UPSC Mains 2022 GS Paper 1 Q14: model answer

About 90% of the world's fish catch comes from coastal and shelf waters, and the richest of these lie where currents bring nutrients to the surface. Currents decide where fishing fleets go.

Primary forces (that set water moving)

• Planetary winds: trades and westerlies drag surface water, creating the major gyres.
• Heating by the sun: warm water expands, raising the sea level slightly near the equator, so water flows down the slope.
• Salinity and temperature differences: cold, salty water is denser and sinks, driving deep thermohaline flow.

Secondary forces (that shape the movement)

• Coriolis force: deflects currents, forming clockwise gyres in the north and anticlockwise ones in the south.
• Gravity and the shape of coasts and the sea floor, which split and divert currents.
• Seasonal winds, which reverse the currents of the north Indian Ocean with the monsoon.

Role in the fishing industry

• Meeting of warm and cold currents mixes water, multiplies plankton and attracts fish:
• Grand Banks off Newfoundland (Gulf Stream and Labrador Current), historically the world's richest cod fishery.
• North-west Pacific off Japan (Kuroshio and Oyashio), one of the largest catches in the world.
• Upwelling along cold-current coasts brings nutrients up: Peru (Humboldt), Namibia (Benguela), California, and the south-west coast of India during the monsoon, which supports oil sardine and mackerel.
• Currents carry eggs and larvae and decide migration routes of tuna and salmon.
• Fog and cool waters at meeting zones also make fish storage easier.

Threats

• El Niño weakens upwelling off Peru, collapsing anchovy catches (1972–73).
• Overfishing collapsed the Grand Banks cod in 1992.
• Ocean warming shifts fish towards the poles.

Currents are the hidden engine of world fisheries; sustainable catch limits and climate monitoring are needed to keep that engine working.