scholarly journals Harbour seals are regaining top-down control in a coastal ecosystem

2018 ◽  
Author(s):  
Geert Aarts ◽  
Sophie Brasseur ◽  
Jan Jaap Poos ◽  
Jessica Schop ◽  
Roger Kirkwood ◽  
...  
Keyword(s):  
North Sea ◽  
Marine Mammal ◽  
Wadden Sea ◽  
Demersal Fish ◽  
Fish Growth ◽  
Harbour Seal ◽  
Top Down ◽  
Harbour Seals ◽  
Seal Population ◽  
The Impact

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 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.

Sources and Fate of PCBs in the North Sea: A Review of Available Data

1991 ◽  
Vol 24 (10) ◽  
pp. 77-85 ◽  
Author(s):  
J. Klamer ◽  
R. W. P. M. Laane ◽  
J. M. Marquenie
Keyword(s):  
Adverse Effects ◽  
North Sea ◽  
German Bight ◽  
Marine Mammals ◽  
Wadden Sea ◽  
Annual Basis ◽  
The North ◽  
Total Input ◽  
Seal Population ◽  
The North Sea

From literature data it is calculated that on an annual basis, 11 to 17 tonnes of PCBs enter the North Sea. Largest sources are the Atlantic Ocean and the atmosphere: together they account for 60-79% of the total input. Sources with greatest impact are the rivers, sewers and sludge. Highest concentrations are found close to the Dutch shore and in the German Bight. The PCB levels result in adverse effects on the seal population in the Wadden Sea. Of the total world PCB production, at least 57% is still in use and their future dispersal into the oceans cannot easily be controlled. If the increase in ocean PCB concentration continues, it may ultimately result in the extinction of fish-eating marine mammals.

Long-term changes in species composition of demersal fish and epibenthic species in the Jade area (German Wadden Sea/Southern North Sea) since 1972

2016 ◽  
Vol 181 ◽  
pp. 284-293 ◽  
Author(s):  
Julia Meyer ◽  
Ingrid Kröncke ◽  
Alexander Bartholomä ◽  
Joachim W. Dippner ◽  
Ulrike Schückel
Keyword(s):  
Species Composition ◽  
North Sea ◽  
Wadden Sea ◽  
Demersal Fish ◽  
Southern North Sea ◽  
Long Term Changes

scholarly journals Different bottom trawl fisheries have a differential impact on the status of the North Sea seafloor habitats

2020 ◽  
Vol 77 (5) ◽  
pp. 1772-1786 ◽  
Author(s):  
A D Rijnsdorp ◽  
J G Hiddink ◽  
P D van Denderen ◽  
N T Hintzen ◽  
O R Eigaard ◽  
...  
Keyword(s):  
North Sea ◽  
Management Strategies ◽  
Demersal Fish ◽  
Time Interval ◽  
The North ◽  
The Status ◽  
Fisheries Conservation ◽  
The North Sea ◽  
The Impact ◽  
Trawl Fisheries

Abstract Fisheries using bottom trawls are the most widespread source of anthropogenic physical disturbance to seafloor habitats. To mitigate such disturbances, the development of fisheries-, conservation-, and ecosystem-based management strategies requires the assessment of the impact of bottom trawling on the state of benthic biota. We explore a quantitative and mechanistic framework to assess trawling impact. Pressure and impact indicators that provide a continuous pressure–response curve are estimated at a spatial resolution of 1 × 1 min latitude and longitude (∼2 km2) using three methods: L1 estimates the proportion of the community with a life span exceeding the time interval between trawling events; L2 estimates the decrease in median longevity in response to trawling; and population dynamic (PD) estimates the decrease in biomass in response to trawling and the recovery time. Although impact scores are correlated, PD has the best performance over a broad range of trawling intensities. Using the framework in a trawling impact assessment of ten métiers in the North Sea shows that muddy habitats are impacted the most and coarse habitats are impacted the least. Otter trawling for crustaceans has the highest impact, followed by otter trawling for demersal fish and beam trawling for flatfish and flyshooting. Beam trawling for brown shrimps, otter trawling for industrial fish, and dredging for molluscs have the lowest impact. Trawling is highly aggregated in core fishing grounds where the status of the seafloor is low but the catch per unit of effort (CPUE) per unit of impact is high, in contrast to peripheral grounds, where CPUE per unit of impact is low.

scholarly journals Vulnerability of harbour seals, Phoca vitulina, to transient industrial activities in the Strait of Dover

2005 ◽  
Vol 85 (4) ◽  
pp. 1015-1016 ◽  
Author(s):  
Laurent J.J. Seuront ◽  
Perrine Prinzivalli
Keyword(s):  
Interannual Variability ◽  
Industrial Wastewater ◽  
Anthropogenic Activities ◽  
Phoca Vitulina ◽  
Harbour Seal ◽  
Industrial Activities ◽  
Dramatic Decline ◽  
Harbour Seals ◽  
Seal Population ◽  
Number Of Individuals

