10. Rich fishing grounds (that is those that have high species diversity and population), such as those
found in Northern Japan or around Iceland, occur where cold and warm ocean currents mix. This is
because:
a. Upwelling of cold currents bringing nutrients to the surface supporting
phytoplankton blooms, providing food for many fish.
b. Warm currents bring nutrients from river run-offs. Cold currents carry fish. Eddies set up
where the currents mix form marine habitats for many fish
c. Cold water sinks, leaving warmer water closer to the surface allowing algae to bloom.
d. Ocean currents act as highways for many fish species, where two or more currents meet
one would expect increased marine life.

Rich fishing grounds with high species diversity and population density, such as those off Northern Japan or around Iceland, primarily occur where cold and warm ocean currents mix due to upwelling that brings nutrient-rich deep water to the surface. This process fuels phytoplankton blooms, the base of the marine food chain supporting abundant fish.​

Option Analysis

Option a: Upwelling of cold currents bringing nutrients to the surface supporting phytoplankton blooms, providing food for many fish.
This is correct. Cold currents cause upwelling, lifting nutrient-rich waters (nitrates, phosphates) from ocean depths to sunlit surface layers, promoting massive phytoplankton growth. These blooms attract zooplankton and fish, creating biodiversity hotspots.​

Option b: Warm currents bring nutrients from river run-offs. Cold currents carry fish. Eddies set up where the currents mix form marine habitats for many fish.
Incorrect. Warm currents typically carry fewer nutrients and do not primarily source from river run-offs; cold currents upwell nutrients, not “carry fish.” Eddies aid mixing but are secondary to nutrient upwelling.​

Option c: Cold water sinks, leaving warmer water closer to the surface allowing algae to bloom.
Wrong. Cold water is denser and sinks, but in current mixing zones, upwelling brings cold, nutrient-laden water up, not sinking. Warmer surface water alone does not drive blooms without nutrients.​

Option d: Ocean currents act as highways for many fish species, where two or more currents meet one would expect increased marine life.
Partially true but incomplete. Currents do migrate fish, but the key driver is nutrient enrichment from mixing and upwelling, not just transport, which explains sustained high populations.​

Key Examples

• Northern Japan: Oyashio (cold) meets Kuroshio (warm), causing upwelling for phytoplankton-rich waters.​

• Iceland: Irminger (cold) and North Atlantic Drift (warm) mix, supporting cod and haddock fisheries.​

This mechanism underscores why these zones sustain global fisheries, vital for food security and economies.​