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A comparative study, using biological indicators, was conducted in Mali where two man-made reservoirs (Selengue and Manantali) are particularly suited for investigating the impact of fishing effort on the fish assemblage: these two ecosystems have relatively similar morphological, edaphic and environmental properties but are subjected to radically different levels of fishing exploitation (low at Manantali, high at Selengue). The comparison is based on a three-month survey of commercial fisheries, focusing on fishing activities and catches on the two reservoirs. The results show that some indicators are useful for evaluating fishing impacts. Among these indicators are: fishing effort which is much higher at Selengue (22 800 fishing trips per month) than at Manantali (3000), catches per unit effort (lower at Selengue than at Manantali following a ratio ranging from 1.5 to 4 according to the gears used), annual yields per ha higher at Selengue (100 kg ha$^{-1})$ than at Manantali (27 kg ha$^{-1})$, the average fish lengths in the catches (16.2 cm at Selengue compared to 23.6 cm at Manantali) and the maximum lengths of the targeted species generally smaller at Selengue (10 to 30 cm) than at Manantali (30 to 50 cm). By contrast, a second class of indicators exhibit values that are contrary to expectations: the species richness (52 compared to 36), the species diversity ($I_{\rm sh}= 4.02$ compared to 3.24) and evenness (0.76 compared to 0.69) were higher at Selengue where 4 species accounted for 50% of the landings compared to only 2 species at Manantali (9 species compared to 7 for 80% of the landings). The trophic structure of the landings is higher (37% primary consumers compared to 49% at Manantali) as well as the mean trophic level (2.74 compared to 2.54 at Manantali). Even if a good understanding of the two fisheries can explain the unexpected trends of the second class of indicators which increase with fishing effort, it clearly appears from this study that only the first class of indicators is robust and can be used for comparative studies across ecosystems. [ABSTRACT FROM AUTHOR]

Results of an annual survey of fishing activities and catches in the Gambia estuary (200 km long), carried out from June 2001 to May 2002, indicated that fishing effort targets mainly (66% of fishing trips) shrimps. As a result of the globalisation of fish markets, the fishery has become a quasi mono-exploitation of shrimp for exporting. At the opposite, fishing activities focusing on fish were low and carried out with large and medium mesh drift nets, gill nets and surrounding nets (17% of fishing trips), handlines and longlines (15%). The annual catch, estimated at 2350 tonnes for the estuarine part of the River Gambia, consisted of 1800 t of fish and 550 t of shrimp. Stownets were the most efficient gear accounting for 50% of the total production (550 t of shrimp and 600 t of fish) while other significant techniques were mainly drift nets (21%), longlines (11%) and gillnets (5%). Fish catches were composed with 55 fish species among which 16 species accounted for 90% of the annual catch. The average catch length of these first sixteen species was 295 mm, clearly demonstrating that fishing for fish was targeting large fish. The effects of the targeted shrimp exploitation resulted in a low fishing effort for fish leading to low fish landings and consequently to an under exploitation of fish resource in the Gambia estuary. [ABSTRACT FROM AUTHOR]

The West-African neighbouring Gambia and Saloum estuaries function in different ways, the first being normal (saltier waters in the lower river, salinity range 0-38) and the second being inverse (saltier waters in the upper river, salinity range 36-90). The common cichlid species Sarotherodon melanotheron was collected in both systems over 16 months (June 2001 to September 2002) at five locations. We quantified reproductive traits from macroscopic examination of the gonads, oocyte counts and measurements, and growth from interpretation and measurements of an otolith sub-sample. There was a clear seasonal cycle of reproduction in the estuaries with a peak at the beginning of the wet season (May-July). Size at maturation was smaller in the more saline environment, the Saloum, for both females (170 mm in Gambia and 131 mm in Saloum) and males (162 and 113 mm respectively). The relative fecundity was lower and the oocyte size was bigger in the freshwater location of the Gambia river. Growth, estimated by counting the annual opaque zones on whole otoliths, was reduced in the hypersaline environment of the Saloum ($L_{\infty }=198$ mm, $K=0.26$). Differences are less obvious between the growth in the Gambia ($L_{\infty }=238$ mm, $K=0.21$) and that in the Saloum with a salinity of less than 60 ($L_{\infty }=244$ mm, $K=0.22$). All these observations suggest that S. melanotheron is able to withstand saltier environments by limiting its growth, reducing the size-at-maturity, and changing its fecundity, but the most profound changes are only visible in hypersaline conditions. [ABSTRACT FROM AUTHOR]