Norderhaug KM et al. (2021), Depletion of coastal predatory fish sub-stocks ...

ECB-ART-49715

Ambio 2021 Jan 01;501:163-173. doi: 10.1007/s13280-020-01362-4.

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Depletion of coastal predatory fish sub-stocks coincided with the largest sea urchin grazing event observed in the NE Atlantic.

Norderhaug KM , Nedreaas K , Huserbråten M , Moland E .

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In this contribution, we propose fishery driven predator release as the cause for the largest grazing event ever observed in the NE Atlantic. Based on the evolving appreciation of limits to population connectivity, published and previously unpublished data, we discuss whether overfishing caused a grazer bloom of the sea urchin (Strongylocentrotus droebachiensis) resulting in overgrazing of more than 2000 km2 kelp (Laminaria hyperborea) forest along Norwegian and Russian coasts during the 1970 s. We show that coastal fisheries likely depleted predatory coastal fish stocks through modernization of fishing methods and fleet. These fish were important predators on urchins and the reduction coincided with the urchin bloom. From this circumstantial evidence, we hypothesize that coastal predatory fish were important in regulating sea urchins, and that a local population dynamics perspective is necessary in management of coastal ecosystems.

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	Fig. 1. Green sea urchins Strongylocentrotus droebachiensis grazing kelp Laminaria hyperborea at Hammerfest (left side, Norway 71° N, photo: Stein Fredriksen) and green sea urchins in a haddock stomach sample (Photo: Hans Kristian Strand)
	Fig. 2. Spatiotemporal development of coastal fisheries for a Atlantic wolffish (A. lupus), b haddock (M. eaglefinus) and c Atlantic coastal cod (G. morhua) shown as regional landings (darker color with increasing latitude) available for the period 1950–2018 (see Methods) within the area where sea urchins (S. droebachiensis) bloomed (Norderhaug and Christie 2009). The green lines show average landings (± SD) before, during urchin overgrazing and after sea urchins started to retreat. During 1970–1980 (dark gray period) urchin populations bloomed and barren ground area coverage peaked. During 1980–1990 (light gray period) sea urchins gradually retracted while kelp recovered in the southernmost part of the barren area. Only total landings data of cod including Barents Sea cod are available during 1970–1983
	Fig. 3. Hypothesized drivers of the regime shift and idealized ecosystem-based adaptive management. a Prior to 1950, the fishing pressure did not deplete coastal demersal predators of sea urchins to levels that threatened functional redundancy. From 1950, an unregulated coastal fishery, rapid technological development of the fleet and increased price of wolffish led to overfishing, serial depletion and loss of ecosystem function (ecosystem overfishing, Murawski 2000). By 1980, the barren ground state reached its maximum spatial coverage (photos from the top: J Thormar, S Fredriksen, E Svensen). b Adaptive ecosystem-based management takes species- and sub-stock-specific vulnerabilities into account. Environmental monitoring (symbolized by gauges) on species, sub-stock, and ecosystem state and function is used to revise and tune management and protection tools (symbolized by fish-hook size). Conservation of sub-stock biomass and size-structure prevent urchin blooms and support healthy ecosystems dominated by kelp forests (photo: J Thormar)

References [+] :

André, Population structure in Atlantic cod in the eastern North Sea-Skagerrak-Kattegat: early life stage dispersal and adult migration. 2016, Pubmed