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High larval and frymortality has been amajor challenge to the commercial culture of the African bony-tongue (Heterotis niloticus). Research indicates inadequate feeding to be a possible cause of the high mortality rates of larvae/fry grown in captivity. An experiment was conducted to first describe the developmental stages of H. niloticus eggs and larvae until schooling, 6 days after hatching (DAH), at 26°C. The morphological development, survival, growth (weight gain and length), and swimming behavior of the larvae were monitored. A follow-up feeding trial was conducted with H. niloticus fry from 6 to 27 DAH involving four treatments: feeding with Artemia nauplii; a combined feed of 50% Artemia nauplii and 50% rotifers (w/w); feeding with rotifers; and no feeding. The activities of three digestive enzymes (trypsin, lipase, and amylase)were assessed in fry exposed to the live feed treatments. At hatching, the larvae had a large, vascularized yolk sac filled with yolk platelets, which occupied approximately one-third of the total body length of the abdominal cavity. Yolk platelet reabsorption started at 1 DAH and was complete at 6 DAH. Metamorphosis was completed at 6 DAH, and schooling behavior was observed. For the feeding trial, the fry that received Artemia nauplii underwent the fastest development until 27 DAH. Similarly, the survival rate was considerably higher in fry fed with Artemia nauplii than in those that received a combination of Artemia nauplii and rotifers or only rotifers. Overall, the survival rates for fry fed with Artemia nauplii and larvae co-fed were 72.7% ± 0.9% and 41.2% ± 3.3%, respectively. Fry did not survive until 27 DAH in the rotifer-fed and unfed treatment groups. Digestive enzyme activity differed significantly (p< 0.05) between treatments and sampling dates. All digestive enzymes were detectable at the onset of mouth opening (5 DAH) and fluctuated through the 7 and 9 DAH. Lower enzyme activity was observed when the fry were subjected to starvation or the rotifer treatment. Results indicate that Artemia nauplii is the best live feed organism for H. niloticus larviculture. [ABSTRACT FROM AUTHOR]

The study assessed the effect of oilseed meal mixtures on the biological value, faecal matter output, ammonia excretion rates and serum biochemistry of Nile tilapia over 63 days. The experimental diets (PPB 1, PPB 2 and PPB 3) were formulated using four selected oilseed meals that were mixed in different protein proportions to contribute 80% of total dietary crude protein. In each diet, either one or two of the oilseed meals were the dominant protein contributor. A commercial tilapia feed was used as the control diet (CTRL). Sex‐reversed Nile tilapia fingerlings (35 g) were stocked at 20 fish per tank in a recirculation system and fed at 3% body weight of their respective diets. At the end of the study, ADCs of dry matter, crude lipid and ash were significantly (p <.05) lower in all the plant‐based diets compared with the control diet. Fish fed the control diet had significantly higher weight gain and SGR compared with the plant‐based diets. Egesta output was 127% higher in the plant‐based diets compared with the control diet. All dietary treatments exhibited a similar trend in changes in ammonia nitrogen. Nevertheless, serum metabolites levels indicated no significant differences among treatments. Although the PPBs did not affect ADC of protein and serum profile, fish growth was reduced while faecal output increased. [ABSTRACT FROM AUTHOR]

The reproductive biology, sexual strategy and sex ratio of the freshwater donacidGalatea paradoxain the Volta River estuary, Ghana, was studied from March 2008 to February 2010. Histological observations of the gonads showed thatG. paradoxais gonochoristic with dominance by females (80%) and a high incidence of hermaphrodites (9.4%) for individuals ranged between 20 and 82 mm in shell length. There was a single spawning event between July and October. Gametogenesis started in December, progressing steadily to a peak in June–July when spawning began, until November when individuals were spent. The dominance of females coupled with the high incidence of hermaphroditism may be a reflection of populations subjected to increasing fishing pressure and over-exploitation. [ABSTRACT FROM AUTHOR]

This study was conducted to assess the optimum clam size and substratum type for the culture of G alatea paradoxa. The experiment was conducted over a 90-day period at the Volta Estuary, Ghana. Three size classes of G . paradoxa categorized as small (27.6 ± 0.4 mm), medium (36.8 ± 0.4 mm) and large (50.0 ± 0.6 mm), were used to ascertain the effect of a sandy and muddy substratum on growth performance and survival. Growth increased from the small-sized clams to the large-sized clams at 2.54, 3.03 and 3.43 g, respectively, over the experimental period. Growth was higher ( P < 0.05) in the muddy substratum (3.58 g) compared with sandy (2.41 g). Similarly, the specific growth rate increased from 0.99% day−1 in the small-sized clams to 1.36% day−1 in the large-sized clams. Survival rates were significantly higher (99.4%) for the large-sized clams compared with the medium (78.8%) and the small-size clams (74.1%). There was significant interaction between the size class and the substratum type with a trend towards increasing survival rate from the small to the large size clams in both substrata. The survival rate was lower in the muddy substratum (71.4%) compared with the sandy substratum (96.7%). The yield was significantly higher in the muddy (2.01 kg m−2 90 day−1) compared with sandy substratum (1.87 kg m−2 90 day−1). The results of this study indicate that the culture of small-sized clams is best practiced on sandy substratum due to their elevated survival rates, whereas the muddy substratum appears most suitable for the culture of larger clams (>40 mm) because of their relatively higher survivorship and better growth performance. [ABSTRACT FROM AUTHOR]