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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]

For effective fishery management, estimated stock sizes, along with their uncertainties, should be accurate, precise, and unbiased. Atlantic salmon Salmo salar stock assessment in England and Wales (and elsewhere across the Atlantic) estimate returning salmon stocks by applying a measure of rod exploitation rate (RER), derived from less abundant fishery-independent stock estimates, to abundant fishery-dependent data. Currently, RER estimates are generated for individual principal salmon rivers based on available local data and assumptions. We propose a single, consistent, transparent, and statistically robust method to estimate salmon stocks that transfers strength of information from "data-rich" rivers, i.e. those with fisheries-independent data, to "data-poor" rivers without such data. We proposed, fitted, simplified, and then validated a Beta–Binomial model of RER, including covariates representing angler and fish behaviours, river flow, and random effects to control for nuisance effects. Our "best" model revealed covariate effects in line with our hypotheses and generalized to data not used to train it. We used this model to extrapolate stock estimates from 12 data-rich to 52 data-poor rivers, together with their uncertainties. The resulting river-specific salmon stock estimates were judged to be useful and can be used as key inputs to river-specific, national, and international salmon stock assessments. [ABSTRACT FROM AUTHOR]

Coastal winds transport water masses and larval fish onshore or offshore which may influence estuarine recruitment, yet our understanding of the mechanism underlying this relationship is limited. Here, we combine datasets from a historical database of larval fish off southeast Australia with a high‐resolution atmospheric reanalysis model to show that normalised abundance of coastally spawned larvae increased with weak to moderate upwelling favourable winds 14 days prior to sampling. The increase in abundance may reflect increased nutrient and plankton availability for larval fish. Normalised larval abundance decreased following strong upwelling favourable winds but increased after onshore (downwelling favourable) winds, due to wind‐driven transport. By combining a commercial estuarine fisheries catch‐rate dataset (4 species, 8 estuaries, 10 years) and the high‐resolution atmospheric reanalysis model, we show that negative effects of upwelling favourable winds during the spawning period can be detected in lagged estuarine commercial fisheries catch rates (lagged by 2–8 years depending on species' growth rates), potentially representing the same mechanism proposed for larval fish. Upwelling favourable winds in the southeast Australian region have increased since 1850 while onshore winds have decreased, which may have reduced larval recruitment to estuaries. Coastal winds are likely an important factor for estuarine recruitment in the southeast Australian region and future research on the estuarine recruitment of fish should incorporate coastal winds. [ABSTRACT FROM AUTHOR]

Understanding salmonid discharge requirements can help inform management to conserve wild populations in a changing climate. This study developed Bayesian hierarchical mixed‐effects models relating 0+ Atlantic salmon (Salmo salar L.) and trout (Salmotrutta L.) densities to different aspects of river discharge. Associations between these densities and nine hydrological variables representing the magnitude, frequency and duration of discharge events were evaluated using historical monitoring data from 36 sites on five rivers in England and Wales. All hydrological variables had weak associations with 0+ salmonid densities. More frequent high discharges between spawning and emergence were positively and negatively associated with 0+ salmon and trout densities, respectively. High discharges might increase spawning site availability for salmon and decrease egg‐to‐fry survival for trout. However, overall, only equivocal evidence was found regarding which discharge aspects affect juvenile salmonid densities. Therefore, a strategic review of juvenile salmonid monitoring programmes integrating environmental data collection is recommended. [ABSTRACT FROM AUTHOR]

Patterns in the Southern Oscillation Index (SOI) affect the life history of many aquatic organisms in the southern hemisphere. We examined the effect of this phenomenon and other factors (i.e. rainfall, river flow and sea surface temperature, SST) on the commercial harvest of the giant mud crab (Scylla serrata) in Australia, given the large inter-annual variations in the catch of this species over the last 15 years, particularly in the north. Regression models were applied to concurrent environmental and catch data for giant mud crab caught from 29 catchments that provided a combined harvest of >20 000 tonnes. Non-metric multidimensional scaling (nMDS) was also used to explore potential regional differences in catch trends. A combination of SOI, SST and rainfall/river flow explained 30-70% of the variability in commercial catches, with mean summer temperature being most influential at higher latitudes. The nMDS revealed distinct groupings of river systems that coincided with biogeographic regions. This work highlights the importance of climatic events on the harvest of giant mud crabs and reinforces the need to adopt a bioregional approach when assessing the performance of fisheries targeting this species. Giant mud crabs are an important food source throughout the Indo-west Pacific and are widely caught in commercial and subsistence fisheries. We explored the impact of environmental drivers on Australian commercial giant mud crab catches and demonstrated that a large proportion of variation in catch rates is due to rainfall and temperature differences. The findings support the general understanding that estuarine fisheries species depend on freshwater runoff. [ABSTRACT FROM AUTHOR]

This review presents a synopsis of the impacts of freshwater flow on fisheries production in estuarine and coastal systems, with particular emphasis on regional examples from eastern Australia and southern Africa. Freshwater flow impacts habitat availability, trophic interactions, and fishers' harvesting behavior in estuarine and coastal systems. Seasonal and interannual variation in freshwater flow influences the distribution and abundance of fish and invertebrates through changes in growth, survival, and recruitment. Episodic flood and drought events have pronounced impacts on fisheries production due to rapid changes in physicochemical conditions modifying species richness and diversity. Many documented reductions in fisheries production have been attributed to river regulation modifying natural variation in freshwater flow. Protecting natural flow regimes is likely to be an effective management strategy to maintain the production of estuarine and coastal fisheries. Understanding the freshwater requirements of estuarine and coastal fisheries will become increasingly important as climate change modifies the hydrological cycle and as human population growth increases demand for water resources. One major challenge for scientists seeking to explore relationships between freshwater flow and fisheries production is to understand how variable flows influence resource availability, fishing activity, and the economic performance of commercial fisheries in estuarine and coastal systems. [ABSTRACT FROM AUTHOR]