The experiments described in this paper relate known target configurations under controlled conditions to acoustic characteristics of multiple moving fish. We wanted to further our understanding of the interactions between targets and the effects these interactions have on the measurement of the number of salmon migrating in rivers. Multiple targets in various configurations were passed through a horizontally oriented 4° × 10° beam from a split-beam echo sounder. The effects on measurements of target strength, detection probability and target location in the beam are presented. The observed target strength was not dependent on target velocity. There was a reduction in target detection due to the single-target selection criteria implemented by the hydroacoustic system. We mimicked the conditions in a river where a close range fish target may modify the beam geometry allowing detection of previously undetected targets. We demonstrated some of the effects resulting from moving targets into radial alignment and we demonstrated shadowing conditions that can cause extinction of target echoes.

Daily growth increments measured in the otolith of Thunnus atlanticus were used to estimate the age and growth of individuals, which were sampled from an artisanal fishery operating around fish aggregating devices (FADs) moored off Martinique Island. In this study, 76 otoliths were analysed for fish ranging from 20 to 68 cm fork lengths (FL). The fish were caught using trolling lines at depths ranging from 0 to 10 m. Counts were made, under a light microscope, on the external part of the transverse section of the sagittae, from the primordium to the ventral edge. The mean coefficient of variation of the age reading was 2.4%. To validate readings, the number of increments for 11 individuals was counted using both optic and scanning electron microscopes. The calculated relationship between readings of both methods was found to be linear, with a slope of 1.03 and R2 = 0.99. Frequency of growth increment formation could not be validated. Growth rates derived from otolith readings were, however, compared to those obtained using a modal progression analysis, which was performed on the length-frequency distributions of the landings over a period of two months. The corroboration between the growth rates was good. One increment was assumed to represent one day. The parameters of the von Bertalanffy growth curve for both sexes were L∞ = 71.4 cm FL, k = 0.002 day−1 and t0 = −80 days. According to our results, there is no evidence that Martinican moored FADs act as ecological traps for blackfin tuna. Instead, young blackfin tuna probably leave the vicinity of Martinican moored FADs to undergo a trophic migration at 7 to 8 month-old, and then come back about 8 months later to breed in the Lesser Antilles area.

The effects of nitrite (NO2–) on the Nile tilapia (Oreochromis niloticus) were investigated, due to their relevance to worldwide aquaculture. Hematological parameters – functional haemoglobin (oxy+deoxy-Hb, %), methaemoglobin (MetHb%), and hematocrit (Hct) – and – plasma osmolality, plasma chloride ([Cl−]) and magnesium ([Mg2+]) concentrations – were analysed. Two experiments were conducted, each with its own non-contaminated control, 8 tilapias for each group. In experiment 1 (48 hours), and the first 48 hours of experiment 2, fish were contaminated with 0.4 mM NO2– (added as NaNO2). In experiment 2, ambient NaNO2– was removed for the second 48 hours. NaNO2– exposure increased plasma [NO2–] to 0.4 mM, which increased MetHb from 10.8 ± 2.5% to 46.8 ± 8.0%, and consequently decreased functional Hb from 89.2 ± 2.5% to 53.2 ± 8.0%. NaNO2 removal led to recovery of both parameters. Both Hct and plasma Mg2+ were lowest in fish recovering from nitrite exposure. Thus, these parameters did not show recovery. Plasma osmolality and [Cl−] were not affected by NaNO2–, probably due to the moderate rise in plasma [NO2–]. O. niloticus was thus sensitive to 48 hours of exposure to 0.4 mM NaNO2, and partially recovered from its effects after 48 hours in sodium nitrite-free water. Cultivators of Nile tilapia should thus consider the toxicity of nitrite, especially in aquaculture systems using recirculating water.

Seasonal changes in nucleic acid concentrations and amino acid profiles in the muscle of juvenile Aristeus antennatus were investigated. RNA content, RNA:DNA and RNA:protein ratios varied significantly between seasons (p < 0.05), being the lowest and the highest values obtained in winter and spring/summer. A significant variation in the total amino acid content (TAA) from winter to summer was observed (p < 0.05). In fact, during this period a significant percent of increase in total non- (NEAA; 20.2%) and essential amino acids (EAA, 18.7%) contents occurred. Concomitantly, a significant decrease in the free amino acid (FAA) content from winter to summer was observed. A higher percentage of decrease with free essential amino acids (FEAA; 56.6%) in comparison to free non-essential amino acids (FNEAA; 34.2%) was attained. The significant increase in RNA and TAA contents from winter to summer may be related with protein synthesis. On the other hand, the lowest values obtained in winter may be due to a reduction in feeding activity; in this period the muscle protein must be progressively hydrolysed, which is evident with the higher FAA content. The liberated amino acids enter FAA pool and become available for energy production.

