Description: Atlantic herring (Clupea harengus), an ecologically and economically important species in the northern hemisphere, shows pronounced seasonal migratory behaviour. To follow distinctive migration patterns over hundreds of kilometers between feeding, overwintering and spawning grounds, they are probably guided by orientation mechanisms. We tested whether juvenile spring-spawning Atlantic herring, caught in the western Baltic, use a sun compass for orientation just before they start leaving their hatching area. Fish were randomly divided into two groups, one of them clock-shifted 6 h backwards, to investigate whether they shift their orientation direction accordingly. Individual fish were placed in a circular bowl and their orientation was tested multiple times with the sun as a sole visual orientational cue. Our results show for the first time that juvenile Atlantic herring use a time-compensated sun compass during their migration. Their swimming direction was impaired, but still present, even when the sky was very cloudy, indicating additional orientation capabilities.

Description: Anthropogenic CO2 emissions cause a drop in seawater pH and shift the inorganic carbon speciation. Collectively, the term ocean acidification (OA) summarizes these changes. Few studies have examined OA effects on predatory plankton, e.g. Hydrozoa and fish larvae as well as their interaction in complex natural communities. Because Hydrozoa can seriously compete with and prey on other higher-level predators such as fish, changes in their abundances may have significant consequences for marine food webs and ecosystem services. To investigate the interaction between Hydrozoa and fish larvae influenced by OA, we enclosed a natural plankton community in Raunefjord, Norway, for 53 days in eight ≈ 58 m³ pelagic mesocosms. CO2 levels in four mesocosms were increased to ≈ 2000 µatm pCO2, whereas the other four served as untreated controls. We studied OA-induced changes at the top of the food web by following ≈2000 larvae of Atlantic herring (Clupea harengus) hatched inside each mesocosm during the first week of the experiment, and a Hydrozoa population that had already established inside the mesocosms. Under OA, we detected 20% higher abundance of hydromedusae staged jellyfish, but 25% lower biomass. At the same time, survival rates of Atlantic herring larvae were higher under OA (control pCO2: 0.1%, high pCO2: 1.7%) in the final phase of the study. These results indicate that a decrease in predation pressure shortly after hatch likely shaped higher herring larvae survival, when hydromedusae abundance was lower in the OA treatment compared to control conditions. We conclude that indirect food-web mediated OA effects drove the observed changes in the Hydrozoa – fish relationship, based on significant changes in the phyto-, micro-, and mesoplankton community under high pCO2. Ultimately, the observed immediate consequences of these changes for fish larvae survival and the balance of the Hydrozoa – fish larvae predator – prey relationship has important implications for the functioning of oceanic food webs.

Description: Highlights: • Microplastics were found both at the surface and at depth within the Baltic Sea. • Microplastic ingestion occurred within both G. morhua and S. sprattus. • Major inflows of water into the Baltic Sea altered G. morhua feeding patterns. • Increase in pelagic feeding in G. morhua increased ingestion of microplastics. This study documents how the abundance of microplastics (〈5 mm) in the Atlantic cod, Gadus morhua, relates to the changes of the fish diet during years with contrasting levels of anoxia for example following years of low or high major Baltic inflows (MBI). A MultiNet Maxi trawl and CTD were deployed annually to collect microplastic samples alongside oxygen, temperature, and salinity conditions. Microplastics were homogenously distributed both within the water column and across years. Gadus morhua diet shifted from dominantly benthic invertebrates (61 %) under oxygenated conditions to dominantly Sprattus sprattus (81 %) under anoxic conditions. The proportion of G. morhua with microplastics in their digestive tract increased when they fed on pelagic fish (38 %) versus on benthic invertebrates (15 %). The proportion of S. sprattus which ingested microplastics (~18 %) did not vary. As anoxia at depth is expected to increase due to climate change, microplastic ingestion by G. morhua will potentially increase.

