4.3 Aquatic Food Production Systems

In July 1992, Canada shut down the northern cod fishery off Newfoundland, the most productive groundfish fishery in the North Atlantic. Spawning biomass had collapsed by roughly 93% in three decades, and officials expected the ban to last about two years while the stock rebuilt.

How long did the shutdown actually last, and had the fishery recovered by the time it ended?

Phytoplankton, microscopic, photosynthetic plankton found throughout oceans, seas and freshwater, form the energy base of most aquatic food webs, the same role grasses and trees play on land. Macrophytes, aquatic plants large enough to see with the naked eye, whether emergent, submerged or floating, add further primary production, particularly in shallower freshwater and coastal systems. Both ultimately support everything further up the food web, from zooplankton through to the fish humans harvest.

Humans consume an enormous range of organisms from both freshwater and marine environments, spanning finfish, shellfish, crustaceans and aquatic plants. Two examples on very different scales:

• Nile tilapia, farmed and wild-caught across freshwater systems in Africa, the Middle East and Southeast Asia, is one of the most widely eaten freshwater fish on the planet and a dietary staple for tens of millions of people, a largely local and regional food source.
• Skipjack tuna, mostly caught in the open ocean and canned, is the single most heavily fished tuna species by volume worldwide, moving through a truly global supply chain from Pacific and Indian Ocean fishing grounds to supermarket shelves on every populated continent.

Global demand for aquatic food is rising for two compounding reasons: population growth, and a shift in dietary preferences toward more animal protein, including seafood, as incomes rise. According to the UN Food and Agriculture Organization, global per-capita consumption of aquatic foods rose from about 9.1 kg a year in 1961 to 20.7 kg in 2022, more than doubling, and growing at nearly twice the rate of world population growth over that period.

Rising demand has encouraged harvesting methods that catch more, faster, at the cost of long-term stock health and wider habitat damage. Match each method below to what makes it destructive, not just intensive.

The Newfoundland northern cod fishery, already covered in full on 1.3 Sustainability and revisited through a tragedy-of-the-commons lens on 3.2, is the standard real-world example of a fishery collapse: the dramatic and lasting decrease of a commercial stock to a point where it can no longer recover on its own. Here, the same collapse anchors the single most important number in fisheries management.

The maximum sustainable yield (MSY) is the highest possible annual catch that can be harvested from a stock indefinitely, without reducing its long-term ability to replace itself. In principle, fishing quotas should be capped at or below the MSY. In practice, harvesting at the true MSY requires much lower fishing effort than most real fisheries have historically used, which is exactly the gap that let stocks like northern cod get harvested past the point of recovery.

Climate change and ocean acidification are placing additional stress on aquatic ecosystems, on top of direct harvesting pressure, and can trigger population collapse in freshwater or marine systems even where fishing itself is well managed.

Ocean acidification, the ongoing drop in seawater pH as the ocean absorbs excess atmospheric carbon dioxide, adds a second, chemical stressor: it makes it harder for shelled organisms to build and maintain calcium carbonate shells and skeletons, a mechanism this subtopic's Skills tab uses to plan an actual investigation.

Unsustainable exploitation of freshwater and marine ecosystems can be mitigated through policy legislation and changes in consumer behavior, acting at every level from a single household's shopping choices to binding international agreements. Common tools include catch permits, fishing quotas, closed seasons, minimum mesh sizes that let juvenile fish escape, protected zones, and sustainability food labeling that lets consumers choose differently.

Marine protected areas restrict or ban fishing across a defined zone, and can support aquatic food chains and sustainable yields well beyond their own boundaries. A protected breeding or nursery ground can act as a source of larvae and juveniles that spill over into surrounding, still-fished waters, effectively subsidizing catches outside the protected zone rather than just removing fishing pressure from inside it.

Aquaculture, the farming of aquatic organisms including fish, molluscs, crustaceans and aquatic plants, is expanding worldwide to increase food supplies and support economic development. It comes with environmental costs alongside its benefits.

One management technique used to reduce these impacts: stocking pens with cleaner fish, species such as wrasse and lumpfish that graze sea lice directly off farmed salmon, reducing reliance on chemical treatments. A second, more capital-intensive option is closed-containment or land-based recirculating systems, which physically separate farmed fish from open water, preventing both escapees and lice transfer at the source, at a significantly higher cost than open net pens.