The Gulf of Riga stock of Baltic herring (Clupea harengus membras L.) has been maintained through several management tools. One such tool is the restriction of vessels’ main engine power (<221 kW). This restriction was implemented in the early 1990s and is based on the vessel types available in the area and on the assumption that the gear size used in trawl fishery depends on the vessel size (power). In the current study, we compared vessels with different engine powers using the same gears currently allowed in the gulf, to identify whether vessel power had any relation to catch structure. The results showed that engine power did not explain the differences in catch structure, which were more dependent on season and depth of water. Easing the power restriction of the trawl vessels in the Gulf of Riga will most likely not have a major negative impact on the sustainable management of the herring population. However, vessels with higher engine power should not use larger trawl gear than is currently used in the gulf.

Maximum sustainable yields and maintaining the intraspecific biodiversity are the aim in fish stock management. Here populations-based management would be the preferred practice. Yet available stock discrimination methods result in establishing the conventional assessment/management units as a compromise. The Baltic herring (Clupea harengus membras L.) exhibits remarkable geographical variability, forming up to12 local populations. Due to reasons mentioned above, only two distinct local populations (Gulf of Riga, (GoR) herring and Gulf of Bothnia herring), are treated as separate assessment and management units. The paper describes the annual workflow of the management process, collection of the biological information from commercial catches and from the research surveys. Management history of GoR herring has revealed a number of challenges. Acoustic estimates are the only fisheries-independent data applied in GoR herring stock assessment. The study highlights the need for methodological changes what are expected to allow enhancing the quality of stock estimates, serving as the basis for the management decisions. We show that while before climate - driven regime shift (prior- 1990s) recruitment was determined by the winter temperature, affecting survival of Young of the year, a new major productivity bottleneck is in increasing spring-summer water temperatures during hatching and larval first-feeding stage.

Hydroacoustic studies using acoustic measurements combined with biological data obtained from trawl hauls remain a challenging task. The main purpose of the present study is to identify sources of uncertainty and compare the calculation methods for a mixed pelagic fish community of Baltic Sea. Most of the calculation methodologies use conventional statistical borders to split the study area, ignoring the possible natural population distribution areas. We compared the proposed “nearest haul” and the “subdivision average” with the conventional “rectangle average” on the Estonian regular acoustic surveys in the northeast Baltic Sea. The abundance estimates of the target species – European sprat Sprattus sprattus (Linnaeus 1758) and Baltic herring Clupea harengus membras (Linnaeus 1758) – with the “nearest haul” were significantly different from the currently used method, which indicates a strong effect of the allocation method. Our results indicate a need for a large scale comparative study to consider alternative calculation methodologies.