5.8 - Impacts of Overfishing
Learning Objectives
Describe the causes and problems that come with overfishing, focusing on both environmental problems (like biodiversity decline) and economic problems (like income loss and lost tourism). The skill here is describing possible approaches or responses to an environmental problem.
I. Fisheries and Fishery Collapse
A fishery is a population of fish used for commercial fishing, or an area where people can fish in large quantities to make a living.
A. Fishery Collapse
Fishery collapse happens when overfishing causes a fish population to decline by over 90%.
Real example (Atlantic cod): Off Newfoundland, Canada, decades of heavy fishing, much of it by huge factory trawlers, drove the northern cod's spawning population down to about 1% of its peak. In 1992 Canada banned commercial fishing for northern cod, and about 30,000 people in Newfoundland and Labrador lost their jobs, the largest layoff in Canadian history. The cod recovered very slowly, and the commercial ban wasn't lifted until 2024.
Example calculation: A fish population was estimated at 800,000 metric tons. Twenty years later it's 56,000 metric tons. of its original size, which is a 93% decline, so this fishery has collapsed (a decline of more than 90%).
Global status: FAO's 2026 report found that about 37.6% of the world's assessed fish stocks are overfished, compared with about 10% in 1974.
B. Long-Term Ecological Vulnerabilities
Once a fish population gets very small, it may never recover. A few reasons why:
- Low genetic diversity: Small populations have low genetic diversity, which makes them less resilient. They may not have the helpful mutations or adaptations they need to bounce back from other ecosystem disturbances.
- Reproductive problems: A very small population can end up with a skewed sex ratio, making it hard for fish to find a mate and reproduce.
- Inbreeding depression: Closely related members end up breeding with one another, which leads to inbreeding depression.
II. Environmental and Economic Impacts
A. Ecological Impacts (Biodiversity and Trophic Effects)
- Direct biodiversity loss: Overfishing can wipe out certain species entirely, lowering the species biodiversity of specific parts of the ocean.
- Trophic cascade: Overfishing large predatory fish like tuna and salmon has massive effects on the trophic pyramid.
- Fishing down the trophic level: As large predatory fisheries get depleted and collapse, commercial fishing shifts to smaller fish lower on the food chain, like herring and sardines. This is called fishing down the trophic level.
- Impact on higher trophic levels: Catching these smaller “forage fish” makes it even harder for the already decimated predatory fisheries to recover, and it also cuts into the food source for large marine mammals and seabirds.
- Impact on lower trophic levels: Removing forage fish (which are predators of zooplankton) causes the zooplankton population to spike. That spike then drives the phytoplankton population (the base of the food web) down. This chain of effects rippling through other species is a trophic cascade.
B. Economic Loss
When a fishery collapses, it can no longer serve as a productive source of income for fishermen. Communities that rely on tourism dollars suffer too. People who came to the area, stayed in hotels, and ate in restaurants because of the good fishing stop showing up, and all those dollars disappear.
- Example data: Extreme overfishing over a period like 1975 to 1985 can produce peak profits (say, $325 million), but this eventually leads to a sharp drop in profitability years later (falling below $100 million), which shows a huge decrease in financial productivity.
- Food security: Fish and other aquatic foods supply at least a fifth of the animal protein eaten by about 3.1 billion people (FAO, 2026). When fish stocks crash, coastal communities, especially in developing countries, lose a main source of protein, not just income.
C. Overfishing as the Tragedy of the Commons
Fishery collapse is a textbook example of the tragedy of the commons. It happens because there are often no laws, regulations, or financial penalties in place to discourage overfishing. The mindset of “if I don’t fish here, somebody else will” encourages people to extract unsustainably.
III. Damaging Fishing Practices
Bottom trawling is an especially damaging practice.
- Mechanism: A net is dragged along the ocean floor, scooping up fish.
- Bycatch: It is effective at catching the target fish, but it also catches bycatch, meaning unintended species like dolphins, whales, and turtles, killing many non-target organisms.
- Habitat destruction: The net physically scrapes up and breaks apart coral reef ecosystems. Coral reefs are home to thousands of species and are hugely beneficial for biodiversity, so destroying them lowers biodiversity.
- Increased turbidity: Bottom trawling stirs up a lot of ocean sediment, which increases the turbidity (cloudiness) of the water. High turbidity is a problem for:
- Photosynthesis: It blocks sunlight from penetrating the ocean surface, limiting photosynthetic algae in coral reefs.
- Predators: It makes life harder for predators that hunt by sight.
- Fish: Sediments can get stuck in fish gills, making it harder for them to breathe.
IV. Proposed Solution to Fishery Depletion
Since the skill asks for a proposed solution, one approach is to put strict regulatory measures and financial consequences in place.
- This would mean establishing fishing quotas (limits on how much fish can be caught) and enforcing fines or penalties for going over those limits.
- Regulations should also ban or strictly limit destructive methods like bottom trawling to protect essential habitats such as coral reefs and cut down on bycatch.
- Marine protected areas (MPAs): Areas of ocean where fishing is limited or banned. Fish inside can grow larger and reproduce, and their offspring spill over into nearby areas that are fished.
- Catch shares: Each fisher or company gets a guaranteed share of the total quota. Because they own a share of future catches, they have a reason to keep the stock healthy, which removes the 'if I don't catch it, someone else will' pressure.
- Bycatch reduction: Gear changes like turtle excluder devices (an escape hatch in shrimp nets) and larger net mesh let non-target animals and young fish escape.
- By making overfishing financially painful for the individual, the practice gets discouraged, which offers a way out of the tragedy of the commons.
Big picture: Overfishing is the aquatic version of the tragedy of the commons, driven by short-term profit. The consequences reach far: destructive methods like bottom trawling physically dismantle underwater cities (coral reefs), while the constant chase for profit pushes humanity to hunt progressively down the food chain, triggering a trophic cascade that destabilizes the whole ocean ecosystem and turns rich fishing grounds into unproductive, fragile populations.