Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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Top-down pressure on a coastal ecosystem by harbor seals
Aarts, Geert ; Brasseur, Sophie ; Poos, Jan Jaap ; Schop, Jessica ; Kirkwood, Roger ; Kooten, Tobias Van; Mul, Evert ; Reijnders, Peter ; Rijnsdorp, Adriaan D. ; Tulp, Ingrid - \ 2019
Ecosphere 10 (2019)1. - ISSN 2150-8925 - p. e02538 - e02538.
demersal fish - diet - harbor seal - impact - intertidaL - Phoca vitulina - predation pressure - sealing - Subtidal - top-down regulation - top predator
Historic 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.
Harbour seals are regaining top-down control in a coastal ecosystem
Aarts, G.M. ; Brasseur, S.M.J.M. ; Poos, J.J. ; Schop, Jessica ; Kirkwood, R.J. ; Kooten, T. van; Mul, Evert ; Reijnders, P.J.H. ; Rijnsdorp, A.D. ; Tulp, I.Y.M. - \ 2018
BioRxiv
Historic 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 again act as a top-down regulatory force on marine ecosystems. Also the harbour seal population in the international Wadden Sea grew at an exponential rate following a ban on seal hunting in 1960's, and the current number ~38,000 is close to the historic population size. Here we estimate the impact of the harbour seal predation on the fish community in the Wadden Sea and nearby coastal waters. Fish remains in faecal 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. Harbour seals from the Dutch Wadden Sea fed predominantly on demersal fish, e.g. flatfish species (flounder, sole, plaice, dab), but also sandeel, cod and whiting. Total fish biomass in the Wadden Sea was insufficient to sustain the estimated prey consumption of the entire seal population year-round. This probably explains why seals also acquire prey further offshore in the adjacent North Sea, only spending 13% of their diving time in the Wadden Sea. Still, seal predation was estimated to cause an average annual mortality of 43% and 60% on fish in the Wadden Sea and adjacent coastal zone, respectively. There were however large sources of uncertainty in the estimate, 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 control by harbour seals on demersal fish. However predation by seals may also alleviate density-dependent competition between the remaining fish, increasing fish growth, and partly compensating for the reduction in fish numbers. This study shows that recovering coastal marine mammal populations could potentially become an important component in the functioning of shallow coastal systems.
Do porpoises have a biological clock?
Mul, Evert ; Smit, Ronald ; Aarts, G.M. ; Biuw, Martin ; Acquarone, Mario ; Scheidat, M. - \ 2017
Impact of harbour seals on declining fish stocks in and around the Wadden Sea
Aarts, G.M. ; Brasseur, S.M.J.M. ; Poos, J.J. ; Schop, Jessica ; Mul, Evert ; Kooten, T. van; Kirkwood, R.J. ; Reijnders, P.J.H. ; Rijnsdorp, A.D. ; Tulp, I.Y.M. - \ 2017
In: Abstracts book - 10th International Symposium Flatfish. - - p. 54 - 54.
While some marine mammals haven't recovered from historic hunting, others have recovered rapidly to presumed pre-exploitation levels. Also harbour sea ls in the Wadden Sea grew at a rapid rate of 12% p.a. following the ban on seal hunting. As a consequence, - 40,000 seals are currently residing in the international Wadden Sea, and collectively, they might act as an important top-down regulatory force. The objective of this study was to estimate the potential impact of predation by harbour seals in The Netherlands on the f ish community in the Wadden Sea and nearby coastal waters. Hard fish remains in faecal samples and estimates on daily energy requirement were used to estimate prey selection and the magnitude of seal predation. GPS tracking data provided information on where they most likely caught their prey. Estimates of abundance and growth of demersal fish species, derived from fish surveys, provided estimates on total prey availability. Harbour seals in the Wadden Sea were found to feed predominantly on flatfish, flounder, sole, plaice and dab, sandeel, fivebearded rockling, whiting, cod, dragonet and bullrout. Given their high daily food requirement, the study suggests there is insufficient food available in the Wadden Sea to sustain the entire harbour seal population. Although harbour seals only spend 10-20% of their time foraging in the Wadden Sea, they may potentially reduce the demersal fish biomass by 50% in the period Sept-June. There are however large sources of uncerta inty, e.g. t he catchability of the fishing sampling gear, particularly for the larger fish specimens, and movement of f ish between the North Sea and Wadden Sea. These resu lts suggest it is important to take the harbour seal prey consumption into account when understanding the functioning of the Wadden Sea ecosystem, which acts as an important nursery area for both seals and several fish species.
Staren naar potvisstaarten
Mul, Evert - \ 2016

Potvissen stranden al eeuwen op de Noordzeekust. Bioloog Evert Mul vergelijkt staartvinnen om de oorzaak te vinden.

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