Unlike most marine fish, herring rely on specific benthic (seabed) habitat to reproduce. They typically spawn in environments with elevated oxygen levels, which lowers egg mortality [1, 2]. This may be areas with strong currents, over coarse substrate (e.g., gravel, small rocks, shingle, coarse sand, broken mollusc shells) and on photosynthesising marine plants [3, 4].
In Scottish waters, herring reproduce in autumn or spring. Autumn-spawning herring spawn primarily near offshore banks in the North Sea and to the west of the Hebrides. Spring-spawning herring spawn in shallower nearshore environments along the west coast, typically in March (see video) . When referring to spawning locations, a “spawning bed” is a discrete patch of seabed where herring eggs are deposited, and a “spawning ground” is a larger geographic area encompassing one or more spawning beds and all the adjoining potential spawning habitat .
Along the west coast of Scotland, spring-spawning herring frequently spawn over beds of coralline red algae, called maerl, a Priority Marine Feature in Scottish seas. Seabed geomorphology and salinity are also important habitat cues for the selection of spawning beds by herring schools . Availability and conservation of essential spawning habitat is key for this benthic spawner, yet knowledge of location and status of spawning beds is scarce in some areas, especially the west coast of Scotland . The ‘West of Scotland Herring Hunt’ project is identifying and producing evidence for the conservation, and potentially restoration, of herring spawning habitat on the Scottish west coast.
Prior to spawning, herring populations gather close to spawning grounds in areas with similar environmental conditions. Adult herring usually return to the spawning grounds where they hatched. Females then deposit their eggs onto the seabed. The eggs stick onto the substrate, forming dense layers. The total number of eggs laid per female varies from 21,000 to 47,000 . Males follow closely and release ribbons of milt which sink and fertilise the eggs. Herring spawn in multiple waves with repeat spawners spawning first, followed by first-time (recruit) spawners . This results in a build-up of egg masses consisting of different development stages. Hatching of the larvae occurs between 7 days to 3 weeks after spawning, depending on temperature. Cold water slows the development of embryos . Light to moderate winds and wave action induce larval hatching due to increased aeration of the spawn .
Larvae of spring-spawning herring hatch with large yolk-sac reserves. This allows them to grow until summer, when plankton is abundant, and they metamorphose into juveniles. Autumn-spawning larvae hatch after the productive summer feeding season. They are capable of surviving, without much growth, over extended periods in cooler, less productive waters until metamorphosing in spring . Larvae are planktonic (floating in the water) and remain near spawning grounds  or passively drift to nursery areas in the north and east of Scotland on the prevailing currents . Once mature, individuals will form large shoals that migrate between feeding grounds, spawning grounds and overwintering areas .
Banner image: Herring eggs on maerl. © Andy Jackson, SubSeaTV; reproduced with permission.
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