Spatial variation in a top marine predator's diet at two regionally distinct sites.

Martha Gosch,Cian Luck,Michelle Cronin,Mark Jessopp,Emer Rogan,William Hunt,Heather M Patterson

doi:10.1371/journal.pone.0209032

Abstract

In ecological studies it is often assumed that predator foraging strategies and resource use are geographically and seasonally homogeneous, resulting in relatively static trophic relationships. However, certain centrally placed foragers (e.g. seals) often have terrestrial sites for breeding, resting, and moulting that are geographically distinct, and associated with different habitat types. Therefore, accurate estimations of predator diet at relevant spatial and temporal scales are key to understanding energetic requirements, predator-prey interactions and ecosystem structure. We investigate geographic variation in the diet of grey seals (Halichoerus grypus), a relatively abundant and widely distributed central place forager, to provide insights into geographic variation in resource use. Prey composition was identified using scat samples collected over concurrent timescales and a multivariate approach was used to analyse diet from two contrasting habitats. Regional differences in prey assemblages occurred within all years (2011–2013) and all seasons (ANOSIM, all p<0.05), apart from in winter. Telemetry data were used to identify core foraging areas and habitats most likely associated with scat samples collected at the two haul-out sites. Regional differences in the diet appear to reflect regional differences in the physical habitat features, with seals foraging in deeper waters over sandy substrates showing a higher prevalence of pelagic and bentho-pelagic prey species such as blue whiting and sandeels. Conversely, seals foraging in comparatively shallow waters had a greater contribution of demersal and groundfish species such as cephalopods and flatfish in their diet. We suggest that shallower waters enable seals to spend more time foraging along the benthos while remaining within aerobic dive limits, resulting in more benthic species in the diet. In contrast, the diet of seals hauled-out in areas adjacent to deeper waters indicates that either seals engage in a more pelagic foraging strategy, or that seals can spend less time at the benthos, resulting in comparatively more pelagic prey recovered in the diet. The substantial differences in prey assemblages over a small spatial scale (<300 km) demonstrates the importance of using regionally-specific diet information in ecosystem-based models to better account for different trophic interactions.

Highlights

Foraging is one of the most important components of individuals’ reproductive success and survival [1]
The majority of prey species in the diet from both sites were rarely observed in scat samples (< 15%F), and when present were typically observed in low proportions (P < 15%) (Fig 2)
As grey seals act as central place foragers, their diet likely reflects the available prey adjacent to haul-outs

Summary

Introduction

Foraging is one of the most important components of individuals’ reproductive success and survival [1]. In free-ranging marine mammals, the energetic costs required during foraging and subsequent prey digestion and assimilation are high [3, 4]. Animals with high-energy requirements have the potential to considerably impact local prey populations and often play important roles in the structure and functioning of communities [5, 6]. Prey behaviour and physical habitat structure may influence predator foraging behaviour [7]. Dynamic physical components of habitat such as tides and upwelling zones, and spatial complexity within the marine environment, dictates the structure of multispecies communities. To understand how top predators and their populations respond to changing ecological and environmental conditions, and to comprehend their functional roles in the marine ecosystem, information on foraging ecology, including diet, is necessary [10,11,12]

Objectives

Methods

Results

Conclusion