Influence of salinity on life history traits of the bonga shad Ethmalosa fimbriata (Pisces, Clupeidae: comparison between the Gambia and Saloum estuaries

Abstract

The common West African bonga shad represents a large part of the fish biomass in 2 neighbouring estuaries that function in different ways. The Gambia estuary has a normal salinity gradient, while the Saloum has an inverse gradient. Bonga shad Ethmalosa fimbriata were collected in both ecosystems during a 16 mo period (June 2001 to September 2002) at 5 locations, to investigate the role of salinity on life history traits. The main traits were studied at a spatio-temporal scale: reproduction from macroscopic examination of the gonads, oocyte counting and measuring, and growth from interpretation and measurements of a sub-sample of otoliths. Analysis of genetic differentiation at 3 intronic and 1 anonymous nuclear gene loci was also carried out to investigate differences between estuaries and among locations. The results did not show any allelic frequency heterogeneity between populations, indicating that populations of both estuaries represent 1 single panmictic unit, and that selection is not significantly acting on these loci. Hence, the response of the different traits to environmental variation may primarily represent phenotypic plasticity. The seasonal cycle of reproduction was clearer in the Saloum, occurring during a long period (January to August). The calculated size at maturity was reduced for both sexes in the upper Saloum, where the salinity was highest. The relative fecundity and the oocyte size were larger in the Saloum. On the otoliths, translucent zones, formed each year at the end of the rains (September to October), were used to estimate the age in months. Growth rates were reduced in the hypersaline environment of the Saloum, whereas growth differences were smaller between the Gambia and the pooled Saloum data, with a salinity < 60 psu. Growth was faster in the lower parts of the Saloum, related to better conditions for fish. The results illustrate that an environment with high salinity (> 60 psu) affects the growth, reduces the size-at-maturity and increases the fecundity of E. fimbriata.

Contribution to the Study of the Growth of the Bonga Ethmalosa Fimbriata (Bowdich) in Senegalese Coastal Waters

“…The performance index of growth and auximetric relationship shows that the results of this study are similar to those of Panfili et al (2004) in Senegal, Moses (1988) and Essen (1995) Nigeria andGerlotto (1976) in Ivory Coast. …”

“…In Senegal, the first studies were from the 50s and 70s. The most recent study of this kind was made by Panfili et al (2004) in the Saloum estuary. It focused on the influence of salinity on life history traits of the species.…”

Using oocyte essential fatty acid composition to assess spawner reproductive potential under hypersaline conditions

“…Monthly sampling was conducted at the coast and inside the estuary from February to October 2014, during Ethmalosa fimbriata's main spawning period (Charles-Dominique 1982, Panfili et al 2004. Three different sites were sampled: Joal (coast, 14°9.1' N, 16°51.7' W), Djifer (river mouth,13°57.8' N,16°4 4.8' W), and Foundiougne (upstream Saloum River, 14°8.1' N, 16°28.1' W) (Fig.…”

“…Spawning of E. fimbriata in Senegalese coastal waters mainly takes place during the upwelling season from February to May (CharlesDominique 1982). In the Sine Saloum estuary, this batch-spawning species exhibits a protracted reproductive period that usually lasts from January to August (Albaret & Gerlotto 1976, Panfili et al 2004. Its physiology, which is adapted to eurysaline conditions allows the species to cope with varying salinities from around 0 to 97 (Panfili et al 2006) and even to reproduce in waters with salinities up to 66 (Charles-Dominique & Albaret 2003).…”

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“…Due to the importance of this species in the landings of artisanal fisheries in the West African sub-region, E. fimbriata has been the subject of many studies focused on its growth and reproductive biology in Senegal (Boëly & Elwertowski, 1970;Scheffers, Conand, & Reizer, 1972;Scheffers, 1973;Scheffers & Conand, 1976;Panfili et al, 2004a;Faye et al, 2014a;2014b). However, no studies on fishing gear selectivity, particularly gillnets, have been conducted in Senegal, whereas the results of stock assessment of this species in the sub-region, demonstrated an overfishing situation and perhaps a recruit ment overfishing due to targeting juvenile fish using small mesh sizes (FAO, 2015).…”