Authors : Francis M. Nwosu
Abstract: Atya gabonensis Giebel (Atyidae) is a freshwater shrimp endemic to West Africa. In the Cross River system, it inhabits the rocky bottom characteristic of the central to northern section of the Cross River. Its reproductive biology and feeding habits were studied from January to June 2006. The onset of spawning was indicated in March by the prevalence of soft-bodied specimens in catch samples. Fecundity of berried females varied with size and month. With a size range of 6.5-11 cm, fecundity varied from 850-12,220 eggs/female during the study period. The stomach content analysis revealed dominance of detritus, indicating that the species could have filtered organic particles from water column and/or near the usually rocky bottom. Conserving the natural habitats of this species, as well as exploring its aquaculture and aquarium potentials are imperative for the improved food-protein intake of the local communities in particular.
Francis M. Nwosu , 2009. Aspects of the Biology of the Gabon Shrimp Atya gabonensis, Giebel 1875 in the Cross River, Nigeria. Journal of Fisheries International, 4: 58-61.
Atya gabonensis (Atyidae) is one of the freshwater shrimp species endemic to West Africa. First reported on Gabon, the species is now known to inhabit freshwater rivers and streams of rocky or non-rocky bottom type from Senegal to Gabon (Powell, 1982). It has been given several common names such as African Fan Shrimp, African Filter Shrimp, African Giant Shrimp, Vampire Shrimp, Blue Rhino Shrimp, Gabon Shrimp and Cameroon Fan Shrimp.
Within the central to northern sections of the Cross River, a separate seasonal fishery exists for the species, lasting from December to about June/July each year, depending on the onset and intensity of the rainy season. In Northern Nigeria, Reed (1967) reported that the species is harvested in small scale, while in the west of Nigeria (e.g., River Osse), Powell (1982) also noted that the species is caught with bamboo poles and artificial shelters. Obande and Kusemiju investigated the food and feeding of this species in Lower River Benue while Okayi and Iorkyaa (2004) studied its length-weight relationship in Mu River, Makurdi, Nigeria.
Studies on other members of the genus include the mating behaviour of A. innocous (Felgenhauer and Abele, 1982) and some aspects of reproduction of A. margaritacea in Pacific Mexico (Martinez-Mayen and Roman-Contreras, 2000). Powell (1982) included another species of this genus Atya africana for the study area but his finding has yet to be confirmed. The present study is a contribution on feeding and some aspects of reproductive biology of the species in the Cross River system, Nigeria.
MATERIALS AND METHODS
The study area: Data for this study were collected at Itigidi situated by coast of central Cross River (Fig. 1). The Cross River has its origin from the Cameroon mountains, initially flowing westward into Nigeria then turning southward and emptying into the Atlantic, a distance of over 600 km. The central to northern segment of this river is characterized mainly by rocky bottom, different from the southerly section which is essentially muddy.
Collection and analysis of samples
Stomach content: A total of 343 stomachs collected from A. gabonensis at the study site (Fig. 1) from January to June 2006. The specimens were caught by artisanal fishermen who dive to the river bed, dipping hands in rock crevices, holes and around objects. These fishers rarely used bamboo traps or similar method for harvesting of the species. Specimens were transported to the lab in a cooler filled with ice bags to reduce posthumous digestion. Stomach content analysis was carried out following the frequency of occurrence method described by Hyslop (1980). Contents of cut stomachs were emptied into a petri-dish and examined using a contrast microscope (Model BM 62B) at x10 magnification. Samples were collected monthly for 6 months.
|Fig. 1:||Map of Cross River State showing the sampling site (Itigidi-shaded area)|
Fecundity estimation: Berried females were not observed in samples until April. This was preceded by occurrence of soft-bodied specimens in March samples. Sampling continued until June when diving method preferred by the local fishermen was considered dangerous as a result of rise in water levels of the Cross River during rainy seasons. At the laboratory, eggs of each berried female were extracted onto an absorbent paper, weighed and placed in sampling bottles containing 4% formalin. Sub-samples of preserved eggs were counted using hand/magnifying lens (Bagenal and Braum, 1978). Three sub-samples were taken from each specimen and a mean was computed. Total number of eggs borne by each shrimp was then obtained by multiplying the average number of eggs by the weight of the egg mass from that specimen.
