Publication Abstract

Power of monitoring programmes to detect the collapse and recovery of rare and Vulnerable fish

D. Maxwell and S. Jennings

1. Many populations have collapsed following fisheries exploitation. Some formerly abundant but vulnerable species are now too scarce to be caught during monitoring surveys. So, when conservation concern is greatest, monitoring may provide little or no information on whether species are further declining or starting to recover in response to management action.

2. We calculated the statistical power of a large-scale annual fish monitoring survey to detect decline and recovery in the abundance of rare and depleted species. For comparison, we also calculated the power of surveys to detect trends in the abundance of the most common species.

3. For rare and depleted species, the power to detect rapid decreases in abundance on time-scales <10 years was poor. Moreover, even if conservation were effective and populations recovered at the maximum potential rate of increase, 5- 10 years monitoring would often be required to detect significant recovery in adult abundance.

4. Given the limited resources to increase replication on surveys, and that improvements to one survey would often be to the detriment of another, it is impossible to reliably monitor the status of most rare fishes affected by overexploitation on a species by species basis.

5. Power can be increased by developing a composite indicator, that tracks trends in the relative abundance of a suite of rare or depleted species with similar life histories. Such an indicator provides a useful overview of the conservation status of large and vulnerable species, and hence the impact of fishing on part of the fish community. However, given the current political focus on species based conservation, the species responsible for trends in this indicator would still need to be identified.

6. We propose that the extent and intensity of fishing activity, rather than the abundance of rare species, should be the favoured short-term (1-5 years) indicators of progress towards conservation targets. This indicator can be reported, and used to judge the success of management, on time-scales consistent with the life spans of Governments and management policies.

7. The abundance of rare or depleted species would be used as a longer-term (5- 10+ year) indicator, to confirm that changes and reductions in the extent and intensity of fishing activity had affected the conservation status of these species.

Reference:

D. Maxwell and S. Jennings (2005) Power of monitoring programmes to detect the collapse and recovery of rare and Vulnerable fish. Journal of Applied Ecology, 42(1): 25-37.