Abstract
Intensive fishing can strongly impact marine ecosystems; among other things, it usually causes the mean trophic level of the catches to decline, an indicator of the occurrence of the ‘fishing down’ (FD) phenomenon. Although FD occurs throughout the world oceans, it can easily be masked by diverse factors, which has misled authors as to its generality. In this contribution, which uses the East China Sea as an example, we explore the masking effect on FD of the taxonomic coarseness of catch data, of assuming that individual sizes remain constant after intensive fishing, and the geographic expansion of fisheries. The result showed that all of these masking factors occur in the East China Sea, where only a few species are reported separately and the bulk of the catch is pooled into non-informative ‘mixed fishes’. Also, the small mesh sizes and intensive fishing have reduced the sizes of fish and their trophic levels, while the fisheries have expanded offshore. Overall, taking the masking factors into account, the fishing down effect, i.e., the decline of the mean trophic level of the catch between 1979 and 2014 is in the order of 0.15 TL per decade, i.e., one of the highest estimates of FD in the world. Some ecological implications are presented.
Highlights
Fishery activities impact the target and bycatch species, and the whole marine ecosystems [1], such as destroying natural habitat, decreasing biomass, and reducing biodiversity [2]
The marine trophic index (MTI) is the Convention on Biological Diversity (CBD)’s name for the mean trophic level (MTL) of fisheries catches, shown by Pauly et al [4] to decline in many fisheries of the world, a phenomenon they called ‘fishing down marine food webs’ (FDMW)
The landings data used here are the fisheries catches in the East China Sea from 1979 to 2014, which originate from China Fishery Statistical Yearbook (CFSY) [20], in which fisheries statistics are presented as annual marine catches of major commercial fish species by the fleets of different provinces or province-level municipalities in China’s seas
Summary
Fishery activities impact the target and bycatch species, and the whole marine ecosystems [1], such as destroying natural habitat, decreasing biomass, and reducing biodiversity [2]. In 2004, the Convention on Biological Diversity (CBD) identified the Marine Trophic Index as an indicator of the biodiversity of large fishes, and by implication, of the impact of fisheries on marine ecosystems [3]. The marine trophic index (MTI) is the CBD’s name for the mean trophic level (MTL) of fisheries catches, shown by Pauly et al [4] to decline in many fisheries of the world, a phenomenon they called ‘fishing down marine food webs’ (FDMW). The fishing down effect means that the intensive fishing pressure leads to changes in catch compositions, which shifts from a dominance of large, high-trophic level species to relatively small, low-trophic level species. A large number of regional and national studies have confirmed the occurrence of fishing down effect, such as in Greece [5], Canada [6], Iceland [7], Uruguay [8], PLOS ONE | DOI:10.1371/journal.pone.0173296 March 7, 2017
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