Conventional wisdom has generally held that a fish’s health is optimally maintained by feeding it a nutritionally complete diet on a daily basis. How- ever, some recent research findings as well as empirical evidence from com- mercial aquaculture have indicated that certain deviations from feeding to satiation on a daily basis may significantly influence fish health and resistance to disease. For example, a common practice in commercial aquaculture of channel catfish is not to feed fish once an infection of a bacterium such
asEdwardsiella ictaluriis manifested (Robinson and Li 1996). Such action generally has been shown to limit the severity of infection.
Another feeding regimen experimentally shown to influence disease re- sistance is an extended period of feed deprivation. In channel catfish reared in ponds, it was found that year 2 fish (∼0.5 kg) had increased resistance to enteric septicemia of catfish caused byE. ictaluri, following a 3-month period of food deprivation (November through January), compared to fish fed throughout this period (Kim and Lovell 1995). However, year 1 fish (initially 22 g) not fed for the same 3-month period had increased mortality when challenged withE. ictaluri. Similar findings were reported in a sepa- rate study by Okwoche and Lovell (1997) in which feed deprivation of year 2 fish for a period of time similar to that in the earlier study caused reduced mortality after challenge withE. ictalurialong with higher antibody produc- tion, while nonfed year 0 fish had increased mortality and reduced antibody production. Nonfed fish in each age group had a reduced phagocytic index.
Differential responses to disease challenge of the two sizes of fish in these studies complicate recommendations for implementing periods of feed de- privation in commercial aquaculture. However, it should be noted that fish previously not fed for 3 months experienced a compensatory gain in the subsequent 3 months (February through April) of feeding such their final weight was similar to that of fish fed throughout the study. Application of feed restriction for enhanced production efficiency and improved health is certainly worthy of additional investigation.
12.4.1. Manipulation of Nutritional Condition
As discussed in the previous section, feeding practices may influence the nutritional status of fish, which may in turn affect their disease resistance by altering immune function or by retarding growth of pathogenic organisms (Lall and Olivier 1993). It was noted that age 2 channel catfish deprived of feed for a 3-month period had a reduced iron content in circulation (Okwoche and Lovell 1997). This may have contributed to their increased resistance to disease when exposed to E. ictaluribecause iron is a critical nutrient sequestered by bacteria from their host to sustain infection.
A study with Atlantic salmon also has demonstrated a relationship be- tween feeding a low-iron diet and increased resistance to disease caused by V. anguillarum (Lallet al. 1996). The low-iron diet did not affect humoral immunity and the complement system but did lower the level of iron in the organs of fish following experimental challenge. Taken together these studies seem to indicate that elevated tissue levels of certain micronutrients may predispose fish to infection and that feeding regimes to lower tissue levels of certain nutrients may improve the disease resistance of fish. How- ever, there is a need for much more research in this area to define clearly
the relationships among feeding regimes, manipulation of tissue nutrient levels, and disease resistance of fish.
Another factor known to influence nutritional condition and potentially health of fish is the dietary composition of energy-yielding nutrients. Fish vary greatly in their ability to metabolize dietary protein, carbohydrate, and lipid, as well as in the distribution and storage of these components in the body. Storage and distribution of lipid between muscle and liver tissues of assorted fish species may vary considerably due to differences in natural his- tory (Sheridan 1988). Variation in nutrient utilization also may influence the health of certain species when given artificial diets. For example, excessive levels of soluble carbohydrate in the diet have been shown to affect adversely the hepatic function of rainbow trout due to glycogen accumulation (NRC 1993). Other species such as the channel catfish maintain hepatic glycogen within more narrow limits even when fed very high levels of soluble carbohy- drate such that liver function and health are not adversely affected. Elevated dietary lipid has been associated with excessive fat deposition in the liver of some species; however, the adverse effects of this condition appear to vary greatly among species. For example, red drum (Sciaenops ocellatus) fed a diet containing 15% lipid had over 25% lipid in the liver, which did not adversely affect liver function (Craiget al. 1999). In contrast, fatty infiltra- tion of the liver has been noted in other fish species fed high levels of lipid (Mosconi-Bac 1990). Thus, the proper balance of energy-yielding nutrients in diet formulations is important not only for sustaining acceptable growth and feed utilization of fish but also for maintaining body composition within certain limits to allow normal functioning and health.