The abundance of the harbour seal (Phoca vitulina) was recorded on a tidal bar in the Dover Strait off Calais, over a six-year period between 1999 and 2004. Despite clear seasonal and interannual variability in the number of individuals hauled out on the bar, underwater activities devoted to the installation of industrial wastewater pipes conducted during seven weeks 1 km away from the bar led to a dramatic decline in the number of seals hauling out. A full 19 months after the end of the operations the harbour seal population had not recovered their initial abundance. The results of this study have critical consequences on the conservation of P. vitulina in areas impacted by anthropogenic activities.

scholarly journals The status of harbour seals (Phoca vitulina) in the United Kingdom

2010 ◽  
Vol 8 ◽  
pp. 117 ◽  
Author(s):  
Dave Thompson ◽  
Callan Duck ◽  
Mike E Lonergan
Keyword(s):  
Survey Methods ◽  
Harbour Seal ◽  
Rocky Shores ◽  
The North ◽  
Harbour Seals ◽  
General Decline ◽  
Seal Population ◽  
The Status ◽  
Thermal Imagers ◽  
The Uk

The UK holds approximately 40% of the European harbour seal population, with the majority found around the coasts of Scotland. Harbour seal populations in the UK have been monitored through a series of repeated aerial surveys of animals hauled out during the annual moult in early August. This moult count is used as a consistent index of population size. Survey methods and frequencies vary. The Scottish and English east coast populations mainly haul out in tidal estuariesand are surveyed annually, using fixed wing aircraft and digital photography. Populations in north and west Scotland often haul out on rocky shores and are surveyed less frequently, using helicopters fitted with thermal imagers. Overall, the most recent minimum estimate of the UK harbour seal population is 24,250 seals of which 19,800 are in Scotland, 3,200 in England and 1,250 in Northern Ireland. The results show that the number of harbour seals in eastern Englandwas increasing before the 1988 and 2002 phocine distemper (PDV) epizootic but has not increased since the end of the 2002 epizootic. There is also evidence of a general decline in most of the large harbour seal colonies around Scotland. The populations along the north and northwest mainland coast were an exception, with numbers appearing to be stable. Between 2001 and 2008, the population in Orkney declined by 67% and Shetland declined by 40%, indicating harbourseals in these areas experienced substantially increased mortality or very low recruitment over this period. The widespread declines, ranging from Shetland to The Wash, suggest that the causes may have been present over a large part of the North Sea and waters off western Scotland.

scholarly journals Population development and status of harbour seals (Phoca vitulina) in the Wadden Sea

2010 ◽  
Vol 8 ◽  
pp. 95 ◽  
Author(s):  
Peter JH Reijnders ◽  
Sophie MJM Brasseur ◽  
Svend Tougaard ◽  
Ursula Seibert ◽  
Thomas Borchardt ◽  
...  
Keyword(s):  
Health Status ◽  
Wadden Sea ◽  
Conservation Status ◽  
Health Condition ◽  
Harbour Seal ◽  
Population Development ◽  
Sex Structure ◽  
Phocine Distemper Virus ◽  
Favourable Conservation Status ◽  
Seal Population

An index for the condition of a population should include a measure of the recuperative power or resilience of the population in question. This measure needs to cover both the demographic and physiological condition of the population. Applied to the harbour seal population in the Wadden Sea we therefore address respectively the population development and distribution, and its health condition, and relate these to environmental conditions. The harbour seal population has been severely depleted by hunting in the first half of the 20th century. After hunting was stopped in the mid-1970s the population recovered gradually. This recovery was twice interrupted by Phocine Distemper Virus (PDV) outbreaks in 1988 and 2002. These PDV-epizootics reduced the population by 57% and 50% respectively. They also lead to changes in age and sex structure of the population, which gradually returned to a stable age-structure. Despite the reduction in population size by respectively 57% and 50%, the population showed a strong recovery with a growth rate close to the considered maximum possible for this species. The observed changes in the distribution of the population over the 4 sub-regions indicate that distribution is not a static phenomenon. Long term field and pathological investigations point out that the general health status of the population has improved, particularly that of newborn seals (0-6months old). The increasing prevalence of parasites in lungs and intestine warrants continued monitoring of the health status of seals. This is especially relevant in view of the exponential increase of the population, which may finally approach the carrying capacity of the area. Concluding, we canstate that the condition of the population in terms of demographic and health parameters is satisfactory. The best guarantee for maintaining such a favourable conservation status is to abstain from human interferences (e.g. rescue, rehabilitation and release) with natural population processes. However, in practice there is increasing exploitation of marine waters. This requires a continuous monitoring of the potential impacts on the population, particularly of the effect on foraging and migratory behaviour.

