Constructing indices to detect temporal trends in discarding

Discards refer to the part of the catch not retained on board during commercial fishing operations, and returned to the sea. With numerous legislation, policies, and resolutions in place that seek to minimize or eliminate discards, performance indicators are required to measure progress towards this objective. Indices of discards were developed for the purpose of illustrating changes in overall discard patterns over time. A discard quantity index provides annual changes in total discard quantity, a discard rate index and discard proportion indices demonstrate how discarding behaviour during fishing operations changes with time. Knowledge of the consistency in the reporting of fishing effort and landings as well as in the sampling programme is important in identifying genuine trends. The indices examined show their potential as useful tools as indicators of changing discard patterns and were most powerful when used in combination. By utilizing data from the observer programme for England and Wales, a reduction of 61% between 2002 and 2008 in the weight of discards could be demonstrated. In this case study, the reduction in discard weight could be attributed to reducing levels of fishing effort and diminishing catch weights.