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Abstract: Forage fish occupy a central position in marine food‐webs worldwide by mediating the transfer of energy and organic matter from lower to higher trophic levels. The lesser sandeel (Ammodytes marinus) is one of the ecologically and economically most important forage fish species in the North‐east Atlantic, acting as a key prey for predatory fish and sea birds, as well as supporting a large commercial fishery. In this case study, we investigate the underlying factors affecting recruitment and how these in turn affect productivity of the North Sea sandeel using long‐term data and modelling. Our results demonstrate how sandeel productivity in the central North Sea (Dogger Bank) depends on a combination of external and internal regulatory factors, including fishing and climate effects, as well as density dependence and food availability of the preferred zooplankton prey (Calanus finmarchicus and Temora longicornis). Furthermore, our model scenarios suggest that while fishing largely contributed to the abrupt stock decline during the late 1990s and the following period of low biomass, a complete recovery of the stock to the highly productive levels of the early 1980s would only be possible through changes in the surrounding ecosystem, involving lower temperatures and improved feeding conditions. To that end, we stress the need for ecosystem‐based management accounting for multiple internal and external factors occurring within the broader context of the ecosystem in which forage fish species, such as sandeel, play an important and integral part. [ABSTRACT FROM AUTHOR]

The commercially and ecologically valuable sandeel (Ammodytes ssp.) make distinct vertical shifts between an inactive stage, during which they seek refuge in the sand, and a pelagic schooling stage, during which they forage. This characteristic discontinuous foraging pattern constitutes a challenge to fishery biologists and has consequences for a wide range of predators ranging from birds and mammals to commercially important species. However, experimental studies that shed light on the primary drivers of foraging activity in fish are rare. In the present study, whole schools of sandeel ( A. tobianus) were caught in August in east Denmark (65°02′30N; 12°37′00E) and kept in large tanks in the laboratory. It was found that the amount of food ingested and memory of past days feeding history are primary drivers of foraging activity at the level of the entire school, whereas external factors such as prey concentration and temperature are merely secondary drivers. [ABSTRACT FROM AUTHOR]

The article presents a study which focuses on the lesser sandeel Ammodytes marinus recruitment, linking it to density dependence and zooplankton composition. It states that the vast majority of sandeels in the North Sea comprises of lesser sandeel ammodytes marinus, and is considered one of the largest fish biomasses. It notes that the abundance of lesser sandeels has great and varied effects on the whole ecosystem and fisheries. However, it is noticed that recruitment of lesser sandeel is failing. Accordingly, the study investigates the factors that influence the decline in recruitment of lesser sandeel. Moreover, the study found out that recruitment of lesser sandeel in the North Sea depends on density regulation and the abundance of calanus finmarchicus.

Disentangling physical–biological interaction processes during early life-stages of fish is crucial for the understanding of fish stock recruitment. Among many individual and environmental aspects affecting mortality during the early life-stages of fish, encountering food at greater than average concentrations is regarded important for survival. Intense aggregations of zooplankton in near-surface waters provide these conditions for larval fish. Simulation studies by individual-based modeling can help understanding of the mechanisms for survival during early life-stages. In this study, we examined how growth and survival of larvae and early juveniles of Lesser Sandeel (Ammodytes marinus) in the North Sea are influenced by availability and patchiness of the planktonic prey by adapting and applying a generic bioenergetic individual-based model for larval fish. Input food conditions were generated by modeling copepod size spectra dynamics and patchiness based on particle count transects and Continuous Plankton Recorder time series data. The study analyzes the effects of larval hatching time, presence of zooplankton patchiness and within patch abundance on growth and survival of sandeel early life-stages in the North Sea. Simulations of patchiness related starvation mortality are able to explain observed patterns of variation in sandeel growth. Reduced prey densities within patches decrease growth and survival rate of larvae and match–mismatch affect growth and survival of larvae with different hatch time due to plankton seasonality. Of general scientific and environmental management interest, the results indicate a steep threshold concentration critical for survival at around 0.04–0.05 no. zooplankton/mL. [Copyright &y& Elsevier]