M.R. REEVE 1
Ministry of Agriculture, Fisheries and Food
Fisheries Experiment Station, Conway, North Wales
Background information with regard to ecology, distribution and method of collection are given for Palaemon serratus, and the published literature is briefly reviewed. The methods of obtaining larvae from mature adults in the laboratory, and the difficulties associated with the maintenance of tank populations of stock prawns are noted. Consideration is given to factors affecting the rearing of young stages, including temperature, salinity, density, cannibalism and food. Growth over several months is followed. Specific points to come out of the study are discussed both on the positive side and also on the negative side, showing where further research is needed. The latter include the need for breeding for specific strains, and the desirability of study of the pathology and biochemistry of the prawn. Scaling-up to a pilot operation is contemplated, as well as economic aspects such as price of the product and cost of rearing. Overall, an optimistic outlook is taken of the biological feasibility of the project, with opinion reserved regarding the economics.
1 Present address: Institute of Marine Sciences, University of Miami, 1 Rickenbacker Causeway, Miami, Florida 33149, U.S.A.
POSSIBILITES D'ELEVAGE DE LA CREVETTE ROSE (Palaemon serratus Pennant) : EVALUATION PROVISOIRE
L'auteur, après avoir exposé les données essentielles touchant l'écologie et la répartition de Palaemon serratus et la technique employée pour recueillir les sujets, et passé brièvement en revue la documentation existante, traite des méthodes adoptées pour obtenir au laboratoire des larves à partir d'individus adultes, et relève les difficultés que pose le maintien en vivier de populations de crevettes. Il étudie les facteurs qui affectent le développement des formes juvéniles : température, salinité, densité, cannibalisme et alimentation. La croissance des animaux est suivie pendant plusieurs mois. Les divers résultats sont examinés sous l'angle tant positif que négatif, de manière à mettre en évidence les domaines nécessitant un complément d'étude. Il apparaît par exemple qu'il faut sélectionner certaines souches et qu'il serait bon d'étudier la pathologie et la biochimie de la crevette. L'auteur envisage la possibilité de passer du stade du laboratoire à celui de l'opération pilote, et se penche sur certains aspects économiques comme le prix du produit et le coût de l'élevage. Dans l'ensemble, il est optimiste quant à la viabilité biologique du projet, mais réserve son opinion pour ce qui est de l'angle économique.
LA ADAPTABILIDAD DEL CAMARON INGLES PALAEMON SERRATUS, PENNANT, PARA EL CULTIVO - DEDUCCIONES PROVISIONALES
Se examinan brevemente los antecedentes de que se dispone con respecto a la ecología, distribución y métodos de recogida de Palaemon serratus, así como la literatura publicada sobre el tema. Se indican los métodos que se siguen en el laboratorio para obtener larvas de adultos maduros, y las dificultades relacionadas con el mantenimiento de poblaciones de camarones en estanques. Se estudian los factores que influyen en la cría de formas jóvenes, como son la temperatura, salinidad, densidad, canibalismo y alimentación. Se sigue el crecimiento a lo largo de varios meses. Se examinan puntos específicos del estudio, tanto desde el punto de vista positivo como negativo, para averiguar en qué sector es preciso realizar nuevas investigaciones. Esto incluye la necesidad de criar razas específicas, y la conveniencia de estudiar la patología y bioquímica del camarón. Se estudia la posibilidad de ampliar las operaciones a escala experimental, y aspectos económicos tales como el precio del producto y los gastos que representa la cría. En general, la viabilidad biológica del proyecto se considera de un modo optimista aunque se reserva la opinión en lo que se refiere a su economía.
The work which forms the basis of this review is reported in detail in Reeve (1969, 1969a) and was largely accomplished over a period of two years ending in the summer of 1966.
Palaemon serratus was chosen for the initial study because it was native to British coasts (and hence was presumably more amenable to laboratory culture than offshore deeper water animals such as Pandalus borealis) and also because it was the largest of the native shrimps and prawns. Its larval life history had been investigated by Gurney (1924) from plankton samples and by Sollaud (1923) in the laboratory. Forster (1951) and Cole (1958) studied its ecology around British coasts. It has also been used extensively in physiological studies.
