Population Dynamics Reveal a Core Community of the Common Bottlenose Dolphin (Tursiops truncatus) in Open Waters of the South-Western Gulf of Mexico

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) IH4 values. 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 Mth indicated 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.

Seasonal population parameters reveal sex‐related dynamics of common bottlenose dolphins in open waters of the southwestern Gulf of Mexico

“…In conclusion, our results support that the dolphin population that uses the coastal waters adjacent to ALS is open and small but emphasize the existence of a core community (as defined by Wells et al, 1987; see also Bolaños‐Jimenez et al, 2021) composed of resident females (Morteo et al, 2014), and males, which show temporal differences in recruitment. This population receives occasional influx of individuals from adjacent areas, with occasional peaks during seasons of higher productivity, supporting the use of this area as a feeding ground (Morteo et al, 2017, 2019) and shelter.…”

“…The number of individuals that were incorporated from the superpopulation was null in over 80% of intervals of our 5‐year study. Thus, the asynchrony in recruitment of individuals points towards differences in habitat use that are consistent with sexual segregation as mentioned before (Morteo et al, 2014) and/or with different residency patterns (Bolaños‐Jiménez et al, 2021), that are not related to seasonal fluctuations of the environment contradicting earlier estimates (Morteo et al, 2017). It is noteworthy that 86% of PF, 91% of PM, and 68% of SND individuals were already present in the population just before the start of our study, which supports the high and long‐term site fidelity for many of these dolphins (Morteo et al, 2014; Bolaños‐Jiménez et al, 2021).…”

“…Thus, the asynchrony in recruitment of individuals points towards differences in habitat use that are consistent with sexual segregation as mentioned before (Morteo et al, 2014) and/or with different residency patterns (Bolaños‐Jiménez et al, 2021), that are not related to seasonal fluctuations of the environment contradicting earlier estimates (Morteo et al, 2017). It is noteworthy that 86% of PF, 91% of PM, and 68% of SND individuals were already present in the population just before the start of our study, which supports the high and long‐term site fidelity for many of these dolphins (Morteo et al, 2014; Bolaños‐Jiménez et al, 2021). Furthermore, Bolaños‐Jiménez et al (2021) found that the dynamics of this population are mainly regulated by the influx of nonresident individuals, which is consistent with the sex‐related dynamics found in this study.…”

“…We identified, cataloged, and sexed individual adult dolphins based on the natural marks on their dorsal fins by using standard photo‐ID techniques (Bolaños‐Jiménez et al, 2021; Rosel et al, 2011; Würsig & Jefferson, 1990). Only long‐lasting marks such as cuts, nicks, and deformities were used for individual identification.…”

“…The detailed survey protocol is outlined in Morteo (2011) and Morteo et al (2014Morteo et al ( , 2017. The full description of the data set, analytical procedures, and spatial and temporal coverage is described elsewhere in Bolaños-Jiménez et al (2021). A detailed definition of methodological terms is presented here in Appendix S1.…”