Jellyfish overtake fish in a heavily fished ecosystem

Over the past half century fishing has led globally to a reduction in the mean trophic level of commercially landed species, with a significant decline from large predatory fish toward plankton-eating pelagic species and low trophic-level invertebrates [1]. An implied endpoint of this ‘fishing down marine food webs’ is a proliferation of previously suppressed gelatinous plankton (jellyfish) [2] thriving on the food no longer consumed by fish. We report here that, in the heavily exploited northern Benguela off Namibia, a transition towards this endpoint has occurred, and jellyfish biomass (12.2 million tonnes (MT)) now exceeds the biomass of once-abundant fish (3.6 MT). This is a profound ecosystem change, with possible consequences from carbon cycling to fish stock recovery. The northern Benguela is a highly productive easternboundary ecosystem fertilised by upwelling, nutrient-rich waters. Historically the region supported large stocks of fish, including sardines (Sardinops sagax) and anchovies (Engraulis encrasicolis), but heavy fishing pressure has reduced stocks, and total landings have fallen from around 17 MT in the late 1970s to just 1 MT now (Figure 1A). Prior to this period of heavy exploitation, large jellyfish (Scyphozoa and Hydrozoa) were not prominent in the Benguela ecosystem: reports of extensive plankton sampling in the 1950s and 1960s do not mention large jellyfish, although numerous small gelatinous species (e.g. ctenophores) were observed (for example [3]). Following early collapses of pelagic fish stocks (in the 1960s), reports of the large Figure 1. Fish and jellyfish in the Namibian Benguela. (A) Time series of total fish landings from the northern Benguela (from FISHSTAT+ www.fao.org/fi/statist/FISOFT/FISHPLUS.asp. Data are Capture Production in the South East Atlantic Major Area 47, Western Coastal Subarea, Divisions 1.3 Cunene, 1.4 Cape Cross, and 1.5 Orange River, covering 15°S to 30°S and from the coast to 10°E). FISHSTAT+ data extend from 1975 only, so do not cover the large sardine crash in the 1960s [11]. (B) Bathymetric map (grey contour lines at 100, 200, 300, 500, 700, and 1000 to 4500 in 500 m increments) and the cruise track (solid red line) followed southward from the Angola–Namibia border to the Namibia–South Africa border. CR, Cunene River; WB, Walvis Bay; CC, Cape Cross; and OR, Orange River. (C–F) maps of distribution of jellyfish and fish from 17°15′S 11°28′E to 28°45′S 15°50′E: (C) Chrysaora hysoscella; (D) Aequorea forskalea; (E) Cape horse mackerel/Cape hake; and (F) clupeids (sardine, anchovy and round herring combined). Colour scale is density, tonnes per nautical mile2.