Trawl Survey

A trawl survey is a fishery-independent research cruise that tows a standardized bottom or pelagic trawl along fixed stations to estimate the abundance and size composition of a fish stock. Because the gear, towing speed, and station design are held constant year to year, the catch rate gives a comparable index that feeds stock-assessment models such as the swept-area biomass estimate and tuning data for age-structured models. ICES and national institutes run long-running surveys (for example the North Sea International Bottom Trawl Survey) whose indices underpin total allowable catch advice. Survey catchability and gear standardization are the main sources of index uncertainty.