Discussions with scientists engaged in the gilthead bream programme have indicated a need to delineate specific research areas within the four major divisions of the general project, i.e., recruitment, nutrition, cultivation, disease or pathobiology. While these areas are unquestionably interrelated and basic to a successful fish culture programme, certain specific facets can be earmarked as priority research projects. The latter are designated in that they represent major problems to be solved that involve definitive goals and finite dates for completion of objectives. Furthermore, these investigations have noteworthy relevance in terms of warm-water marine fish culture in countries other than Israel Aspects are closely related to aquaculture endeavours in the United States in its widespread fish culture programme.
Information reported below, in the main, is from discussions with Dr. Hillel Gordin of the 10LR staff and deals with his area of expertise in control of the life cycle of S. auratus. Recruitment activities are directed along a broad front as illustrated as follows:
Successful induction of spawning has been achieved, operative now in December rather than in mid-February. This makes it further possible that the spawning season may be extended three months into March. Apart from induced spawning using HCG, the environmental triggers or determinants of spawning are generally unknown. Knowledge of endocrinology assumes additional importance in view of the bisexual polymorphism of Sparus auratus. Functional sexual dimorphism begins after year one.
Information is needed in the following closely related areas:
Understanding of the sexual cycle, i.e., egg maturation, especially the effect of the absence of male specimens.
Time of hormonal injection, using a morphological basis or analyses of blood and/or urine system for optimisation of dose injection.
Understanding of the mechanism of operation of the gonadotropin hormone, i.e. doses related to various stages of maturation. This aspect and that of (2) are closely interrelated to environmental or “triggering” factors.
A major problem is the availability of male fish throughout the spawning season. Males have active sperm through October; however, females have eggs responsive to hormonal treatment one and a half months later. Thus, at the end of the season, females are still spawning while males have terminated their sperm production. This necessitates preservation of active sperm, possibly through liquid nitrogen treatment. Furthermore, there is an absence of males beyond the two year age, which necessitates determination of conditions for a proper male/female proportion of animals. This is required for development of a proper breeding programme of Sparus auratus.
Aforementioned areas of relevance to aquaculture in general include, among others, sperm preservation, understanding of the hormonal cycle, and development of methodology for rapid determination of time of hormone injection via biochemical analysis. Control of spawning will permit ultimate establishment of superior genetic lines, especially fast-growing, disease-resistant stock. Furthermore, the programme can serve as a model or template for establishment of a spawning/breeding programme in warm-water marine fisheries comparable to that established in the salmon fishery. Academic/research personnel in biochemistry and endocrinology in Israeli universities are prepared to assist in this project on an in-depth basis. Participation already is in progress on analyses of gonadal development in S. auratus at various stages of animal growth and maturation.
The Elat facility has the capability of developing a superior line of genetic stock. However, as noted, the male problem must be solved, that is, keeping a two year male as a male for the third year. The secual/social/environmental aspects noted are relevant to other fishes. Programmatic projection is for a three to five year period overall, in which finite evaluation of induced versus natural spawning and analyses of induced spawning on a commercial hatchery basis can be achieved.
Various larval rearing problems are evident, several of which can be noted:
Design of proper facilities for optimal rearing of larval, i.e., water flow plus other physical parameters. Technical problems also exist in application of particulate food to system.
Replacement of currently used live food with artificial diets coupled with the design of experimental set-up for analysis of animal/food response on an individual basis. S. auratus requires a smaller particle, i.e., <100 μ , than most fishes, especially fresh-water types, in addition to which different food sizes are required as the animal develops.
Nutritional studies are needed in all three general areas of fish development, i.e. larval, post-hatchery (stages of fry growth) and grow-out. However, primary attention can be directed toward early stages of growth where natural food is used (larval animals) or where mortality may be high and growth increments poor (post-hatchery fry stages).
Replacement for currently used natural foods is a priority item in hatchery programmes. Major effort here must be directed toward evaluation of capsulated products and other water-stable food delivery systems in particle or flake form. Preliminary evidence suggests that shrimp-flavoured flake formulations are effective although water circulation patterns must be regulated to allow maximal flake distribution with minimal disturbance of the fragile young animal. Development of a useful flake product, even as a supplement to Artemia in larval fish culture, has significant implications in fish culture in general.
A comparison is needed of growth and survival rates of wild versus hatchery-reared S. auratus fry on specific flake and/or particulate diets. Aspects such as protein levels, energy requirements, vitamin needs and possible growth-promoting factors are largely unknown and need to be resolved. Preliminary data suggest that mortality of wild fry can be significantly reduced to a high-protein diet, together with a noteworthy increase in development. Proposed work should include analyses of growth increment with time, with effort directed toward establishing the minimum time required to bring fry from post-hatchery to stockable fry. Rapid growth, together with a nutritionally sound regime, may be further reflected in rapid weight gain under pond/cage grow-out conditions.
As discussed in the initial report, both cage (open sea) and pond cultivation appear most promising in terms of S. auratus culture at Elat.
Pond Cultivation
Marine ponds at Elat are unique in that they afford an opportunity to evaluate such operations in a warm arid climate with high evaporation rates coupled with seepage characteristic of highly sandy strata. Both pond aspects and fish growth, including optimal density, water flow rates, degree of natural productivity, etc., can be critically examined.
