Options for modulating intra-specific competition in colonial pinnipeds: the case of harbour seals (Phoca vitulina) in the Wadden Sea.

Rory P Wilson,Nikolai Liebsch,Victoria J Hobson,Andrew Bone,Agustina Gómez-Laich,William P Kay,Ursula Siebert

doi:10.7717/peerj.957

Abstract

Colonial pinnipeds may be subject to substantial consumptive competition because they are large, slow-moving central place foragers. We examined possible mechanisms for reducing this competition by examining the diving behaviour of harbour seals (Phoca vitulina) after equipping 34 seals (11 females, 23 males) foraging from three locations; Rømø, Denmark and Lorenzenplate and Helgoland, Germany, in the Wadden Sea area with time-depth recorders. Analysis of 319,021 dives revealed little between-colony variation but appreciable inter-sex differences, with males diving deeper than females, but for shorter periods. Males also had higher vertical descent rates. This result suggests that males may have higher overall swim speeds, which would increase higher oxygen consumption, and may explain the shorter dive durations compared to females. Intersex variation in swim speed alone is predicted to lead to fundamental differences in the time use of three-dimensional space, which may help reduce consumptive competition in harbour seals and other colonial pinnipeds.

Highlights

IntroductionCompetition is a major driver explaining patterns of space use in animals (MacArthur, 1958; Pianka, 2011) and it is considered severe in central-place foraging (sensu Orians & Pearson, 1979) and colonial breeders, such as seabirds or pinnipeds, because the density of animals around the breeding site leads to correspondingly high local pressure on resources (e.g., see Birt et al, 1987; Lewis et al, 2006; Gaston, Ydenberg & Smith, 2007; Breed, Bowen & Leonard, 2013)
Competition is a major driver explaining patterns of space use in animals (MacArthur, 1958; Pianka, 2011) and it is considered severe in central-place foraging and colonial breeders, such as seabirds or pinnipeds, because the density of animals around the breeding site leads to correspondingly high local pressure on resources
Dive durations During dives, both sexes spent a similar amount of time underwater (LMM, Sex effect F1,31 = 2.4, p = 0.1) (Fig. 2B) with no differences observed between the three locations (LMM, Site effect F1,31 = 0.07, p = 0.8)

Summary

Introduction

Competition is a major driver explaining patterns of space use in animals (MacArthur, 1958; Pianka, 2011) and it is considered severe in central-place foraging (sensu Orians & Pearson, 1979) and colonial breeders, such as seabirds or pinnipeds, because the density of animals around the breeding site leads to correspondingly high local pressure on resources (e.g., see Birt et al, 1987; Lewis et al, 2006; Gaston, Ydenberg & Smith, 2007; Breed, Bowen & Leonard, 2013). These relate primarily to different individuals taking either different types of prey (Forero et al, 2002; Ishikawa & Watanuki, 2002; Garthe et al, 2007) or the same prey in different spaces (e.g., Wilson, 2010). Both of these are fostered by differences in area use (Thompson et al, 1998), depths (Ishikawa & Watanuki, 2002; Laich et al, 2012) and even the timing of foraging bouts (Harris et al, 2013). Though, that spatial and temporal segregation in resource use does not necessarily guarantee a reduction in competition (Wilson, 2010)

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Discussion

Conclusion