What Makes a Habitat a Home: Understanding Settlement and Recruitment Variation in European Sea Bass, Dicentrarchus labrax

Sea bass stocks in the UK are in decline as a result of increased fishing pressure and variable inter-annual recruitment. Recruitment variation is driven by survival in the early life stages; therefore, nursery habitats are thought to be able to stabilize recruitment through providing optimal growth conditions for juvenile fish. A thorough understanding of the factors that drive juvenile sea bass survival is needed, however, our understanding of what constitutes quality nursery habitat for juvenile sea bass is weak, with current knowledge based almost solely on saltmarshes. Juvenile sea bass were sampled using conventional seine and fyke nets across estuarine habitats, alongside dietary DNA metabarcoding to assess their distribution diet and condition, using measures of abundance, condition, stomach fullness, and diet. To determine whether the mechanism of larvae entering estuarine nurseries is an active or passive process the vertical distribution patterns of larval sea bass were compared across tidal cycles. Finally, over-winter survival was predicted based on energy budget modelling and temperature-dependent growth experiments, based on in-situ measurements of winter temperatures. Juvenile sea bass did not differentially select high tide habitats, but saltmarshes and sand provided increased foraging success. At low tide, however, sea bass were more abundant in complex habitat with lower foraging success. Diets mainly consisted of decapods and polychaete worms across habitats, but there was evidence of increased planktivory over mud. Larval sea bass did not show evidence of flood tide transport and likely rely on passive tidal forcing to migrate into estuaries, or they are trying to retain to deeper water. According to our models, winter thermal minima resulted in complete cohort loss in all scenarios on the East coast. The results of this study suggest that multiple habitats along the estuarine mosaic are important for juvenile sea bass at some point, and that a seascape approach to management is necessary, however, winter temperatures likely present a more extreme bottleneck to recruitment.