Pressures on marine life
0 of 4 hands-on
01 Introduction
The sea is under pressure from many directions at once, and the pressures add up. Knowing what they are helps divers recognise them on a dive and understand which actions matter most. This lesson is a map of the main ones.
Objectives
- Name the main human pressures on the sea and the habitats they affect
- Explain coral bleaching in simple terms
- Describe how plastic, runoff and fishing gear harm marine life
02 Watch
Many pressures at once
5 min · Video in production
- 1
A fishing boat hauling a full net, then a turtle caught in a discarded net on a reef.
- 2
Runoff after heavy rain staining the water brown at the edge of a bay, with a reef behind it.
- 3
A coral colony bleaching in time-lapse, with white skeleton showing and then algae growing over it.
- 4
A hand reaching into the water to pick up plastic bottles from the sand, then a healthy reef at the end.
03 Learn
The main pressures
| Pressure | What it is | What it does |
|---|---|---|
| Overfishing | Catching fish faster than they can reproduce | Removes top predators and large breeders, upsets food webs. More than a third of assessed fish stocks are fished beyond sustainable levels. |
| Destructive fishing | Bottom trawling, blast fishing, poison | Flattens or poisons habitats, and kills everything nearby |
| Bycatch and ghost gear | Unwanted animals caught, and lost or abandoned nets and lines | Turtles, sharks, rays and mammals are caught, and lost gear keeps catching for years |
| Pollution | Plastic, sewage, fertiliser, chemicals, noise | Entanglement, swallowed plastic, algal blooms, low-oxygen zones, disturbed behaviour |
| Coastal development | Building, dredging, removing mangroves and seagrass | Silt smothers reefs, and nursery habitats disappear |
| Climate change | Heat waves, acidification, stronger storms | Bleaching, slower reef growth, shifting species |
| Invasive species | Animals and plants moved by ships or releases | Native species are outcompeted or eaten |
| Tourism and diving | Physical contact, anchors, crowding, waste | Broken coral, stressed animals, disturbed habitats |
Coral bleaching
Reef-building corals live in partnership with tiny algae in their tissue. The algae make food from sunlight and give the coral its colour. When the water stays too warm for too long, the coral expels the algae and turns pale, showing its white skeleton. This is bleaching. As little as 1 degree Celsius above the usual summer maximum, held for several weeks, can trigger it.
A bleached coral is not dead yet. If the water cools soon enough, the algae can return and the coral can recover. If the heat lasts, the coral starves and dies. The most recent global bleaching event, which began in 2023, has been the most widespread on record.
Predict first
You dive a reef where many corals have turned white. What is the most accurate description?
Plastic, line and ghost gear
Millions of tonnes of plastic enter the sea every year. Large pieces entangle animals. Small pieces are mistaken for food, and they break into microplastics that spread through the whole food web. Lost fishing nets, traps and lines, called ghost gear, keep fishing without a fisher. Divers often meet them on wrecks and reefs.
Runoff and nutrients
Fertiliser, sewage and sediment washed from the land feed algae growth. Too much algae can smother corals and use up oxygen when it decays. Silt reduces light and clogs the feeding structures of corals and sponges. Heavy rain, cleared land and coastal building all increase the load.
Take a guess first
A branching reef coral can grow several centimetres a year, but many large, boulder-shaped corals grow only about 1 cm a year. At that rate, how many years does a boulder coral take to grow 50 cm across?
Pressures combine
Pressures rarely act alone. A reef weakened by runoff and overfishing will be less able to recover from a heat wave. A reef with healthy fish populations and clean water has better odds. That is why local action still matters even when the global changes seem too large to influence.
04 Explore
Try it yourself
Match each pressure with the harm it causes.
Match each pressure with the harm it causes.
- Overfishing
- Ghost gear
- Runoff
- Bleaching
- Invasive species
05 Apply
Scenario
On a wreck dive you find a large fishing net draped over a section of the wreck, with a small fish caught in it. You are at 24 m with a buddy, you are at about half of your gas, and you do not have a cutting tool or any entanglement training.
What is the best response?
06 Check
07 Summary
Key points
- Main pressures are overfishing, destructive fishing, bycatch and ghost gear, pollution, development, climate change, invasive species and tourism.
- Bleaching means corals expel their algae under heat stress. It is not always fatal if conditions improve.
- Plastic and ghost gear entangle and harm animals, and nets are hazardous to divers too.
- Pressures combine, so reducing local ones helps reefs survive global ones.
Before you apply this in the water
- Have I been trained by a qualified instructor for this?
- Am I medically fit to dive?
- Do I have a buddy and a plan?
- Am I diving within my training and limits?
Answer every question in the Check section to mark this lesson complete.