The present study examines and deals with the growth, haemato-immunology and physiology of four fish species (Hybrid tilapia, mullet, Oreochromis mossambicus and white seabream sp.) in captivity. Several previous studies dealt with fish meal (FM) replacement and deduced inferences on the basis of growth response. However, these experiments have generally ignored the influence of such protein substitute on the immune functions in fish. The experiments presented in this thesis were conducted at coastal regions of Sindh and at Centre of Excellence in Marine Biology. In 1st chapter, the world’s aquaculture status, contribution and fish species selection criteria in aquaculture are discussed. Highlights of the status of aquaculture in Pakistan, its contributions and issues are also included. In 2nd chapters, the effects of optimal salinity, dietary protein levels and stocking density on physiological, haematological changes and growth of fin fish. The respected salinity were 0, 10, 20 and 30 ‰ (represented by S1, S2, S3 and S4 respectively) with 30% (P1) and 40% (P2) dietary protein levels. The growth rates were not significantly (P > 0.05) differed in connecting S2-S3/P1 or between S2- S3/P2. The food consumption rates, muscular hydration and the conversion ratio significantly (FCR) (P < 0.05) decreased with increasing salinity. No obvious changes were recorded in haematocrit (Hct %) level, haemoglobin (Hb), plasma protein, muscles and liver protein levels in all treatments. Also, the plasma cortisol and glucose levels increased slightly with salinity but were not significantly (P > 0.05) different among treatments. The present study suggest that the salinity ranging from 10 to 20‰ and 30% dietary protein level were better for growth performances of hybrid tilapia. In 3rd chapter, the fishmeal was replaced by algal meal, soybean meal and shrimp protein hydrolysate meal in the fed and its effect were assessed on mullet (Mugil cephalus). This study clearly shows that the optimum growth activity, physiological, haemato- immunological parameters and tissue aminoacids profile were achieved at lowest fish meal or algal-meal inclusion levels. The optimal inclusion levels of shrimp head protein hydrolysate showed excellent performance on growth and physiological activities. However, the increasing inclusion level of algal meal decreased the proteolytic enzyme activity and other bio-physiological activates (e.g. specific growth rate, liver, hepatosomatic index etc.). This study also showed that the pure plant products may not be fed to fish owing to its antiprotease activity which would reduce the growth performance of fish (mullet species). In 4th chapter, we conversed to evaluate the effect of rearing capacity (SD) on growth, physiology and production of grey mullet (Mugil cephalus). This study conducted in net cages at Ketibunder, Sindh. The juveniles were introduced at three duplicated groups [30, 60 and 120 fish m-3 (ST1, ST2 and ST3 respectively)]. The growth rates were significantly (P < 0.05) high at lowers stocking density. The lowest coefficient variation (Cv) was observed at ST1 and ST2. The HSI (hepatosomatic index), liver glycogen and body protein level were also significantly (P ˂ 0.05) higher at ST1 and ST2 than ST3. The total proteolytic enzyme activity, ash and moisture were slightly increased at ST1 and ST2 but were not (P ˃ 0.05) significantly different among all treatments. The results of this study obviously indicate that the growth and physiological parameters were influenced by stocking densities. In 5th and 6th chapters, effect of stocking density and shrimp head protein hydrolysate was assessed. The fish were stocked at 20, 40 and, 80/50L. The dietary protein ranging (30% - 40%) was maintained by shrimp head protein hydrolysate (SHPH) in the fed. At the end of the study, the blood samples were also taken for the physiological assessments. The aforementioned results of 5th and 6th chapters obviously indicate that the lower stocking density (20 fish/ 50L) and lower (30%) dietary protein did not show any unfavorable effects on growth and biological activities of hybrid tilapia. However, the high blood glucose and plasma cortisol levels indicate that the highest stocking density (80 fish/ 50L) may exert negative pressure on growth and physiological commotions. In 7th chapter, we also discussed the effect of Ketotestosterone (11KT), fish meal protein (FMP), and shrimp head protein hydrolysate (SHPH) on growth and immunity of tilapia (Oreochromis mossambicus). The fish were firstly stocked at 10 fish/ 50L in four dealings (FMP, SHPH, +FMP and +SHPH) with three replicates. The 5 mg/kg of 11KT was added by spray (+FMP, +SHPH) and without spray of 11KT (- FMP, - SHPH). The growth rates and food conversion efficiencies of fish fed on (+FMP and +SHPH) were significantly better than those fed on FMP (control) diet. The haematoimmunological parameters (e.g. RBCs, white blood cells, haemoglobin and albumin etc.) and physiological parameters (visceral and hepatosomatic index) were improved in fish fed +FMP and +SHPH supplemented diets. The plasma (triiodothyronine: T3) levels were significantly improved in males than females whereas (thyroxine: T4) levels were insignificant in both sexes. The plasma 11KT levels were significantly (P < 0.05) higher in males than females. Plasma estrogen (E2) levels were also insignificant in females of fed on all test diets. These findings clearly point out that the supplemented ketotestosterone to fishmeal diets enhanced growth, plainly moving the blood levels of 11KT without producing any estrogenic activates. In 8th chapter, deals with the effect of fish meal protein FMP substitute with shrimp head protein hydrolysate (SHPH), poultry by-product protein concentrate (PBPC), soybean protein (SBP) and combination of SHPH+PBPC on growth activity of seabream fish (Diplodus sargus). This was comparison study between the wild and farmed fish. The isoenergetic and isonitrogenous diets with 30% protein were given to fish supplemented either by SHPH, PBPC, SPH+PBPC or SBP. The growth performance and haemato-immunological parameters (e.g. leukocytes, erythrocytes, albumin, globulin etc.) of fish fed on SBP, PBPC and control (FMP) were considerably (P < 0.05) poles apart. In addition, these bio-physiological parameters were found had higher values in fish fed on SHPH or SHPH+PBPC diets. Similarly, we compared these whole growth and physiological limits between farmed and wild fish population of white seabream. The present study suggests that in farmed fish supplied diets where fish meal protein (FMP) was replaced by SHPH, PBPC and SBP meal had adverse effects on growth performance, haemato-immunology. The aforementioned results clearly indicate that the growth behavior and biological activities were significantly improved in farmed fish than wild population of white seabream fed on substituted experimental diets. The aforementioned results obtained in these trials clearly demonstrate that the most favorable protein level, sources of dietary protein and physico-chemicals parameters of water (e.g. DO, pH, ammonia, nitrite, light intensity, salinity level, etc.) must be considered and maintained at optimal level to achieve healthy growth of fish in culture.