Freshwater temperature in the state of Rio Grande do Sul , Southern Brazil , and its implication for fish culture

In this study we verified data of water temperatures collected by CORSAN-RS from 1996 to 2004 in several cities of the state of Rio Grande do Sul, southern Brazil, and analyzed the possibility of raising the most cultivated fish species in Brazil. The water temperature from 1996 to 2004 was 16 to 28oC in summer, 17 to 23oC in fall, 14 to 17oC (down to 9oC in the coldest months) in winter and 14 to 21oC in spring. Native species of this state, such as silver catfish (Rhamdia quelen), traíra (Hoplias malabaricus), dorado (Salminus brasiliensis), pintado (Pimelodus maculatus), as well as carps (family Cyprinidae), are resistant to the low winter temperatures. These species have a lower growth rate in coldest months (winter/spring) but a good development in warmer months (summer/fall), reaching a satisfactory performance throughout the year. In the periods of more intense cold, mortality of some introduced species, such as surubim from Amazon Basin (Pseudoplatystoma sp.), pirapitinga (Piaractus brachypomus), pirarucu (Arapaimas gigas), pacu (Piaractus mesopotamicus), tambaqui (Colossoma macropomum) and Nile tilapia (Oreochromis niloticus) may occur. In addition, as most tropical species have a thermal range for growth and reproduction between 20 to 28oC, some species may have poor development even in fall. Therefore, water temperature in this state should be considered in the choice of fish species to be cultivated.


Introduction
The culture of freshwater fish in Brazil was continuously increasing up to 2001, but during 2002-2004 there was almost no increase in production.However, in the south region and Rio Grande do Sul state the latest data presented a stagnation pattern with even a tendency towards decreased production in [2005][2006] (Baldisserotto, in prep.).
Temperature limits a great variety of biological processes, from the speed of simple reactions to the ecological distribution of fish species (Arana, 2004).Most fishes are ectothermic, with their body temperatures being determined by water temperature (Hazel, 1993).Consequently, this parameter can also interfere with fish growth and feeding rate (Laevastu & Hayes, 1984), and significant changes in water temperature may cause a serious challenge to the maintenance of physi-ological function (Hazel, 1993).Different fish species possess different ranges of temperature preference (Baldisserotto, 2002).Thus, to improve productivity or simply identify which species should be cultivated in a certain environment it is important to know the water temperature range in the area.Therefore, the objective of this study was to analyze the mean freshwater temperatures from 1996 to 2004 in the state of Rio Grande do Sul, southern Brazil, collected by a state agency, and to relate it to the culture of fish species in Brazil proposed for cultivation.

Material and Methods
The Companhia Riograndense de Saneamento (CORSAN-RS) measures the water temperature in their water treatment stations (WTS) on a daily basis in all the state of Rio Grande do Sul, Brazil.The collection of data was made using graduate glass thermometers submerged into the water as it arrived in the WTS, pumped from rivers or lakes.Mean monthly data collected in the WTS for nine years (1996 to 2004) of several cities of this state (Alegrete, Alvorada, Bento Gonçalves, Fig. 1.Mean water temperature in several cities of the state of Rio Grande do Sul from 1996 to 1998 (Source CORSAN/RS). of water temperature were produced with software Surfer 8 (Golden Software, Inc.), relating the geographic localization of the cities to their respective mean water temperatures.

Results
The water temperature in the state of Rio Grande do Sul is in agreement with the geographical position and altitude of the studied cities.The mean water temperature in the summer of 1996 to 2001 remained in the 21 -27ºC range (Figs.1a and  2a), and in the summer of 2002 to 2004 there was a wider mean temperature range, from 16 to 28ºC (Fig. 3a).The lowest water temperature in the summer of this period occurred from the central to north and northwest regions (Figs. 1a,2a and 3a).The lowest mean monthly temperature in the summer was recorded in Passo Fundo (12ºC) and the highest in Alegrete (32ºC).
The mean water temperature in the fall from 1996 to 2004 remained in the 17 -25ºC range and the lowest temperatures found in this period were in the central (Caçapava do Sul, Cachoeira do Sul, Santiago, and Santa Maria) and north (Frederico Westphalen, Palmeira das Missões, Passo Fundo, Carazinho, and Três Passos) regions (Figs. 1b,2b and 3b).The lowest mean monthly water temperature in the fall of this period (12ºC) was recorded in the cities of Carazinho and Passo Fundo and the highest (28ºC) in the cities of Quaraí, São Borja, Torres, and Cachoeira do Sul.
The mean water temperatures in winter from 1996 to 2004 presented low variation in all sampled places (from 14 to 17ºC).The area that presented the highest mean temperature in the winter of this period extends from Uruguaiana to São Borja up to the north of the state (Frederico Westphalen, Palmeira das Missões, Passo Fundo, Carazinho, Três Passos, and Santa Rosa), bordering the Uruguay River (Figs. 1c, 2c and  3c).The lowest mean monthly temperature (10ºC) was recorded in Alegrete, Bento Gonçalves, Passo Fundo, Quaraí, Santa Maria, and São Marcos and the highest (20ºC) was observed in Passo Fundo and Três Passos.
The mean water temperature in spring from 1996 to 2004 was in the 14 -21ºC range, and in this season from 1999 to 2001 there was a lower variation of temperature, from 17 to 20ºC (Figs.1d, 2d and 3d).The lowest mean monthly water temperature in this period (9ºC) was registered in Frederico Westphalen and Passo Fundo, and the highest (32ºC) was measured in Quaraí.