The importance of the North Sea for winter dispersal of harbour seals Phoca vitulina from the Wadden Sea

1997 ◽  
Vol 81 (1-2) ◽  
pp. 97-102 ◽  
Author(s):  
Mardik F. Leopold ◽  
Bert van der Werf ◽  
Edith H. Ries ◽  
Peter J.H. Reijnders
Keyword(s):  
North Sea ◽  
Wadden Sea ◽  
Phoca Vitulina ◽  
The North ◽  
Harbour Seals ◽  
The North Sea

scholarly journals Status of the harbour seal (Phoca vitulina) in Southern Scandinavia

2013 ◽  
Vol 8 ◽  
pp. 77 ◽  
Author(s):  
Morten Tange Olsen ◽  
Signe May Andersen ◽  
Jonas Teilmann ◽  
Rune Dietz ◽  
Susi Manuela Clermont Edrén ◽  
...  
Keyword(s):  
Harbour Seal ◽  
Current Status ◽  
Harbour Seals ◽  
Seal Population ◽  
Southern Scandinavia ◽  
Original Size ◽  
The Baltic ◽  
Further Development ◽  
Long Term Survey

The harbour seal population in Southern Scandinavia has experienced repeated declines caused by hunting and epizootics. These events have shaped the current distribution and abundance of the population. This paper assesses the current status of the population. We estimate trends in abundance of harbour seals from long term survey data, compare these with historic trends inferred from previously published material, and discuss past and potential threats to the harbour seal population of Southern Scandinavia. It is evident that harbour seals have disappeared from haulout areas along the Danish shores of Kattegat and in the westernmost part of the Baltic Sea, where they were previously numerous. In the 1920-30s, when abundance was at its lowest, the population is estimated to have been only a fraction of its original size. Following 30 years of protection the population is currently approaching historic abundance and might have reached the carrying capacity in some areas. Further development depends largely on effects of future epizootics, anthropogenic disturbance, and availability of suitable haulout sites.

scholarly journals Sealworm (Pseudoterranova decipiens) infection in the benthic cottid (Taurulus bubalis) in relation to population increase of harbour seal (Phoca vitulina) in Skagerrak, Sweden

2001 ◽  
Vol 3 ◽  
pp. 47 ◽  
Author(s):  
Sven-Gunnar Lunneryd ◽  
Karl Inne Ugland ◽  
Paul Eric Aspholm
Keyword(s):  
Gadus Morhua ◽  
Final Host ◽  
Infected Fish ◽  
Harbour Seal ◽  
Population Increase ◽  
Threshold Size ◽  
Pseudoterranova Decipiens ◽  
Harbour Seals ◽  
Seal Population ◽  
Benthic Fishes

In Koster archipelago (northern Skagerrak, Sweden) the harbour seal population increased from approximately 350 to more than 1000 individuals between 1988 and 1998. During the same period, sealworm (Pseudoterranova decipiens) abundance in the most heavily infected fish species, bullrout (Myoxocepahulus scorpius) and sea scorpion (Taurulus bubalis), did not increase. Since harbour seals do not normally feed on those cottids an infection route via cod (Gadus morhua), which consume cottids, is proposed. The abundance of sealworm in the final host is therefore related to the probability of cod preying on infected cottids prior to being preyed upon by seals. Our model predictsthat the abundance of larval sealworm in benthic fishes is not related to the number of seals when the colony is over a specific threshold size.

scholarly journals Monitoring trends in the abundance of harbour seals (Phoca vitulina) in Icelandic waters

2010 ◽  
Vol 8 ◽  
pp. 227 ◽  
Author(s):  
Erlingur Hauksson
Keyword(s):  
Coastal Area ◽  
East Coast ◽  
Aerial Survey ◽  
Phoca Vitulina ◽  
Harbour Seal ◽  
Northwest Coast ◽  
Coastal Regions ◽  
Harbour Seals ◽  
Seal Population ◽  
Icelandic Waters

Harbour seal (Phoca vitulina) numbers along the coast of Iceland were monitored by aerial survey in the period 1980-2006. Trends in the abundance of the harbour seal population on the whole coast and in coastal regions of Iceland waters were estimated using ANCOVA on the survey counts, corrected for the influence of several covariates. Harbour seals were found in every coastal area, but were most abundant in Faxaflói, Breiðafjörður and on the northwest coast inthe beginning of this study. Harbour seal numbers declined significantly at a rate of rest = -0.04 (SE 0.005) yr-1 during this period. Decline was highest in Faxaflói and at the south coast (≅7%), while the east coast experienced a significant but lesser (≅1%) decline. Other coastal areas did not show significant trends. The northwest coast was the richest harbour seal area in Iceland in 2006. In Icelandic waters seals are commercially harvested, and unreported but probably high numbers of harbour seals are killed intentionally by shooting and accidentally in fishing geareach year. These factors likely contributed to the overall observed decline in seal numbers.

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