Can population surveys show if the Mediterranean monk seal colony at Cap Blanc is declining in abundance?

1. The Mediterranean monk seal is an endangered marine mammal with only a f ew populations thought to be viable. The largest aggregation of this specie s is found at the Peninsula of Cap Blanc, on the western Sahara, where a co lonial structure is maintained. The size of this colony is estimated by ann ual capture-recapture surveys, but it is unclear if the attained level of p recision is enough to detect changes in numbers in the short term. 2. In this study, simulated capture-recapture experiments were used to inve stigate the relationship between population estimates and abundance. The st atistical power necessary to detect a population decline with ongoing surve ys was estimated, and the requirements for an improvement in precision were evaluated. Additionally, changes in abundance over consecutive surveys wer e examined for comparison with an alternative method of Bayesian inference. 3. Results indicated that the power to detect trends from the actual survey s was lower than should be required for the conservation of a small populat ion, such as the colony at Cap Blanc. A 5% annual rate of decline in abunda nce would only be detected with high power after 12 years, with a populatio n of 317 seals being reduced to 172. If the annual rate of decline was 10%, 8 years would be required to detect a reduction of 317 to 136 seals. 4. Capture-recapture surveys can produce reliable data but do not identify moderate or low population declines. The increase in precision of surveys i s costly and time consuming, and other monitoring methods are needed to det ect early signs of a decline. Bayesian methods cannot provide a better resu lt because they are not robust to small sample size and heterogeneity in ca pture probabilities. Despite the need for alternative methods for earlier d etection, capture-recapture surveys are still required to estimate the size of the colony, and precision in population estimates can be evaluated in r elation to cost effectiveness.