Overview of possible spawning grounds of the central Baltic herring stock and the seasonality of spawning (Ad hoc report)

Atlantic herring (Clupea harengus) plays a crucial ecological and economic role in the Baltic Sea, but its populations are vulnerable to overfishing and environmental changes, requiring careful management for sustainability. The Central Baltic herring stock, spanning ICES subdivisions 25–29 and 32, is a key resource for regional fisheries, with substantial economic importance. However, stock assessments indicate a long-term decline in spawning biomass, with fishing pressure fluctuating above sustainable levels in recent decades, highlighting the need for continued management efforts to ensure its sustainability.

Herring is a highly adaptable species with diverse physiological traits that allow it to thrive across various salinity levels, contributing to its broad geographic distribution and ecological success. The species exhibits a unique reproductive strategy among clupeids, with ecotypes capable of spawning during different seasons and utilising highly specific spawning grounds where benthic eggs are laid and attached to a variety of substrates. Combined with high fecundity and strong site fidelity, the herring’s life history supports a high reproductive potential and enhances population resilience to environmental fluctuations. The Central Baltic herring stock consists of multiple genetically distinct spawning components with variations in migration patterns, growth, and reproductive traits, though their specific contributions to stock dynamics remain unclear. This complexity, combined with mixing with adjacent stocks and uncertainties in historical spawning distributions, poses challenges for assessing genetic diversity, resilience, and long-term productivity.

The literature review conducted in this study provides a synthesis of information on the spatial and temporal patterns of Central Baltic herring stock spawning, incorporating findings from various studies. The goal is to summarize current knowledge about the distribution and timing of herring spawning in different areas of the stock distributions. The Central Baltic herring stock is divided into smaller regions based on hydrological or bathymetric characteristics. The spawning time, depth, and substrate preferences of the spring and autumn herring ecotypes are reviewed.

In this study, historical maps representing potential spawning grounds were digitized and overlaid to facilitate direct comparisons, synthesis and consolidation of information scattered across literature. The maps used were sourced from studies employing various methods and data types, including direct mapping from inventory or monitoring data, interviews, literature-based mapping, and modelling techniques. As a result, the precision and resolution of the digitized data vary, and the sources must be interpreted with caution, taking into account their respective strengths and limitations.

The review and mapping of herring spawning grounds in the Baltic Sea reveal distinct patterns for spring and autumn spawners. Spring-spawning herring are found in shallow coastal areas, with spawning times varying from February to July depending on the region. Habitat quality, including vegetation and substrate type, is a key factor in determining spawning site selection. Key spawning substrates include rocks, gravel, sand, and various types of algae. Many well-documented spawning grounds for spring-spawning herring have been found in the littoral zones throughout the entire distribution area of the studied stock.

Autumn-spawning herring, once dominant in the Baltic, have significantly declined, leading to gaps in current knowledge. These herring typically spawn in deeper waters from August to November. Historical records and indirect evidence suggest important autumn spawning grounds around Bornholm and the Central Baltic Proper. The decline has made direct observation challenging, and much of the existing data is based on older literature and indirect methods.

The analysis of commercial catch data shows a strong seasonal peak in the proportion of spawning fish during spring but no clear signal during autumn. Results indicate that spring-spawning herring predominantly spawn between April and June, with some activity extending into the summer months. The catch data aligns well with the literature, indicating a southwest-northeast gradient in spawning times. Both sources agree that spring spawners are most commonly found in shallow coastal areas. Further, the presence of spawning fish in the autumn catches is low but, despite the scarcity of data, they appear consistently distributed in the southern parts of the stock distribution area, aligning with the specific spawning locations indicated in the literature (e.g., grounds around Bornholm and the Central Baltic Proper). While some signals of autumn spawning are identified, they are not always captured by commercial catch data, highlighting gaps and the need for more targeted studies.

The limitations of using digitized maps from various sources are highlighted, including differences in resolution, methodology, and temporal coverage. The accuracy of these maps cannot be independently validated, and some information may be outdated. More precise and up-to-date studies are needed to better understand the current fine scale distribution and conditions of herring spawning grounds in the Baltic Sea.