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The spawning bed characteristics of the Baltic herring (Clupea harengus membras L.) were investigated during the spawning season in the Airisto Inlet (Northern Baltic Sea) in SW Finland. Within the area, 46 transects were investigated by scuba diver; 37 transects were randomly selected and 9 were known to be spawning beds. Data on the substratum and vegetation were analysed by principal component analysis and discriminant analysis which revealed 10 parameters describing the spawning beds. Two groups were identified; the first contained most of the known spawning beds as well as those resembling them in many parameter values. These beds were situated near the deepest part of the area and contained broad and rich vegetation zones growing on hard bottoms. Herring eggs were not found on all of these beds, but analysis showed they were all potentially suitable for spawning. The other group consisted of sites with bottoms covered by soft sediments, even near the shore line, with the vegetation zone becoming narrower and poorer in plant species. Only two spawning beds were found in this group. [ABSTRACT FROM PUBLISHER]

A collapse of the littoral fish populations was found in the middle archipelago zone near the Island of Seili (northern Baltic Sea) when fish populations were studied in summer 1996 in a comparable way withearlier studies done in the early 1970s and 1980s. A similar comparison was made in the outer archipelago (the islands off the island of Uto), but no evident change was found. Reasons for the decline in littoral fish stocks are discussed, but no single-cause effect can be pointed out. Among possible explanations is a substantial increase of eutrophication in the middle archipelago together with increasing ferry traffic which both cause structural changes in the littoral environment of the study area. [ABSTRACT FROM AUTHOR]

During 2014-2019, the prevalence of Corynosoma spp., a parasite species in great cormorants ( Phalacrocorax carbo spp.) and in Baltic herring ( Clupea harengus membras ), was studied in the Archipelago and the Bothnian Seas of the northern Baltic Sea. These results suggest that cormorants may act as a definitive host for these acanthocephalan parasites. Adults were more infected with the parasites than juveniles, which could be due to their larger size. A lower prevalence of Corynosoma spp. in juveniles may be because smaller cormorants eat smaller fish that have less parasites. We found that the most abundant corynosoma species in both the Baltic herring and cormorants were Corynosoma semerme , whereas only a few individuals of C. strumosum and only one C. magdaleni were found. The prevalence of corynosoma in herring increased from 2014 to 2018, and individuals in the Bothnian Sea were infected less frequently than herring in the Archipelago Sea. Results also showed that infected herring individuals were generally larger than non-infected individuals, which could be explained by their size and their feeding habits. Currently, the changing environment of the Baltic Sea may cause an effect on the herring making them more susceptible to infections. Our results, therefore, emphasize the importance of the regular monitoring of infections and the parasite-host relationships in the Baltic Sea., Competing Interests: I declare that there are no conflicts of interest regarding the publication of this manuscript., (© 2023 The Authors.)

Due to heavy anthropogenic influence and variation of the environmental conditions in the Baltic Sea, reproductive disorders are becoming a major environmental concern. We show here an increasing prevalence of gonadal malformations in the Baltic herring (Clupea harengus membras), a key species of the Baltic ecosystem and important in commercial fishery. During 1987-2014, the spawning herring population in the Archipelago Sea (AS) (North Baltic Sea, Finland) was monitored annually and analyzed for gross morphology of the gonads [total number (n) of analyzed fish = 38 284]. Four different types of malformations were repeatedly found and named as asymmetric, rudimentary, segmented, and branched gonads, but also hermaphroditic gonads and miscellaneous (unidentified) disorders were recorded. In 2013, additional samplings (n of fish analyzed = 541) showed similar malformations in herring from the Bothnian Sea. In some gonad types, histological examination revealed disintegration of seminiferous tubules and hyperplasia of the interstitial tissue. In 2014, the overall prevalence of malformations was still relatively low in the AS (frequency = 0-3.4 %; n = 750) and had apparently minimal effect on population recruitment. However, an increasing trend in the time-series (GLM; F = 32.65; p < 0.001) and a significantly higher prevalence in the Bothnian Sea (frequency = 0.7-5.0 %; n = 541; χ (2) = 6.24; p < 0.05) suggest that gonadal malformations may become a new threat for fish in the Baltic Sea. The observed gonad atrophies may be due to environmental endocrine disruption; however, also other explanations may exist and potential explanations are discussed.

Substantial ecological changes occurred in the 1970s in the Northern Baltic during a temporary period of low salinity (S). This period was preceded by an episodic increase in the rainfall over the Baltic Sea watershed area. Several climate models, both global and regional, project an increase in the runoff of the Northern latitudes due to proceeding climate change. The aim of this study is to model, firstly, the effects on Baltic Sea salinity of increased runoff due to projected global change and, secondly, the effects of salinity change on the distribution of marine species. The results suggest a critical shift in the S range 5-7, which is a threshold for both freshwater and marine species distributions and diversity. We discuss several topics emphasizing future monitoring, modelling, and fisheries research. Environmental monitoring and modelling are investigated because the developing alternative ecosystems do not necessarily show the same relations to environment quality factors as the retiring ones. An important corollary is that the observed and modelled S changes considered together with species' ranges indicate what may appear under a future climate. Consequences could include a shift in distribution areas of marine benthic foundation species and some 40-50 other species, affiliated to these. This change would extend over hundreds of kilometres, in the Baltic Sea and the adjacent North Sea areas. Potential cascading effects, in coastal ecology, fish ecology and fisheries would be extensive, and point out the necessity to develop further the "ecosystem approach in the environmental monitoring".

In studying the relevance of the predation hypothesis as an explanation for the vertical migration of zooplankton two predictions were tested: 1) The egg sacs of Eurytemora are sufficient to cause different predation pressure in food selection experiments with visually hunting planktivores as predators and ovigerous vs. non-ovigerous Eurytemora females as prey. 2) If copepods avoid predation by vertical migration, there should occur differences in the vertical distribution pattern of ovigerous and non-ovigerous females according to selective predation. The results showed that, when ovigerous and non-ovigerous females were presented to predators, a significant preference for ovigerous females was found in high densities of prey. No preference was found in experiments with males vs. non-ovigerous females and at low copepod densities. In a field study we found that non-ovigerous females distributed throughout the water column preferred the deeper parts. Ovigerous females were also abundant in deeper waters but almost totally avoided the surface layer above 20 m. Thus our results support the predation hypothesis when the adaptive value of vertical migration is considered.On the basis of our results we deduced testable predictions on the evolutionary effects of predation: 1) In heavily foraged communities there is strong coevolution among the prey to resemble each other in terms of which are critical in selective predation. 2) If the carrying of an egg sac is considered as parental care, it is likely that parental care occurs more seldom in environments with high predation.