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Following general life history theory, immediate reproductive investment (egg mass × fecundity/body mass) in oviparous teleosts is a consequence of both present and past environmental influences. This clarification questions the frequent use of season-independent (general) fecundity formulas in marine fish recruitment studies based on body metrics only. Here we test the underlying assumption of no lag effect on gametogenesis in the planktivorous, determinate-fecundity Atlantic herring ( Clupea harengus ) displaying large plasticity in egg mass and fecundity, examining Norwegian summer-autumn spawning herring (NASH), North Sea autumn-spawning herring (NSAH), and Norwegian spring-spawning herring (NSSH). No prior reproductive information existed for NASH. Compared with the 1960s, recent reproductive investment had dropped markedly, especially for NSAH, likely reflecting long-term changes in zooplankton biography and productivity. As egg mass was characteristically small for autumn spawners, although large for spring spawners (cf. different larval feeding conditions), fecundity was the most dynamic factor within reproductive investment. For the data-rich NSSH, we showed evidence that transient, major declines in zooplankton abundance resulted in low fecundity over several subsequent seasons, even if Fulton's condition factor (K) turned high. Temporal trends in K slope ( K on total length) were, however, informative. These results clarify that fecundity is defined by ( i ) dynamics of primary (standing stock) oocytes and ( ii ) down-regulation of secondary oocytes, both processes intimately linked to environmental conditions but operating at different timescales. Thus, general fecundity formulas typically understate interannual variability in actual fecundity. We therefore argue for the use of segmented fecundity formulas linked to dedicated monitoring programs.

Buoyancy acting on plankton, i.e. the difference in specific gravity between plankton and the ambient water, is a function of salinity and temperature. From specific gravity measurements of marine fish eggs salinity appears to be the only determinant of the buoyancy indicating that the thermal expansions of the fish egg and the ambient seawater are equal. We analyze the mechanisms behind thermal expansion in fish eggs in order to determine to what extent it can be justified to neglect the effects of temperature on buoyancy. Our results confirm the earlier assumptions that salinity is the basic determinant on buoyancy in marine fish eggs that, in turn, influence the vertical distributions and, consequently, the dispersal of fish eggs from the spawning areas. Fish populations have adapted accordingly by producing egg specific gravities that tune the egg buoyancy to create specific vertical distributions for each local population. A wide variety of buoyancy adaptations are found among fish populations. The ambient physical conditions at the spawning sites form a basic constraint for adaptation. In coastal regions where salinity increases with depth, and where the major fraction of the fish stocks spawns, pelagic and mesopelagic egg distributions dominate. However, in the larger part of worlds' oceans salinity decreases with depth resulting in different egg distributions. Here, the principles of vertical distributions of fish eggs in the world oceans are presented in an overarching framework presenting the basic differences between regions, mainly coastal, where salinity increases with depth and the major part of the world oceans where salinity decreases with depth. We show that under these latter conditions, steady-state vertical distribution of mesopelagic fish eggs cannot exist as it does in most coastal regions. In fact, a critical spawning depth must exist where spawning below this depth threshold results in eggs sinking out of the water column and become lost for recruitment to the population. An example of adaptation to such conditions is Cape hake spawning above the critical layer in the Northern Benguela upwelling ecosystem. The eggs rise slowly in the onshore subsurface current below the Ekman layer, hence being advected inshore where the hatched larvae concentrate with optimal feeding conditions.

Hypoxia [O 2 < 2.0 mL L -1 (87 μ mol kg -1 )] and severely hypoxic water masses [O 2 < 0.5 mL L -1 (21.8 μ mol kg -1 )] are increasing in coastal marine ecosystems due to eutrophication and warming. Here, we investigate the response of the suboxic-tolerant endemic fish, Sufflogobius bibarbatus , to variations in the thermal and oxygen environment, as well as to predation pressure, using 22 yr worth of satellite and in situ data. We show that environmental variation and predation pressure affect the goby population, which has expanded over the last decade while that of horse mackerel has contracted. These changes co-occurred with a general warming in the north and central shelf areas (north of 24.5°S). Spring warming positively affected both goby and hake abundances, but not the horse mackerel, suggesting different responses to surface temperature. The goby habitat contracted when predators were abundant, particularly in the north, which is the fringe of its distributional area. The implications of the differential tolerance of gobies and their predators for climate variations are discussed.