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Knowledge of the adequacy of field sampling protocols is critical for detecting species and assessing biological conditions. Several studies have been conducted to determine the appropriate electrofishing distance for estimating fish assemblage richness in temperate North American streams. We tested whether electrofishing 40 times the mean wetted channel width was sufficient for estimating species richness and developing precise metrics for Atlantic Forest streams in southeastern Brazil. We sampled 32 sites with differing gradients, substrates, and anthropogenic pressures. Our results show that 40 channel widths were not sufficient to estimate species richness in those systems, presumably because of the high number of rare species. However, 40 channel widths were sufficient for applying other metrics of assemblage condition (e.g., number of common species, percentage of tolerant individuals, percentage of Characiform species, percentage of Siluriform species, percentage of water column species, percentage benthic species, Shannon diversity, dominance). This suggests that 40 channel widths are an appropriate sampling distance for applying environmental assessment protocols to Atlantic Forest streams. el conocimiento sobre la pertinencia de protocolos de muestreo es un aspecto clave para detectar especies y evaluar condiciones biológicas. Se han llevado a cabo numerosos estudios encaminados a determinar la distancia más adecuada para la electro-pesca, aplicada a la estimación de riqueza de ensambles de peces en ríos templados de Norte América. En esta contribución se probó si la electro-pesca aplicada a una distancia de 40 veces el ancho promedio del canal de inundación, era suficiente como para estimar la riqueza específica y desarrollar medidas precisas para los ríos del bosque tropical del Atlántico, en el sureste de Brasil. Se muestrearon 32 sitios con diferentes gradientes, sustratos y grados de presión antropogénica. Nuestros resultados muestran que en esos sistemas, usar 40 veces el ancho del canal no era suficiente como para estimar la riqueza, presumiblemente porque el alto número de especies raras. Sin embargo, 40 veces el ancho sí fue suficiente para conocer otros indicadores de la condición de los ensambles (e.g. número de especies comunes, porcentaje de individuos tolerantes, porcentaje de especies Caraciformes, porcentaje de especies Siluriformes, porcentaje de especies que habitan la columna de agua, porcentaje de especies bentónicas, diversidad de Shannon, dominancia). Esto sugiere que 40 veces el ancho del canal de inundación es una distancia apropiada de muestreo para aplicarla en protocolos de evaluación ambiental en ríos de los bosques tropicales del Atlántico.

Fish abundance and environmental data collected over ten years (1980–1989) from the middle Thames estuary, England, were analyzed to detect temporal trends in fish populations and relationship with environmental parameters, and to assess water quality. Fish were collected from the cooling water intake screens of West Thurrock power station, situated 35.5 km below London Bridge, in the mid-estuary. Marine species abundance were highly seasonal, with peaks in December–March for herring (Clupea harengus), sprat (Sprattus sprattus), 3-spined-stickleback (Gasterosteus aculeatus), and poor cod (Trisopterus minutus); July–August for flounder (Platichthys flesus); and September–December for sand goby (Pomatoschistus minutus), whiting (Merlangius merlangus), bass (Dicentrarchus labrax), plaice (Pleuronectes platessa), and dab (Limanda limanda). Bimodal seasonal patterns of peaks or unclear seasonality in abundance characterized marine estuarine-dependent sole (Solea solea), Nilsson's pipefish (Syngnathus rostelattus) (April/May and September/October), and pouting (Trisopterus luscus) (May and November/December); the estuarine smelt (Osmerus eperlanus) (October and January) and the catadromous eel (Anguilla anguilla) (June and October). There was substantial variation in the abundance of common species over the period of ten years, with herring, sand goby, flounder, and plaice showing a stable abundance in 1980–1984, increasing sharply in 1985–1986, and then decreasing successively through the remainder of the decade (1987–1989). The first half of the decade was a period of higher abundance for less tolerant species such as smelt, sprat, and poor cod, while the second half showed higher abundances of species tolerant to harsh environmental conditions such as sand goby, flounder, eel, and plaice. A general pattern of stable fish populations with a slight trend of deterioration was found to emerge over the years, related to the number of species and quantities of common species. Multivariate techniques of principal component and canonical correspondence ordinations were used for assessing relationships between fish populations abundance and environmental variables. The most significant environmental variables correlated with fish species were temperature and dissolved oxygen. High abundances of flounder were associated with high temperature, while high abundance of poor cod, sprat, herring, and 3-spined-stickleback were associated with high dissolved oxygen, flow, ammonical nitrogen, and low temperature. Plaice, whiting, sand goby, bass, and dab were preferentially found in high salinity and suspended solids, while smelt and sole were likely to prefer average values or showed no clear preferences.