Sensitivity analysis of residency and site fidelity estimations to variations in sampling effort and individual catchability

Abstract

Mark-recapture techniques are fundamental for assessing marine mammal population dynamics and individual temporal patterns. Since biases imposed by field conditions are generally unknown, we simulated variations in sampling effort (m) and maximum individual catchability (rmax) to analyze their effects on residency levels measured through the number of recaptures (occurrence, O), duration of stay (permanence, P), and average recurrence (periodicity, I) relative to a reference level of exhaustive daily sampling frequency. The number or recorded individuals (Dr) was also used to determine the performance of the simulations. Results for standardized (s) parameters showed that occurrences (Os) were proportional to m and were not influenced by rmax. Individual permanence (Ps) and individual periodicity (Is) were 8-49% and 3-11.74 times lower than expected, respectively, depending on m and rmax. Also, Os, Ps, and Is were not influenced by study duration, thus inter-study comparisons are feasible if m and rmax are similar. Dr was 68-92% (rmax= 0.01) and 1-8% (rmax= 1.0) lower than expected depending on m. Longer studies were more accurate but greater effort did not significantly increase Dr estimates. The use of bimonthly sampling frequencies (m= 0.07) was barely accurate and predictions for incomplete datasets were poor. Survey field data were also analyzed from 14 published studies on 4 dolphin species and compared to daily sampling frequencies; resulting values for Os, Ps, and Dr were 62.4-93.3%, 11.6-66.4%, and 2.4-33.8% lower than expected, respectively; also Is was 2.3-7.3 times lower than expected. The model produced Dr values that were similar to population estimates from empirical data, and bias was smaller than 15% in 87.5% of the cases, thus simulation accuracy was deemed acceptable.