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Fisheries Indicators for the Open Ocean

A school of fish (Bigeye Jacks), Clipperton Island.

Photo credit: Clifton Beard, CC-BY-NC 2.0, some rights reserved

Lead author: Daniel Pauly and Vicky W.Y. Lam

The open ocean is increasingly under threat of increasing exploitation of fish stocks due to inadequate and/or ineffective management. In fact, the fisheries catch in the open ocean contributed 12% of the annual average global marine catch in the decade from 2001 to 2010. Fisheries indicators are useful to provide information about the states of fisheries and ecosystem over the past 50 years in the open ocean. This is important for assessing the vulnerability of the fish stocks to human impacts.

Four indicators were developed using the Sea Around Us database:

  • Catch from bottom-impacting gear;
  • Marine Trophic Index;
  • Fishing-in-Balance Index and the projected catch potential under climate change in the 2030s and 2050s;
  • Demersal fishing effort.
Though the proportion of global fish landings in the Open Ocean coming from bottom impacting gear has stabilised in the last 60 years, some regions of the Open Ocean – in particular the southwestern and northwestern parts of the Atlantic Ocean – have very high proportions of fish landings coming from bottom impacting gear in the past decade

Bottom impacting gear types

Time series data of annual landings (1950-2006) from bottom-impacting gear (such as dredges and bottom trawls) were used to assess the human impact on the marine ecosystem. Bottom impacting gears cause substantial damage to vast areas of bottom habitats. This type of fishing can lead to direct mortality of benthic invertebrates (organisms that live on the bottom of a water body, or in the sediment, and have no backbone). It can also destroy the three-dimensional complexity of the seabed and hence reduce the species diversity and biomass of benthic communities. Therefore, this indicator allows an assessment of the impact of human drivers on the vulnerability of fish stocks.

Marine Trophic and Fishing-in-Balance Indexes

When a fishery begins in a given area, it usually targets the largest of the accessible fish. Once these species decline, the fisheries turn to less desirable, and generally smaller fish. To detect this pattern and assess changes in species composition of the fisheries in the open ocean, Marine Trophic and the Fishing-in-Balance Indexes were developed.

The Marine Trophic Index measures where different species of fish sit in the food web. It is an indicator used by the Convention on Biological Diversity and it corresponds to the mean trophic level of the fisheries catch in a given area. It reflects the ‘fishing down the food web’ phenomenon, associated with the decline in the mean trophic level of fisheries landings. It is generally expected that a decline in Marine Trophic Index (fishing for increasingly smaller fish) may indicate a decline in the biodiversity of top predators, linked to overexploitation.

The Fishing-in-Balance Index measures a geographic expansion, showing an increase when a fisheries expands, and is best analyzed jointly with the Marine Trophic Index. These two indexes not only allow an understanding of the status of the stocks, but also enable an assessment of changes in the ecosystem and potential impacts on human wellbeing.

The mean Fishing-in-Balance (FiB) index has increased steadily since the 1950s, suggesting both a geographical expansion of the fisheries and, jointly with trends of Marine Trophic Index (MTI), the occurrence of the ‘fishing down’ phenomenon in some areas, notably the Southwest Atlantic Ocean

Demersal fishing effort

Fishing effort is any activity, such as the deployment of vessels, used to catch fish during a conventional period, for example, for one year. The cumulative power of the engine of fishing vessels is used as a measure of nominal effort, which is converted to effective effort through multiplication by an annual technological factor reflecting improvements in locating fish (for example, through echosounders and navigation (GPS)). Demersal effort was quantified by multiplying the power of the vessels’ engines and the numbers of days at sea in a year. A database of the nominal fishing effort deployed by the world’s maritime countries was created by Anticamara et al. (2011) which was spatialized by (Watson et al. 2013), and which was used to estimate demersal fishing effort from 1950 to 2006.


The four indicators provide important information on the status of the marine resources and allow further assessment of the vulnerability of fisheries to human-induced impacts. The data used for the above indicators and analyses are essentially landings data as reported by FAO, based on voluntary reporting by member countries. These data are known to be incomplete, and generally biased downward due to all catch components not being included in official records, and thus being inadvertently substituted with an effective zero catch.