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This study provides information on the use of shrimp head silage protein hydrolysate (SPH) as an alternative protein source for tilapia feeding. Six diets (28% protein, 12% lipid) were prepared where ®shmeal protein was replaced at levels of 0 (control), 10, 15, 20, 25 and 30% with the hydrolysate. The diets were supplied to Nile tilapia fry (338 mg initial weight) stocked in plastic recirculating 20 l tanks (10 animals per tank), with three replicates per treatment. After an 8 week experimental period, ®sh fed the diets containing 10 and 15% SPH showed signi®cantly better performance in terms of ®nal body weight, weight gain (%), mean daily weight gain (mg day À1 ), speci®c growth ratio and feed conversion ratio than those fed the control diet (®shmeal as protein source) and higher-SPH diets. It is concluded that shrimp head hydrolysate is a promising alternative protein source for tilapia feeding, improving growth ratio at dietary inclusion levels as high as 15%. In addition, the diets with added shrimp silage protein were well accepted by the ®sh, which avidly consumed the feed during the experiment.

In Mexico, the tilapia Oreochromis provides one of the most important sources of animal protein and income in a wide variety of communities throughout the country, however, their culture and production face severe problems because of lack of management information. Evaluated in the present study is the degree of morphological, meristic and allozyme variation of two tilapia strains, two species and one hybrid from two important reservoirs in Mexico, two tilapia fry production farms (TFPF) and an experimental production system (EPS), to diagnose the actual status and degree of variation among them. Twenty-two presumptive loci were utilized to determine the genetic variation, structuring and distances among samples. Multivariate analyses of 11 meristic and 33 morphometric characters were also evaluated, including the distance from the end of the mouth opening to the most anterior dorsal fin (0.9894), distance from the anal fin base to the anterior part of the caudal fin (0.9845) and the base length of the dorsal fin (0.9839) which contributed to the separation of groups. The canonical discriminant functions for the morphometric and meristic variables show that the correct classification of the organisms in percentages and in the sites of origin was on average 72%. The tilapia from the reservoirs and the experimental production system had higher genetic variations than tilapia from the fry production farms (average H e 0.310, 0.062, 0.151 for Metztitlan, Infiernillo and EPS, respectively, vs 0.024 and 0.000 for Los Amates and Zacatepec, respectively). The genetic data indicate that fishes of the TFPF should be monitored closely, as they are the main source of dispersion to the reservoirs. Present results show that these data could be a fast and reliable aid to the fisheries and management of tilapia in Mexico.

A preliminary assessment of growth performance and yield of subadult Pacific red snapper Lutjanus peru raised in floating sea cages was conducted by measuring fish length, weight, feed conversion, and survival rate during a 120‐d grow‐out trial at Punta el Caballo Beach, Nayarit State, Mexico. Nine floating cages (12.5‐m3 capacity) were used as experimental units. Fish were stocked in triplicate treatment cages at 30, 50, and 70 fish/m3 (1.9, 3.3, and 4.4 kg/m3, respectively) using more than 5,600 wild subadults (mean initial weight ±SD = 63.9 ± 1.4 g) as initial fish stock. Fish were fed twice per day with a sinking commercial pellet, and dissolved oxygen, temperature, pH, transparency, and ammonia nitrogen in each cage were recorded weekly. After 120 d, the mean individual weight at harvest was inversely related to stocking density. Mean final weight, weight gain, and specific growth rate differed among groups, with the highest values recorded for the 30‐fish/m3 density (233.4 ± 5.3 g [mean ± SD], 1.4 g/d, and 1.1% per day, respectively). Mean final body length, feed conversion, condition index, and survival did not differ among density treatments. Net yield (mean ± SD) ranged from 5.0 ± 0.2 kg/m3 (30‐fish/m3 treatment) to 7.9 ± 0.3 kg/m3 (70‐fish/m3 treatment) and differed among the treatments. Considering the initial biomass and density, the 70‐fish/m3 treatment produced a higher total biomass (mean ± SD = 152.9 ± 2.4 kg) but a lower average weight than the 50‐ and 30‐fish/m3 treatments. Our findings suggest that at all stocking densities tested, subadult Pacific red snapper will grow in floating sea cages without significant mortalities. A benefit–cost analysis must be achieved to define the best treatment in economic terms.

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