What makes a habitat a home? Habitat associations of juvenile European sea bass, Dicentrarchus labrax, in estuarine nurseries.

Selection of nursery habitats by marine fish, such as European sea bass (Dicentrarchus labrax), is poorly understood. Identifying and protecting the full range of juvenile nursery habitats is vital to supporting resilient fish populations and economically important fisheries. We examined how the condition, stomach fullness, and diet of juvenile European sea bass, along with their abundance, differ at high or low tide between the following estuarine habitats: saltmarsh, oyster reefs, shingle, sand, and mud edge habitats. Using a combination of fyke and seine netting we found no difference in sea bass abundance or condition across high-tide habitats, suggesting that rather than differentially selecting between them, juvenile sea bass use all available shallow habitats at high tide. Stomach fullness was significantly higher on saltmarsh and sand compared to mud, and thus these habitats may support better foraging. Dietary DNA metabarcoding revealed that sand and saltmarsh diets mostly comprised Hediste polychaetes, whereas zooplanktonic taxa dominated diets over mud. At low tide, sea bass abundance was highest in shingle and oyster reefs, where stomach fullness and condition were lowest. This may indicate a potential trade-off between using habitats for foraging and refuge. Although sea bass abundance alone does not capture productivity, the high abundance across all estuarine habitats at high tide suggests that it is important to consider the protection of a mosaic of interconnected habitats to support nursery functions rather than focus on individual habitat types.

Oil and gas platforms degrade benthic invertebrate diversity and food web structure.

Oil and gas exploitation introduces toxic contaminants such as hydrocarbons and heavy metals to the surrounding sediment, resulting in deleterious impacts on marine benthic communities. This study combines benthic monitoring data over a 30-year period in the North Sea with dietary information on >1400 taxa to quantify the effects of active oil and gas platforms on benthic food webs using a multiple before-after control-impact experiment. Contamination from oil and gas platforms caused declines in benthic food web complexity, community abundance, and biodiversity. Fewer trophic interactions and increased connectance indicated that the community became dominated by generalists adapting to alternative resources, leading to simpler but more connected food webs in contaminated environments. Decreased mean body mass, shorter food chains, and the dominance of small detritivores such as Capitella capitata near to structures suggested a disproportionate loss of larger organisms from higher trophic levels. These patterns were associated with concentrations of hydrocarbons and heavy metals that exceed OSPAR's guideline thresholds of sediment toxicity. This study provides new evidence to better quantify and manage the environmental consequences of oil and gas exploitation at sea.