ALBERT

Description: Due to atmospheric accumulation of anthropogenic CO2 the partial pressure of carbon dioxide (pCO2) in surface seawater increases and the pH decreases. This process known as ocean acidification might have severe effects on marine organisms and ecosystems. The present study addresses the effect of ocean acidification on early developmental stages, the most sensitive stages in life history, of the Atlantic herring (Clupea harengus L.). Eggs of the Atlantic herring were fertilized and incubated in artificially acidified seawater (pCO2 1260, 1859, 2626, 2903, 4635 μatm) and a control treatment (pCO2 480 μatm) until the main hatch of herring larvae occurred. The development of the embryos was monitored daily and newly hatched larvae were sampled to analyze their morphometrics, and their condition by measuring the RNA/DNA ratios. Elevated pCO2 neither affected the embryogenesis nor the hatch rate. Furthermore the results showed no linear relationship between pCO2 and total length, dry weight, yolk sac area and otolith area of the newly hatched larvae. For pCO2 and RNA/DNA ratio, however, a significant negative linear relationship was found. The RNA concentration at hatching was reduced at higher pCO2 levels, which could lead to a decreased protein biosynthesis. The results indicate that an increased pCO2 can affect the metabolism of herring embryos negatively. Accordingly, further somatic growth of the larvae could be reduced. This can have consequences for the larval fish, since smaller and slow growing individuals have a lower survival potential due to lower feeding success and increased predation mortality. The regulatory mechanisms necessary to compensate for effects of hypercapnia could therefore lead to lower larval survival. Since the recruitment of fish seems to be determined during the early life stages, future research on the factors influencing these stages are of great importance in fisheries science.

Description: Highlights: • Belt Sea cod, plaice and flounder differentiate in their specific egg densities. • Ontogenetic egg density increase in stage IV cod eggs elevates modeled egg mortality. • Drift model indicates retention in western Baltic in cod and flatfish yolk sac larvae. • No eastward transport to Arkona Sea or Bornholm Sea until end of yolk sac stage. • Ambiguity in flounder egg density could reflect more complex population structure. Abstract: Vertical distribution is an important feature of pelagic fish eggs and yolk sac larvae impacting their survival and dispersal, especially in heterogeneous and highly variable estuarine environments like the Baltic Sea. Egg densities determining the vertical distribution pattern were experimentally ascertained for cod (Gadus morhua), plaice (Pleuronectes platessa) and flounder (Platichthys flesus) from the western Baltic Sea. Plaice eggs floated at lower mean (±standard deviation) density range (1.0136 ± 0.0007 g cm−3) compared to cod (1.0146 ± 0.0009 g cm−3) and flounder eggs (1.0160 ± 0.0015 g cm−3), which floated on the highest density level. In flounder egg diameter was significantly related to egg density and in cod a weak correlation could be found between egg dry weight and density. All other relationships between female size, egg size, egg dry weight and egg density were not significant for any of the species. Available egg density data for Baltic Sea cod, plaice and flounder are summarized considering ICES subdivisions and stock management units. A hydrodynamic drift modeling approach was applied releasing drifters in the Belt Sea continuously from December to May, covering the species’ spawning seasons. The model implemented experimentally derived egg density ranges and included ontogenetic egg density modifications for cod eggs, increasing egg density from a late egg development stage to first hatch. A drifter was removed from the model, i.e. considered dead, when its initially prescribed density value exceeded the density range available at the temporally resolved geographical positions along the drift trajectories. Highest survival occurred during releases in April and May but no cohorts survived if they were drifted east into the central Arkona Basin or the central Baltic Sea, irrespective of whether a major Baltic Inflow (1992/1993) or a stagnation-year (1987/1988) was simulated. The dispersal characteristics of the surviving yolk sac larvae of all three species reflected retention within the Belt Sea or northwards transport through the Great Belt into the Kattegat and partly into the Skagerrak. There was no successful transport to more eastern Baltic areas past a hypothetical line from the island of Moen (Denmark) to Kap Arkona on Rügen Island (Germany).

Description: The high mortality during fish early life stages is a major bottleneck in aquaculture. Therefore, the establishment of methods to prevent and control diseases, to ensure efficient growth and to reach maximal survival rates is mandatory to optimize the productivity. A promising solution can be the early activation of the immune system by administration of probiotics as nutritional supplements. In our study we assess the effect of the probiotic candidate Bacillus subtilis on the innate and adaptive immune response of juvenile European sea bass (Dicentrarchus labrax). Therefore, Artemia nauplii were used as live carriers to feed B. subtilis to 3-month-old sea bass over a period of 2 weeks. Subsequently, the juveniles were fed another week without administering B. subtilis in order to estimate the bacterial mucus-binding ability. During the course of the experiment, we evaluated direct effects on the cellular immune response by fluorescence-activated cell sorting analysis and on survival. As a next step we will determine profiles of immune gene expression. To estimate cellular stress, the expression level of metabolism- and stress-related genes will be measured. Furthermore, the RNA/DNA ratio as an indicator of growth will be analysed.