The species is found throughout the Mediterranean and only enters British waters at the extreme north of its range, where it has in the past been fished along the south coast of England, Wales and Ireland. In 1964, however, when the study began, prawns were scarce in the areas around the laboratory at Conway in North Wales, and the live mature animals needed had to be transported from the south coast of England. The journey of up to 350 mi (563 km) was made in the minimum possible time, using vehicles which could accommodate bins containing sea water through which oxygen was bubbled.
The initial study, the results of which will be referred to below, was concerned mainly with the location of sources of stock prawns and the procurement and rearing of larvae and post-larvae.
It was known from the work of Cole (1958) that the period of egg carriage in nature is directly related to temperature. On arrival at the laboratory, berried females (those which had spawned and were carrying their eggs) were therefore gradually accustomed to warm water of up to 20°C. In this way, the duration of egg carriage could be reduced from up to five months in nature to 28 days at 20°C in the laboratory. The fecundity of a prawn was dependent on its size, the largest animals producing over 3000 eggs. The larvae hatched as zoea and did not pass through a stage where they required phytoplankton suspensions for food; they could be separated from the adults by making use of their attraction to light. Maturing females which were taken from the natural environment were able to mate and spawn successfully in laboratory tanks and eventually to produce viable larvae.
Considerable difficulty was experienced in keeping stocks of adult prawns over long periods in laboratory tank populations. One very obvious problem was the susceptibility of a soft newly-moulted prawn to attack from its companions. Although growth was rarely observed in adult stocks, prawns nevertheless continued to moult regularly, the frequency depending on the temperature; at 20°C the mean intermoult period was 21 days. In tanks in which the density of animals was reduced, the survival rate over a period increased. Various forms of cover were offered in which soft animals could take refuge, but these were of little use. Survival rates were higher in stocks of berried females, because they did not moult during the period of egg carriage.
Apart from the mortality of stocks of adults due to cannibalism, mass mortalities also occurred from microbial infection. Prawns were very susceptible to attack of the cuticle, presumably by chitiniferous bacteria. Another symptom known as “white spot”, which appeared to be a progressive tissue breakdown from the tail forwards, normally caused death and spread through a population. No detailed work has yet been done on the pathology of these effects, through they appear to be associated with the crowded conditions of laboratory culture, since they were not encountered in commercial catches. Other mortalities which occurred in apparently healthy prawns just before moulting might be associated with feeding deficiences. They were mostly fed on fresh mussel, Mytilus edulis, on the shell.
In most of the experiments to be described in this section, larvae were reared in containers with a capacity not exceeding two litres, in sea water which had been filtered and was maintained at a temperature of 20°C and salinity of 32.5‰. The nauplii of Artemia were provided as food. Larval growth was measured by increase in length, since the number of larval stages was found not to be constant under varying conditions (Reeve, 1969).
The optimum temperature for larval survival and growth and for metamorphosis, was between 20 and 25°C, varying from brood to brood. At this maximum rate of growth metamorphosis was achieved within about three weeks, and after about seven larval stages had been passed through. Growth rate at any one temperature varied very much between broods. Larvae kept at 30°C did not survive to metamorphosis, and those kept at temperatures below 10°C showed no significant growth.
Salinity to lerance was wide, a 50 percent survival rate resulting after four days following direct transference of larvae from normal sea water to that at salinities ranging from 15–45‰. In long-term low-salinity experiments survival was little affected above 23‰, although there was some improvement in growth in salinities up to that of normal sea water.
Larvae actively consumed each other, although direct observation could not determine finally whether cannibalism or merely the scavenging of the dead and dying animals was occurring. Indirect evidence, such as the high rates of survival of isolated larvae, and the decimation of the smaller animals in populations of two sizes confirmed, however, that active cannibalism of potentially viable larvae was taking place. An effort to prevent this was made by introducing vigorous aeration in order to evenly distribute and circulate the larvae in the container. The effort failed because anything but the gentlest aeration itself caused high mortality rates.
Provided that there was an adequate supply of food, however, larval cannibalism was not as serious as might have been expected. Even at densities as high as 400/l a survival rate of 28 percent after 26 days from hatching could be achieved, by which time larvae were about to metamorphose and were 8 mm in length. At 20/1 the survival rate was 60 percent. Over 90 percent of a population of individuals isolated from each other would survive from hatching through metamorphosis.
