Introduction

Fish often experiences various environmental stressors under natural and culture conditions, which results into temporary or chronic disturbance of homeostasis. Water temperature plays an important role in cold-blooded animals. Temperature has long been known as a key environmental factor controlling the physiology, distribution and behavior of fishes [1–4]. Temperature directly influences metabolism affecting all physiological processes viz. food intake, metabolism and nutritional efficiency [5, 6]. Temperature below or above the thermal limit can induce alterations in the fish immune response [7] and influence the development of infectious diseases [8]. Therefore, the basic understanding of thermal physiology of a species is most essential to develop their culture technique. Moreover, in the current scenario, this is also essential to assess the biological effects of different thermal phenomena such as climate change, El Niño, La Niña, overwinters, etc. [9–13].

In India, there is a wide fluctuation of environmental temperature throughout the year. Summer temperature in some areas, especially in northern part is around 45 °C, whereas during winter the mercury drops below 2 °C in these areas. Most of the teleostean species have developed their own specific adaptive mechanisms which enable them to cope up with fluctuations of water temperature and the species survive in the stressful environment [14]. It is assumed that juvenile fishes living at cooler latitudes should have some capacity to adapt to changing thermal conditions, but this is unclear whether the tropical populations possess sufficient acclimation capacity to accommodate further temperature increase [15]. The understanding of tropical fish responses to temperature change is rudimentary [16]. The lowering of temperature adversely affects the physiological responses of air breathing catfish, Clarias batrachus (magur). The food consumption rate and digestive enzyme activities were reduced in the fish at temperature below 25 °C [17, 18].

Catla catla (catla) is one of the most economically important carp species and used in composite culture throughout India. Its natural distribution seems to be governed by temperature dependency rather than latitude and longitude. It is a eurythermal species that grows best at water temperature between 25 and 32 °C. Early-stage larvae remain in surface and sub-surface waters and are strongly phototactic. Adults feed only in surface and mid-waters [19]. There is lack of information related to the larval stages. In the present investigation, survival rate of Catla larvae exposed to various temperatures was evaluated using suitable statistical method. This may help to understand the adaptability of this species to various temperatures.

Material and Methods

Experimental System and Maintenance of Larvae

Glass aquarium (50 l each) covered with transparent fiber glass sheet was used as fish culture unit. Each aquarium was connected with a filtration unit (Sera fil bioactive 130, Germany) for the maintenance of reduced level of nitrogenous products in the fish culture unit. The cooling/heating (HAILEA Chiller HC-300A, China/Sera Aquarium Heater 300, Germany) unit was attached with each aquarium to maintain the desirable temperature. Indian major carp Catla catla and its larvae were procured from Chatterjee Brothers’ Fish Farm, West Bengal having induced breeding facility. Fish were kept in outdoor cemented tanks (165 l) at water temperature 28 ± 2 °C. The fish (0.167 ± 0.02 g) were randomly divided into seven groups and were introduced into experimental aquaria (50 fish/aquarium) maintained in wet laboratory facility. The larvae were acclimated at 28 °C for 7 days. Fish were fed with diet (40 % protein) at the rate of 5 % of body weight throughout the study period. The whole amount of food was divided into two parts and was fed daily at 9.00 a.m. and 5.00 p.m. pH and dissolved oxygen level of water were monitored daily using portable multiple water quality measuring meter (HACH, HQ40 d, USA). pH ranged from 7.60 to 8.04, 7.61 to 8.17, 7.805 to 8.37, 7.87 to 8.41, 7.76 to 8.74, 7.60 to 8.45, 7.62 to 8.47, and dissolved oxygen ranged from 5.57 to 6.15, 5.30 to 6.10, 5.21 to 6.25, 5.56 to 6.18, 5.06 to 6.25, 5.12 to 6.35 and 5.05 to 6.10 mg/l at 10, 15, 20, 25, 30, 33 and 35 °C treatments, respectively throughout the study period.

Experimental Temperature

Seven different temperatures (Table 1) maintained simultaneously under laboratory conditions were 10, 15, 20, 25, 30, 33 and 35 °C. One group of fish was kept in outdoor condition. Three replicates were used for each temperature. Each experimental temperature was achieved at a rate of change of 1 °C per 12 h (2 °C/day) starting from acclimation temperature of 28 °C. This required 1–9 days to reach the assigned temperature. Duration of experiment was 25 days. The range of temperature selected in the present experiment was based on the environmental temperature which prevailed in various parts of India in different seasons.

Table 1 Planned and measured water temperature (±SE) in each treatment during experiment
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Statistical Analyses

Mortality of Fish

Dead fish were collected at regular interval from each tank and their number was recorded. Data were compiled as mean ± S.E. All data were analyzed by using one-way analysis of variance (ANOVA). Statistical significance was accepted at P < 0.05 level.

