Capture Fisheries
Capture Fisheries tests stock assessment and management: how effort, catch, and mortality are converted into advice on exploitation. Expect CPUE, catch curves, mortality partitioning, MSY, selectivity, and regulation design.
Core rules
- Fishing effort must be standardized (gear type, vessel size, soak time, horsepower) before comparing CPUE across fleets.
- Total mortality Z from a catch curve is the negative slope of ln(number) against age; Z = F + M, and exploitation rate E = F/Z.
- Maximum sustainable yield under the logistic Schaefer model occurs at half the unfished biomass and about half the unfished fishing mortality.
- Length-frequency data (ELEFAN-type methods) estimate growth and mortality when ageing is impractical.
Worked example
A catch curve gives Z = 0.8 per year; an independent estimate of natural mortality is M = 0.4. Then F = Z − M = 0.4, and E = F/Z = 0.4/0.8 = 0.50. If the target is E = 0.40, the stock is overexploited and effort should be reduced by roughly 20% (0.40/0.50). Confirm with yield-per-recruit and biomass data before recommending a cut.
Traps that cost marks
Dropping the sign of the catch-curve slope; comparing unstandardized CPUE; assuming M is a universal constant; equating MSY with maximum economic yield; and treating a single year of high catch as evidence of a healthy stock.