Publication Abstract

Reference points and reference directions for size-based indicators of community structure

S. Jennings and N.K. Dulvy

Size-based community and ecosystem metrics, such as mean body mass and slopes of size spectra, have been proposed as indicators to support the Ecosystem Approach to Fisheries Management (EAFM). These metrics show relatively consistent responses to size-selective exploitation, and 'unexploited' reference points can be predicted with models of size-structured food webs. In practice, however, metrics derived from monitoring surveys size-species-abundance data cannot be compared with reference points because survey gears are highly selective and because species, size, time and area-related catchability are not known. For this reason, and given the undesirable effects of fishing already observed in most fish communities, reference directions rather then reference points provide better medium term management targets. We show that the power of surveys to detect trends consistent with reference directions will depend on the range of body size-classes included in the analysis, and we demonstrate a simple approach for predicting the signal to noise ratio in survey species-size-abundance data. Selection of different body-size ranges will weight metrics to respond to the release of small fishes from predation, the depletion of larger individuals due to exploitation, or both. Such weightings will not be consistent over time since the differential vulnerability of larger species, within-population changes in mean body size and life history, genetic changes in life history, predator-prey relationships and the effects of competition will depend on contemporary rates of fishing mortality and the history of exploitation. The power of surveys to detect trends consistent with reference directions is poor on time-scales of less than 5- 10 years. Thus size-based indicators would be used as surveillance indicators, rather than indicators to support year on year management decisions. Trends counter to reference directions would trigger analyses to identify causes of the undesirable trends and relevant management actions. Trends in size based indicators consistent with the reference direction would demonstrate that target species management has concomitant benefits for the fish community, consistent with the EAFM

Reference:

S. Jennings and N.K. Dulvy (2005). Reference points and reference directions for size-based indicators of community structure. ICES Journal of Marine Science, 62(3): 397-404.