4.3

Phytoplankton

Microscopic, photosynthetic organisms (autotrophs) that form the foundation of aquatic food webs. Primary producers.

Macrophytes

Large, visible aquatic plants (e.g., seaweed, kelp). Act as primary producers in coastal ecosystems, providing energy and habitat.

Trophic Levels (Aquatic)

An organism's position in a food chain representing its role in energy flow. Example: Phytoplankton (T1) → Zooplankton (T2) → Small Fish (T3).

Aquatic vs. Terrestrial Food Production

Key differences: Humans harvest from higher trophic levels in aquatic systems (tuna vs. cows). Aquatic food chains are typically longer. Energy transfer can be more efficient (less inedible bone).

Drivers of Demand for Aquatic Food

Human population growth and rising incomes, which lead to dietary shifts that favor fish as a protein source.

Maximum Sustainable Yield (MSY)

The highest possible annual catch of a species that can be sustained over time without depleting the stock. The point where the harvest rate equals the population growth rate.

MSY Curve Dynamics

A model showing yield vs. fishing effort. The apex represents MSY (fully fished). To the right, harvest exceeds reproduction (overfished), causing stock decline. To the left, harvest is below reproduction (underfished).

Overexploitation (of fisheries)

Harvesting a fish stock at a rate greater than its Maximum Sustainable Yield (MSY), leading to population depletion and potential collapse.

Fishery Collapse

Severe depletion of a fish stock to a point where it is no longer commercially viable and recovery is slow or uncertain. Example: Grand Banks cod in the 1990s.

Capture Fisheries

The harvesting of fish and other organisms from wild populations (wild-caught) in oceans, lakes, or rivers.

Bottom Trawling

Dragging a large, weighted net along the seafloor. Causes significant habitat destruction and has very high levels of bycatch.

Purse Seine Fishing

Encircling a school of fish (e.g., tuna) with a large net that is then closed at the bottom like a drawstring bag. Often associated with high bycatch.

Longline Fishing

Using a main fishing line up to 100 km long with thousands of baited hooks attached. Poses a major bycatch risk to sharks, turtles, and seabirds.

Gillnets

Vertical panels of net that entangle fish by their gills as they swim through. A passive method that often results in high bycatch of non-target species.

Aquaculture

The farming of aquatic organisms (fish, molluscs, crustaceans, plants) with human intervention in the rearing process to enhance production.

Benefits of Aquaculture

Can reduce pressure on wild stocks, provides a reliable food source, and supports economic development and livelihoods for local communities.

Negative Impacts of Aquaculture

Includes pollution (waste, antibiotics), habitat loss (e.g., mangroves), spread of disease to wild stocks, and high demand for wild-caught fish as feed (fishmeal).

Marine Protected Areas (MPAs)

Defined marine regions where human activities are restricted to protect ecosystems. Act as breeding refuges, helping to replenish fish stocks in surrounding areas.

Fisheries Management Strategies

Policies to prevent overexploitation, including: setting catch quotas (based on MSY), issuing fishing permits, creating seasonal closures, and regulating gear types.

Ocean Acidification

The ongoing decrease in ocean pH caused by the absorption of excess atmospheric CO2, which forms carbonic acid when dissolved in seawater.

Impact of Ocean Acidification

Reduces the availability of carbonate ions, hindering the ability of marine organisms (e.g., corals, shellfish) to build and maintain their calcium carbonate shells and skeletons.

Coral Bleaching

The expulsion of symbiotic algae (zooxanthellae) from coral tissues due to stress, primarily from increased ocean temperatures. This starves the coral, turns it white, and can lead to its death.