Impact of Exploitation of the Oceans » Effects on Target Species

What you'll learn this session

Study time: 30 minutes

How commercial fishing affects target fish populations
The concept of overfishing and its consequences
Methods used to measure fish stock depletion
Case studies of collapsed fisheries
Sustainable fishing practices and management strategies
The economic and ecological impacts of target species exploitation

Effects of Ocean Exploitation on Target Species

Humans have been fishing the oceans for thousands of years, but modern industrial fishing has dramatically increased our ability to catch fish. This has led to serious consequences for many marine species that are directly targeted by fishing operations.

Key Definitions:

Target species: Marine organisms specifically sought by fishing operations for commercial purposes.
Overfishing: Catching fish faster than they can reproduce and grow.
Maximum Sustainable Yield (MSY): The largest catch that can be taken from a species' stock over an indefinite period without harming its population.
Stock collapse: When a fish population falls to less than 10% of its historic levels.

★ Commercial Fishing Methods

Modern fishing uses highly efficient methods that can capture massive quantities of fish:

Trawling: Dragging large nets through the water or along the seabed
Purse seining: Encircling schools of fish with nets that close like a purse
Longline fishing: Using lines with thousands of baited hooks
Gillnetting: Setting mesh nets that trap fish by their gills

★ Direct Impacts on Fish Populations

Commercial fishing affects target species in several ways:

Reduction in overall population size
Changes in age structure (fewer older, larger fish)
Reduced genetic diversity
Disruption of spawning patterns
Evolutionary changes (fish maturing at smaller sizes)

Understanding Overfishing

Overfishing occurs when fish are harvested at rates faster than they can reproduce. This leads to declining populations and can eventually cause fishery collapse. About 33% of global fish stocks are currently being fished at unsustainable levels.

The Overfishing Cycle

Overfishing typically follows a predictable pattern:

→ Stage 1: Discovery

A productive fishing ground is discovered, leading to initial high catches with minimal effort.

→ Stage 2: Expansion

More fishing vessels join in as word spreads about the profitable area. Technology improves to increase catches.

→ Stage 3: Decline

Catch rates begin to fall as fish populations can't reproduce quickly enough to replace those being caught.

→ Stage 4: Intensification

Fishers invest in more advanced technology to maintain catch levels despite declining stocks.

→ Stage 5: Collapse

Fish populations fall below critical levels where commercial fishing becomes unprofitable.

→ Stage 6: Abandonment

Fishers move to new areas or target different species, sometimes leaving devastated ecosystems behind.

Measuring Fish Stock Depletion

Scientists use several indicators to assess the health of fish populations:

♦ Scientific Indicators

Catch Per Unit Effort (CPUE): The amount of fish caught relative to the effort expended (e.g., hours spent fishing)
Size and age distribution: Healthy populations have a mix of ages; overfished populations show fewer older fish
Spawning stock biomass: The total weight of all sexually mature fish in a population
Recruitment rates: The number of young fish entering the fishable population each year

♦ Warning Signs of Overfishing

Declining average size of caught fish
Increasing effort needed to catch the same amount
Shifts in species composition (fishing down the food web)
Collapse of predator populations
Fishers travelling further to find fish
Targeting of previously "trash" fish species

Case Study Focus: North Atlantic Cod Collapse

The collapse of the North Atlantic cod fishery off Newfoundland, Canada, is one of the most dramatic examples of overfishing:

For centuries, cod was so abundant that ships reported being able to catch fish simply by lowering baskets into the sea
In the late 20th century, industrial fishing with factory ships and advanced technology intensified
Scientists' warnings about declining stocks were ignored in favour of short-term economic gains
By 1992, stocks had collapsed to 1% of historical levels, forcing Canada to declare a moratorium on cod fishing
Over 35,000 fishers and plant workers lost their jobs
Despite the fishing ban, cod stocks have shown limited recovery even 30 years later
The ecosystem has fundamentally changed, with different species now dominating

Biological Effects on Target Species

Intensive fishing creates powerful evolutionary pressures on fish populations, leading to measurable changes:

♥ Evolutionary Impacts

Fishing selectively removes certain individuals from populations, causing evolutionary changes:

Earlier maturation: Fish evolve to reproduce at younger ages and smaller sizes
Reduced growth rates: Energy diverted to reproduction rather than growth
Behavioural changes: Fish becoming more elusive or changing migration patterns
Genetic bottlenecks: Loss of genetic diversity limiting adaptability to environmental changes

♥ Population Structure Changes

Fishing alters the natural age and size structure of fish populations:

Removal of large, old individuals (which are often the most fertile)
Disruption of social structures in schooling fish
Changes in predator-prey relationships
Altered sex ratios in species where size determines gender
Reduced resilience to environmental stressors like climate change

Economic Consequences of Target Species Depletion

The depletion of target species has far-reaching economic impacts beyond just the fishing industry:

Fishing community collapse: When fisheries fail, entire communities that depend on them can collapse economically
Increased fishing costs: As fish become scarcer, the cost of catching them rises
Food security issues: Many coastal communities rely on fish as their primary protein source
Market impacts: Fish prices increase as supply decreases, making protein less affordable
Shift to aquaculture: As wild fisheries decline, farm-raised fish production increases

Case Study Focus: Bluefin Tuna

The Atlantic bluefin tuna illustrates how high-value species can be driven to the brink of extinction:

Prized for sushi and sashimi, a single bluefin tuna can sell for over £1 million
Population has declined by over 97% from historic levels in some regions
Average size of caught fish has decreased dramatically
International Commission for the Conservation of Atlantic Tunas (ICCAT) has struggled to implement effective conservation measures
Illegal, unreported and unregulated fishing continues to threaten recovery
Shows how luxury demand can drive unsustainable fishing practices

Sustainable Management Approaches

Several strategies can help prevent target species depletion and promote recovery:

✓ Fishing Quotas

Setting limits on how much fish can be caught, based on scientific assessments of population health.

✓ Marine Protected Areas

Creating "no-take" zones where fish can reproduce and grow without fishing pressure.

✓ Gear Restrictions

Regulating fishing equipment to reduce catch of undersized fish and limit bycatch.

✓ Seasonal Closures

Prohibiting fishing during spawning seasons to protect breeding populations.

✓ Certification Schemes

Programs like the Marine Stewardship Council that certify sustainable fisheries.

✓ International Agreements

Coordinated management of fish stocks that cross national boundaries.

Conclusion

The exploitation of target species in our oceans has profound consequences for marine ecosystems, human food security and coastal economies. Understanding these impacts is crucial for developing effective conservation strategies. While many fish stocks are currently overexploited, there are promising examples of recovery when science-based management is implemented. The future of our oceans depends on balancing human needs with the biological limits of marine ecosystems.

🧠 Test Your Knowledge!