Aquaculture,91 (1990)349-358

Elsevier Science Publishers B.V., Amsterdam

Physiological stress in striped bass: effect of

acclimation temperature

Kenneth B. Davisa and Nick C. Parkerb,*

Ecological Research Center, Department of Biology, Memphis State University,

Memphis, TN38152, USA

bU.S. Fish and Wildlife Service, Southeastern Fish Cultural Laboratory, Marion, AL 36756, U.S.A.

(Accepted 2 May 1990)

ABSTRACT

Davis, K.B. and Parker, N.C., 1990. Physiological stress in striped bass: effect of acclimation temperature. Aquaculture, 91: 349-358.

Physiological responses to acclimation temperature and confinement stress were studied in yearling striped bass, Morone saxatilis. Temperature significantly affected plasma cortisol, glucose, chloride, and hematocrit but not osmolality in fish acclimated to 5,10,16,21,25 and 30 C. Close confinement for 12 min in a net resulted in changes of these characteristics in some or all of these groups. Generally, plasma cortisol and glucose increased, and plasma chloride, osmolality, and hematocrit decreased due to net confinement, and returned toward resting levels after release from the net. The physiological changes during stress were greatest and the recovery from these changes was slowest at 5 and 30C. Further, the quantitative response was least and the recovery most rapid in fish stressed at 10 and 16C. Acclimating and moving striped bass within this temperature range should decrease stress-related responses and mortality and thus improve yield after stocking.

INTRODUCTION

Fishing pressure for striped bass, Morone saxatilis, has increased markedly over the last 15 years (Axon and Whitehurst, 1985). The increase has been made possible largely by the artificial propagation techniques developed by Stevens (1966), in which mature fish are taken from inland or coastal waters to hatcheries, where they are induced to ovulate with hormones; young fish are then released in lakes and reservoirs. Artificial environments in the hatchery, crowding, transporting, and stocking in the new environment are stressful conditions that endanger the viability of the fish.

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*Present address: Texas Cooperative Fish and Wildlife Research Unit, Texas Tech University, Lubbock, TX 79409-2125, USA.

Freshwater fish respond to most stressful stimuli with a predictable pattern of physiological changes. The type of stressor includes external factors such as poor water quality, fish density or handling disturbance (Wedemeyer, 1972; Tomasso et al., 1981; Davis et al., 1984). The typical physiological stress response is generally independent of the type of stressor and includes a rapid increase in plasma cortisol and glucose, and frequently a delayed decrease in electrolytes and osmotic pressure (Carmichael et al., 1984a,b).

Stress has been described as an energy drain (Barton and Schreck, 1987a); energy that might be used for growth is thus channeled into catabolic uses. The quantitative aspects of physiological changes induced by stressors depend on the acclimation temperature and the intensity and duration of the stressor (Strange, 1980; Carmichael et al., l 984a; Davis et al., l 984). Culture practices that limit the stress response improve survival and growth because these responses can increase disease susceptibility, decrease osmoregulatory capacity, decrease growth, and generally disrupt homeostasis (Davis et al., 1985; Barton et al., 1987).

Sensitivity of fish to stress and the interaction with temperature differ markedly among species. The physiology of stress has been studied in most detail in the Salmonidae and Ictaluridae - groups that vary greatly in optimum temperatures and in tolerance to stress. Striped bass are produced primarily by spawning brood stock caught from natural waters, and raising the fry. Young fish are very sensitive to handling stress and frequently many die due to the conditions under which they are maintained. Perhaps because of their sensitivity, only limited data are available on stress physiology in striped bass. This study was designed to quantify some of the physiological responses in yearling striped bass acclimated to a wide range of temperatures and stressed by close confinement in a net.

MATERIALS AND METHODS

Thirty-five yearling striped bass, Morone saxatilis, were stocked into each of six indoor 1600-1 tanks supplied with flowing well water at 21C. These experiments were done in June with an ambient photoperiod. A random sample of 46 fish weighed 110.4 36.6 g (X s.d.) and were 19.1 2.1 cm in standard length. Food was withheld for at least 14 days prior to sampling. Water temperature in each tank was adjusted during the first 6 days to slowly establish test conditions of 5, 10, 16, 21, 25 and 30C; these experimental temperatures were maintained for 8 days before sampling began.

Fish were sampled in groups of six and were anesthetized with 0.02% tricaine methanesulfonate before they were bled. The first six fish were transferred quickly to anesthetic and bled immediately. Bleeding of each group was completed within 3 min. These animals constitute the resting group (R), and were sampled at 13.00 h. All remaining fish were rapidly dipped into a net and confined for 12 min. During confinement in the net, fish were held in the water so that they were crowded and were unable to maintain equilibrium but were completely covered with well-oxygenated water. After 12 min. a second group was sampled and the remaining animals were released into the tank and sampled at 6, 24, and 48 h after release from the net.

