Habitat Suitability for Juvenile Flatfish of the Inner Danish Waters: Section for Ecosystem Based Marine Management

Coastal fish play important roles in ecosystem functioning in addition to their disproportionately large contribution to fisheries. By inhabiting coastal areas, fish are exposed to a wide range of human activities over and above their direct exploitation. This thesis illustrates the extent of these indirect anthropogenic impacts by reviewing evidence of human activities effects on exploited, coastal fish species from the northeast Atlantic, where ~92% of those species were impacted in at least one life-history stage.

Furthermore, there is evidence of anthropogenic impacts acting on 78% of the juvenile life-history stages as they utilise coastal habitats. With a clear need to understand the role that coastal habitats play in provisioning juveniles for replenishing exploited fish populations, this thesis continues by quantifying habitat suitability for juvenile flatfish of the inner Danish waters and establishing a method to trace their contributions to fished adult populations.

Two approaches are used to quantify habitat suitability, one obtains fish data and environmental data from a targeted survey of near-shore habitats, the other utilises existing surveys of juvenile fish and pairs it with modelled environmental data. Juvenile habitat suitability models are presented describing juvenile density and growth responses to changes in the physical environment.

The results of both approaches are critically assessed and used to make interpolative maps of predicted habitat suitability across the inner Danish waters. The species-specific trends described in these models and predictions are discussed with reference to previous empirical findings from across their ranges.

These two studies propose potential mechanisms creating the observed patterns of density and growth and discuss them in the context of current theories of density dependence and population connectivity. The fourth work included in this thesis, documents the efficacy of otolith chemistry to differentiate between contiguous coastal juvenile habitats of the inner Danish waters, to enable future work on the connectivity between juvenile and adult habitats in this area as well as the neighbouring North and Baltic Seas.

Together, the four key studies that comprise the majority of this thesis provide both information for preliminary advice on the importance of juvenile habitats of the inner Danish waters, and a foundation for future work to further develop juvenile habitat models and describe the life-history connectivity of important fisheries species.