World fisheries Revision Notes

Ocean currents are continuous, directed movements of seawater generated by factors such as wind patterns, Earth's rotation, and differences in water density. Significant ocean currents exist in all major oceans and can be broadly classified as warm or cold currents based on their temperature relative to surrounding waters.

Warm currents typically originate near the equator and flow toward the poles. For example, the Gulf Stream in the North Atlantic carries warm water from the Caribbean toward northern Europe. Conversely, cold currents generally flow from polar or subpolar regions toward the equator, such as the California Current along the west coast of North America.

These currents form large-scale circulation patterns that influence climate and marine ecosystems. Warm currents tend to raise coastal temperatures, while cold currents often cool coastal regions. The location of these currents is generally along continental margins and open ocean pathways, shaping marine environments over vast areas.

• Warm currents flow poleward from equatorial regions.
• Cold currents flow equatorward from polar regions.
• Major currents are found along continental coasts and open oceans.
• Examples include the Gulf Stream (warm) and California Current (cold).

Marine fish populations are not evenly distributed across the oceans. A significant concentration of fish occurs in shallow waters over continental shelves, which are submerged extensions of continents with depths generally less than 200 metres. These areas receive ample sunlight, supporting photosynthesis and productive food webs.

The continental shelf environment provides abundant nutrients and habitats such as seagrass beds, coral reefs, and rocky substrates that support diverse fish species. The relatively shallow depth allows for higher primary productivity compared to the open ocean, which in turn sustains larger fish populations.

Because of these favourable conditions, many commercial fisheries target species found on continental shelves. The accessibility of these areas also facilitates fishing activities.

• Continental shelves are shallow, nutrient-rich zones extending from continents.
• Sunlight penetration supports high primary productivity and food chains.
• Fish populations are dense due to abundant food and habitat variety.
• Most commercial fisheries operate on continental shelves.

Ocean currents affect marine ecosystems by transporting water masses with different temperatures and nutrient levels. Cold currents often bring nutrient-rich waters from polar regions toward the equator. These nutrients support plankton growth, forming the base of the marine food web and supporting abundant fish populations.

For example, the Humboldt Current off the west coast of South America is a cold current that delivers nutrients, sustaining one of the world's richest fisheries. In contrast, warm currents tend to have lower nutrient concentrations because they promote stratification, which limits the upward movement of nutrients from deeper waters.

As a result, fish species composition and abundance differ between areas influenced by cold and warm currents. Cold current regions typically support large populations of commercially important fish, while warm current areas may have fewer nutrients and different species adapted to those conditions.

• Cold currents transport nutrient-rich waters, boosting plankton and fish populations.
• Warm currents usually have lower nutrient levels due to water stratification.
• Fish species and abundance vary according to current temperature and nutrient availability.
• Example: Humboldt Current supports rich fisheries due to nutrient upwelling.

The El Niño Southern Oscillation (ENSO) is a recurring climate pattern involving changes in sea surface temperatures and atmospheric pressure across the tropical Pacific Ocean. During an El Niño event, surface waters in the eastern Pacific Ocean become unusually warm, disrupting normal oceanic and atmospheric circulation.

This warming weakens the cold Humboldt Current along the Pacific coast of South America, reducing the upwelling of nutrient-rich waters. Consequently, plankton populations decline, leading to a decrease in fish stocks that depend on this food source.

ENSO events typically occur every 2 to 7 years and can last from several months to over a year. The disruption caused by ENSO has significant economic impacts on fisheries, as reduced fish catches affect livelihoods and food security in coastal communities.

• ENSO involves periodic warming of eastern Pacific surface waters (El Niño phase).
• Warming disrupts cold currents and nutrient upwelling along South America's Pacific coast.
• Reduced nutrients cause declines in plankton and fish populations.
• Fisheries experience lower catches and economic hardship during ENSO events.
• ENSO cycles occur irregularly every 2–7 years.

Fisheries along the Pacific coast of countries such as Peru and Chile rely heavily on fish species supported by the nutrient-rich Humboldt Current. During ENSO events, the reduction in nutrient upwelling causes a sharp decline in fish populations, particularly small pelagic fish like anchovies and sardines.

This decline leads to lower fish catches, affecting commercial fishing industries and local communities dependent on fishing for income and food. The disruption also forces fishers to travel farther offshore or target different species, increasing operational costs and ecological pressures.

Understanding ENSO's impacts helps fisheries managers develop strategies to mitigate risks, such as adjusting quotas or diversifying target species during El Niño periods.

• ENSO reduces fish populations by disrupting nutrient supply.
• Commercial catches of anchovies and sardines decline sharply during ENSO.
• Economic and social impacts affect coastal fishing communities.
• Fisheries management must adapt to ENSO variability to sustain stocks.