The marine silverside <b><i>Odonthesthes regia </i></b>is an atherinid fish highly appreciated in South America, especially in Peru and Chile where it is considered a fish of first quality. This thesis reports research into basic biological aspects that could be useful to the establishment of future cultures of this species. Capture of specimens in the wild, transportation, acclimatization and conditioning to confined systems are detailed. <b><i>Odonthesthes regia </i></b>adapts easily to confinement; conditioning of the brood stock is achieved with photoperiod stimulation, and a ripe female can produce between 4500 and 7500 eggs. Incubation period lasted 31.51 days or 391.397 ATU (Accumulated Thermal Units) with an average temperature of 12.43 C^o. No significant differences in weight and length were observed between larvae fed with enriched and non-enriched <b><i>Artemia</i></b>. Larvae showed better performance in terms of growth and survival in salinities below the values for fully marine environments (5 o/oo to 15 0/00). They are morphologically adapted to consume artificial diet pellets, with particle diameter no greater than 0.6 mm, from day 7 after hatching and can be placed under a feeding regime based only on artificial diets from day 30 after hatching. Juveniles reared in a sea cage system performed well and adapted rapidly to this type of confinement. Values for growth and mortality obtained over a period of 12 months are similar to those observed in other marine fishes cultivated at industrial scales. The main obstacles to the future cultivation of this fish are related to the possibility of producing an artificial feed less dependent on fishmeal, and especially formulated for its nutritional and energetic requirements, and to the development of demanding markets to stimulate the business.

The Chilean scallop <i>Argopecten purpuratus </i>is a commercially important cultured bivalve in both Chile and Peru with annual yields of approximately 37,000 tons. At present no studies have been carried out to investigate the population genetics of this species and therefore genetic management cannot currently be successfully practised. The aims of this study were to provide baseline data for genetic improvements of stocks and the conservation of genetic resources. This was to be achieved by assessing the genetic diversity using microsatellites and mitonchondrial DNA genetic markers. A species-specific mitochondrial DNA primer pair was successfully developed using sequence amplified from PCR primers developed for the clam complex Lasaea. PCR primers were developed for three microsatellite loci successfully isolated from <i>A. purpuratus</i> and one locus was variable and therefore suitable for phylogenetic analysis. The genetic variation of over 150 individual scallops from 5 populations were assayed and a variety of tests were performed on the allele frequencies and sequences. The sequence analysis results provide evidence supporting the existence of two separate, genetically differentiated, stocks in Chile: The Tongoy Bay stock (consisting of the Coquimbo and La Serena samples) and the Antofagasta/Concepcion/Valparaiso stock. It is hypothesised that the anthropogenic introduction of scallops from Tongoy Bay and the Mejillones and the Rinconda stocks into newly established cultures around Puerto Montt combined with the hydrology of the region have acted to cause an extreme form of range expansion. The differentiation observed between the stocks, is considered to result from the replacement of original wild scallops in Tongoy Bay by individuals descended from larvae from cultured individuals. In addition, tests of population expansion suggest recent large increases in scallop numbers. This finding supports the hypothesis of large periodic El Niño induced expansions of <i>A. purpuratus </i>numbers, previously only documented through increases in scallop catches (up to 60% growth).

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Several diseases were studied in the shore crab, <i>Carcinus maenas, </i>and the edible crab, <i>Cancer pagurus. </i>The most commercially important disease included a novel, systemic disease of <i>C. maenas,</i> (christened “milky disease”). Infected crabs displayed white haemolymph with reduced clotting abilities, and was infected with small Gram-negative bacteria. This was accompanied by changes in serum chemistry, and cell necrosis in the gill and hepatopancreas. The present dynamics of another commercially important syndrome, shell disease, were studied after recent improvement in local sewage treatment. <i>C. pagurus</i> were surveyed in 2003 in Langland Bay (post-sewage treatment change) and compared to similar data collected in 1997 (pre-sewage treatment change). Significant results included a decrease in shell disease lesion severity in certain anatomical areas, and in small crab class sizes in 2003-4, compared with 1997-8. Cost effective feeds were optimised for adult <i>C. maenas,</i> which incorporated a range of possible immunostimulants (chitin powder, yeast extract, polychaete extract). Chitin fed crabs showed significantly reduced tissue bacterial load, possibly because of the bacterial purging effect of the chitin as it passed through the gut. This resulted in significantly less mortality in experimental animals.  Yeast fed animals experienced significantly increased haemocyte counts and haemocyte phagocytic activity. This may be due to the immunosimulating nature of the β-1, 3-glucans found in yeast extract. Crabs fed polychaete extract showed significantly increased phenoloxidase activity and granular cell counts. Observations of an enhanced feeding response suggest that polychaete extract has beneficial nutritive value, and has the possibility to enhance feed palatability and reduce feed wastage.

