Beschreibung: Understanding the drivers behind fluctuations in fish populations remains a key objective in fishery science. Our predictive capacity to explain these fluctuations is still relatively low, due to the amalgam of interacting bottom-up and top-down factors, which vary across time and space among and within populations. Gaining a mechanistic understanding of these recruitment drivers requires a holistic approach, combining field, experimental and modelling efforts. Here, we use the Western Baltic Spring-Spawning (WBSS) herring (Clupea harengus) to exemplify the power of this holistic approach and the high complexity of the recruitment drivers (and their interactions). Since the early 2000s, low recruitment levels have promoted intense research on this stock. Our literature synthesis suggests that the major drivers are habitat compression of the spawning beds (due to eutrophication and coastal modification mainly) and warming, which indirectly leads to changes in spawning phenology, prey abundance and predation pressure. Other factors include increased intensity of extreme climate events and new predators in the system. Four main knowledge gaps were identified related to life-cycle migration and habitat use, population structure and demographics, life-stage specific impact of multi-stressors, and predator–prey interactions. Specific research topics within these areas are proposed, as well as the priority to support a sustainable management of the stock. Given that the Baltic Sea is severely impacted by warming, eutrophication and altered precipitation, WBSS herring could be a harbinger of potential effects of changing environmental drivers to the recruitment of small pelagic fishes in other coastal areas in the world.

Beschreibung: Eutrophication-driven harmful algal blooms (HABs) can have secondary effects on larval fishes that rely on estuaries as nurseries. However, few studies worldwide have quantified these effects despite the global rise in eutrophication. This study presents a novel approach using biochemical body condition analyses to evaluate the impact of HABs on the growth and body condition of the larvae of an estuarine resident fish. Recurrent phytoplankton blooms of Heterosigma akashiwo occur in the warm-temperate Sundays Estuary on the southeast coast of South Africa. The response in body condition and assemblage structure on larval estuarine roundherring (Gilchristella aestuaria) was measured in conjunction with bloom conditions, water quality and zooplanktonic prey and predators. Larvae and early juveniles were sampled during varying intensity levels, duration and frequency of hypereutrophic blooms. This study demonstrated that extensive HABs could significantly impact larval roundherring, G. aestuaria, by decreasing larval nutritional condition and limiting their growth, resulting in poor grow-out into the juvenile phase. Poor condition and growth may likely affect recruitment success to adult populations, and since G. aestuaria is an important forage fish and zooplanktivore, poor recruitment will hold consequences for estuarine food webs.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 2014-2019: Since September 2014, temperature, salinity, pH (only 2014-2015) and oxygen data were additionally logged in 10-minutes intervals at the GEOMAR pier (54°19'48.8N 10°08'59.6E) (AANDERAA oxygen sensor 3835 & SEABIRD SBE 37-SI MicroCAT CT(D)). The sensor system is mounted to a floating platform so that a continuous depth of 1 m is guaranteed at every time point. Oxygen data were corrected for salinity, temperature and depth following the manual for Aanderaa Optodes using the salinity and temperature measurements from the SEABIRD SBE 37-SI MicroCAT CT(D) sensor. pH was also corrected for salinity, temperature and depth following Martz et al. (2010). After cleaning and other re-boots of the sensor package, temperature, salinity and oxygen data tend to deviate from true values. Hence, 60 minutes of data after any re-boot (after sensor servicing with re-deployment, data download or power failure) were deleted. Furthermore, salinity data lower than 8 and pH data lower than 5 and larger than 10 were removed from the data set as these values were identified as outliers. On May 22nd 2018 as well as on May 26th (22:00) til 27th (15:00) 2019 oxygen data were identified as outliers and removed from the data set. The logged oxygen data were plotted against titration data (doi:10.1594/PANGAEA.930974) to check for drifts in the optode's data. But no drift pattern could be detected and the fit of the regression was very good R2adj. = 0.673, p 〈 0.001)

Beschreibung: 2005-2019 CTD (48M, Sea & Sun Technology GmbH, Trappenkamp, Germany) measurements and water samples at the surface (0 m depth), at 7 m and 18 m depth were taken biweekly between 2005 and 2019 at the “Wittlingskuhle” a bit off the GEOMAR pier in the Inner Kiel Fjord (N 54°19.69, E 10°09.06). The oxygen concentration of these water samples was measured by the Winkler- iodometric titration method (Winkler 1888) in mg/L and are converted to oxygen-saturation values by correcting for temperature, salinity and pressure.

Beschreibung: Ovigerous females were collected by scraping from inner Kiel Fjord in July -August 2017 and kept under control constant temperature room (~20°C) until egg hatching. Newly hatched larvae were reared under different salinity treatments in order to estimate their salinity tolerance,. Experiments were conducted until larvae reached the megalopae stage (last larval stage) or died, The results of such study could give us a primary outlook on the dispersal ability of H. takanoi larvae along the Baltic Sea, and insights about the life cycle of H. takanoi specifically in the south-western Baltic.

Beschreibung: Crabs were collected from inner Kiel Fjord by scraping in November 2018, in order to study their tolerance to salinity and consequently their ability to establish a population along the Baltic Sea, in addition, their feeding rates were estimated under laboratory condition. Experiments were conducted in constant climate temperature room (~12 °C) at GEOMAR during December 2018.

Beschreibung: Salinity in Kiel Fjord was recorded on mostly-weekly cruises of the research vessel FK Polarfuchs from 2005 to 2018 using a CTD48M probe (Sea and Sun technologies, Trappenkamp, Germany) at a station in front of the GEOMAR Pier (“Wittlingskuhle”, Position: 54°19'69 N, 10°09'06 E). These measurements are important for better understanding the salinity as one of the important abiotic factor determine the reproduction, establishment and has an effect on many species in the Baltic Sea in general and the fjord specifically.