Description: Atlantic herring (Clupea harengus), an ecologically and economically important species in the northern hemisphere, shows pronounced seasonal migratory behaviour. To follow distinctive migration patterns over hundreds of kilometers between feeding, overwintering and spawning grounds, they are probably guided by orientation mechanisms. We tested whether juvenile spring-spawning Atlantic herring, caught in the western Baltic, use a sun compass for orientation just before they start leaving their hatching area. Fish were randomly divided into two groups, one of them clock-shifted 6 h backwards, to investigate whether they shift their orientation direction accordingly. Individual fish were placed in a circular bowl and their orientation was tested multiple times with the sun as a sole visual orientational cue. Our results show for the first time that juvenile Atlantic herring use a time-compensated sun compass during their migration. Their swimming direction was impaired, but still present, even when the sky was very cloudy, indicating additional orientation capabilities.

Description: Sprat (Sprattus sprattus) is one of the most commercially exploited fish species in the Baltic Sea and expresses a pronounced seasonal migration pattern. Spawning takes place, among other places, in the Kiel Bight and Kiel Fjord in early summer. Juvenile sprat leave the nursery areas in late summer/early autumn to move to their feeding and overwintering grounds. What kind of orientation mechanisms sprat use for migration is not known yet. This study shows that juvenile sprat can use a time-compensated sun compass, heading towards the northeast, in the direction of their proposed overwintering grounds in Bornholm Basin. The sprats tested at the end of August oriented themselves in the predicted direction, whereas the sprats tested at the beginning of August only showed a random orientation. For the first time, this demonstrates the onset of migratory readiness in juvenile sprat, indicating the preparation for starting their migration.

Description: Highlights: • Mnemiopsis leidyi is capable of catching and digesting herring yolk-sac larvae. • Predation on herring larvae is decreasing with prey age and increasing with predator size. • Predation of M. leidyi on herring larvae is not affected by the presence of alternative natural prey represented by the copepod Acartia tonsa. • Substantial predation of M. leidyi on yolk-sac herring larvae may occur in the field, when both overlap spatially and temporarily. Western Baltic spring spawning herring (Clupea harengus, L.) is a commercially important fish stock currently suffering a strong decline. Larval survival is essential for stock recruitment and can be substantially decreased by predation. The comb jelly Mnemiopsis leidyi A. Agassiz, 1865, is a lobate ctenophore which is invasive to the Baltic Sea and a known ichthyoplankton predator. However, predation on herring larvae in the Baltic Sea by M. leidyi has not been studied since its initial establishment in 2006. To address this knowledge gap, we conducted feeding experiments to investigate (1) the predation capability of M. leidyi on herring yolk-sac larvae, and (2) the influence of larval age, (3) predator size and (4) the presence of alternative prey on the clearance rate of M. leidyi on herring yolk-sac larvae. Our results showed that M. leidyi exhibited the ability to capture and digest herring larvae. The clearance rates of M. leidyi on herring larvae decreased with larval age and increased with predator size, while the presence of alternative prey had no effect on clearance rates. This finding suggests that M. leidyi can efficiently consume herring yolk-sac larvae under laboratory conditions. However, further investigations using mesocosm or field studies are necessary to evaluate the potential impact of M. leidyi on the mortality and recruitment of herring yolk-sac larvae under Baltic Sea field conditions.

Description: To evaluate the influence of ocean acidification on predatory plankton, e.g. Hydrozoa and fish larvae as well as their interaction in complex natural communities, we deployed eight pelagic mesocosms for 53 days (Mai to July) in Raunefjord, Norway, and enclosed 60 m³ of local seawater containing a natural plankton community under post-bloom conditions. Four mesocosms were manipulated to simulate extreme pCO2 levels of 2069 µatm while the other four served as untreated controls. To investigate the interaction between Hydrozoa and fish larvae influenced by OA ee studied OA-induced changes at the top of the food web by following ≈2000 larvae of Atlantic herring (Clupea harengus) hatched inside each mesocosm during the first week of the experiment, and a Hydrozoa population that had already established inside the mesocosms. Organisms of both taxa inside and outside the mesocosms were measured over the course of the experiment in regular intervals. The data stems from 55µm and 500µm Apstein net hauls, subsequent microscopic analyses as well as carbon to nitrogen measurements. Under OA, we detected 20% higher abundance of hydromedusae staged jellyfish, but 25% lower biomass. At the same time, survival rates of Atlantic herring larvae were higher under OA (control pCO2: 0.1%, high pCO2: 1.7%) in the final phase of the study.

