Eastern Baltic cod (Gadus morhua) has experienced a dramatic decline in biomass, distribution, size and condition over recent decades, related to a combination of fishing pressure and environmental stressors. Reduced parental size and condition are known to impact reproductive success, affecting egg quality, fertilization and hatching success and larval survival. However, how these reproductive traits vary throughout the spawning season at the population level remains poorly understood for this stock. To address this knowledge gap, this study investigated egg and larval characteristics of captive Eastern Baltic cod over an entire spawning season. Fish from three different size classes were kept in three separate tanks and allowed to spawn naturally. Eggs were collected daily from April to August, with peak spawning observed in early to mid-June. We assessed egg and larval size, deformities, thiamine concentration, daily survival and hatching success over time. Our findings revealed high variability in these reproductive traits both across the spawning season and between size groups, without consistent temporal trends. Notably, smaller fish produced the largest eggs with the highest total thiamine concentrations, which contrasts with previous studies linking larger egg size with larger females. At the same time, these eggs showed the lowest survival and hatching rates. The only consistent pattern across all tanks was a positive relationship between egg and larval size, though unrelated to survival. These results suggest higher than expected complexity of reproductive dynamics in Eastern Baltic cod and raise the question whether the stock has undergone a shift in reproductive strategy, possibly reflecting an increased allocation towards egg quality over quantity during the recent period of stock decline.

ABSTRACT Plankton monitoring often assumes spatial homogeneity, sampling only a few sites across large areas. However, environmental parameters such as salinity, temperature, and depth can vary strongly over short distances and can create multiple distinct ecological zones, which support different mesozooplankton assemblages. We investigated mesozooplankton assemblages in three Central and Eastern Baltic Sea bays to assess variability on different spatial scales: between and within bays. Our results revealed notable differences in assemblage composition both between and within bays, associated primarily with salinity and temperature gradients. Higher salinity conditions in Kappelshamnsviken were associated with greater abundances of the calanoid copepods Pseudocalanus sp. and Temora sp., whereas rotifers were more abundant in the less saline bays of Tvären and Baggensfjärden. Even within single bays, spatial differences of less than 1 km produced distinct assemblages. These findings demonstrate that assuming homogeneity can overlook important local variability. Incorporating multiple sampling locations at fine spatial scales is therefore essential for accurately capturing the full range of environmental niches and associated mesozooplankton assemblages in coastal monitoring programs.

The Eastern Baltic cod (Gadus morhua) stock has declined severely due to decades of overfishing, habitat degradation, and eutrophication. Restocking has been proposed as one potential measure to support recovery, but effective evaluation of such efforts requires reliable larval marking techniques. This study investigated three otolith marking methods; strontium immersion, alizarin complexone immersion (30 and 50 mg/L), and thermal marking, for newly hatched Eastern Baltic cod larvae. We assessed mark visibility as well as potential effects on larval survival, length, and deformities. Strontium produced clear and detectable marks, assessed using micro-XRF analysis, without adversely affecting larval development. Alizarin also created distinct marks at both concentrations, but the 50 mg/L treatment showed a possible tendency toward reduced survival and increased deformity severity. Thermal marking failed to produce visible otolith marks under light microscopy and was associated with lower survival and elevated developmental issues. This study identifies safe and effective marking methods for early life stages, offering crucial guidance for developing traceable and biologically sound stocking strategies for the vulnerable Eastern Baltic cod stock. The results highlight strontium and low-dose alizarin as effective and biologically safe options for marking larvae, with strontium emerging as the most promising method for large-scale application. These tools provide practical means to evaluate restocking success in future management strategies.

Fish communities in the coastal Baltic Sea are currently monitored using passive fishing gears, such as gillnets. In recent years, eDNA approaches have gained popularity for fish detection thanks to their non-invasive nature, faster processing, and more precise identification, although such methods have been underutilized in comprehensive fish community assessments in the Baltic Sea. This study reports fish diversity using eDNA metabarcoding within different depth profiles for the first time in temperature-stratified coastal bays in the Baltic Sea, while also offering some comparison with traditional net-based approaches. Comparing samples above and below the thermocline revealed exclusivity in fish species at both depths, emphasizing the importance of vertical sampling in capturing a comprehensive understanding of fish distribution patterns in such systems. Results indicated that eDNA captured more fish taxa per sample compared to gillnet sampling, with similar or higher fish diversity, although variations occurred between bays. This study highlights the importance of incorporating eDNA metabarcoding, alongside traditional survey methods, to aid assessment of fish communities in aquatic environments.

Decades of overfishing and unsustainable management, together with habitat degradation and eutrophication, depleted the cod (Gadus morhua) stocks in the Baltic Sea. Accompanying severe oxygen deficiency and decreased salinity in their spawning grounds restricted successful spawning today to the Bornholm basin, resulting in decreased recruitment. In order for the species to recover, several different measures are required and proposed, among others restocking. We therefore investigated the possibility of producing laboratory-reared cod larvae acclimatized to the current environment in the Baltic Sea. For this, cod were reared from newly fertilized eggs to non-feeding yolk-sac larvae, testing the effect of different salinity reduction treatments during early development on mortality, hatching success, and neutral buoyancy. The results show that a sudden ambient salinity decrease after hatching has no strong effect on survival or hatching (around 60% and 95%, respectively), while it decreased neutral buoyancy of larvae from 18 to minimum 12.5 psu. Lowest buoyancy was reached in treatments with a salinity change in the early egg stage. Gradual salinity decrease starting early in the egg stage yielded to significantly increased mortality and reduced hatching success, but also lowest buoyancy of 12 psu. We showed that a decrease of ambient salinity enables the production of yolk-sac cod larvae with reduced buoyancy, which are potentially better acclimatized to survive in current environmental conditions in the Baltic Sea.

