**Background:** Ocean warming is a major driver of change in marine ecosystems, and fish embryos are particularly vulnerable due to limited thermal tolerance and behavioral responses. Atlantic herring (Clupea harengus) is a socio-economically important species with multiple spawning components, but the winter-spawning Downs herring in the eastern English Channel (EEC) is understudied compared to Baltic or Norwegian stocks. This study aimed to evaluate the impact of temperature on embryonic traits and explore potential parental effects on offspring quality.
**Methods:** Wild ripe herring were collected from commercial fishers in the EEC on December 8 and December 17, 2021. Twenty females (mean standard length 25.5 ± 1.4 cm, mean wet weight 202.4 ± 36.9 g) and 60 males were used for artificial fertilization. Eggs from each female were fertilized using three randomly chosen males and incubated at three temperatures: 8°C, 10°C (current conditions), and 14°C (future scenario under SSP5-8.5). Three identical recirculating systems maintained temperature, with daily monitoring of pH, salinity, and oxygen. Key embryonic traits assessed included fertilization rate (at 1 dpf), mean egg diameter at eyed stage, survival rate at eyed stage, hatching rate, developmental rate (half-days to eyed stage and hatching), larval total length, myotomal depth, dry weight, yolk sac volume, and developmental stage at hatching. Maternal traits measured included total length, standard length, wet weight, Fulton's condition factor (CF), gonadosomatic index (GSI), hepatosomatic index (HSI), age (via otoliths), von Bertalanffy growth parameters (TL∞ and K), oocyte diameter and ellipticity, and muscle RNA:DNA ratio. Statistical analyses used linear mixed models and generalized linear mixed models with family, batch, and incubator as random factors.
**Key Results:** Temperature had a statistically significant effect on eight of eleven functional traits. Fertilization rate decreased with increasing temperature (χ² = 50.80, df = 2, p < 0.001), with inter-family variation from 0 to 40.37%. Mean egg diameter at eyed stage decreased with temperature (F = 28.82, df = 2, p < 0.001; range 1.47–2.08 mm). Survival rate at eyed stage was significantly higher at 10°C compared to 8°C or 14°C (χ² = 19.93, df = 2, p < 0.001; range 0–63.11% across families). Hatching rate was lower at 10°C and 14°C compared to 8°C (χ² = 13.94, df = 2, p < 0.001; range 0–94.11%). Development was faster at higher temperatures for both eyed stage (χ² = 26.25, df = 2, p < 0.001) and hatching (χ² = 32.97, df = 2, p < 0.001). More larvae reached stage 1b at 14°C compared to control temperatures (χ² = 11.31, df = 2, p < 0.05). Yolk sac volume was larger at control temperatures (F = 3.04, df = 2, p = 0.05). No significant differences were found for larval length (F = 1.42, p = 0.24), myotomal depth (F = 2.08, p = 0.13), or dry weight (F = 2.56, p = 0.08) at hatching. Multiple regression models showed that maternal traits explained 31–70% of variance in embryonic traits. Fertilization rate was positively associated with standard length (β = 10.10, p < 0.001), condition factor (β = 5.92, p < 0.001), and TL∞ (β = 2.48, p < 0.001), and negatively with HSI (β = -0.32, p < 0.05) and 14°C (β = -0.53, p < 0.001). Eyed-stage survival was higher in larger females (β = 25.45, p < 0.001) and at 10°C (β = 0.64, p < 0.001), and negatively affected by K (β = -2.49, p < 0.001). Mean egg diameter and hatching rate were both higher in older females (β = 0.18, p < 0.001 and β = 3.04, p < 0.001, respectively).
**Clinical Implications:** This study provides the first experimental evidence of temperature impacts on Downs herring embryos, showing that warming to 14°C negatively affects fertilization, hatching, and yolk reserves while accelerating development. The strong inter-family variability suggests potential adaptive capacity within the stock, but the overall negative effects could reduce recruitment success. The finding that older, larger females produce higher quality eggs supports the BOFFFF (big old fat fecund female fish) hypothesis and underscores the importance of maintaining age-structured populations for fisheries management. These results are particularly relevant given that North Sea herring has experienced below-average recruitment since 2015, and the Downs component now contributes approximately 25% of total spawning stock biomass. Further research should investigate multi-stressor effects (e.g., acidification, hypoxia), temperature variability, and transgenerational effects to better predict climate change impacts on this stock.