Ecology of aquatic organisms
Ecology is the study of how living things live with each other and with their home.
Life cycle
Most fish lay many eggs. The eggs hatch into tiny larvae that drift as plankton. Most are eaten. Survivors become young fish (juveniles) in a safe nursery, such as a mangrove or reef. Later they become adults that breed. Some fish, such as salmon, live in sea and river both.
What decides how many fish
- Food: more plankton, more fish.
- Space and safety: nursery places matter.
- Temperature and water quality.
- Predators and fishing.
Fishery resources
A fishery resource is any water animal or plant we use: fish, shrimp, crab, squid, shellfish, seaweed. A stock is one kind of fish in one area that breeds together.
Renewable but limited
A stock is renewable, because fish breed. It is also limited, because the sea has only so much food and space. Each year the stock adds new fish; this is its growth. Growth is small when the stock is tiny (few parents) and small when the stock is crowded (little food). It is biggest in the middle.
The biggest catch that can be taken every year for ever is called the maximum sustainable yield (MSY). In our model, growth is biggest when the stock is half full.
Fisheries management
Fisheries management means making rules and plans so that fishing can go on for ever. First we need data: catch records, surveys and fish sizes. Then we choose rules.
Common tools
- Catch limit (quota): a fixed total weight for a season.
- Closed season or closed area: no fishing while fish breed or in nursery places.
- Mesh-size and gear rules: a bigger mesh lets young fish escape.
- Minimum landing size: small fish must be put back.
- Effort limits: fewer boats or fewer days at sea.
- Protected areas and stock enhancement: release young fish from hatcheries, build artificial reefs.
Rules work best when fishers help make them. Fishing communities that manage their own area often keep stocks healthy.
Try it
In the 3D: step 5. Set the catch to 20 and watch the bar. Then set 8. Then tick the rules box.
Predict first: what catch can the stock carry for ever? (Hint: look at the growth number in the text.) Check your guess.
At home with coins: 50 coins is the stock. Each round, add 25% of the coins you hold (growth), then remove 10. Do 5 rounds. Now try removing 20 each round.
Key formulas and definitions
- Next year stock = stock + growth − catch
- Model growth = 0.5 × S × (1 − S ÷ 100), S = stock (biggest at S = 50)
- Sustainable catch ≤ yearly growth; overfishing: catch > growth
Worked examples
1. A stock has S = 50. Growth = 0.5 × S × (1 − S/100). What is the growth, and what is the stock next year if 10 are caught?
Growth = 0.5 × 50 × (1 − 0.5) = 12.5. Next year = 50 + 12.5 − 10 = 52.5. The catch is smaller than growth, so the stock rises a little.
2. The same stock (S = 50) loses 20 each year. What happens in the first year? Is this sustainable?
Growth = 12.5. Next year = 50 + 12.5 − 20 = 42.5. The stock shrinks. With less stock, growth gets smaller too, so the fall speeds up. This is not sustainable; it is overfishing.
3. Why does a closed season during breeding help the stock?
Fish that are not caught lay their eggs. More eggs mean more young fish, so the stock grows back faster. Fishing breeding fish removes the very parents that would make next year's stock.
Common mistakes
- Thinking "renewable" means "unlimited". A renewable stock only recovers if enough parents are left.
- Thinking a bigger boat or net always means a better future catch. It can deplete the stock sooner.
- Thinking small fish are worthless, so catching them does no harm. They are the next year's adults.
- Mixing up catch and growth. Catch is what we remove. Growth is what the stock adds.