The historical occurrence of the angelshark Squatina squatina and common skate Dipturus batis in Dutch coastal waters

• Main Authors: Bom, R.A., van de Water, M., Brader, A., van der Veer, H.W., van Leeuwen, A.
• Format: Book
• Language: English
• Published: 2019
• Subjects: Common skate; Dipturus batis
• Online Access: https://www.vliz.be/imisdocs/publications/45/352145.pdf

The North Sea is known for its natural richness.The area is also among the most intensively exploited seas in the world, and there is a rising concern of fisheries impact, as many fish species have disappeared or populations are marginalized. With the loss of fish biodiversity there is a growing call to restore the natural richness of the North Sea. However, it is often unclear what this natural richness is, because our baseline often dates back one or two human generations. Elasmobranchs (sharks, skates and rays) arean example of a group of species of which the populations in the North Sea have marginalized over the past century. Two Elasmobranch species,the angelshark Squatina squatina and commonskate Dipturus batis are now locally extinct and are globally critically endangered. Here we review the historic abundance and some importantlife-history aspects of these species in the Dutch part of the area, and the adjacent Dutch inlet seas. Palaeontological sources dating back to the origin of the North Sea (2.5 million year ago)suggest that angelshark and common skatehave always been present in the North Sea. Written historical sources from the 16th to the 19th century suggest that the angelshark used tobe fairly common and that the common skate was common or very common in the Dutch waters. Catch reports indicate that the angelsharkstarted to decline at the onset of the 20th centuryand went locally extinct in the Dutch NorthSea in the 1970s. Time series (landing data andfishery-independent data) of common skateshow a similar picture. We see that the start ofthe decline of both species coincides with the onset of steam-powered fisheries whereas the demise follows on the intensification of diesel-powered beam-trawlers. Historical reports and catch data indicate that the angelshark mainly occurred in the summermonths in shallow waters close to the coast.Only adults, sometimes gravid females, andnew-born individuals were caught, suggesting that the species used the area as a nurseryground. Common skates were mainly caughtin winter months and these were mainly young individuals, below the maturation size. This suggest that this species used the area as a nurseryground, although catches of freshly laid eggs show that the species also used to reproduce inthe area. Our overview shows that species that are now lost from the Dutch waters were once an abundant part of the North-Sea food web. If we base our ideas of a North Sea ecosystem on a baseline view after 1900 we may have overly low expectations of its potential natural richness. That means that we may underestimate the value of restoration and reduction of exploitation.