1. The fraction of a fish population which lives in regions where it is susceptible to fishing during a given fishing season. This fraction receives recruits from or becomes mingled with the non-available part of the stock at other seasons, or in other years (any more or less completely isolated segment of the population is best treated as a separate stock); 2. The proportion of a stock which is susceptible to fishing. Fish become "available" to fisheries through migration, growth and /or change of behavior (from pelagic to demersal). If an available section of a stock is sufficiently isolated from the rest of the stock, it could be considered a management unit and be managed as such; 3. Describes whether a certain kind of fish of a certain size can be caught by a type of gear in an area.