Closely correlated is the concept of brackish ponds utilizing underground water or that from desalinization projects. In installations in the vicinity of Elat, water inflow is at 3 000 – 4 000 mg C1/litre while waste discharge is 6–8 mg C1/litre (approximately one-third to one-half that of sea water). In the Elat area, as much as 3 – 5 × 106 cu m/year of such water is available for pond culture, raising possibilities of varies types of poly-culture. Such fish as Tilapia have variable tolerant salinity gradients while certain marine species, i.e. Sparus auratus. can withstand as much as 15 ppt C1 with adaptation. The implications of brackish water pond culture as an adjunct to freshwater/marine systems is considerable.
Cage or Open-Sea Culture
In view of the relevance of this approach to warm-water marine fish culture in general, it is highly desirable to solve extant “aquatechnical” problems of two general types:
Optimization of the system, including materials and methods involved in construction of enclosures and mechanisms for delivery of dry feed to the culture unit.
Biological/feeding problems, including examination of density and adaptation of fish to enclosure. A major area of study involves that of feeding itself since approximately 50 – 75 percent of the food introduced is lost. The concept of a floating pellet needs to be examined. Another subject area is that of handling of fish and size differences, especially in view of the specific disease problems inherent in warm waters. These aforementioned comprise essential components of the entire open sea farming operation.
The significance of disease in cultivation of S. auratus is seen in the following research proposed by Dr. Ilan Paperna of the H. Steinitz Marine Biology Laboratory at Elat. A major problem area is that of visceral granuloma of Sparus auratus as well as other cultivated fishes. Correlations are noted between-disease or symptom enset and high ratios of fish meal in the diet. Etiological agents appear to be present in the spoiled fish meal, possibly rancidity products or other breakdown products of autolysis/deterioration.
STUDY OF THE ETIOLOGY, CAUSES AND PREVENTION OF VISCERAL GRANULOMA, A DIET - RELATED DISEASE IN FISH FED ON DRY PELLETED FOOD IN SEAWATER CULTURE
Introduction
A relation apparently exists between a change from “wet” to dehydrated trout diets in salomid culture in Europe and the United States and severity and frequency of diet-related diseases such as hepatoma, visceral granuloma and lipid liver degeneration. Whereas hepatoma in trout was found to be associated with mycotoxicosis, caused by the presence of an aflotoxin in pelleted food, the causes for the other two diseases still remain unknown. Viral, fungal and mycobacterial etiologies were suggested as causes, but none was confirmed. Circumstantial evidences, on the other hand, based on field observations and feeding experiments, suggest that the pathological conditions in the fish are diet-related diseases.
Pathological conditions similar to those of visceral granuloma in trout have been found in Spams auratus culture in seawater in Elat, Israel. Histological changes in the viscera of the affected fish were far more extreme than those described from trout, and resulted in growth retardation, deformations, blindness and death. Affected fish frequently sucoumbed to secondary bacterial septicemia, the extensive necrotic changes leading to destruction of the hematopoietic tissue in the kidneys, spleen and thymus apparently impaired the immune response system of the fish. Observations as well as preliminary experiments clearly suggest association between pelleted diet containing a high concentration of fish meal 30 – 60 %) and the development of visceral granuloma. Incidence and severity of the disease in S. auratus increased with rising concentration of fish meal in the diet. However, some evidence also suggests a possible relation between the quality (e.g., “freshness” and the source of the fish meal) and the development of the disease. Preliminary experimental data suggest that the factor in fish meal causing visceral granuloma is an antimetabolite. Fortified vitamin mix and addition of mineral prevented visceral granuloma in fish fed on diet containing 15–30 percent fish meal, while it had no effect in fish fed on 100 percent fish meal. The observed visceral granuloma is apparently not necessarily specific to spariid fish because similar pathological changes were seen also in grey mullet fed on the same pelleted food. In summary, there are two critical problems to consider. First, visceral granuloma is not specific to particular fish species and it is likely to affect a wide range of cultured fish. Second, intense marine and freshwater culture require the use of high protein feeds, by the use of high concentrations of fish meal in diets, thus the risk of visceral granuloma in such fisheries is likely to increase, therefore, there is urgent need for better understanding of the etiology of this disease.
Objectives of the Proposed Research
Determination of the visceral granuloma causative factor in fish meal, its nature (lipid or protein), its mode of action on the fish (antimetabolite, toxicant or ?), and its possible association with fish meal source (herring, whitefish, etc.), quality (heat or cold processed) and the degree of its deterioration due to storage in inadequate conditions.
Study of the etiology and pathology of visceral granuloma in fish, and its relation-with fish age and developmental stage as well as with the growth conditions in the fish farm.
Formulation of preventive measures and curative procedures for visceral granuloma disease in fish.
Plan of Study
Diets containing different ratios of fish meal of (1) several sources and and qualities (2) exposed to various degrees of deterioration under different storage conditions, or exposure to humidity, heat, etc, will be tested in series of feeding experiments with Sparus auratus, grey mullet and also Tilapia aurea (freshwater-brackish water fish) of different ages.
Fish meal which contains the visceral granuloma factor will be selected for further studies: Fish meal will be broken to lipid and protein fractions and each fraction will be fed to fish, in order to determine with which fraction this factor is associated; different ratios of vitamins at different combinations, with and without minerals, will be fed to fish to determine whether the visceral granuloma factor is an antimetabolite.
Fish fed on fish meal containing granuloma factor will be maintained at different growth conditions and will be exposed to different stresses. Pathological processes and hematological values will be studied in affected fish.
