Seasonal and spatial variations in the composition and relative contribution of the harpacticoid species in the Mondego estuary (western Portugal) were studied based on a monthly sampling along a salinity gradient. These benthic harpacticoids were collected in the water column by means of a 63-µm plankton net. The influence of hydrological parameters (temperature, salinity, oxygen dissolved concentration, pH, turbidity, chlorophyll-a, total suspended solids and nutrient concentrations) was analysed by means of a redundancy data analysis. In addition, this is the first study to provide a checklist of benthic harpacticoids from this estuary. In total, 13 species plus six species not yet identified but known to belong to the genus Canuella, Microsetella, Ectinosoma, Mesochra, Harpacticus and Parapseudoleptomesochra were identified. Copepodites and adults of Euterpina acutifrons and Paronychocamptus nanus were most abundant in this harpacticoid community. While P. nanus dominated in winter and spring at upstream stations, E. acutifrons dominated more downstream. Although this species occurred along the whole salinity gradient, its highest abundances were found in the north arm of the estuary, except in autumn. This distribution may be due to the adaptability of E. acutifrons to different environments and the intermediate position between marine and estuarine conditions assigned to this species. Paraleptastacus cfr. spinicauda showed a relative occurrence of 5–10% in all stations, except at the mouth of the estuary (M). Tachidius discipes was found in low densities in Mondego estuary in spite of the large numbers of copepodites of this species in the southern arm, characterized by a high level of total suspended solids. The northern arm of the estuary was characterized by a higher numerical occurrence of harpacticoid organisms throughout the study years, with the higher densities towards the freshwater part (upstream areas). Canuella sp., Ectinosoma sp. (copepodite), Ectinosoma melaniceps, Leptocaris brevicornis, Phyllognathopus viguieri, Microsetella norvegica (copepodite) and Macrosetella gracilis (copepodite) were considered rare species. Nonetheless harpacticoid species were represented by a higher number of adults in the northern arm, and juveniles in the southern arm.

Freshwater fishing is highly developed in the Amazonian basin, and the problems of managing the fish resource are becoming increasingly acute. The global model of exploited stock dynamics has proved inadequate for explaining the variations observed in catches from one year to the next. Following the results of recent studies in maritime fishing, this work aims to identify the contribution of environmental variables (hydrological parameters have been selected) to the explanation of variations in the abundance of the exploited fish community. An abundance index is calculated based on the catch per unit of effort statistics and the surface of the considered basin. Using a 12-year time series, the results show that the effect of the fishing effort on the index of abundance is negligible when viewed alongside the flood regime parameters for the years prior to the catch. The effects revealed are, in chronological order of appearance: 1) a positive effect of the water level during the flood regime peak three years previous to the catch, probably associated with recruitment for a number of species; 2) the effect of the water level during its rise two years before the catch, which may well be related to competition phenomena; and 3) the effect of severe low-water levels two years before the catch, possibly due to mass natural mortality during the severest periods of low-water. Because there are complex migratory movements of fish populations either side of low-water periods, only the estimated biomass during the flood appears to be an accurate reflection of the mean biomass in the environment. Linear models are presented, where the abundance index is a function of the hydrological parameters previously identified. The model with 3 variables corresponding to the 3 evidenced effects, explains more than 83% of the variations in the annual abundance indices.