# Scale-dependent foraging behaviour and habitat associations of two sympatric marine top predators

## TL;DR

This study shows how two types of seals adjust their foraging strategies at different scales in the North Sea, revealing new insights into their habitat use and competition.

## Contribution

The paper introduces the first use of hidden Markov models to detect multi-scale foraging behavior in marine predators.

## Key findings

- Both grey and harbor seals exhibit nested broad-scale and focused foraging behaviors.
- Accounting for scale increased inferred foraging activity by up to 46% in harbor seals.
- Foraging strategies vary regionally and species-specifically across the North Sea.

## Abstract

Theoretical research has considered how animals should optimise foraging strategies to maximise fitness, adapting search scale to exploit different habitats and minimise competition. Empirical studies have described multi-scale area-restricted search (ARS) strategies for some species, but the physical and biological mechanisms underpinning such behaviour are rarely studied.

Our objectives were to quantify the presence, prevalence, and habitat associations of scale-dependent foraging for two sympatric seal species, accounting for regional variation across the seascape.

We analyse a GPS telemetry dataset of 116 grey (Halichoerus grypus) and 325 harbour seals (Phoca vitulina) tracked throughout the North Sea. We test the existence of multi-scale ARS, comparing hidden Markov models (HMMs) with two ARS states against more conventional HMMs (one ARS state). We quantify regional variation and examine the scale-dependence of foraging habitat associations using post-hoc “use-encounter” models.

Both species exhibited nested broad-scale and focussed ARS. Accounting for scale resulted in increases of up to 25% and 46% in inferred ARS for grey and harbour seals respectively. The prevalence and habitat associations of different ARS scales varied in a regional species-specific manner.

We demonstrate the first application of HMMs to capture multi-scale ARS from animal-borne tracking data. Overlooking scale-dependence may mask individual variation and underestimate ARS, with consequences for ecological understanding and conservation applications. We hypothesise that seals employ different search scales for different habitats, competition levels and/or prey types. We call for further research to elucidate the prevalence and ecological significance of this phenomenon in other aquatic predators.

The online version contains supplementary material available at 10.1007/s10980-025-02281-z.