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The current status of global aquaculture production was reviewed with a special emphasis on Africa and in particular Egypt. The main species of interest in this study were tilapia Oreochromis niloticus and African catfish Clarias gariepinus which are gaining popularity and are of considerable importance in the market of farmed fish in this continent and of economic relevance to Egypt and other Middle Eastern countries. Research was principally directed to establishing the suitability of specific feed ingredients and materials that could be included in balanced diets for both species. Various animal and plant by-products were selected to evaluate their nutritional value for either species. The experimental protocols, materials and methods and techniques employed are described for nutritional investigations with tropical freshwater fish. These included the various parameters assessed in the growth and digestibility studies relevant to the species in question. These include Specific Growth Rates (SGR), Feed Conversion Ratios (FCR), Protein Efficiency Ratio (PER) and Apparent Net Protein Utilisation (ANPU). An initial investigation to determine the coefficients of digestibility of protein, amino acids and energy was first undertaken using tilapia as the model warmwater fish species. This investigation was able to provide useful data and information as a prelude for successive growth trials with both tilapia and catfish. Fishmeal, soyabean meal, corn gluten meal, poultry by-products including feathermeal and blood meal were all tested at a variety of inclusion levels in successive trials. Apparent digestibility coefficients (ADC %) for tilapia fed diets containing 60% LT 94 fishmeal and 40% of each ingredient are reported. ADC of dry matter (DM) and protein (CP) and energy (E) for the reference fishmeal diet were 83.99 DM; 92.60 CP; and 93.31E respectively. For each test ingredient, these values were as follows; 1- PBM (56.99 DM; 69.30 CP &amp; 73.47 E), 2- Feathermeal (54.09 DM, 45.53 CP &amp; 49.11 E), 3- Blood meal (76.13 DM; 85.79 CP and 75.96 E), 4- Solvent extracted soyabean meal (85.83 DM; 93.46 CP &amp; 82.16 E), 4- Full fat soyabean meal (75.86 DM; 86.99 CP &amp; 74.84 E). The amino acid availability coefficients reflected the same trends as protein digestibility, and these varied from &gt;87% on average for the essential amino acids in fishmeal, 83% for maize gluten and 85% for solvent extracted soyabean meal with an average of 63% for feathermeal and only 61% for poultry meat meal. The importance of plant protein sources and especially soyabean meal was the focus of a complete nutritional study with juvenile tilapia, The influence of full fat soyabean meal (FFSB) inclusion on growth performance, feed utilisation and the gastrointestinal digestive enzymes was also measured in this experiment. It was found that soyabean meal levels above 50% could reduce growth performance and adversely affect gut enzyme activities. Tilapia fed a series of diets with FFSB (58,63 and 63% + DLmethionine did not perform as well as the control group. SGR values ranged between 2.42 to 2.12, and ANPU between 39.41-34.46. Supplementation of the diet with methionine did not restore performance. Hepatic trypsin and amylase enzyme activity was affected with FFSB (from 12.64-1.43 Units and 4.99-2.76 Units respectively). No affects were detected on general proteolytic activity for stomach, intestine and liver. For studies with African catfish, it was first necessary to assess the different grades of fishmeal that could be employed in suitable reference diets for this species. A Poultry by-product meal (PBM) was further evaluated as a fishmeal replacement source (0- 100%) for this species. Catfish fed dry and wet diets of two types of fishmeal showed significant differences in growth performance. Catfish fed dry diets performed better than those receiving wet diets for both LT94 and white fishmeal sources. SGR were (2.80 and 2.75 dry) and (2.46 and 2.57 wet). FCR (0.97 and 0.80 dry) and (1.30,1.30 wet), ANPU (41.85,52.94 dry) and (31.43,30.9 wet) for LT94 and White fishmeal respectively. The PBM fed catfish showed significant differences in weight gain and feed utilisation. SGR was between 3.57 to 2.83, FCR between 1.61 to 2.25 and ANPU fell from 28.90 to 18.82 for groups' fed the control fishmeal diet towards the maximum level of PBM substitution. Histological examination of liver tissue showed alterations in hepatic morphology with respect to sinusoids and fat accumulation for catfish fed higher amounts of PBM. A restricted inclusion of up to 40% poultry by-product meal could therefore be suggested for practical diet formulations. Further investigations were undertaken to assess the potential for either maize gluten meal (MG) or soyabean meal as substitute protein sources for the African catfish Catfish fed higher inclusions of MG displayed SGR's ranging between 5.28 to 2.79, FCR between 0.81 to 1.53 and ANPU values from 52.33 to 24.99%. All lower performance data were obtained for 75% MG substitution of LT94 fishmeal protein. Further histological examination of liver tissue revealed alterations in hepatic structure associated with higher levels of MG. It was suggested that no more than 25% substitution of fishmeal with maize gluten meal is feasible under the present conditions. In a separate study, catfish fed diets containing different levels of FFSB (58,63 and 63% + DL-methionine) at the expense of fishmeal (LT94), showed significant differences in weight gain. SGR ranged between 3.11 to 2.78, FCR 0.82-0.83 and ANPU between 54.48 to 48.60. Also trypsin activities for intestine ranged between 2.75 to 1.71 Units, liver 1.37 to 1.05 Units and stomach 4.09 to 2.29 Units of activity for increasing levels of FFSB. Hepatic amylase was also reduced from 4.49 to 2.46 Units. General proteolytic activities however, did not show any significant differences between catfish fed different levels of FFSB for the stomach, intestine and liver. The conclusions from each of the nutritional trials were considered and comparisons between the response of tilapia and catfish were made. The advantages of plant based protein concentrates was stressed due to the problems currently existing for animal sources and the expense of fishmeal There were many similarities for the tilapia and catfish and it would seem that both fish species could greatly benefit from improved diet formulations that may meet with their nutritional requirements whilst minimising cost of production. A future strategy of research is presented that includes further work to identify more feed ingredients for potential use in these species.

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