Rapid recovery of Dutch gray seal colonies fueled by immigration

Abstract

Gray seals were first observed breeding in the Dutch Wadden Sea in 1985, after centuries of absence. The breeding colony there is now the largest on the European continent. We describe the changes in gray seal numbers and their geographical expansion, and estimate how these processes were influenced by immigration from other colonies. Counts of hauled out animals were carried out between 1985 and 2013, monitoring three different periods of the seals' annual cycle. Using priors determined for the UK population, a Bayesian demographic model was fitted to pup numbers to estimate the population parameters driving the growth. This included immigration of subadults into the breeding population, which contributed to an average growth rate in the pup counts of 19%/yr, much higher than expected in a closed population. This immigration may account for approximately 35% of the total annual growth. In addition, at least 200 gray seals from the UK visit the area temporarily. Recovery of the population in the Netherlands occurred more than 50 yr after gray seals were protected in the UK. These time scales should be taken into account when studying long living marine mammals, e.g., in impact and conservation studies.

Changes over 50 years in fish fauna of a temperate coastal sea: Degradation of trophic structure and nursery function

“…Recovery of the benthic community takes a number of years (Van der Veer et al, 1985;Newell et al, 1998;Van Dalfsen & Essink, 2001) and often the community structure differs from the original due to a shift in sediment type (Desprez, 2000;Wilber and Clarke, 2001). Finally, a link with the abundance and hence consumption of top predators seems likely: cormorants and seals have increased considerably since the 1990s (e.g., Brasseur et al, 2014) and this also holds true for the harbour porpoise (Camphuysen, 2004).…”

Stranding and rehabilitation in numbers: population development and stranding data on the Dutch coasts 1990-2016 : analysis of new data from a public database

“…In the Wadden Sea both harbour and grey seal populations grew in absence of the hunt, and partially fuelled by the influx from seals in other North Sea countries (Brasseur et al submitted 2017, Brasseur et al 2015. In the Delta region recovery was initially slower and clearly, as for both species births are too low to explain the recent growth, immigration is most probably the underlying drive.…”

“…In 2017 over 4000 grey seals were counted in the Wadden Sea region during the moult ( Figure 4) and almost 800 pups were born in the breeding season of 2016-2017 . The numbers have grown at such a high rate that this could only be explained by a continuous influx of young animals from the UK into the Dutch breeding population (Brasseur et al 2015). Additionally, there is continuous exchange of animals between the Dutch coasts, and other North Sea haul outs in France, Germany and Denmark, but most importantly the UK.…”

“…Grey seals had almost completely disappeared from the Dutch coast in the Middle Ages, assumingly as a result of the hunt by humans that recently had colonised the area (Reijnders et al 1995, Härkönen et al 2007, Brasseur et al 2015. As opposed to harbour seals, grey seals remain on land for over a month during breeding and are as such a relatively easy prey for hunters.…”

“…There are essentially two monitoring sets: a) In the Wadden Sea aerial surveys have been carried out since the 1960's (Brasseur et al 2018, Reijnders et al 2003, Van Haaften 1975, Cremer et al 2017. The surveys were initially aimed at surveying the harbour seals, but from 2001 onwards, grey seals which had been surveyed by boat were included (Brasseur et al 2015). Aerial surveys were carried out from a fixed-wing aircraft, flying at elevations of 500-1000 ft. (150-300 m) and speeds of 160 to 220 km/h.…”