Many coastal ecosystems, such as coral reefs and seagrass meadows, currently experience overgrowth by fleshy algae due to the interplay of local and global stressors. This is usually accompanied by strong decreases in habitat complexity and biodiversity. Recently, persistent, mat-forming fleshy red algae, previously described for the Black Sea and several Atlantic locations, have also been observed in the Mediterranean. These several centimetre high mats may displace seagrass meadows and invertebrate communities, potentially causing a substantial loss of associated biodiversity. We show that the sessile invertebrate biodiversity in these red algae mats is high and exceeds that of neighbouring seagrass meadows. Comparative biodiversity indices were similar to or higher than those recently described for calcifying green algae habitats and biodiversity hotspots like coral reefs or mangrove forests. Our findings suggest that fleshy red algae mats can act as alternative habitats and temporary sessile invertebrate biodiversity reservoirs in times of environmental change.

With its geographically isolated location and geological history, the Mediterranean Sea harbors well-known biodiversity hotspots, such as Posidonia oceanica seagrass meadows. Recently, long-living mats formed by the fleshy red alga Phyllophora crispa have been described to be associated with a high diversity of sessile invertebrates in the Tyrrhenian Sea. One of the key taxa among these sessile invertebrates are bryozoans: their abundance, diversity, and spatial distribution in P. crispa mats represent a gap in scientific knowledge. Thus, we conducted a pilot study on bryozoan assemblages associated with P. crispa mats around Giglio Island (Tuscan Archipelago, Italy) in 2018, followed by a comparative study on four sites distributed around the island in the subsequent year, 2019. We compared these findings to bryozoan abundance and diversity on P. oceanica shoots and leaves during the second expedition. The findings revealed more than 46 families, with a significantly higher number of taxa identified in P. crispa mats (33) than in P. oceanica meadows (29). The Shannon diversity index was similar between P. crispa and P. oceanica shoots, while Pielou’s evenness index was lower in P. crispa mats. The most abundant families reported across all habitats were Crisiidae, Aetidae, and Lichenoporidae; but the most abundant family on P. crispa was Chlidoniidae (Chlidonia pyriformis). The assemblages associated with P. crispa differed among sites, with higher abundances but lower diversity on the exposed southernmost site. The total bryozoan abundance was significantly higher on P. crispa (average 2.83 × 106 ± 1.99 × 106 colonies per m2 seafloor) compared to P. oceanica meadows (average 0.54 × 106 ± 0.34 × 106 colonies per m2 seafloor). Our results show a high diversity of bryozoans on P. crispa thalli compared to P. oceanica meadows, which was consistent throughout the study. These findings confirm the value of the red alga-generated habitat for associated bryozoans and may have implications for future biodiversity assessments and conservation measures.

In the Mediterranean Sea, the fleshy red alga Phyllophora crispa forms dense mats of up to 15 cm thickness, mainly located on rocky substrates in water depths below 20 m. Because of the observed density of these mats and some first observations, we hypothesize that P. crispa is a yet undescribed ecosystem engineer that provides a multitude of ecological niches for associated organisms along small-scale environmental gradients. Therefore, we conducted an in-situ pilot study in the Western Mediterranean Sea to assess potential influence of the algae mats on the key environmental factors water movement, temperature and light intensity. We comparatively and simultaneously measured in P. crispa mats, in neighboring Posidonia oceanica seagrass meadows, on neighboring bare rocky substrates without algae mats, and in the directly overlying water column. We used several underwater logging sensors and gypsum clod cards. Findings revealed that P. crispa significantly reduced water movement by 41% compared to the overlying water column, whereas water movement was not affected by P. oceanica meadows and bare rocky substrates. Surprisingly, P. crispa increased the water temperature by 0.3°C relative to the water column, while the water temperature in P. oceanica and on bare rocky substrates was reduced by 0.5°C. Light intensity inside the red algae mats was reduced significantly by 69% compared to the water column. This was similar to measured light reduction of 77% by P. oceanica . These findings highlight the strong influence of the dense red algae mats on some key environmental factors. Their influence is obviously similar or even higher than for the well-known seagrass ecosystem engineer. This may be a factor that facilitates associated biodiversity similarly as described for P. oceanica .

Brown shrimp, Crangon crangon, inhabit highly productive sandy and muddy grounds of the southern North Sea. The stomachs of the shrimp contain variable and often high numbers of sediment grains. The function of sediment grains inside the stomach and the purpose of their ingestion are only poorly understood. We tested in laboratory experiments whether sediment and associated organic material complement the natural food of C. crangon or if sand grains may be used by the shrimp to support trituration and maceration of ingested food. The shrimp showed no notable preference for sediment with natural organic content over sediment with reduced organic content, limited ingestion of sediment upon starvation, and no additional uptake of sand grains after feeding. Instead, C. crangon took up sediment only while feeding on regular food, suggesting that sand grains are not ingested intentionally but rather incidentally as a side effect of hasty gobbling. This conclusion is supported by the highly variable uptake of sand grains among individuals. Under experimental conditions, sand grains from sediments do not seem to have a crucial function in food processing and digestion in brown shrimp.

Raw data set for manuscript entitled "Fleshy red algae mats act as reservoir for sessile invertebrate biodiversity".