Net Gain

Net Gain: sustainable harvesting and fisheries management for Cambridge IGCSE Marine Science

This free game is part of our Cambridge iGCSE Marine Science (0697) course for exams in EXAM YEARS 2027-2029.

Ten years as a herring skipper

Net Gain is a 3D game about one of the biggest questions in marine science: how can we keep catching fish without the fish running out? It covers the Cambridge IGCSE Marine Science (0697) points on why a growing human population puts fisheries in danger of being over-harvested, on outlining and evaluating strategies for sustainable harvesting, and on why sustainable harvesting keeps fisheries going for future generations and keeps fish stocks at levels that do not affect food chains.

You skipper a herring trawler out of Port Kelda, a fishing town on the North Sea, for ten years. You steer the boat, shoot your net, tow it through the shoals and haul in the catch. Every fish caught is gone from the sea. Adult herring spawn on a gravel bank in late summer, their eggs stick to the gravel, the young fish grow up in a sheltered nursery bay and the next year they join the adult shoals. A fisheries scientist, Iona, explains each idea as you meet it.

Watch a fish stock crash

The first three years are a free-for-all. The town grows, more people want fish and more boats join the fleet every year. Nobody limits the catch, young fish are caught before they can breed and the boats fish the adults while they gather to spawn. After every year a report shows the herring stock and the catch on a chart, and you watch the stock fall below the danger level. The gannets, seals and porpoises that eat herring leave too: overfishing breaks the food chain.

Bring in the rules, one a year

• Licences and boat size: you choose which boats get one of the few fishing licences. Give one to a big trawler or a supertrawler and you find out why restricting boat size matters.
• Net types and mesh size: you pick your net. With a larger mesh you can see the young herring slip out through the holes, so they can grow up and breed.
• Closed season: in August and September the herring spawn. You keep your net up while the biggest shoals of the year gather on the gravel bank, and watch the eggs that will become next year's fish.
• Quotas: you set the fleet's total catch with a slider, using the scientists' advice on how much the stock can replace. Haul too late and the fish over your quota are thrown back dead as discards.
• Marine Protected Area: you drag a no-fishing zone onto the best place, the herring nursery, and watch fish spill over its edge.
• International agreement: the herring migrate into a neighbour's waters, where foreign boats fish with no rules, until both countries sign one set of shared rules.

Evaluate every strategy

Exam questions ask you to outline and evaluate these strategies, so the game shows the cost of each one as well as the benefit. Fewer licences mean some fishers lose their income. A closed season means months without earnings. Quotas cause discards and are hard to enforce. An MPA leaves less sea to fish, so fishers may catch less at first and travel further. International agreements only work if every country signs and sticks to them. The recap at the end lists each strategy with what it does and a drawback, next to the chart of your own ten years.

A game takes about 5-10 minutes, your best score is saved and there are three quick checks along the way, so it works well as a starter, a revision task or homework alongside the lesson.