Discussion
In southern Brazil the climate is subtropical with a wide water temperature range (up to 32ºC in summer and below 9ºC in some cities in winter).The north and northeast regions of Rio Grande do Sul are located in the highest altitudes and usually the lowest air temperature.Consequently, these regions also present the lowest water temperatures (Machado, 1950;Sartori, 2003).
Fishes have cutaneous and hypothalamic thermal sensors to detect water temperature change, but as ectothermic animals, they rely essentially on behavioral thermoregulation (Bicego et al., 2007).Metabolism of freshwater fishes in dependent of temperature acclimation: an increase in water temperature induces an increase on oxygen consumption (Debnath et al., 2006) and ATP hydrolysis in some tissues (Lermen et al., 2005).The thermal range of fishes from temperate waters is around 4 to 25ºC, much wider than tropical (25 to 28ºC) and cold water fish (4 to 15ºC) (Parker & Davis, 1981;Frascá-Scorvo et al., 2001).Native species such as silver catfish (Rhamdia quelen), dourado (Salminus brasiliensis), traíra (Hoplias malabaricus), and exotic species such as common carp (Cyprinus carpio), grass carp   1) and are, therefore, well adapted to the low winter water temperatures in southern Brazil.Northern Brazil has a tropical climate and throughout the year mean water temperatures are above 24ºC.Some species that live in these ecosystems such as pacu (Piaractus mesopotamicus), pirarucu (Arapaima gigas), tucunaré (Cichla sp.), pirapitinga (Piaractus brachypomus), surubim (Pseudoplatystoma coruscans) and tambaqui (Collossoma macropomum) are not able to tolerate temperatures lower than 16ºC, which usually occur in the coldest months in southern Brazil (Table 1).Pacu and surubim are cultivated in southern Brazil (specimens of surubim are bought from producers from the Pantanal Basins and not collected from the rivers of southern Brazil), but their growth and reproduction are affected due to lower water temperatures, and usually mortality of these fishes occurs in the coldest months ( Araújo-Lima & Gomes, 2005;Urbinati & Gonçalves, 2005).In juvenile Nile tilapia (Oreochromis niloticus), temperatures below 16 to 18ºC inhibit the immune response and reduce growth rates (Cyrino & Conte, 2004).Consequently, the system for tilapia production requires the use of plastic greenhouses in a tunnel with a rigorous control of the water temperature as suggested by Zimmermann & Fitzsimmons (2004).Similar systems may be used for other tropical species, but as this system increases the production cost an economical analysis of the viability of this procedure in southern Brazil is advised.On the other hand, as the best growth rate of rainbow trout (Oncorhynchus mykiss) is between 15 and 17ºC (Logan & Johnston, 1992), this species can be raised only in regions of higher altitude in Rio Grande do Sul, and its growth rate probably will be reduced in the warmer months.
The spawning of most freshwater fishes is directly related to the water temperature (Ferraz de Lima et al., 1984).Carps tolerate a wide temperature range, but gonadal maturation can be significantly retarded when exposed to temperatures below 16°C (Woynarovich & Horváth, 1984), and induced spawning after administration of pituitary carp extract only occurs when fish are maintained in water with temperature above 18°C (Table 2).However, for most species raised in Brazil the best temperature range for reproduction is 24-28 o C (Table 2), which is slightly higher than the mean temperatures observed in the spring of the state of Rio Grande do Sul.Consequently, fish farmers have to postpone the induced spawning of these species or use reproducers in non-optimal conditions.
It can be concluded that water temperature in the state of Rio Grande do Sul should be considered in the choice of fish species to be raised.Native species from this state (silver catfish, traíra, dorado) and carps require a temperature range suitable for cultivation, but pacu, matrinxã, surubim (originated from Pantanal Basin), pirapitinga, pirarucu, tambaqui (Amazon basin) and tilapia are not recommended species, as a high mortality is very likely to occur in the colder months.

Fig. 2 .
Fig. 2. Mean water temperature in several cities of the state of Rio Grande do Sul from 1999 to 2001 (Source CORSAN/RS).

Fig. 3 .
Fig. 3. Mean water temperature in several cities of the state of Rio Grande do Sul from 2002 to 2004 (Source CORSAN/RS).

Table 1 .
Temperature ranges (ºC) for freshwater fishes raised in Brazil.* Water temperature from the natural habitat and proposed by the authors (growth experiments not performed).ND -Not determined.

Table 2 .
Maximum, minimum and ideal temperature range (ºC) for reproduction and egg incubation for different fish species.ND -Not determined.