Thermal tolerance of herring (Clupea harengus) early embryogenesis in relation to temporal temperature dynamics in the Baltic Sea

Early embryonic development is perhaps the most vulnerable phase in the life cycle of fish with supposedly the narrowest thermal tolerances and may represent an ontogenetic bottleneck, particularly in view of global warming. We investigated the thermal window for early embryogenesis in herring from the Baltic Sea and integrated experimental data to assess implications of warming for herring reproduction in shallow coastal waters. Eggs of six females were incubated at ∼2-20°C to determine survival rates until end of gastrulation. Using Bayesian parameter estimation, optimal temperature ( T o pt =9.5°C) and thermal optimum range where survival exceeds 90% ( T range =3.0-15.1°C) were determined and validated with literature data on viable hatch. Our results suggest that early development shows a broader temperature tolerance than subsequent stages. Maternal effects such as body length, weight, and condition factor, on embryo survival were minimal overall but greater variations in survival were recorded at critical temperatures, suggesting that maternal traits play a more significant role under extreme conditions. Additionally, the annual dimension of the optimal spawning period was estimated along a two-decade time series of in situ data from a major spawning area. Here, results indicate a broadening of the optimal spawning period over the years correlating with increased field abundance of herring larvae. The simultaneous decline in recruitment over the same period indicates that stages after early embryogenesis are experiencing major mortality bottlenecks. Projections under future warming scenarios indicate that while lethally high temperatures are unlikely soon, the optimal spawning window may occur earlier in the season and shorten in duration, potentially increasing the risk of larval mortality e.g., due to temporal mismatch with suitable feeding conditions.