Time Series Analysis

Assuming the ambient temperature (28 °C), simple linear regression of cumulative mortality against the range (Maximum–Minimum) temperatures was performed separately for the “increase” and “decrease” shoulders of temperature/mortality curve, to determine the corresponding slope factor β, or percentage changes of mortality per 1 °C difference of temperature:

$$ Ln\left( M \right) = Const + \beta \left( {T_{max} - T_{min} } \right) +\upvarepsilon $$

where, Ln (M) = Log-transformed mortality, β = slope factor, Tmax and Tmin = maximum and minimum temperature, respectively and \( \upvarepsilon \) = residual term

“Decrease” temperatures were defined as reducing T from 28 °C, while the “Increase” implied raising temperature from 28 °C. Log-transformed mortality was used to produce a normal distribution of the dependent variable. In the above regression equation, the temperature intervals between 28 and 25 °C, 28 and 20 °C, 28 and 15 °C, 28 and 10 °C (Table 2) and above 28, i.e. 28–30 °C, 28–33 °C, 28–35 °C (Table 2) were studied separately, because their correlations between Ln (M) and T became pronouncedly concave as the absolute value of beta increased.

Table 2 Mortality of Catla catla versus range of water temperature
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Results and Discussion

Mortality of Fish

In 10, 15, 20 and 25 °C temperature groups, the water temperature gradually decreased from the acclimation temperature of 28 °C. In 10 °C temperature group, as the temperature reached 16 °C, mortality started; 17 % mortality was recorded at 14 °C and 34 % larvae died immediately as the water temperature became 10 °C. All larvae (65 %) died within 2 days after reaching 10 °C temperature. In 15 °C temperature group, 4 % mortality was recorded at temperature 24–18 °C. As the water temperature further dropped to 15 °C, 16 % mortality of larvae was recorded immediately and the mortality was continued up to 11 days in this group. In 20 °C temperature group, 2 % mortality was found as water temperature became 24 °C; 9 % larvae died immediately as water temperature dropped to 20 °C. Mortality of larvae continued up to 11 days of reaching this temperature (Fig. 1). In 25 °C temperature group, 2 % mortality was recorded on day-4 of reaching 25 °C water temperature and mortality of larvae continued up to 18 days. In 30 °C temperature group, 1 % mortality of larvae was found on day-10 of reaching this water temperature. In 33 °C temperature group, 1 % mortality was recorded within 24 h of reaching this temperature, and further 1 % mortality was found after 4 days. In 35 °C temperature group, 1 and 2 % mortality were recorded on day-2 and day-3, respectively after water temperature become 35 °C. There was no mortality of Catla kept in outdoor condition during the study period.

Fig. 1
figure 1

Mortality of Catla catla exposed to the temperature 10, 15 and 20 °C

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Among all these treatments, significantly (P < 0.05) higher cumulative mortality of Catla was found at lowest temperature of 10 °C as compared to the other groups. Cumulative mortality rates of Catla were 89, 43, 24, 18, 1, 2 and 3 % in 10, 15, 20, 25, 30, 33 and 35 °C temperature groups, respectively (Fig. 2). Though 1–3 % mortality of Catla was recorded in groups exposed to temperature above the acclimation one, there was no significant (P > 0.05) difference among these groups. This indicated that the lowering of temperature from 28 °C was more stressful as compared to the raising of temperature for this size group of Catla. Temperature tolerance of fish is dependent upon acclimation temperature [20]. There was 18 °C difference between the acclimation temperature (28 °C) and the lowest temperature (10 °C) at which the larvae were exposed, whereas there was 7 °C difference between the highest temperature (35 °C) exposed group and the acclimation temperature. As the minimum temperature tolerance limit of adult Catla is ~14 °C, the mortality was highest in larvae exposed at 10 °C. The highest temperature of 35 °C used in the present study was 3 °C higher as compared to the optimum range of temperature of 25–32 °C [19]. The present study showed that at 10 °C temperature all larvae died within 2, whereas at 15 and 20 °C, mortality continued up to 11 days; it continued up to 18 days in 25 °C temperature exposed group. In a similar study, adult Clarias batrachus were exposed to a temperature range of 10–35 °C; significantly (P < 0.05) higher mortality of fish was recorded at 10 °C as compared to the other temperatures. At 10 °C, 50 % fish died within 5 days of exposure [17]. Lower environmental temperature affected both cellular and humoral responses in fish [21].

Fig. 2
figure 2

Cumulative mortality of Catla catla (n = 3) exposed to various temperatures

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With simple regression analysis for the temperature range (T < 28 °C and T > 28 °C), percentage changes of mortality in each fall and increase of ΔT = 1 °C was calculated in the framework of log-linear regression model (Table 3). When temperature was reduced from 28 °C, the cumulative increment of mortality in each 1 °C fall of temperature amounted to e.109 = 1.115 (P < 0.05). High R-square value was observed indicating a high variation (96.8 %) in log-transformed mortality due to difference in temperature. Further, beta coefficient was observed less steep when temperature increased beyond the acclimation temperature of 28 °C. The cumulative mortality of e.075 = 1.077 (P > 0.05) was obtained for each 1 °C increase of the temperature beyond 28 °C. In cold and temperate regions, the temperature-mortality rises as temperature falls below or rises above this “point of maximum comfort” i.e. 28 °C. The above discussion also attempted to establish the range of linear increase in mortality that occurred with each 1 °C drop of outdoor temperature and the effect of cold spells on mortality.

Table 3 Regression results of temperature ranging less and greater than 28 °C
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Conclusions

The present study showed that 15 °C temperature was stressful to the larvae of Catla as mass mortality occurred. One degree fall of temperature from 28 °C resulted into 10.7 % mortality of larvae. Being a tropical species, Catla showed adaptability towards higher temperature. Temperature of aquatic environment is important for ensuring survival and failure to adapt temperature fluctuations is generally ascribed to the inability of fish to respond physiologically resulting its mortality.