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Van alle zeeorganismen vormen sponzen de rijkste bron aan mariene natuurlijke producten inzetbaar als geneesmiddel. Ze worden ook wel de “darmen van de oceaan” genoemd omdat ze grote aantallen micro-organismen (zoals bacteriën) bevatten die het dier beschermen tegen vijanden. Aardig meegenomen is dat de stoffen die ze aanmaken ook in tal van geneeskundige toepassingen bijzonder nuttig blijken! Zo zijn er vandaag al twee bioactieve stoffen uit sponzen op de markt waarmee kanker bij mensen kan worden bestreden.

During the Second World War (1939-1945) herring fishing effort in the North Sea was greatly reduced. The traditional autumn herring fisheries by Belgian fishers on the Flemish banks around the spawning areas in the southern North Sea however, continued during WWII. Data presented here show a 10-fold increase in the Belgian ‘autumn spawning herring’ catches and a 5-fold increase in landings per unit of effort of Downs herring during WWII. It is argued that these increased catches are explained by a combination of factors including the sustained effect of a major increase in catch power, the effects of strong pre-WWII year classes and the effects of decreased fishing mortality during the 6-years cessation in fishing on the herring feeding grounds in the central North Sea and in the English Channel. The data and information presented here represent a period in which a gap exists in time-series of herring catches from the North Sea and in the stock assessment time series which currently go back to 1947. The results are also discussed in the context of the recovery of the autumn spawning Downs herring. The recovery of stock components and the spatial diversity in the spawning areas of the herring stock in the North Sea is a crucial aspect of fisheries management that aims to enhance stock resilience to changes in environmental conditions and direct effects of anthropogenic activities.

Plaice (Pleuronectes platessa, L.) and dab (Limanda limanda, L.) are among the most abundant flatfishes in the north-eastern Atlantic region and the dominant species in shallow coastal nursery grounds. With increasing pressures on commercial flatfish stocks in combination with changing coastal environments, better knowledge of population dynamics during all life stages is needed to evaluate variability in year-class strength and recruitment to the fishery. The aim of this research was to investigate the complex interplay of biotic and abiotic habitat components influencing the distribution, density and growth of plaice and dab during the vulnerable juvenile life stage and to gain insight in spatial and temporal differences in nursery habitat quality along the west coast of Ireland. Intraspecific variability in plaice diet was observed at different spatial scales and showed a link with condition, recent growth and morphology. This highlights the effect of food availability on habitat quality and the need to consider small scale variation when attempting to link habitat quality to feeding, growth and condition of juvenile flatfish. There was evidence of trophic, spatial and temporal resource partitioning between juvenile plaice and dab allowing the co-existence of morphologically similar species in nursery grounds. In the limited survey years there was no evidence that the carrying capacity of the studied nursery grounds was reached but spatial and interannual variations in fish growth indicated fluctuating environments in terms of food availability, predator densities, sediment features and physico-chemical conditions. Predation was the most important factor affecting habitat quality for juvenile plaice and dab with crab densities negatively correlated to fish condition whereas shrimp densities were negatively associated with densities of small-sized juveniles in spring. A comparison of proxies for fish growth showed the advantage of Fulton’s K for routine use whereas RNA:DNA ratios proved less powerful when short-term environmental fluctuations are lacking. This study illustrated how distinct sets of habitat features can drive spatial variation in density and condition of juvenile flatfish highlighting the value of studying both variables when modeling habitat requirements. The habitat models generated in this study also provide a powerful tool to predict potential climate and anthropogenic impacts on the distribution and condition of juveniles in flatfish nurseries. The need for effective coastal zone management was emphasized to ensure a sustainable use of coastal resources and successful flatfish recruitment to the fishery.

The growing importance of defining high quality nursery grounds for juvenile fish as a result of changing nearshore environments has resulted in the use of different indicators for fish condition and growth. This field study aimed to compare and evaluate a morphometric (Fulton’s K condition factor) and biochemical (RNA:DNA ratio) condition index in juvenile flatfish to assess habitat quality in four sandy beach nursery grounds in Galway Bay, Ireland. Juvenile plaice (Pleuronectes platessa), and dab (Limanda limanda) were surveyed monthly from June - September in 2008 and 2009 using a 1.2 m beam trawl. Sediment samples were collected using Van Veen grabs for substrate characterization and physicochemical measures and predator densities were recorded. Gut content analyses revealed recent feeding history of both species. Generalized Additive Modelling (GAM) was used to investigate non-linear relationships between condition indices and habitat characteristics and diet features. The results of the study revealed a lack of correlation between the morphometric and biochemical condition indices of plaice and dab. Fish growth, indicated by RNA:DNA ratios and Fulton’s K, displayed considerable spatio-temporal variability. Site-related patterns in Fulton’s K for plaice and dab were consistent between years whereas RNA:DNA ratios displayed annual and interspecific variability among nursery habitats. This indicates a higher sensitivity of RNA:DNA ratios to short-term environmental fluctuations which is not apparent in Fulton’s K measurements of juvenile flatfish. Density of predators, sediment grain size and salinity were the most important predictors of both condition indices. Diet features did not contribute to the explained variability in the models predicting RNA:DNA ratios whereas certain prey groups significantly improved Fulton’s K of plaice and dab. These findings aid understanding of the biological and physical mechanisms promoting fast growth and high survival which will help to identify high quality nursery areas for juvenile plaice and dab.