A wide variety of foods was offered to newly-hatched larvae of 4 mm in length, ranging from coarse resuspensions of dried Chlorella to chopped ox liver. The most satisfactory food - both from the point of view of ease of preparation and tank maintenance, and also with respect to growth and survival - was undoubtedly the nauplii of Artemia, the brine shrimp. The optimum concentration of live Artemia in the medium was found to be as high as 5–10/ml. During 1966 trouble was experienced with the food, in that certain stocks of brine shrimp eggs, which appeared to originate from Utah, failed to allow the larvae to successfully complete metamorphosis. The nature of the presumed deficiency or possibly toxicity of these Artemia nauplii has not been resolved. It is known that other workers have recently experienced similar problems with Artemia.
On some occasions larval populations were reared beyond metamorphosis. In one experiment, a population was reared at 20°C beyond 150 days from hatching. Following metamorphosis the diet was gradually changed over to fresh mussel. At 150 days, survival had decreased to 13 percent and the mean wet weight had increased from 0.24 mg to 900 mg. This represents a growth rate at least twice as rapid as that encountered in nature around British coasts.
The information which appears above summarizes some of the points of the study which are comprehensively reported elsewhere (Reeve, 1969, 1969a). On the one hand, in some cases relatively simple solutions have been found to certain potential problems, but on the other hand there are areas requiring further study, and these are brought into sharper focus by the results of the present work.
On the positive side of the balance sheet, the following points may be made. In the first place, all phases of the life-cycle have been shown to be capable of continuance in the laboratory, including the maturation of the gonads of adults, copulation, spawning, egg-carriage, hatching and growth and metamorphosis of larvae, and the growth of postlarvae and juveniles. The complete cycle has not so far been attempted in one population. Even if this proves difficult in the future, it should still be possible to arrange a supply of larvae from adults out of the natural environment throughout most of the year, by taking advantage of the long breeding season. This would be achieved by accelerating the development of maturity and shortening the period of egg-carriage at the beginning of the season, and by slowing down the process towards the end of the breeding season, by raising or lowering the water temperature in which the animals were kept after being brought into the laboratory. The maintenance of high temperatures in the laboratory sea water supply also facilitated the speeding up of the growth rate of the young stages, by provision of a year-round growing season. The net result is the attainment, within six months, of a size which would require over a year in nature.
The palaemonids' habit of carrying their spawned eggs confers certain advantages to a would-be culturist which are not offered by penaeids. The condition and number of the expected hatching may be readily checked and the course of development followed very closely. The resultant larvae hatch directly into a predatory zoea stage, which eliminates the necessity for costly and time-consuming preparations of phytoplankton suspensions. These advantages may be offset to some extent by the fact that fewer eggs are carried - the larger eggs of Palaemon are measured in thousands rather than in the hundred-thousands of the penaeids. Even so, this in turn would appear to be offset by the very high mortalities encountered by penaeid larvae as they pass through a greater number of larval stages. Palaemon larvae may be reared at very high densities with surprisingly good survival rates. It is possible that with improved feeding techniques, the cannibalistic tendencies of the young animals can be considerably reduced even at very high densities. One approach might be to provide larger-sized Artemia nauplii for the late larvae and postlarvae.
As is inevitable in any biological study, the initial work suggests many more avenues of enquiry. Some of the directions which it is important that the research should take in the near future may be briefly touched upon. One of the most interesting and possibly the most rewarding may be the development of genetical strains. It is likely and also desirable that successive generations from the original parents will be capable of being reared through in the laboratory. It is already clear that there is considerable variation between broods, and this could be exploited. A primary requirement would be a strain which combines a fast growth with a high survival rate. The latter property will probably be connected with uniform growth rates within a brood, because it is in situations where some larvae rapidly outgrow others that cannibalism is likely to be more pronounced. A further growth rate requirement is a strain which attains its maximum growth rate at the lowest possible temperature and experiments have suggested that the optimum temperature for growth varies between broods.
