EU request on overview of the possible spawning grounds of the central Baltic herring stock and the seasonality of the spawning

Spring-spawning herring in the Baltic

The spawning seasons and grounds of spring-spawning herring in the Baltic Sea are well-documented, showing a southwest-northeast gradient in timing. Spawning begins in the southwestern regions from February to May at temperatures of 4°C and salinity of 4. Moving eastwards along the southern coasts, the spawning period shifts to March through June, with temperatures ranging from 7–14°C. In the northern Hanö Bight, spawning occurs between April and June at temperatures of 5.5–15°C, while in the Trosa Archipelago, it takes place from May to July at temperatures of 4–15°C. In Finnish waters, spawning starts in May and continues until midsummer at temperatures around 6–7°C.

Spawning grounds are generally located in shallow littoral zones, with depths varying up to 20 meters depending on the area. Substrates include rocks, mixed gravel and sand, and various forms of vegetation, including red algae (e.g., Furcellaria, Ceramium, Polysiphonia), seaweed, and submerged plants, suggesting a wide range of suitable habitats.

Autumn-spawning herring in the Baltic

The decline of the Baltic autumn-spawning herring has resulted in significant gaps in knowledge and high uncertainties about this ecotype. Once the dominant ecotype in the Baltic Sea during the early to mid-20th century, autumn-spawning herring supported important fisheries. However, from the 1950s to the 1970s, the population dramatically declined, likely due to overexploitation. Today, stock assessments primarily focus on spring-spawning herring, assuming that the autumn-spawning component is negligible.

The decline of autumn-spawning herring has created significant challenges in obtaining accurate, current information about their spawning behavior. Most of the existing knowledge is based on older studies from over 50 years ago, when the population was more abundant. With the autumn-spawning herring now at critically low levels, direct research has become difficult, increasing uncertainty. Existing literature suggests that spawning grounds may be found along steep coastal slopes exposed to currents or intense vertical mixing, with substrates such as sand, gravel, and stones. Offshore deeper waters are also indicated in other regions like the North Sea. However, in the Baltic, spawning grounds have primarily been inferred from indirect observations, such as mature or spent fish distributions and small school locations. Multiannual ichthyoplankton surveys in the Bornholm Basin (2002–2019) have shown potential significance for autumn-spawning herring, with patterns in larval abundance and distribution. Despite this, the lack of modern research tools limits full understanding of this ecotype.

Analyses of commercial samples

The findings from commercial catch data align closely with those from the literature review. Both indicate that herring primarily spawn in spring, between April and May, with some spawning extending into summer. The timing of spawning varies geographically, with northern areas typically experiencing later spawning.

Both sources agree that spring spawners are most commonly found in shallow coastal areas, while autumn spawners are more often located in offshore areas. The commercial catch data reveals a weak signal for autumn spawning, with a slight increase in activity observed in ICES Subdivision 26 during the 2000s. This may be due to challenges in detecting autumn spawning, such as changes in fishing patterns, low fish abundance, or the short spawning period.

The literature review and digitized maps identify specific autumn spawning locations, including areas around the island of Bornholm and the central Baltic Proper, where herring spawn between August and November. Historical records further support the presence of autumn spawning in multiple locations, including the northern Baltic Sea, highlighting the spatial extent of this phenomenon. However, autumn spawning is not always captured by commercial catch data, emphasizing gaps in available data and the need for more targeted studies.