Acid-base disturbances and effects on oxygen uptake rates in Nile tilapia (<i>Oreochromis niloticus</i>) following acute and prolonged CO₂ exposure

High levels of dissolved carbon dioxide (CO2) occur nightly in earthen ponds characterized by high respiration rates. Exposure to high CO2 conditions (hypercapnia) leads to acidosis in fish, which can be compensated by an accumulation of HCO3− to recover intra- and extracellular pH levels, with a capacity that appears to be species-specific. For Nile tilapia, a freshwater tropical teleost traditionally produced in earthen ponds, little information is available on the tolerance to dissolved levels of CO2 and associated acid-base disturbances. Here, we investigated first the effects of acute and progressively increasing CO2, from normocapnic conditions to 60 mg CO2 L−1, on oxygen uptake rates (MO2). This was followed by exposure to three concentrations of CO2; 10, 30, and 60 mg L−1 (equivalent to pCO2 of 5.4, 16.2, and 32.4 mmHg) against a normocapnic control (pCO2 0.3 mmHg), to investigate acute (1 h) or prolonged (24 h) effects on standard (SMR) and maximum metabolic rates (MMR), haematology, and extra- and intracellular acid-base status in adult Nile tilapia (mean BM 435 ± 16 g ± SE). Acute exposure to hypercapnia led to concentration-dependent decreases in both SMR and MMR. Fish were able to fully or partially recover MMR and metabolic scope (MS) after 24 h, while depression of SMR persisted at all CO2 levels. Acute exposure to CO2 caused intra- and extracellular pH levels to decrease by up to 0.5 units in a concentration-dependent manner. Only the lowest hypercapnic treatment (pCO2 5.4 mmHg) was able to fully recover within 24 h. Changes in haematological variables appeared minor, being restricted to increasing haematocrit, haemoglobin concentration, and mean cell volume in the highest CO2 treatments after 24 h exposure. Although the Nile tilapia is generally considered a species able to tolerate poor water quality, the modest or slow acid-base regulation following hypercapnic exposure suggests sensitivity to hypercapnia

Feasibility of extensive, small-scale mud crab farming (Scylla serrata) in East Africa

Small-scale farming of mud crabs (Scylla serrata) has been suggested as an alternative income for resource poor coastal communities in East Africa. However, it and it is unclear if the present culture methods are profitable and ecologically sustainable at larger scales. Here we assess the two dominant culture methods (crab fattening in cages and grow-out farming in ponds) using economic and ecological analyses in Kenya and Tanzania.Cost-revenue analyses of crab-fattening cultures showed negative results at all scales in both countries. High labor costs and poor survival and growth make crab fattening an unprofitable and unsustainable livelihood in its present form. Grow-out cultures of small juvenile crabs showed a positive return at prices above $US 3.4 kg-1, which is achievable in Kenya and Mozambique, but difficult in Tanzania today. Analyses showed that it would be more profitable to farm smaller commercial crabs, and to increasing the size of the pond culture. However, the availability of seed-crabs and local feed sources limit the size of sustainably crab farms in a coastal community to approximately 500 commercial crabs per year. Before grow-out farming can become a profitable alternative income in East Africa, survival rates need to be improved, and market conditions and profits to local crab farmers enhanced