Among other qualities to be sought would obviously be resistance to disease, and the whole problem of the pathology of infections in crowded tanks must be investigated from the standpoints both of prevention and cure. This may be associated with other problems of a physiological and biochemical nature, such as an analysis of the variations between the stocks of Artemia in their suitability as food. It will be imperative to know whether food which proves to be of poor quality can be enriched in some way or whether it is actually toxic, whether the quality of the food can be tested other than by bio-assay, and whether food from a particular geographical source may always be relied upon. This sort of work suggests a complete biochemical study of the digestive potential of the prawn. It was clear from the initial work that the adult prawns were not being adequately fed in the laboratory. An analysis of the digestive enzymes should indicate the range of chemical materials capable of being digested and also suggest the composition of a possible synthetic food; for the older animals, at least, this might be desirable, since it might prove impossible to provide vast quantities of a naturally-occurring biological food such as fresh mussel.
The routine rearing of juveniles is still to be considered in detail, and their requirements, like their behaviour patterns, may well be rather different from those of the larvae and early postlarvae. A future need will be the scaling-up of experiments, from laboratory to pilot operations, on a continuous rather than a discontinuous basis. This will necessitate the development of techniques to minimize cost and handling. One possibility in the scaling-up process might be semi-automation of the rearing of the young stages. In one experiment Artemia nauplii were allowed to grow in water enriched with algae, while they were at the same time being grazed down by prawn larvae. It should be possible to decide by experimentation on the initial number of Artemia nauplii per prawn larva necessary in the original culture, in order to provide continuously available food for the larvae through metamorphosis and perhaps for a month or more after that time. One of the advantages of the method would be that the food organisms would themselves be increasing in size and would consequently continue to provide suitable food for much larger prawns. Even fully-grown prawns can catch and consume adult Artemia.
It is of course too early to be committed at this stage to the attainment of a commercial farming operation. There are other points of an economic rather than biological nature to be considered. In the first place, there is the question of whether the animal currently being studied is the best possible choice. It seems unlikely that a more suitable local species is to be found. At first sight the larger, fastergrowing and highly fecund foreign species such as Macrobrachium and Penaeus appear to be commercially much more attractive propositions, and this might well be the case if they existed in the same geographical region as Palaemon. However, at this stage the maintenance of a species which would be costly and difficult to obtain far away from its natural habitat would probably create many more problems. Much investigative work in the region of origin of the foreign prawn would be an essential prerequisite.
A further consideration in any commercial operation would be the cost of maintaining the high temperatures required for the fastest growth of larvae and juveniles in a hatchery. The transference of late juveniles to fatten off in outside ponds whose temperature was controlled by the climate would inevitably slow down their growth considerably at most times of the year. The problem might not be so serious if it is assumed, as is probably the case, that marketable prawns can be reared within a year under optimum conditions. Such conditions might be the use of the winter period for the indoor hatching and rearing of the young to about half-size or less, followed by the fattening-off period in the spring and summer in outside ponds which might reach a temperature of 20°C in sheltered locations. Another possibility is in the use of the warm-water effluent from electricity generation stations.
In short, although the culture of Palaemon serratus is probably biologically feasible, there are many factors which await resolution before a prediction of its economic feasibility can be accurately judged. It might be considered that with the projected world food shortage, the biological feasibility should take precedence. This possibility aside, however, the fact that intensive cultivation of any marine fish or shellfish is still (with notable exceptions) largely unexplored, would indicate that continued work will provide valuable experience in the field of crustacean cultivation, even if the particular project itself does not mature.
Cole, H.A., 1958 Notes on the biology of the common prawn Palaemon serratus (Pennant). Fishery Invest., Lond.(2), 22(5):1–22
Forster, G.R., 1951 The biology of the common prawn, Leander serratus Pennant. J.mar.biol. Ass.U.K., 30:333–60
Gurney, R., 1924 The larval development of some British prawns (Palaemonidae) Leander longirostris and Leander squilla. Proc.zool.Soc.Lond., pp.961–82
Reeve, M.R., 1969 Growth, metamorphosis and energy conversion in the larvae of the prawn, Palaemon serratus. J.mar.biol.Assoc.U.K., 49(1):77–96
Reeve, M.R., 1969a The laboratory culture of the prawn Palaemon serratus. Fishery Invest., Lond.(2), 26(1):1–38
Sollaud, E., 1923 Le développement larvaire des Palaemoninae. Bull.biol.Fr.Belg., 57:509–603