No abstract

This study was conducted (1) to evaluate the effects of photoperiod (fixed vs. decreasing light), fish size (136 vs. 220 mm), dissolved ions (hardness and salinity) and diet (menhaden oil vs. coconut oilbased) on the tolerance (survival) of Nile tilapia, Oreochromis niloticus, to low temperatures (decreased by approximately 0.5 8C per day) and (2) to evaluate the effect of dietary fatty acid composition on selected physiological characteristics of Nile tilapia exposed to decreasing temperatures. Size significantly affected mortality, with smaller fish being less tolerant to low temperatures than larger fish. Results were equivocal in the photoperiod, dissolved ion and dietary lipid experiments, and were dependent on the method of data analysis employed. Diet significantly affected plasma osmolarity, with higher values in fish fed the menhaden-based diet. Haematocrit, serum glucose, sodium and cortisol concentrations, serum and splenic lysozyme activities, lymphocyte count and hepatosomatic index were not affected by diet. Haematocrit, white cell count and serum glucose and sodium concentrations were significantly affected by temperature, but serum and splenic lysozyme content, hepatosomatic index, and serum cortisol concentrations were not. The results of this series of experiments indicate that altering the environment or diet has little effect on the ability of Nile tilapia to survive low temperatures.

Small (4.4 ± 1.50 g; mean ± SD) Nile tilapias Oreochromis niloticus were more tolerant of nitrite than large (90.7 ± 16.43 g) fish. The 96‐h median lethal concentration of nitrite‐N to small fish was 81 mg/L (95% confidence interval = 35–127 mg/L) compared with 8 mg/L (4–11 mg/L) for large fish. Addition of chloride to test water (as either calcium chloride or sodium chloride) protected both small and large fish from nitrite. Sodium chloride and calcium chloride appeared to be similarly effective in inhibiting nitrite toxicity.

Reduction or elimination of fish meal and fish oil from aquaculture diets can help to reduce the potential for contamination and dependence of the industry on pelagic fisheries while improving economic competitiveness. However, fish oil provides important omega‐3 (n‐3) fatty acids (FAs) essential to shrimp health and beneficial to humans. This study evaluated an organic, plant‐based diet formulated to replace fish meal and fish oil with plant proteins and docosahexaenoic acid (DHA) produced by algal fermentation. Shrimp cultured in replicate outdoor ponds at 25/m2 were fed either a diet composed of organically produced plant ingredients or a conventional commercial fish‐meal‐based feed. No significant differences were found in production parameters between the conventional fish‐meal‐based diet and the plant‐based diet (production: 4594 and 4592  kg/ha; harvest size: 18.7 and 19.2  g; survival: 93 and 88%; and feed conversion ratio: 1.4 and 1.3, respectively). At harvest, shrimp were analyzed for 147 chemical contaminants and 71 FAs. Contaminant levels were negligible for shrimp raised on both diets. The fish meal and fish oil diet provided significantly higher quantities of eicosapentaenoic acid and DHA than the plant‐based diet, and the shrimp fed the conventional diet reflected this with higher levels of these beneficial FAs in edible tissues. Differences between feeds and shrimp tissues suggest that essential n‐3 FAs may accumulate in shrimp tissues over time or that natural pond productivity may play a role in providing supplemental nutrition. Shrimp raised on the two diets and wild‐caught shrimp are clearly distinguishable by their FA profiles. Compared to alternative protein sources like beef, pork, or chicken, differences in lipid profiles of shrimp raised on either diet may be insignificant because both offer increased human health benefits.

The salinity and temperature tolerances of black sea bass Centropristis striata juveniles (46 ± 10.3 mm total length [mean ± SD]) were estimated to assist in decisions concerning location of culture facilities and types of production systems. At 21°C, all exposed fish survived for 7 d in salt concentrations of 10, 20, or 35 g/L. All fish exposed to salt concentrations of 5 g/L died within 3 d. Temperature extremes were estimated after acclimation of fish to 24–25°C. Half of the juveniles exposed to a gradual increase (+0.65°C/d) or decrease (–0.51°C/d) in temperature (at a nominal salt concentration of 20 g/L) were dead when the water temperature reached 33.3°C and 3.7°C, respectively. Results of this study indicate that this species requires saline water at or above a salt concentration of 10 g/L and that it may not be practical to keep black sea bass over winter or over summer in shore‐based outdoor facilities located in temperate climates.