This study describes the feeding habits of plaice Pleuronectes platessa and dab Limanda limanda during early juvenile development and relates differences between nursery grounds and sampling years to spatial and temporal variabilities in macrobenthic prey availability. The main prey taxa of both species were copepods, bivalves, amphipods, polychaetes and oligochaetes and size-related variation in diet was found. Despite considerable similarity in the prey items, differences in food composition between the two species were observed and spatial variability in diet confirmed their opportunistic feeding behaviour. A high degree of dietary overlap was found in June and decreased steadily throughout the season. The prey composition in the guts of both species largely reflected the composition of the main macrobenthic taxa in the sediment. The overall data suggest that resources were not limiting in the littoral sandy nursery areas in the west of Ireland and no indications were found that exploitative competitive forces upon the benthic prey assemblages occurred between P. platessa and L. limanda. Feeding success, Fulton's K condition and dietary overlap, however, showed spatial and temporal variations, and were probably affected by the availability and density of macrobenthic prey.

On sandy beach nursery grounds along the west coast of Ireland, 0 year-group turbot Scophthalmus maximus were found to consume six types of crustaceans, in addition to polychaetes. The 0 year-group brill Scophthalmus rhombus fed almost exclusively on mysids, even though nine taxonomic prey groups were identified in the sediment across the investigated beaches. Both species avoided non-motile organisms such as gastropods and bivalves, which were present in high abundances in the sediment and their growth and condition was not significantly related to the quantity or type of prey consumed, temperature or salinity. A high incidence of feeding was detected for both species over the duration of the study, suggesting that food was not limiting on west of Ireland nursery grounds. Temporal partitioning of settlement was detected between S. maximus and S. rhombus, indicating that inter-specific competition for food does not occur between these two flatfish species on west of Ireland nursery grounds.

European flounder (Platichthys flesus. L.) is a euryhaline flatfish species which can actively migrate towards and cope with low salinity environments. A laboratory experiment was undertaken to analyse the effect of salinity on condition and growth of metamorphosing European flounder. The working hypothesis was that flounder, which preferentially settle in low salinity habitats, would display accelerated development and/or enhanced growth and condition at lower salinities. The fish used in the experiment were in the late stages of metamorphosis. At the end of the 21 day laboratory rearing period no significant difference in ontogenetic development was found between exposures (salinity of 0, 10, 20 and 30). No significant differences in somatic growth rate, somatic condition or standard length were observed between treatments. There was no correlation between RNA:DNA ratio and somatic condition. Contrary to expectations, mean RNA:DNA ratios (measure of short-term well being) tended to increase with salinity and were significantly higher in the 30 salinity exposure compared to the 0 salinity exposure. The working hypothesis was, therefore, rejected. The results demonstrate that laboratory observations can fail to capture the complex ecological interactions at play in field environments. The preference for low salinity environments may be driven by other environmental factors such as predator/competition avoidance and food supply.

Many juvenile flatfish species are dependent on shallow water coastal nurseries for obtaining food, shelter and rapid growth during this vulnerable life stage. The quality of nursery habitat is commonly assessed by measuring the growth of juvenile fish. Higher growth rates result in successive improvements in feeding, predator avoidance and ultimately survival. The present study compares 3 different growth indices for juvenile plaice and dab in nursery habitats in Galway Bay, Ireland. Sampling was carried out in the summers of 2008 and 2009 using a beam trawl for the collection of flatfish which were immediately stored in liquid nitrogen. The three indices examined were: condition derived from length weight residuals, otolith size at age and nutritional condition expressed as the RNA: DNA ratio. Significant temporal and spatial variation in condition was noted and linked with variation in biotic and abiotic factors. Growth rates also revealed differences between flatfish species. The value of the three indices in representing different facets of the history of juvenile plaice and dab growth is discussed in relation to their use as indices of nursery ground quality.

This project aims to develop a model of juvenile flatfish habitat requirements using field surveys and controlled experiments. Distribution and abundance of juvenile flatfish is assessed in relation to biotic and abiotic habitat features within the greater Galway Bay area. GIS and statistical models are used to create habitat maps and characterise “essential fish habitat” for these species. Habitat features associated with optimal growth are identified using otolith growth rates and morphometric condition indices. To date the analysis has revealed that small-scale variation in density and condition of plaice and dab is mostly explained by physical factors (depth, organic content, substrate, temperature). Predator/prey abundances and the interaction between physical and biological factors also affect habitat choice and condition.