Blood was taken with heparinized syringes from the caudal vessels in the hemal arch. It was separated by centrifugation, hematocrit was determined, and the plasma was stored frozen. Plasma concentrations of cortisol were determined by radioimmunoassay (Serono Diagnostics, Braintree, MA, U.S.A.), glucose by colorimetry (Sigma Diagnostics, St. Louis, MO, U.S.A.), chloride by amperometric titration with a chloridometer (American Instrument Company, Silver Spring, MD, U.S.A.), and osmolality by a vapor pressure osmometer (Wescor Inc., Logan, UT, U.S.A. ). The radioimmunoassay technique was verified by spiking known cortisol amounts to striped bass plasma with low cortisol concentrations. Intra- and interassay coefficients of variation from this spiked sample were 3.62 and 4.84%, respectively.

Data were analyzed by analysis of variance followed by Duncan's multiple range test where significance (P<0.05) was indicated. Statistical tests were performed by release 6.03 of the SAS System for Personal Computers.

RESULTS

All resting plasma characteristics except osmotic pressure were significantly affected by acclimation temperature (Table l). Cortisol was higher in fish held at 30 C than at any other temperature and hematocrit was highest above 16 C. Plasma glucose concentrations generally decreased with increasing temperature, and chloride concentrations were significantly higher at the two lowest temperatures.

All fish were alive after release from the net. After 6 h of recovery 11 fish were dead - eight in the 30 C group, one in the 25 C group and two in the 21 C group.

Blood hematocrit was unchanged by stress in fish held at 5 C and only slightly changed after 24 h of recovery at 10 C ( Fig. 1). Hematocrit was significantly elevated after 12 min of net stress in fish held at 16, 25 and 30 C. Although no significant increase occurred at 21 C, hematocrit decreased significantly during recovery at all four temperatures above 10 C.

Plasma osmotic pressure increased significantly only in fish stressed at 25 C (Fig.2). Osmotic pressure decreased at some point during recovery in fish at all temperatures except 10 C. Only in fish held at 10 C was there no change during the experiment.

Plasma chloride did not change after 12 min of net stress in fish held at any of the temperatures tested (Fig. 3). However, concentrations of chloride decreased during the recovery period at all temperatures, but the decrease was not significant in fish held at 16 and 25C.

 

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TABLE 1. Resting blood characteristics of striped bass acclimated to six different temperatures. Data are presented as means, s.e., and na

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Fig. 1. Hematocrit changes due to confinement stress in striped bass acclimated to different temperatures. Values are means s.e. before stress (R), after 12 min of net confinement (S), and 6, 24, and 48 h after release from the net, in striped bass acclimated to temperatures of 5, 10, 16, 21, 25 and 30 C. Data are means of five or six fish except where shown by number inside the bar. All fish held at 30 C were dead by 48 h after release from the net. Different letters by each mean represent statistically different (P< 0.05) subsets by Duncan's multiple range test.

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Fig. 2. Plasma osmolality changes due to confinement stress in striped bass acclimated to different temperatures. Experimental details are the same as those described in Fig. 1.

 

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Fig. 3. Plasma chloride concentration changes due to confinement stress in striped bass acclimated to different temperatures. Experimental details are the same as those described in Fig. 1.

 

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Fig. 4. Plasma glucose concentration changes due to confinement stress in striped bass acclimated to different temperatures. Experimental details are the same as those described in Fig. 1.

 

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Fig. 5. Plasma cortisol concentration changes due to confinement stress in striped bass acclimated to different temperatures. Experimental details are the same as those described in Fig. 1.

Glucose concentrations were not significantly changed during the experiment in fish held at 5 or 16 C (Fig. 4). A 12-min net stress caused a small but significant increase only in fish held at 25 and 30 C, and recovery to resting levels was rapid. Glucose increased in fish held at 21 C but the response was delayed and recovery rapid. In fish held at 10 C, a delayed increase of plasma glucose occurred 24 h after release from the net and levels remained high at 48 h. In general, plasma glucose changed very little during or after the net stress.

The 12-min net stress increased cortisol concentrations in all fish held at 16 C and higher (Fig.5). Although recovery was rapid in fish held at 16 and 21 C, resting levels were never regained in fish held at 25 and 30 C. Cortisol did not change significantly in fish held at 10 C and a delayed increase at 6 and 24 h of recovery occurred in fish held at 5 C.

DISCUSSION

The highest and lowest experimental temperatures appeared to be detrimental for holding or handling striped bass. Compared with other groups of fish, the group of fish acclimated at 30 C had greater changes after confinement in hematocrit, osmolality, chloride, and cortisol; recovered to a lesser degree; and had higher mortality. Striped bass acclimated to 5 C seemed to lose some metabolic regulatory ability - particularly for glucose - and changes due to net stress were quantitatively lower and temporally slower. For instance, the characteristics that changed after net stress did not become statistically different until after 6 h (cortisol) or 24 h (chloride and osmolality) of recovery.

