Brown crab and European lobster size composition data was collected by researchers onboard commercial crustacean potting vessels across Wales. Core measurements taken include: string number, number of pots per string, the abundance and size of each lobster and crab caught in the string, the sex of each animal, GPS location of string and whether the animal was landed or discarded. Additional measurements sometimes taken include: Abdomen width of lobsters, whether the animal was berried, pot type information (e.g., presence/absence of escape gaps, entrance size, pot size, top entry or side entry), soak time, moult stage, condition index and damage. Trips were sporadic and dependent on weather and fisher availability. Trips occurred more frequently March-August in line with the seasonality of pot fishing in Wales.