Investigating fine-scale spatio-temporal predator-prey patterns in dynamic marine ecosystems: a functional data analysis approach

1. Spatial management of marine ecosystems requires detailed knowledge of spatio-temporal mechanisms linking physical and biological processes. Tidal currents, the main driver of ecosystem dynamics in temperate coastal ecosystems, influence predator foraging ecology by affecting prey distribution and ecology. The mechanistic links between tidal currents and how they influence predatorprey behaviour and interactions at a fine scale are poorly understood. 2. Studies of fine-scale changes in oceanography, prey and predator behaviour with tidal currents require repeated surveys of the same location over brief time-scales. Such data are highly temporally and spatially autocorrelated and require appropriate analytical tools. 3. We used functional data analysis (FDA), specifically functional principal component analysis (FPCA), to analyse repeated, fine-scale, survey data collected in the North Sea. FPCA was used to explore the relationship between the behaviour of an important North Sea prey species (sandeel Ammodytes spp.) and a vulnerable surface-foraging predator (black-legged kittiwake Rissa tridactyla) with fine-scale tidally driven changes in bio-physical characteristics (temperature stratification and maximumsubsurface chlorophyll concentration). 4. The FPCA indicated that sandeels were aggregated close to the surface at maximum ebb (ME) currents. Surface-feeding kittiwakes were also found in highest numbers during ME in locations of both high subsurface chlorophyll concentration and shallow sandeel aggregations. We suggest that the combination of a well-stratified water column with the movement of tidal currents over uneven topography results in surface aggregations of sandeels which kittiwakes exploit. 5. Synthesis and applications. Functional Data Analysis provides a useful tool for examining spatio-temporal patterns in natural ecosystems. In combination with fine-scale repeated survey design, we identified the importance of tide in driving prey behaviour and hence predator foraging behaviour. This has implications both for critical marine habitat identification for Marine Protected Area selection and for fisheries stock assessments. We therefore recommend that tidal aspects should be taken into account when designing marine surveys in temperate coastal ecosystems both to ensure the best identification of critical marine habitat and to improve the accuracy of fish stock assessments.