Population dynamics of the Giant African River prawn <i>Macrobrachium vollenhovenii</i> Herklots 1857 (Crustacea, Palaemonidae) in the Cross River Estuary, Nigeria

Abstract

Growth, mortality, recruitment, yield-per-recruit and present rate of exploitation of the African river prawn Macrobrachium vollenhovenii were studied in the Cross River Estuary, Nigeria. The research was based on length-frequency data collected between January 1997 and June 1998, and FAO-ICLARM Software (FiSAT) routine runs. Growth parameters obtained from the seasonalized von Bertalanffy growth function were L∞ 213.63 and 198.99 mm, K = 1.24/year and 1.24/year; C = 0.95 and 0.95; WP = 0.6 and 0.45, for M. vollenovenii males and females, respectively. Using the seasonalized length-converted catch curves, the instantaneous rate of total mortality Z was estimated as 3.93/year and 6.85/year for males and females. Natural mortality M, fishing mortality F, and exploitation rate E of both sexes were 2.21/year and 2.27/year, 1.72/year and 4.58/year, and 0.44 and 0.67, respectively. The length at first capture L c estimated from the probability of capture routine was 44.25 mm and 73.41 mm for males and females. One recruitment peak was observed for males, and two of unequal strength for females. Length-weight relationship gave 'b' values of 3.483 and 3.329 for males and females, respectively. The relative yield-per-recruit analysis predicted maximum exploitation rates (E max ) of 0.42 and 0.47 as against current exploitation rates of 0.44 and 0.67 for males and females and suggest that only female M. vollenhovenii are being over-exploited. Hence, it is proposed that fishing on females during the reproductive season is reduced, and that the return of egg-bearing females into the waters is enforced.

Artisanal Fishery of the Exploited Population of Macrobrachium vollenhovenii Herklot 1857 (Crustacea; Palaemonidae) in the Asu River, Southeast Nigeria

“…Gross dissimilarity in Φ′ among a number of stocks of the same species or related species is an indication of the unreliability of the estimated growth parameters. The growth performance index (Φ′) for M. vollenhovenii in the Asu River was 2.80, which compares favourably with the value 2.93 recorded by Gabche and Hockey (1995) in the Lobe River, Cameroon and 2.75 reported by Nwosu and Wolfi (2006) for the Cross River estuary, Nigeria. This is an indication that our data were suitable for the length-frequency analysis and that the estimated growth parameters are reliable.…”

“…For instance, the K (1.14 year -1 ) value obtained in this study compares well with the values 0.91 and 1.24 recorded by Etim and Sankare (1998) and Nwosu and Wolfi (2006) in the Fahe Reservoir, Cote d'Ivoire and the Cross River estuary, Nigeria respectively, although it was higher than the value 0.55 obtained by Falaye and Abohweyere (2008) in the Lagos-Lekki Lagoon. The asymptotic length (23.65 cm) recorded in this study was higher than values recorded for this species previously in other regions.…”

“…Longevity estimates showed that their lifetime ranges from 0.9 to 3 years (Gabche & Hockey 1995;Enin 1995;Etim & Sankare 1998;Nwosu & Wolfi 2006). The longevity of 2.10 years estimated in this study falls within this range.…”

“…The asymptotic length (23.65 cm) recorded in this study was higher than values recorded for this species previously in other regions. Etim and Sankare (1998) recorded 18 cm, Falaye and Abohweyere (2008) 18.81 cm and Nwosu and Wolfi (2006) 21.36 cm for males in the Cross River estuary. The large size of the species in the Asu River may be attributed to the reduced number of natural predators and food availability.…”

“…As a consequence, this species is abundant in rivers that are directly or remotely connected to the ocean. Surprisingly, studies on the biology and ecology of Macrobrachium in West Africa are mainly focused on coastal and estuarine populations (Bello-Olusoji & Somers 1997;Nwosu & Wolfi 2006;Nwosu et al 2007;Falaye & Abohweyere 2008).…”