Optimum temperature conditions for juvenile striped bass have been estimated by growth dynamics to be about 24 C and the greatest bioenergetic efficiency in fluctuating temperatures between 14 and 22 C (Cox and Coutant, l 981). Coutant et al. (1984) showed a seasonal change in selection temperature from 24-27 C during spring and summer to 20-25 C in fall. Larger fish (adults and subadults) followed by radiotelemetry selected cooler (22-26 C ) refuges in reservoirs during summer (Moss, 1985). These studies were based on behavioral characteristics and complex metabolically related features such as growth and food conversion. The optimum temperature for handling fish is the temperature at which homeostatic concentrations in blood characteristics diverge least, both during and after the disturbance. This may be judged quantitatively (the amount of change) or temporally (the rapidity of return to pre-stress levels). Internal control systems for the physiological characteristics measured here probably reflect different mechanisms. Chloride and osmolality are affected by the diffusion rates of electrolytes between the fish and the environment and by osmoionoregulatory processes of the gills and kidney. Plasma glucose, cortisol, and hematocrit levels are determined more by the effect of temperature on metabolism than by diffusion. Because diffusion is a passive process, as temperature increases so will the movement of diffusing molecules. Metabolic rate is probably greatest at some optimum temperature and decreases above and below that optimum. It seems reasonable that optimum temperatures for handling fish would be lower than optimum temperatures for growth and food conversion.

The pattern of changes that occurred due to net stress in striped bass is similar to that described in catfish (Davis et al., 1984); largemouth bass, Micropterus salmoides (Carmichael et al., 1984a,b); and salmonids (Strange and Schreck, 1978; Barton et al., l 980; Specker and Schreck, 1980; Davis and Parker, 1983). Temperature profiles of stress in striped bass are more similar to those of largemouth bass and to a lesser degree catfish than they are to salmonids. This relationship probably reflects the optimum temperature similarities of these fishes. The data presented here suggest that the best temperature at which striped bass can be transported would be between 10 and 16 C, which is 10- 12 C lower than the optimum temperature for bioenergetic efficiency (Cox and Coutant, 1981). The optimum handling temperature probably represents the best balance of a somewhat reduced metabolic rate, which compensates for the decrease in diffusion rate of the ions. An additional advantage of low handling temperatures is higher oxygen saturation and lower metabolic waste production. The metabolic rate of stressed steelhead (Oncorhynchus mykiss) was twice that of unstressed fish, and may reduce by one-quarter the energy available for other metabolic needs such as ionoregulation or recovery from ion loss during stress (Barton and Schreck,1987b).

Further, suppression of the cortisol response has additional benefits to the fish. Elevations of cortisol in salmonids and catfish are associated with longer term effects such as decreased disease resistance due to a suppression of the immune system (Wedemeyer, 1970; Ellsaesser and Clem, 1986). All of these stressors could result in reduced fitness of the fish (Strange and Schreck, 1978) and may result in high post-stocking mortality. The lowest temperature, 5 C, probably slows metabolism to such a degree that adequate homeostasis cannot be maintained. Further, even though the changes in osmolality, chloride, and cortisol were delayed after the net stress, physiological recovery was also delayed at this temperature.

Our data suggest that the response was least (quantitatively) and recovery most rapid (temporally) in striped bass handled at 10 and 16 C. Transporting fish within this temperature range should diminish the stress response, decrease long- and short-term mortality and increase handling efficiency associated with stocking. However, gradual acclimation of fish to the new thermal environment is very important if the temperature of the receiving water is not close to that of the transporting water, since temperature shock is also a stressor (Carmichael et al., 1984a). An additional benefit might be gained by lightly anesthetizing fish (Strange and Schreck, 1978; Tomasso et al., 1980) and by adding salts to the water. Sodium chloride, calcium chloride, and other salts have been shown to promote hauling success (Strange and Schreck, 1978; Tomasso et al., 1980; Davis et al., 1982; Carmichael et al., 1984b) in fish. The addition of these compounds to water between 10 and 15 C should result in the best conditions for transporting striped bass.

ACKNOWLEDGEMENTS

We thank James Crawford and Mary Anna Davis for technical support with data collection; Dewey Tackett for statistical support; and Melvin Beck, Paul Eschmeyer, Charles Lessman, Michael Redding, and Bill Simco for their helpful review of the manuscript.

REFERENCES

Strange, R.J. and Schreck, C.B., 1978. Anesthetic and handling stress on survival and cortisol concentration in yearling chinook salmon (Oncorhynchus tshawytscha). J. Fish. Res. Board Can., 35: 345-349.