Top‐down pressure on a coastal ecosystem by harbor seals

Geert Aarts,Jan Jaap Poos,Peter Reijnders,Jessica Schop,Roger Kirkwood,Tobias Van Kooten,Adriaan D Rijnsdorp,Sophie Brasseur,Evert Mul,Ingrid Tulp

doi:10.1002/ecs2.2538

Abstract

AbstractHistoric hunting has led to severe reductions of many marine mammal species across the globe. After hunting ceased, some populations have recovered to pre‐exploitation levels and may have regained their prominent position as top predator in marine ecosystems. Also, the harbor seal population in the international Wadden Sea grew at an exponential rate following a ban on seal hunting in 1960s, and the current number ~38,000 is close to the historic population size. Here we estimate the impact of the harbor seal predation on the fish community in the Wadden Sea and nearby coastal waters. Fish remains in fecal samples and published estimates on the seal's daily energy requirement were used to estimate prey selection and the magnitude of seal consumption. Estimates on prey abundance were derived from demersal fish surveys, and fish growth was estimated using a Dynamic Energy Budget model. GPS tracking provided information on where seals most likely caught their prey. Harbor seals hauling‐out in the Dutch Wadden Sea fed predominantly on demersal fish, for example, flatfish species (flounder, sole, plaice, dab), but also on sandeel, cod, and whiting. Although harbor seals acquire the majority of prey further offshore in the adjacent North Sea, and only spend 14% of their diving time in the Wadden Sea, seal predation was still estimated to cause an average annual mortality of 43% of the remaining fish in the Wadden Sea and 60% in the nearby shallow coastal waters (<20 m). There were however large sources of uncertainty in the estimated impact of seals on fish, including the migration of fish between the North Sea and Wadden Sea, and catchability estimates of the fish survey sampling gear, particularly for sandeel and other pelagic fish species. Our estimate suggested a considerable top‐down pressure by harbor seals on demersal fish. However, predation by seals may also alleviate density‐dependent competition between the remaining fish, allowing for increased fish growth, and partly compensating for the reduction in fish numbers. This study shows that recovering coastal marine mammal populations could become an important component in the functioning of shallow coastal ecosystems.

Highlights

Large-scale historic whaling and sealing led to a severe global decline of many marine mammal species (Clapham et al 1999, Baker and Clapham 2004)
The impact of seals on fish biomass in the Wadden Sea and adjacent coastal zone For the demersal fish species remaining in the Wadden Sea and nearby shallow coastal zone (
This high estimated predation pressure can be explained by the relatively large estimated annual fish consumption by harbor seals (i.e., ~17,500 tons between September 2015 and 2016) compared to the recent low prey fish standing stock biomass observed in the Dutch Wadden Sea (1100 tons) and adjacent Wadden coastal zone (3600 tons)

Summary

Introduction

Large-scale historic whaling and sealing led to a severe global decline of many marine mammal species (Clapham et al 1999, Baker and Clapham 2004). While some marine mammal populations have not fully recovered after hunting ceased (Baylis et al 2015), or have even continued to decline (Springer et al 2003), others have gone through rapid increases (Brasseur et al 2018) reaching or exceeding presumed pre-exploitation levels (Roman et al 2015). This raises the question how such recoveries influence the food web regulation in marine ecosystems. The population is approaching estimated pre-1900 levels with approximately 38,000 individuals regularly hauling-out in the international Wadden Sea (Galatius et al 2017), of which approximately 10,000 in the Dutch part of the Wadden Sea (Fig. 1)

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