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 (r max) 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 (D r) was also used to determine the performance of the simulations. Results for standardized (s) parameters showed that occurrences (O s) were proportional to m and were not influenced by r max. Individual permanence (P s) and individual periodicity (I s) were 8-49% and 3-11.74 times lower than expected, respectively, depending on m and r max. Also, O s , P s , and I s were not influenced by study duration, thus inter-study comparisons are feasible if m and r max are similar. D r was 68-92% (r max = 0.01) and 1-8% (r max = 1.0) lower than expected depending on m. Longer studies were more accurate but greater effort did not significantly increase D r 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 O s , P s , and D r were 62.4-93.3%, 11.6-66.4%, and 2.4-33.8% lower than expected, respectively; also I s was 2.3-7.3 times lower than expected. The model produced D r 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.

Development of a site fidelity index based on population capture-recapture data

“…This matrix has the sampling occasion represented as columns and individuals as rows, which only contain zeroes and ones, indicating the presence of a particular individual in a sampling occasion. The three most common indicators used in studies that describe site fidelity are the Occurrence, Permanence and Periodicity, which were defined below ( Morteo, Rocha-Olivares & Morteo, 2012 ; Balmer et al, 2008 ; Simões-Lopes & Fabian, 1999 ; Ballance, 1990 ) (see Data S1 for example of indicators’ application).…”

“…First, the index performance was studied using simulated populations. An algorithm was designed to generate simulations of mark-recapture surveys ( Morteo, Rocha-Olivares & Morteo, 2012 ; Seber, 1982 ), in which each individual could be identified without error. The simulations were performed using R software by writing their algorithm ( R Development Core Team, 2015 ; ).…”

“…The simulations were based on daily surveys, in which the first occasion was considered as a capture and the other instances as recaptures. The simulations assumed that the ideal conditions were met: (1) capture equitability: each individual is recognizable, and has the same probability of being marked and captured; (2) sampling independence: marking or capture of an individual does not affect the probability of being recaptured; (3) observer independence: all individuals keep their marks, and all observed individuals are recorded without fail; (4) geographic independence: the entirety of study area has the same probability of being sampled, and individuals may be found anywhere (spatial homogeneity); (5) social independence: the probability of marking an individual does not depend on the presence of others; (6) closed population in equilibrium: there is no permanent migration and the number of deaths is equal to births; (7) binomial probability of capture: the Bernoulli process determines whether individuals are observed (probability of success) or not (probability of failure) at time t ( Morteo, Rocha-Olivares & Morteo, 2012 ).…”

“…There is a variety of indicators that are used for site fidelity metrics, but the most commonly used metrics are Occurrence, Permanence and Periodicity ( Morteo, Rocha-Olivares & Morteo, 2012 ; Simões-Lopes & Fabian, 1999 ). For example, Storz et al (2000) , in their study of dispersion and site fidelity of fruit bats ( Cynopterus sphinx ), used the proportion of days in which an individual was resident in a particular site as an indicator for site fidelity permanence.…”

Distribución espacial, estructura social y amenazas de conservación de una pequeña comunidad de delfines nariz de botella, Tursiops trunca (Odontoceti: Delphinidae) en Ecuador

“…Fidelidad al área de estudio. Se estimó mediante el índice de ocurrencia (IO), determinado por el número de veces en las cuales los individuos fueron vistos en el total de ocasiones de recaptura posibles en un período determinado (Morteo, Rocha-Olivares, & Morteo, 2012b). Un valor de 1 indica que un animal fue visto siempre en cada ocasión.…”