Assessing fish stocks at Higher Level: This statement is Higher Level only. It shows how scientists count the fish in a stock and how inspectors check what is caught, because sustainable quotas depend on both.
Practise this as you read
- Describe one method of measuring a fish stock.
- Describe one method of monitoring harvest rates.
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You cannot manage what you cannot count: To fish sustainably, managers need to know how many fish there are and how many are being taken. A fish stock is counted by scientists; the harvest is watched by inspectors.
The points to remember
- Catch limits (quotas) are set from an estimate of the stock: how many fish, and how fast they breed.
- If the estimate is too high, the quota is too high and the stock is overfished.
- If it is too low, fishers lose income and food for no reason.
- Two jobs: measure the stock (how many fish are there?) and monitor the harvest (how many are caught?).
- Fish are hard to count: they move, live out of sight and change in number every year.
Remember it as: Count the fish in the sea, then count the fish on the boat.
Measuring the stock
- How many fish are in the sea?
- Acoustic surveys, mark-recapture, catch per unit effort.
Monitoring the harvest
- How many fish are taken out?
- Logbooks, landings, satellite tracking, observers.
Real example: the cod off Newfoundland, Canada, collapsed in 1992 partly because the stock was estimated to be bigger than it really was, so the catch limits were set too high.
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Out at sea, most stocks are measured by acoustic surveys. It works best for fish that swim in shoals in open water.
Method 1: an acoustic survey
- A research ship sails set lines (transects) across the area.
- An echosounder sends sound pulses down; shoals of fish send echoes back.
- The strength of the echoes shows how many fish are under the ship.
- Trawl samples show which species and sizes make the echoes.
- The results are scaled up to the whole area to estimate the stock.
Remember it as: Ping, echo, sample, scale up.
Survey
Every autumn, Norwegian and Russian research ships survey the capelin of the Barents Sea with echosounders.
Rule
No fishing is allowed if next spring's spawning stock is likely to be under 200,000 tonnes.
Result
The surveys found too few capelin, so the catch limit was zero for 2025, and zero was advised again for 2026.
Strengths
- Covers a huge area in a few weeks.
- Counts the fish directly, not from catches.
Limits
- Misses fish close to the seabed or the surface.
- Costly ships; storms can stop it (in 2025 one cut the survey short).
In a lake or a small bay, the stock can be counted by mark-recapture.
Method 2: mark-recapture in a lake
- Catch fish with nets or traps; choose a mesh that catches adults only.
- Mark them (a tag or a fin clip) and release them; give them time to mix.
- Catch a second sample and count how many are marked.
- Lincoln index: N = (n1 × n2) ÷ m.
- Repeat a year later: the increase is the natural increase, the most that can be caught sustainably (the MSY).
Remember it as: Mark some, mix them, catch again, count the marked.
| Perch in Lake Kerrin | This year | Next year |
|---|---|---|
| First catch, marked and released (n1) | 120 | 140 |
| Second catch, a week later (n2) | 150 | 175 |
| Marked fish in the second catch (m) | 30 | 35 |
| Population N = n1 × n2 ÷ m | 600 | 700 |
In Lake Kerrin, this year: 120 × 150 ÷ 30 = 600 perch. Next year: 140 × 175 ÷ 35 = 700. The stock grew by 100 adults in a year, so about 100 perch a year is the most that can be taken without the stock shrinking.
It works only if: Marks do not fall off or harm the fish; the marked fish mix evenly; and no fish are born, die, arrive or leave between the two catches. In the open sea fish move too far, so tags are used to track movement instead.
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A third way uses the fishing fleet's own records. If boats work just as hard but catch fewer fish, there are probably fewer fish.
Method 3: catch per unit effort
- Catch per unit effort (CPUE) = catch ÷ effort, e.g. tonnes per day at sea.
- It uses the fleet's own records, so it is cheap and covers the whole year.
- If CPUE falls, the stock is probably falling: the same work catches fewer fish.
- Trap: fish crowd into a few shoals and sonar finds them, so CPUE can stay high while the stock falls.
Remember it as: Same work, fewer fish: fewer fish in the sea. Unless they are all in one shoal.
1980s
- Big trawlers with sonar fished the Grand Banks. Their catch per day stayed high.
Shoals
- As the stock shrank, the last cod crowded into a few dense shoals, which sonar found easily.
Mistake
- The high catch rates were read as a healthy stock, so the catch limits stayed high.
1992
- The stock collapsed and Canada stopped the fishery.
A quota protects a stock only if catches are checked. Monitoring the harvest also feeds the next stock estimate.
Monitoring harvest rates
- Logbooks: the crew record what, where and when they catch; now often electronic.
- Landings checks: fish are weighed and recorded at the port when sold.
- Satellite tracking (VMS): each boat's position is sent to the authorities, so no-fishing zones can be checked.
- Observers and cameras on board record the real catch, including fish thrown back.
- Comparing the harvest with the quota shows when to close the fishery.
Remember it as: Write it, weigh it, track it, watch it.
Tracking
In the European Union every fishing boat over 12 m must send its position by satellite (a vessel monitoring system).
Records
Catches must be recorded electronically, so inspectors can compare them with the landings weighed at the port.
Cameras
Under rules agreed in 2023, high-risk boats over 18 m will have to carry cameras, which show any fish thrown back at sea.
Limits: Crews can under-report catches or throw unwanted fish back. Observers are costly, and boats far out at sea can switch off their trackers.
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How this comes up: Paper 2, Section B (a) [4]: outline methods. Section B (b) [7]: describe procedures for estimating a stock and its sustainable yield, or evaluate monitoring methods. Paper 1: data on a fishery.
Norway and Russia share the capelin stock of the Barents Sea and set a catch limit for it every year.
Outline one method of measuring the size of a fish stock and one method of monitoring its harvest rate.
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