Thermal windows supporting survival of the earliest life stages of Baltic herring (Clupea harengus)

Projecting climate-driven changes in marine systems will require knowledge on how thermal windows affect the vital rates of key species. To examine the potential, direct effect of climate-driven warming on southwest Baltic herring, we quantified the survival, development, and biochemical condition of embryos (eggs and yolk-sac larvae) at ten temperatures between 2.9 and 21.7 degrees C. Viable hatch was highest from 7 to 13 degrees C, <20% at 2.9 degrees C and 0% at 21.7 degrees C. Between 5 and 19 degrees C, increasing temperature (T) decreased the time to 50% hatch (H-t, h,): H-t = 4461.9 x T - 1.24 (r(2) = 0.98, p < 0.0001). Using degree-days [degrees d = T (degrees C) x age (d)] could normalize some (but not all) thermal effects. Most hatching occurred 90-120 degrees d post-fertilization, unfed larvae lost 0.33 mu g dry mass (DM)degrees d(-1), larvae did not survive >160 degrees d post-hatch. RNA-DNA ratios rapidly decreased between 50 and 80 degrees d post-hatch, whereas DNA x DM-1 increased throughout the yolk-sac phase and likely provides a stronger indicator of irreversible starvation. The critical, mixed feeding stage is likely 60-100 degrees d post-hatch. The broad thermal tolerance of herring embryos makes direct, negative effects of warming unlikely; however, a lack of common methods among studies makes it difficult to project how climate warming will affect embryos of different fish populations and species.