Culturing African Lungfish (Protopterus sp) in Uganda: Prospects, Performance in tanks, potential pathogens, and toxicity of salt and formalin

Abstract

Culturing species resilient to drought and stressful water quality conditions may be a significant part of the future of African aquaculture. Air breathing fishes potentially have a role in low-management culture systems for small farms because dissolved oxygen does not threaten the fish crop. The African lungfish (Protopterus sp) is advantageous because it is: an indigenous fish with good flesh quality, an air-breather, and a biocontrol agent against schistosome vector snails. Wild lungfish stocks are declining and national strategies to protect its natural population are lacking. Lungfish is highly valued as food, has certain nutraceutical benefits and supports livelihoods of many communities in Uganda. A variety of lungfish products on markets include fried pieces (54%), cured/smoked fish (28%), whole fresh gutted fish (10%) and soup (8%). Lungfish products are increasingly found alongside tilapia and Nile perch in rural and urban markets with cured products being exported to Kenya, DRC and Southern Sudan. Its fingerlings are now sold as bait in the Nile perch fishery. Women not only consume lungfish but are actively engaged in its trade. However, some countervailing sociocultural beliefs continue to deter some fish consumers from eating lungfish. Culture performance of African lungfish fingerlings (9.58 ─ 9.95g) fed at three commercial diets was evaluated. Experimental fish accepts exogenous sinking pellets but marginal increases in average body weight were observed. Mean (± SE) final weight (15.86 ± 0.80g) for fish fed the commercial diet-3 was significantly higher (p < 0.05) than fish which fed on diet-1 and diet-2. Specific growth rates (SGR) for diet-2 (0.50 ± 0.06%/d) were significantly higher (p< 0.05) than diet-1 (0.27 ± 0.03%/d), and marginally more (p < 0.05) than diet-2 (0.37 ± 0.04%/d). Feed conversions were similar for fish fed diet-1, 2 and 3. Survivals after an 11-week culture period were relatively low (< 60%) but generally increased (R2 = 0.667, P = 0.0071) with increasing dietary proteins. Diet-3 (57.50 ± 2.85%) had a significant higher survival rate (p < 0.05) than diet-1 (45.83 ± 3.44%) and diet-2 (40.84 ± 2.10%). All water quality parameters were within recommended aquaculture ranges. Poor growth and high mortalities experienced in this study may be due to i) sub-optimum dietary protein levels, ii) cannibalism, iii) disease infections, iii) density, iv) contaminants in the feed and, iv) wrong management protocols. Unless these factors are adequately addressed this fish is not a good aquaculture candidate. Additional studies will be needed to assess the culture potential of African lungfish due to high rates of mortality that occurred in the present study. Common diseases encountered include; bacteria (Aeromonas sp., Flavobacterium columnare and Pseudomonas sp.), fungus (Fusarium spp., Aspergilus sp and Saprolegnia sp) and parasite (Dactylogurus sp, Trichodina sp., Tetrahymena sp, Heterorchis sp. and Cestodes). However, about 60% fungal infections mostly occurred compared to monogenes (9%), tapeworms (25%) and bacteria (6%). Moribund fish were infected with fungal and bacterial infections of the liver, spleen, dermal layer and gastro-intestinal tract. Skin erosion and dermal mycosis evident in most infected fish but with some indication of regeneration. African lungfish fingerlings (7.78 ± 1.47 g) appear to be sensitive to saline conditions having a LC50 of 2.59 and 1.84 for 24 and 96h, respectively. Lungfish behaves normally at low salt concentrations (0 ─ 1.6 g/L) but become lethargic within 4h when concentrations reach 4 g/L. Juveniles are tolerant to formalin having LC50 of 220.8 and 193.8 mg/L for 24 and 96h, respectively.