Description: To evaluate the influence of ocean acidification on predatory plankton, e.g. Hydrozoa and fish larvae as well as their interaction in complex natural communities, we deployed eight pelagic mesocosms for 53 days (Mai to July) in Raunefjord, Norway, and enclosed 60 m³ of local seawater containing a natural plankton community under post-bloom conditions. Four mesocosms were manipulated to simulate extreme pCO2 levels of 2069 µatm while the other four served as untreated controls. To investigate the interaction between Hydrozoa and fish larvae influenced by OA ee studied OA-induced changes at the top of the food web by following ≈2000 larvae of Atlantic herring (Clupea harengus) hatched inside each mesocosm during the first week of the experiment, and a Hydrozoa population that had already established inside the mesocosms. Organisms of both taxa inside and outside the mesocosms were measured over the course of the experiment in regular intervals. The data stems from 55µm and 500µm Apstein net hauls, subsequent microscopic analyses as well as carbon to nitrogen measurements. Under OA, we detected 20% higher abundance of hydromedusae staged jellyfish, but 25% lower biomass. At the same time, survival rates of Atlantic herring larvae were higher under OA (control pCO2: 0.1%, high pCO2: 1.7%) in the final phase of the study.

Description: To evaluate the influence of ocean acidification on predatory plankton, e.g. Hydrozoa and fish larvae as well as their interaction in complex natural communities, we deployed eight pelagic mesocosms for 53 days (Mai to July) in Raunefjord, Norway, and enclosed 60 m³ of local seawater containing a natural plankton community under post-bloom conditions. Four mesocosms were manipulated to simulate extreme pCO2 levels of 2069 µatm while the other four served as untreated controls. To investigate the interaction between Hydrozoa and fish larvae influenced by OA ee studied OA-induced changes at the top of the food web by following ≈2000 larvae of Atlantic herring (Clupea harengus) hatched inside each mesocosm during the first week of the experiment, and a Hydrozoa population that had already established inside the mesocosms. Organisms of both taxa inside and outside the mesocosms were measured over the course of the experiment in regular intervals. The data stems from 55µm and 500µm Apstein net hauls, subsequent microscopic analyses as well as carbon to nitrogen measurements. Under OA, we detected 20% higher abundance of hydromedusae staged jellyfish, but 25% lower biomass. At the same time, survival rates of Atlantic herring larvae were higher under OA (control pCO2: 0.1%, high pCO2: 1.7%) in the final phase of the study.

Description: To evaluate the influence of ocean acidification on predatory plankton, e.g. Hydrozoa and fish larvae as well as their interaction in complex natural communities, we deployed eight pelagic mesocosms for 53 days (Mai to July) in Raunefjord, Norway, and enclosed 60 m³ of local seawater containing a natural plankton community under post-bloom conditions. Four mesocosms were manipulated to simulate extreme pCO2 levels of 2069 µatm while the other four served as untreated controls. To investigate the interaction between Hydrozoa and fish larvae influenced by OA ee studied OA-induced changes at the top of the food web by following ≈2000 larvae of Atlantic herring (Clupea harengus) hatched inside each mesocosm during the first week of the experiment, and a Hydrozoa population that had already established inside the mesocosms. Organisms of both taxa inside and outside the mesocosms were measured over the course of the experiment in regular intervals. The data stems from 55µm and 500µm Apstein net hauls, subsequent microscopic analyses as well as carbon to nitrogen measurements. Under OA, we detected 20% higher abundance of hydromedusae staged jellyfish, but 25% lower biomass. At the same time, survival rates of Atlantic herring larvae were higher under OA (control pCO2: 0.1%, high pCO2: 1.7%) in the final phase of the study.