofzero salinity which determines the upper limit of theestuary, should move towards the sea, reducingthe area of influence of the salt and the brackishwater that is present in the southern region ofPatos Lagoon (Costa et al. in press). Still, it isexpected that this process shall be intensified whenincreasing the length of the rocky man-made pair ofjetties located in the mouth of the estuary (Fernandeset al. 2005).An increase of the outflow of the estuaryis also expected because there is an increase inthe volume of displaced water into the lagoon.In parallel, the speed at which the water exitsthrough the rocky jetties should increase, resultingboth from the volume of the outflow andrenovation work that is done on them (Fernandes etal. 2005).TemperatureAir temperature is expected to rise in theregion. The INPE model points to an elevation ofbetween one and three degrees Celsius, dependingon the season and model that is analyzed. Althoughthere are no historical data on water temperature inthe lagoon and estuary, the low average depth andresidence time of the water there indicate that it willfollow the trends that have been verified for the air(i.e. increasing temperature).The time series (Fig. 4) shows an upwardtrend in temperatures in the region, reinforcing theconditions outlined in these models.Estuary of Patos Lagoon - 30 yearsIn light of the data presented above, it canbe predicted that, in 2040, the estuary would besmaller, less salty and warmer, with stronger outflowand weaker inflow currents at the mouth of theestuary.Response of fish stocksOver time, the evolution of a fish cohort thatis not fished behaves according to the conceptualmodel in figure 6. The number of individuals in thecohort is maximal at time zero, i.e., immediatelyafter larval eclosion. As these individuals grow, thecohort biomass increases, while mortality reducesthe number of individuals. The cohort biomass peakoccurs when the natural mortality removes a largeenough number of individuals that the growth of thesurvivors can no longer compensate the biomass ofthe cohort. This point occurs at the intersection ofthe curves for number and weight of individuals(Fig. 6).When fishing is added to the model (Fig. 7),the curve representing the number of individualsdeclines even more due to increased mortality fromdeath by fishing. The biomass and numbers of acohort decrease with the increase in fishingmortality.Pan-American Journal of Aquatic Sciences (2010), 5(2): 320-330