Topic 6: Human Impact and Resource Management
How humans affect the ocean and what we can do to protect it.
🗺️ Topic 6 Mind Map
6.1 Overfishing and Fish Stock Management
Overfishing occurs when fish are caught faster than they can reproduce, causing population decline.
Destructive Fishing Methods
| Method | How It Works | Problems |
|---|---|---|
| Bottom trawling | Heavy net dragged along seafloor | Destroys seabed habitats, high bycatch |
| Drift nets | Large nets left to drift in open water | Catches non-target species (dolphins, turtles) |
| Longlining | Long line with many baited hooks | High bycatch of seabirds and sharks |
| Dynamite fishing | Explosives used to stun/kill fish | Destroys coral reefs and kills all organisms |
| Cyanide fishing | Poison sprayed to stun fish for aquarium trade | Kills coral and non-target species |
Effects of Overfishing
- Fish populations decline below sustainable levels
- Disruption of food webs — removing one species affects others
- Loss of biodiversity
- Economic impact — fishing communities lose their livelihood
- Trophic cascades — e.g., removing predators causes prey populations to boom
6.2 Sustainable Fishing Practices
Sustainable fishing means catching fish at a rate that allows populations to maintain themselves — at or below the maximum sustainable yield (MSY).
Management Strategies
| Strategy | How It Works |
|---|---|
| Fishing quotas | Limit total catch allowed per species per season |
| Minimum mesh sizes | Juvenile fish escape through larger mesh; grow to breed |
| Closed seasons | Ban fishing during breeding/spawning periods |
| No-take zones / MPAs | Areas where fishing is banned, allowing recovery |
| Size limits | Must return fish below a minimum size |
| Gear restrictions | Ban destructive methods like bottom trawling or drift nets |
| Boat/licence limits | Restrict the number of boats allowed to fish an area |
| Aquaculture | Farm fish to reduce pressure on wild stocks |
6.3 Pollution
Types of Marine Pollution
| Type | Sources | Effects |
|---|---|---|
| Plastic | Single-use items, fishing gear, industrial waste | Entanglement, ingestion, microplastics in food chains, habitat damage |
| Chemical | Industrial discharge, pesticides, fertilisers | Toxicity, bioaccumulation, biomagnification in food chains |
| Oil | Spills from tankers/rigs, routine shipping | Coats organisms, destroys habitats, toxic compounds |
| Sewage | Untreated waste from coastal towns | Eutrophication, bacterial contamination, oxygen depletion |
| Noise | Shipping, sonar, construction | Disrupts whale/dolphin communication and navigation |
Microplastics
Microplastics (<5mm) come from breakdown of larger plastics and from products like cosmetics. They enter food chains when ingested by plankton and filter feeders, and accumulate through biomagnification.
Bioaccumulation vs Biomagnification
- Bioaccumulation: Build-up of toxins in ONE organism over its lifetime
- Biomagnification: Increasing concentration of toxins at EACH trophic level up the food chain
6.4 Climate Change Effects on Oceans
Burning fossil fuels increases atmospheric CO₂, causing global warming. This has three major effects on oceans:
| Effect | Cause | Impact on Marine Life |
|---|---|---|
| Ocean warming | Absorbed heat from atmosphere | Coral bleaching, species migration towards poles, disrupted breeding |
| Ocean acidification | Absorbed CO₂ → carbonic acid | Dissolves calcium carbonate shells/skeletons (corals, molluscs, urchins) |
| Sea level rise | Thermal expansion + ice melt | Flooding of coastal habitats, loss of nesting sites, mangrove/saltmarsh loss |
Species Range Shifts
As oceans warm, many marine species are moving towards the poles to find cooler water. This disrupts existing ecosystems and food webs in both the areas species leave and the areas they colonise.
- Ocean pH has dropped from ~8.2 to ~8.1 since pre-industrial times (~30% more acidic)
- Sea level has risen ~20cm in the last century
- Thermal expansion = warming water takes up more volume (main cause of sea level rise)
6.5 Marine Conservation and Protected Areas
Marine conservation aims to protect and restore ocean ecosystems through:
- Marine Protected Areas (MPAs): Restrict human activities in designated zones
- No-take zones: Complete ban on resource extraction — allows full recovery
- Legislation: Laws banning harmful practices (e.g., dumping, destructive fishing)
- International agreements: Countries cooperate on shared ocean issues
- Education and awareness: Teaching communities about sustainable practices
- Habitat restoration: Replanting mangroves, coral reef restoration projects
Benefits of MPAs
- Fish populations recover and spill over into surrounding areas
- Protect breeding and nursery grounds
- Maintain biodiversity and ecosystem health
- Support sustainable tourism (e.g., diving)
6.6 Renewable Marine Energy
The ocean can provide renewable energy — energy from sources that are naturally replenished and won't run out.
| Source | How It Works | Advantages | Disadvantages |
|---|---|---|---|
| Offshore wind | Wind turbines in the sea | Strong, consistent winds offshore; no land use | Expensive to build/maintain; visual impact; bird strikes |
| Tidal energy | Barrages or underwater turbines capture tidal flow | Highly predictable and reliable | Disrupts estuarine habitats; expensive; limited suitable sites |
| Wave energy | Devices on the surface capture wave motion | Abundant energy source; renewable | Technology still developing; storm damage risk; expensive |
- Renewable = naturally replenished, won't run out
- Non-renewable = finite supply (oil, gas, coal)
- Marine energy reduces CO₂ emissions and dependence on fossil fuels
- All sources have some environmental impact — weigh benefits vs costs
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