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By looking at how much fish is caught? It's pretty easy to measure: https://en.wikipedia.org/wiki/Overfishing#/media/File:Surexploitation_morue_surp%C3%AAcheEn.

• justin66 9y ago
  Surveying the presence or absence of fish is an activity for which a strong economic incentive exists. It's somewhat different than things like sediment depth, etc. which the gp comment mentioned. That stuff will have been mapped to the extent necessary for military operations and determining the viability of offshore drilling.
• sametmax 9y ago
  That could be because fish are moving into other areas or learn to avoid the nets. Fishing yields can't be the only source of data.
  • Cacti 9y ago
    How do you think they do the surveys in the first place? ;) They use nets.
  • nate_meurer 9y ago
    Modern factory fishing fleets use sonar and track movements of fish populations over vast areas. It's highly unlikely fish have learned to evade sonar, or nets.