RESULTS AND DISCUSSION
Stomach contents analysis: The stomach contents of A. gabonensis with size range of 6.5-14 cm (total length) examined in this study constituted of detritus and mud throughout. Whereas, specimens ranging in size from 6.5-11.5 cm had only detritus, larger specimens of 12-14 cm had detritus and mud. No other food item was identified. Of the 343 stomachs examined, 12 (or 3.5%) were empty. Table 1 shows the result of the gut content analysis for different size groups.
Fecundity estimation: A total of 147 egg-carrying females were sampled from April to June 2006. While 18 (of 48) or 37.5% were sampled in April, 93 (of 112) or 83.04% and 36 (of 65) or 55.38% were sampled in May and June, respectively (Fig. 2). Figure 3 shows the relative importance of berried females during the sampling months. Peak of reproductive activity was in May. No berried female was observed in samples from January to March. However, 8 (of 25) or 32% sampled in March were soft-bodied.
Number of eggs (or fecundity) per unit specimen sampled varied with size of the specimen. Within a size range of 6.5-11 cm, fecundity range was 850-12,220. Table 2 shows the size range and corresponding number of eggs estimated. Felgenhauer and Abele (1982) observed that reproduction in Atya margaritacea was continuous based on the presence of ovigerous females in samples throughout the year.
|Table 1:||Size groups and food items collected from A. gabonensis in the Cross River|
|Table 2:||Relationship between size (total length in cm) and number of eggs in A. gabonensis females|
This observation contrasts from the situation of A. gabonensis in the present study where no ovigerous female was found in samples until April. However, their report of maximum reproductive period during the rainy season (June to October) is similar to findings in the present study. Peak of reproductive activity fell within the rainy season month of May. This result is however could not be conclusive since sampling was terminated in June when it became unsafe for the divers due to rise in water levels as the rainy season in the area intensified. It is considered probable that maximum reproductive period could lie within any of the other rainy season months not sampled. Besides the rise in water levels of the Cross River, heavier rains associated with this period is usually accompanied by massive runoff and flooding of rivers which decreased visibility and made fishing by the diving method difficult. Although, no berried female was encountered in samples until April, the occurrence of soft-bodied specimens in the preceding month (March) was considered as consistent with pre-reproduction moulting associated female shrimps.
|Fig. 2:||Percentage of berried females in each monthly sample|
|Fig. 3:||Relative importance of berried females of A. gabonensis for each sampling month|
Most atyids are known to filter water column and also scrap substrate for food, an exception though is A. abelei from the Pacific slope of Panama which feeds by passive filtration only (Felgenhauer and Martin, 1983). Generally, prawns are said to be omnivorous, consuming anything from algae, aquatic insects and their larvae, mollusks, crustaceans and fish to detritus and mud (Kurian and Sebastian, 1982; New, 1990; Adetayo and Kusemiju, 1994; Khan et al., 2001; Job and Nyong, 2004; Job, 2006).
The reports of these researchers indicated that most prawns/shrimps consume detritus at some stage in their life cycle. In fact, Obande and Kusemiju reported the occurrence of detritus in 79.1% of stomachs examined in A. gabonensis from the Benue River, algae and detritus forming the most important food items. However, only detritus and mud particles were observed in the present study. The possible explanation for the occurrence of only detritus and mud could be the fact that food preference for most species changes with season (availability) and size (suitability). Larger specimens (12-14 cm TL) consumed detritus and mud while in medium sized specimens (6.5-11.5 cm TL), only detritus was observed in stomachs examined. Environmental conditions of this habitat resulting in the abundance or scarcity of one food item or another could be responsible. However, this result is still open to further investigations.
The students, Bessie Peter and Utitofonidara Ekong are acknowledged for assisting with data collection.