The first internal transcribed spacer (ITS-1) of nuclear ribosomal DNA and mitochondrial DNA 16S rRNA were amplified by Polymerase chain reaction (PCR) using genomic DNA extracted from adult tissue of four species of Scylla spp. and the first zoeal stages of S. serrata, S. paramamosain and S. olivacea as template. Using the ITS-1 region, variation in product fragment length was found to be useful for distinguishing S. serrata and S. olivacea from two other species. The other two species (S. paramamosain and S. tranquebarica) could be identified using the restriction endonuclease Hha I. Using 16S rDNA, all four species were identified using PCR-restriction fragment length polymorphism (RFLP) by double digestion with DraI and HindIII. These genetic markers can be used for hybridization breeding studies and in field studies of larval and juvenile mud crabs of the genus Scylla.

The Gambia Estuary, a “normal” estuary with a decreasing salinity gradient from the mouth towards the head, is moderately exploited by small-scale fisheries and does not receive any severe pollution from either agriculture or industrial activities. Neither the Gambia River nor its estuary are artificially impounded. As the last large West African estuary free of major human disturbance, it is of considerable interest for comparative studies on the effect of major environmental perturbations in West African estuarine ecosystems. The aquatic environment and fish communities of the Gambia Estuary (about 250 km long) were studied by purse seine sampling at different periods in the river cycle, covering all hydro-climatic seasons that are characteristic of West African estuaries. Emphasis was placed on the diversity, composition, structure and distribution of fish assemblages in relation to fluctuations in physico-chemical factors such as water temperature, salinity and turbidity. Results on the aquatic environment, mainly the salinity range (from freshwater to 39) and dissolved oxygen (never a limiting factor for fish in the estuary) and on the main characteristics of the fish fauna (high diversity of life cycles, all the ecological categories represented) indicated that the Gambia Estuary was free of major climatic perturbation and reinforced the choice of this system as a reference for the study of the effects of major perturbations on estuarine tropical fish communities.

The Gambia Estuary is one of the last estuaries in West Africa not to have been affected by strong environmental changes and adverse human disturbances. In order to describe the spatio-temporal change in fish biomass in this estuary, and the seasonal changes in fish size classes, five research surveys have been conducted at characteristic periods in the river's water regime and meteorological seasons. The fish assemblages were sampled by two vertical echosounding protocols: mobile surveys (zigzag transects) in the estuary and moored stations. The two sampling methods produced a similar image of the fish assemblages in terms of the distribution of total biomass and target strength distributions. Large fish aggregations were detected in the most marine area at the end of the wet season, and fish biomass increased through the dry season in the upstream zone. Fish were smaller at the beginning of the wet season and they were always larger in downstream areas.

A fyke net survey of the juvenile and small fish assemblage of the Gambia estuary was conducted on three sampling occasions during the maximum recruitment season, from May to November 2002. This is the first detailed description of that particular component of the fish community in the Gambia estuary. The overall fyke net fish assemblage comprised 51 species, with a majority of young individuals of large and medium size species and the adults of small species. This assemblage is defined as the “small fish component of the mangrove channels”. The assemblage is dominated by true estuarine and certain marine species, all able to complete their life cycle within the estuary. The sea spawning species whose juveniles use the estuary mainly as a nursery ground, are not very diverse or abundant during the recruitment season. They are limited to the lowest part of the estuary due to the low salinity prevailing in the middle and upper reaches during the rainy season. This suggests that extreme seasonal variability (river flooding) in certain West African tropical estuaries at the time of maximum recruitment by certain marine species may reduce the nursery ground value for these taxa.

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∞ = 198 mm, K = 0.26). Differences are less obvious between the growth in the Gambia (L∞ = 238 mm, K = 0.21) and that in the Saloum with a salinity of less than 60 (L∞ = 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.

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.