A long-term view on recent changes in abundance of common skate complex in the North Sea

Abstract

Following decades of declines, populations of large fish recently started to increase in the North Sea, presumably due to reduced fishing pressure. However, population recovery may be too readily claimed, since standardised sampling of fish stocks commenced only in the 1970s, well after many species had already collapsed. A true recovery must be seen from a long-term perspective. The critically endangered common skate (Dipturus batis, Rajidae) species complex is an example of a large-bodied fish that mostly disappeared before standardised monitoring took place. Here, we put the recent increase in population size into a 120-year perspective, throughout three geographical divisions in the North Sea. We analysed a large range of mostly undisclosed historical data and contemporary sources. A reconstruction of Dutch commercial landings data confirms that the species used to be very abundant between 1901 and 1920, and shows how it steadily declined from 1920 onwards until it got extirpated around 1970. Based on a quantitative analysis of standardised catch numbers from fishery-independent surveys time, we conclude that the current abundance of the species is still below historical baselines and represents a local recovery at most. We further demonstrate a prominent and consistent pattern in size distribution, with larger (mature) individuals only occurring in the northern North Sea. A large dataset on historical stomach contents from the central North Sea confirmed the diet of young common skate, which consisted predominantly of shrimps. Our review exemplifies the importance of marine historical ecology to deduce the natural richness of the North Sea.