Abstract
The presence of transient and temporary individuals in capture-mark-recapture studies may violate the assumption on equal catchability, and thus yield biased estimates. We investigated the effects of residency patterns on population parameters of bottlenose dolphins inhabiting the coastal waters off the Alvarado Lagoon System (ALS), Veracruz, Mexico. We hypothesized that this population is open but there exists a “core community” that behaves as a closed population. Between 2006 and 2010, we conducted 75 photo-identification surveys and recorded 263 dolphin group encounters, in which 231 dolphins were identified. Individuals present during only one season, classified as transients (n= 85), were excluded from the study, and a standardized residency index (IH4) was computed for each dolphin that remained in the sample (n= 146). We used the K-means clustering method to split the sample into groups based on individual (seasonal, annual) IH4values. These clusters were named as regular residents (RR,n= 55), occasional residents (OR,n= 45), and occasional visitors (OV,n= 46). The cumulative frequency of newly identified individuals displayed an asymptotic trend for the whole sample and all clusters, indicating that most of the individuals present in the study area during the study period were identified. The assumption of demographic closure was tested to define the core community, and was rejected for the whole sample and the OV cluster (p< 0.001 in both cases), indicating that the population is open. The closure assumption was not rejected for RR and OR clusters (χ2= 6.88, DF = 13,p= 0.91, and χ2= 17.8, DF = 16,p= 0.33, respectively), indicating that these clusters were demographically closed over the 5-year period. Thus, we defined this aggregation of individuals as the “core community”. The closed population model Mthindicated that the total abundance of this core community was 123 individuals (95% CI: 114–133). Our results provide quantitative evidence of the existence of a core community in open waters of the Gulf of Mexico, and points toward residency pattern as a main driver of population dynamics. These results highlight the importance of considering residency patterns when dealing with heterogeneity in the sample of a highly mobile species.
Highlights
In most wildlife populations, sex and age have profound effects on the chances of an individual dying, or producing offspring (Neal, 2004), these factors are routinely considered when studying population structure and dynamics (Ojasti and Dallmeier, 2000)
site fidelity (SF) has been defined as the “tendency to return to a previously occupied location” (Greenwood, 1980; Switzer, 1993), whereas philopatry is the tendency to stay in a familiar environment, mainly related to the natal habitat (Begon et al, 1995)
As the average group size for the full study period was fairly small (9.0 ± 11.2 SD) and the standard deviation of the difference between extreme field estimates was less than one individual, we considered that our group size estimates are highly accurate, and representative of the total number of animals in the group
Summary
Sex and age have profound effects on the chances of an individual dying, or producing offspring (Neal, 2004), these factors are routinely considered when studying population structure and dynamics (Ojasti and Dallmeier, 2000). SF and residency patterns are evaluated using information on the number of recaptures, the duration of stay, and/or the average recurrence of individuals (see Ballance, 1990 and Morteo et al, 2012b). In this regard, Ballance (1990), stated that “a high number of resightings, a long period time between the first sighting and the last resighting, and a short time interval between adjacent sightings describes an animal with a high degree of residence.”. In this regard, Ballance (1990), stated that “a high number of resightings, a long period time between the first sighting and the last resighting, and a short time interval between adjacent sightings describes an animal with a high degree of residence.” individual residency patterns can be considered as a manifestation of SF and philopatry
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