Thermal conditions and age structure determine the spawning regularities and condition of Baltic herring (Clupea harengus membras) in the NE of the Baltic Sea

Abstract

Baltic herring (Clupea harengus membras) is a total spawner with a group-synchronous ovarian organization. Age polymodality in total spawners is considered an important factor in assuring that a strong population is sustainable under an intensive harvesting regime and different climatic conditions. In the present study, we investigated the seasonal and inter-annual variation in spawner age structure and the effect of preceding winter thermal conditions on the start of the herring spawning and larvae retention period. Herring spawning season in the Gulf of Riga starts up to six weeks later after colder winters compared to milder winters. Significantly older individuals dominated at the beginning of the spawning season, and thus herring mean age gradually decreased towards the end of the spawning season from 1999-2015. On an annual scale, this pattern was obvious after cold winters, while after mild winters the pattern did not continue, indicating a more homogenous maturation cycle and spawning period, despite the age and size of the herring population in mild winters. Further, herring condition factor was studied in relation to age and spawning season following different winter thermal conditions. Young, 2- and 3- year old first-spawning herring experienced significantly lower conditions after cold winters compared to older ages, indicating an age-dependent effect of preceding winter on herring maturation cycle, condition and spawning time.

Keywords: Age structure; Baltic herring; Condition factor; Preceding winter; Spawning time.

Figure 1. Rectangles showing the area of herring commercial catches and data sampling.

Rectangles showing the area of herring commercial catches and data sampling to characterize herring spawning. Stars denote stations of larval herring collection and water temperature measurements.

Figure 2. Gulf of Riga herring spawning (black bar) and larvae (grey bar) distribution seasons.

Gulf of Riga herring spawning (black bar) and larvae (grey bar) distribution seasons in 1999–2015. Red dots denote the time when water temperatures exceed 17 °C, which is considered critical for normal embryonic development (Ojaveer, 1981).

Figure 3. The linear regression between the start of herring spawning season and sum of mean monthly winter air temperature.

The linear regression between the start of herring spawning season and sum of mean monthly winter air temperature in 1999–2015. Error bars = 1 s.e.

Figure 4. Dynamics of spawner biomass and abundance of age 4-10+.

The temporal dynamics of the Gulf of Riga herring spawning stock biomass, Spawning stock biomass (A) and age 4-10+ individuals abundance (B) in spawning stock in 1999–2015 (ICES, 2018).

Figure 5. The temporal dynamics of Baltic spring spawning herring mean age during the spawning season.

The temporal dynamics of Baltic spring spawning herring mean age during the spawning season. Each data point in particular calendar week (cw) denote average age for 1999–2015. Error bars = 1 s.e.

Figure 6. Box plots of the Fulton condition factor (K) of young (2–3 year) and old (4 − 10 + year) herring after cold and warm winter (25% and 75% percentiles of winter air temperatures over the years in 1999–2015).

Vertical boxes denote quartiles, the line inside the box median, whiskers 10th and 90th percentiles, and dots show extreme values. Different letters indicate p < 0.05.

Figure 7. Box plots of the Fulton condition factor (K) of young (2–3 year) and old herring (4 − 10 + year) in early (prior cw 20) and late (after cw 21) spawning season.

Box plots of the Fulton condition factor (K) of young (2–3 year) and old (4 − 10 + year) herring after cold and warm winter (25% and 75% percentiles of winter air temperatures over the years in 1999–2015). Vertical boxes denote quartiles, the line inside the box median, whiskers 10th and 90th percentiles, and dots show extreme values. Different letters indicate p < 0.05.