The ideal fishery model accounts for several categories of fish encountered by fishing gear and the resulting mortalities. 'Catch' means all the fish taken by the gear. 'Landings' means all the fish brought to land. The difference between landings and catch forms the 'discards': fish caught and thrown back. The probability of death of a discarded fish (Pd) depends on many factors, including its biology and the mode offishing. If100 fish are discarded, the removals (total deaths due to fishing) are then the landings plus 100Pd. In certain fisheries, other categories of fishing-related deaths are distinguished.
Reliable estimation of discards and the discard-mortality fraction Pd is difficult. The former may require an at-sea observer program, and the latter, field and laboratory experimentation. Because of these difficulties, models that theoretically require data on total removals are often used with landings data instead. This imparts a bias to many analyses, which is most troublesome when the fraction of the catch discarded is not constant, as under changing regulations, increasing abundance, or highly variable recruitment.
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