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Shallow shore zones are generally considered to provide juvenile habitats for many invertebrate and fish species and additionally serve as spawning grounds for important components of oceanic food webs and fishery resources such as herring (Clupea spp.). Herring attach their demersal eggs to benthic substrates, rendering reproduction success vulnerable to environmental changes and local habitat alterations. However, little information is available on the effects of different substrates on the survival of demersal eggs. Hypothesizing that the structural complexity of spawning substrates generally affects herring egg survival and that the effect magnitude depends on the suitability of ambient environment, field experiments were conducted on a major spawning ground of C. harengus in the Southwestern Baltic Sea. Herring eggs were artificially spawned on substrates of different structural complexities and incubated in situ under differing temperature regimes, at the beginning and the end of the natural herring spawning season, to include the full suite of stressors occurring on littoral spawning beds. Results of this study indicate a positive relation between high structural complexity of spawning substrates and herring egg survival. Mean egg mortality was three times higher on substrates of lowest complexity than on highly complex substrates. These differences became even more prominent under unfavorable conditions that appeared with rising water temperatures later in the spawning season. Although the mechanisms are still unclear, we conclude that structural complexity, particularly formed by submerged aquatic vegetation, provides a crucial prerequisite for the successful reproduction of substrate spawning marine fishes such as herring in the Baltic Sea. [ABSTRACT FROM AUTHOR]

During their spring migration, Atlantic herring ( Clupea harengus) populations in the Baltic Sea rely on shallow transitional waters, such as estuaries, bays, and lagoons for spawning. Such inshore spawning grounds are ecologically important by providing suitable substrates for demersal egg deposition. These habitats are often highly impacted by multiple anthropogenic threats. Decades of eutrophication have caused a decline in depth distribution of submerged aquatic vegetation, the main herring spawning substrate in the Baltic Sea. Nowadays, spawning beds are limited to the shallow littoral zone (≤3 m depth). Accordingly, macrophytes are increasingly exposed to mechanic forcing due to storm-induced wave action. Generally, reproductive success and year class strength of the Western Baltic herring population is strongly determined by the survival of early life stages such as eggs and larvae in local nursery areas. However, explicit mechanisms by which local stressors might affect overall recruitment are currently not well understood. Hypothesizing that aquatic vegetation limited by water depth causes high herring egg mortality due to increased exposure to storm-induced hydrodynamics, we performed a combination of field studies investigating the impact of storm events on herring egg loss. Results of an egg loss experiment revealed a total egg loss of 29% in one single spawning bed during a storm event within the spawning season and the quantification of eggs attached to macrophyte litter on the shoreline emphasize the potential for regional weather extremes such as storm events to act as influential stressors for herring reproduction. [ABSTRACT FROM AUTHOR]

Fish species classified as demersal substrate spawners attach their eggs on macrophytes or hard substratum. As a result of consecutive events of egg deposition and successive waves of spawning adults, the formation of multiple egg layers on spawning substrate was already observed for demersal spawning fish species, such as the Atlantic herring (Clupea harengus, Linnaeus 1758). However, empirical information on the effects of egg concentration on herring egg survival are scarce. Hypothesizing that high egg concentration has a negative effect on herring egg survival, we experimentally investigated egg mortalities in artificially spawned egg clusters of 1–10 vertical layers. Moreover, potential effects of egg packing density (loose and dense) in one single layer set-up were investigated. We found decreasing herring egg survival in clusters of increasing egg layers. Although egg mortality was lowest in surface layers, a layerwise analysis did not reveal a stepwise increase of egg mortality from outermost to innermost eggs. Furthermore, egg mortality in surface layers significantly increased with increasing egg layer number underneath, indicating that the extent of layering is affecting egg mortality also in surface layers. This implies a reproductive disadvantage not only for eggs being spawned in the beginning of consecutive spawning events but also for latest spawned eggs. However, different egg packing densities in single layer spawn did neither influence the fertilization success nor mortality of herring eggs. Increased thickness of egg layers may result from the ongoing reduction and fragmentation of coastal vegetation due to eutrophication and coastal modifications. Our results indicate that such multiple egg layer deposition has the potential to negatively impact hatching success in demersal spawning fish species, such as the herring. [Display omitted] • We investigated density-dependent herring egg mortality via laboratory experiments. • The total egg mortality increases with the number of layers within batches of spawn. • Surface layer mortality is lowest but increases with numbers of egg layers underneath. • Monolayers result in best egg survival but require more area of spawning substrate. • Habitat loss related increase in spawn concentrations pose a threat for egg survival. [ABSTRACT FROM AUTHOR]

In fishery science, early life-stage survival and development are regarded as major factors driving the population dynamics of marine fishes. During the last century, the main research focus has been on the spatio-temporal match of larval fish and appropriate food ( bottom-up processes). However, these field studies are often criticised for their limited capability to disentangle their results from mortality caused by predation since these top-down mechanisms are rarely studied. We examined the predation on herring ( Clupea harengus ) larvae in a Baltic inshore lagoon by investigating the spatio-temporal overlap of larval herring and their potential predators such as the dominant threespine stickleback ( Gasterosteus aculeatus ) in distinct habitats (sublittoral and littoral areas) using a set of different gears and sampling techniques. Despite significant spatial and temporal predator-prey overlap, stomach analyses suggested that very few larvae were consumed by sticklebacks, even if projected to the entire study area and season. Other well-known predators of clupeid larvae such as gelatinous plankton occur later in the year after young herring have migrated out of the system. The observed predation on herring larvae was much less than expected and appears being a minor factor in determining herring reproduction success in our study area, particularly if compared to other causes of mortality such as egg predation. Providing a relatively good shelter from predation might be a key element making transitional waters valuable nursery grounds for the offspring of migrating marine fish species. [ABSTRACT FROM AUTHOR]