12.4.2. Seasonal Feeding Regimes
As indicated in the previous section, manipulation of diet formulations and/or feeding schedules as functions of fish size or production objective has not been as well developed in the culture of various fish species as it has with terrestrial animals. Thus considerable opportunity exists for further refinement in these areas to increase the production efficiency in aquacul- ture. Seasonal factors also may influence feeding regimes in certain types of aquaculture, particularly where fish are cultured in systems in which environ- mental conditions vary seasonally. It is well established that the prevalence of certain infectious diseases varies with the season, as most pathogenic bac- teria have specific temperature ranges in which their virulence in greatest (Plumb 1994). It also is thought that a fish’s immunity may be altered along with its metabolism as a result of seasonal temperature changes. Therefore, certain modifications of feeding schedules and diet composition in relation to season have been evaluated to a limited extent. One such modification seen in pond culture of channel catfish in the southeastern United States is overfortification of diets with vitamin C during the spring of the year. This
modification is intended to enhance the disease resistance of fish by provid- ing higher than normal levels of this vitamin during the time of the year when some of the most serious bacterial epizootics are prevalent. The marginal efficacy of such practice has limited its continued application in the industry today. Experiences in Chile with jaw deformities in rapidly growing salmon smolts transferred to the sea during periods of high water temperature sug- gest that extra dietary supplementation with vitamin C before and after the smolt transfer period is beneficial. However, further research on the sea- sonal application of some of the immunostimulants discussed in previous sections appears warranted.
12.4.3. Use of Medicated Feeds
Effective control of disease-causing organisms is a primary concern in aquaculture. Currently there are only a limited number of government- approved chemotherapeutics in the United States which can be used to treat diseases of fish intended for human consumption (Noga 1996); however, many other countries have less stringent rules governing use of chemothera- peutics in aquaculture. Administration of antibiotics in the diet is one of the most effective means of treating numerous fish in large volumes of water.
Two antibiotics which are approved in the United States for treatment of Aeromonas salmonicidainfections in salmonids andEdwardsiella ictaluriin cat- fish are Romet and Terramycin. Romet, produced by Hoffmann-LaRoche, Inc., is a potentiated sulfonamid consisting of sulfadimethoxine and orme- toprim. Terramycin is the trade name for oxytetracycline hydrochloride produced by Pfizer.
Several factors can limit the effectiveness of antibiotic treatment. One such factor is that sick fish generally do not feed vigorously and therefore may not consume enough medicated feed to ensure proper dosage. In ad- dition, some antibiotics may have limited palatability to certain species. The palatability of Romet was increased for channel catfish by increasing fish meal to approximately 16% of the diet (Robinsonet al. 1990). In addition, reducing the concentration of antibiotic in the diet in conjunction with feed- ing at a higher rate to provide the recommended dosage has been shown to be effective (Robinson and Li 1996). Romet is heat stable and thus can be used in diets manufactured by extrusion processing. Terramycin is a broad- spectrum antibiotic that is effective against a variety of bacteria but is heat labile such that its use is restricted to diets manufactured by compression pelleting.
Another potential limitation to antibiotic treatment is the development of resistance by certain strains of bacteria. Such antibiotic resistance requires proper isolation of the disease-causing bacteria and determination of its sen- sitivity to various antibiotics. Such diagnosis must be completed in a timely
manner so that treatment with an appropriate antibiotic can be initiated before the infection progresses.
Antibiotics also have been used as growth promoters for various pro- duction animals including some fish species. It is generally believed that positive responses to antibiotic supplementation in diets are due to altering the microflora of the gastrointestinal tract. Enhanced digestibility of some unsaturated fatty acids was observed in rainbow trout fed diets containing chloramphenicol, oxolinic acid, and oxytetracycline (Cravediet al. 1987).
In three separate studies conducted in aquaria (Ahmed and Matty 1989), cages (Viola and Arieli 1987), and ponds (Viola et al. 1990), weight gain of common carp was generally improved with inclusion of the antibiotic Virginiamycin in the diet. In contrast, neither Romet nor oxytetracycline inclusion in the diet improved the weight gain of channel catfish (Rawles et al. 1997). Due to the inconsistent improvements in performance of fish fed antibiotics, this practice generally has not been implemented in commercial aquaculture. The potential for antibiotic resistance and residues in the fish also has restricted the use of antibiotics in fish diets. Other com- plications also may result from prolonged administration of antibiotics in the diet. For example, spinal deformities were produced in Atlantic salmon but not in Arctic charr after prolonged feeding of Terramycin (Toften and Jobling 1996).
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