Dominant drivers of larval survival are considered to include oceanographic dispersal, sea temperatures, and food availability in the phase of first-feeding. However, research progress on larval herring survival dynamics indicates that multiple factors might act on differing larval developmental stages. Hypothesizing that in inshore systems of the western Baltic Sea bottlenecks of herring development occur before the point of first-feeding, we analysed an extensive time-series of weekly abundances of early stage larvae in Greifswald Bay, an important spawning area for Western Baltic herring. Additionally, we investigated whether distinct hatching cohorts contribute differently to established survival indices on the level of (i) later larval stages in Greifswald Bay and (ii) 1+ group juveniles in the overall western Baltic Sea. Results revealed that abundances of the earliest larval stage explain 62% of the variability of later stage larvae and 61% of the variability of surviving juveniles, indicating pre-hatching survival bottlenecks. Hatching cohorts occurring later during the spawning season contribute most to the surviving year class. Earlier hatching cohorts were not found to result in significant amounts of growing larvae, indicating a bottleneck phase at the critical period when larvae start feeding.

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Shallow estuaries, bays, and lagoons are generally considered hot spots of ocean productivity that often adjust rapidly to seasonal variations in atmospheric temperatures. During spring when biological reproductive processes begin in the temperate zones, regional climate variability can be immense and uncovering a non-linear biological response, such as fish recruitment to changing temperature regimes might be challenging. Using herring as a paradigm for a response of coastal spring productivity to regional climate drivers, we demonstrated how the annual timing of spawning periods can significantly affect the reproductive success of spring-spawning herring (Clupea harengus) in the western Baltic Sea. An investigation of spawning phenology in consecutive years indicated a temperature threshold range of 3.5–4.5°C triggering initial spawning in the coastal zone. Based on this finding, we analyzed the timing of larval hatching peaks, larval survival and recruitment to the adult population relative to multi-decadal time-series of seasonal sea-surface temperatures. The results revealed that the late seasonal onset of cold periods the corresponding elongation of the period where larvae hatch from the eggs and early larval hatching peaks significantly reduced larval production in a coastal nursery area and finally lead to a reduced abundance of juveniles in the entire distribution area. Using a combination of field research and time series analysis, we presented precedence for shifting regional winter regimes providing a present-day stressor to reproductive capacity of a central component of the coastal food web.

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We analyzed the predation of threespine stickleback (Gasterosteus aculeatus) on the spawn of Atlantic herring (Clupea harengus) in a mesohaline lagoon of the Baltic Sea, hypothesizing a significant predation effect of a resident estuarine fish on the recruitment of an anadromous oceanic species. A predator exclusion field experiment using artificially spawned experimental units was combined with tank feeding experiments to estimate the effects of temperature and prey density on the herring egg consumption by estuarine G. aculeatus. The predator exclusion experiment showed a significant mortality of herring eggs caused by estuarine predatory fish species. A strong increase in the consumption of herring eggs by sticklebacks was observed between 11uC and 15uC (Q 10 5 3.15). Additionally, we found a significant positive correlation between egg concentration per area and predation intensity. Nearly all eggs were consumed at concentrations $ 25 eggs cm 22 , but predation was less intense at egg concentrations below that threshold. Field data on herring egg concentrations, stickleback abundances, and stomach contents were combined with the findings of the experiments to estimate the percentage of spawned herring eggs that is consumed by the local stickleback (M PS ). The highest M PS (11.4%) was estimated for a week in April in the second half of the spawning season. We conclude that stickleback predation on herring eggs potentially affects the local herring year class strength.Transitional waters connecting temperate river tributaries with the coastal ocean worldwide represent nutrient-rich, mesohaline, and seasonally highly variable environments. These characteristics pose unique challenges for euryhaline faunal communities populating those ecosystems and also for scientists investigating the ecology of these waters (Elliott and Whitfield 2011). Fish communities of temperate estuaries and coastal lagoons are often subject to significant changes during the course of a year (Thiel et al. 1995). This variability is caused by seasonal changes in environmental conditions, the specific migration behaviors, and the reproductive cycles of the different species. Ecological studies of qualitative and quantitative aspects of estuarine fish assemblages therefore often represent merely a temporary status of an otherwise highly dynamic system. These snapshots are difficult to generalize, e.g., in order to construct predictive multispecies model approaches for ecosystem productivity or energy transfer. Few studies include changes in estuarine fish assemblages caused by seasonal immigration of oceanic species, and even fewer consider the actual extent and importance of trophic links generated in the context of this migratory behavior.Western Baltic spring-spawning herring (Clupea harengus) undergoes an extensive annual migration (Aro 1989).

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After the major loss of eelgrass habitats in the 1930s the snake pipefish Entelurus aequoreus was considered to be rare in the Wadden Sea (south-eastern North Sea, German Bight). We hypothesize that a sudden increase in pipefish abundance observed in the area since 2004 is related to new habitats provided by the introduced Japanese seaweed Sargassum muticum. SCUBA observations conducted near the Island of Sylt during July 2006 showed that approximately every second S. muticum thallus was inhabited by E. aequoreus, whereas no pipefish could be found on sedimentary areas dominating the Wadden Sea. In a field experiment, we removed thalli from a S. muticum bed and transplanted them to an unvegetated site and to the donor S. muticum bed, respectively. Nine days after the transplantation, abundances of pipefish per thallus were almost equally high within all transplanted S. muticum units at vegetated and originally unvegetated sites. Abundances of E. aequoreus in transplanted thalli did not significantly differ from those found in unaffected control thalli located in the donor S. muticum bed. To explain high pipefish densities within S. muticum beds we sampled the zooplankton community inside and outside the S. muticum bed and compared it with gut contents of pipefish. Zooplankton densities were significantly higher inside the S. muticum bed. Harpacticoid copepods were the dominant group associated with S. muticum and also represented the major prey of E. aequoreus. We conclude that stocks of E. aequoreus are promoted by S. muticum beds since they benefit from higher food supply during the spawning season, probably in addition to increased shelter by habitat complexity. Results of the present study suggest that introductions of habitat-forming species such as S. muticum may cause beneficial effects on abundance and diversity of native species.

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