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Return rates of Atlantic salmon (Salmo salar) from the sea to European rivers have declined in recent decades. The first months at sea are critical for growth and survival; recent evidence suggests that reduced food availability may be a contributory factor to the observed declines. Here, zooplankton abundance data are used to derive a measure of prey energy available to forage fish prey of salmon during early marine migration. This zooplankton prey energy has significantly and dramatically declined over much of the northeast Atlantic, and specifically within key salmon migration domains, over the past 60 years. Marine return rates from a set of southern European populations are found to exhibit clustering not entirely predictable from geographical proximity. Variability in grouped return rates from these populations is correlated with zooplankton energy on a range of scales, demonstrating the potential use of zooplankton energy as an indicator of salmon marine survival. Comparison with environmental variables derived from ocean model reanalysis data suggests zooplankton energy is regulated by a combination of climate change impacts on ecosystem productivity and multi-decadal variability in water mass influence along the migration routes. [ABSTRACT FROM AUTHOR]

Abstract: Commercial catch-per-unit-effort (CPUE) data from nine estuaries were related to hydrological variation in eastern Australia. Relationships between drought declaration, rainfall, freshwater flow and fisheries catch rates were assessed from 1997 to 2007. Estuaries varied from 0.5 to 2.0×106 ML of mean freshwater inflow per annum. Monthly CPUE data from gillnetting were used to infer the abundance of yellowfin bream (Acanthropagrus australis), dusky flathead (Platycephalus fuscus), luderick (Girella tricuspidata), sand whiting (Sillago ciliata) and sea mullet (Mugil cephalus). CPUE for all species examined, except yellowfin bream, increased in proportion to freshwater flow and decreased during periods of drought. Freshwater flow may affect CPUE by stimulating migration and schooling due to salinity fluctuations altering habitat availability. Minimum and maximum flows were important determinants of CPUE. Freshwater flow per se may not be as important in influencing CPUE as extremes in the hydrological continuum. Seasonal flows were consistently the most important aspect of the flow regime that explained the highest proportion of variability in CPUE. Seasonal freshwater pulses proximate to critical reproductive periods may influence catchability by triggering seaward spawning migrations in estuarine-dependent fish. [Copyright &y& Elsevier]