Abstract
During the last 20 years, a series of studies has suggested trends of increasing jellyfish (Cnidaria and Ctenophora) biomass in several major ecosystems worldwide. Some of these systems have been heavily fished, causing a decline among their historically dominant small pelagic fish stocks, or have experienced environmental shifts favouring jellyfish proliferation. Apparent reduction in fish abundance alongside increasing jellyfish abundance has led to hypotheses suggesting that jellyfish in these areas could be replacing small planktivorous fish through resource competition and/or through predation on early life stages of fish. In this study, we test these hypotheses using extended and published data of jellyfish, small pelagic fish and crustacean zooplankton biomass from four major ecosystems within the period of 1960 to 2014: the Southeastern Bering Sea, the Black Sea, the Northern California Current and the Northern Benguela. Except for a negative association between jellyfish and crustacean zooplankton in the Black Sea, we found no evidence of jellyfish biomass being related to the biomass of small pelagic fish nor to a common crustacean zooplankton resource. Calculations of the energy requirements of small pelagic fish and jellyfish stocks in the most recent years suggest that fish predation on crustacean zooplankton is 2–30 times higher than jellyfish predation, depending on ecosystem. However, compared with available historical data in the Southeastern Bering Sea and the Black Sea, it is evident that jellyfish have increased their share of the common resource, and that jellyfish can account for up to 30% of the combined fish-jellyfish energy consumption. We conclude that the best available time-series data do not suggest that jellyfish are outcompeting, or have replaced, small pelagic fish on a regional scale in any of the four investigated ecosystems. However, further clarification of the role of jellyfish requires higher-resolution spatial, temporal and taxonomic sampling of the pelagic community.
Highlights
Increases in cnidarian and ctenophore biomasses have been reported for a number of major ecosystems[1,2,3,4], and various qualitative reviews[5,6,7] and meta-analyses[8] suggest numerous localized increases worldwide
With regards to hypothesis H1 - jellyfish consume a larger part of the common resource than small pelagic fish, the estimated energy requirements (E) for the recent time-periods suggest that small pelagic fish require on average 2–30 times more energy per year compared with their respective jellyfish competitors, depending on ecosystem (Fig. 1b and Table 1)
For the Southeastern Bering Sea (RACE bottom trawl survey, 1982–2012) and the Black Sea, where data were available before the apparent increases in jellyfish biomass, it is evident that both the biomass and energy requirements of the jellyfish have increased relative to those of small pelagic fish (Fig. 1b and Table 1)
Summary
Increases in cnidarian and ctenophore (hereafter collectively termed jellyfish) biomasses have been reported for a number of major ecosystems[1,2,3,4], and various qualitative reviews[5,6,7] and meta-analyses[8] suggest numerous localized increases worldwide. Small pelagic fish, (2) jellyfish and small pelagic fish are competing for a limited resource, and (3) jellyfish are significant predators upon early life stages of small pelagic fish, and constrain recruitment of the latter. For each of these statements, we have formulated a hypothesis. These were tested using the best available time-series data of small pelagic fish, jellyfish and crustacean zooplankton biomass for three major ecosystems: the Southeastern Bering Sea, the Northern California Current, and the Black Sea (Supplementary Fig. S1a–c). 1012 kJ year−1 SD have been suggested to be important competitors of various forage fishes in the Southeastern Bering Sea[18,24] and the Northern California Current[3,25,26,27], of sardines in the Northern Benguela[1,17] and of anchovy in the Black Sea[2,28,29]
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