Inter-individual association levels correlate to behavioral diversity in coastal bottlenose dolphins (Tursiops truncatus) from the Southwestern Gulf of Mexico

Abstract

Las asociaciones entre individuos se correlacionan con la diversidad de las conductas en delfines costeros (Tursiops truncatus) del Sureste del Golfo de MéxicoInter-individual association levels correlate to behavioral diversity in coastal bottlenose dolphins (Tursiops truncatus) from the Southwestern Gulf of Mexico Introduction: Societies in mammal species are influenced by intrinsic and extrinsic factors that affect the temporality of the associations among individuals. Coastal bottlenose dolphins generally live in small fluid aggregations variable in composition, but the nature of their associations is commonly unknown. Our goal was to determine if school size was influenced by dolphins' behavior, and if individuals associated to develop particular activities within the coastal waters of Alvarado, Mexico. Methods:In total, 80 boat-based surveys were conducted (2002 -2003 and 2006 -2009), where group size, behavior, and photo-identification data were collected. From 237 sightings and 2,021 dolphins the mean school size was 8.5 animals (s. d. = 8.6), but individuals and pairs were observed more frequently (33 %).Results: Temporal differences in school sizes and behavior conveyed with habitat seasonality (P < 0.05), but were inconsistent across years; thus short-term factors such as marine traffic and fisheries intensity were deemed important. Dolphins were commonly feeding (29 %) corresponding to groups of 4 -6 animals, whereas solitary individuals typically showed evasion (P < 0.01). Discussion and Conclusions:Association coefficients computed for 89 of the 232 identifiable dolphins proved not random only in 6 % of the 3,915 combinations (P < 0.05), and these coefficients were positively correlated to the diversity of activities developed by each dyad (P < 0.01), thus the nature of their associations in most cases (93 %) became richer with time. The community is likely formed by very small units that frequently exchange members; this may be due to large food availability and low predator abundance in the area, but also to avoid detection and threats posed by local artisanal fisheries.

Influence of artisanal fisheries on the behaviour and social structure ofTursiops truncatusin the South-western Gulf of Mexico

“…Group size in coastal bottlenose dolphins usually varies between 5 and 15 individuals (Morteo, 2011); however, smaller groups (<5 individuals) were found in the coastal waters of Alvarado. This seems typical for the studied population (Morteo, 2011), thus it has been hypothesized to constitute a strategy that reduces detectability by fishers while also decreasing the chances of negative outcomes for the dolphins (García-Vital et al ., 2015); the latter as a response to the pressure by fishers that this dolphin population has suffered for decades, which was evidenced during this study, including violent retaliation to individuals that interacted with their gear (Del Castillo-Olvera, 2010; Morteo, 2011; Morteo et al ., 2017; Rechimont et al ., 2018). Conversely, in other study areas, where human-dolphin competition for fish is not as noticeable, dolphin group sizes observed around fishing activities are mostly related to the biomass of prey captured in the gear (Lauriano et al ., 2004; Rocklin et al ., 2009).…”

“…The adaptation of strategies for food acquisition (Sargeant & Mann, 2009), where adult male dolphins moving alone or in small groups are more frequently associated with fishing gear depredation (Adimey et al ., 2014; Morteo et al ., 2014; Labadie et al ., 2018; Rechimont et al ., 2018) has been linked to their curious and predatory behaviour, as well as their easy habituation to the varying conditions in their habitat (Adimey et al ., 2014; Morteo et al ., 2014, 2017; La Fauci, 2017). Many of these interactions have been attributed to adult males and point to the development of a sex-biased strategy to take advantage of captured fish (Morteo et al ., 2012, 2017; Garcia-Vital et al ., 2015; Rechimont et al ., 2018). Furthermore, female individuals within the population are more resident and thus experienced (Morteo et al ., 2014), and especially nursing or lactating individuals are known to avoid fishing areas (La Fauci, 2017), since these present a higher risk of injury (Srinivasan et al ., 2017), due to fishers' antagonistic responses to the encounters with bottlenose dolphins (Morteo et al ., 2012; Rechimont et al ., 2018).…”

“…It is noteworthy that frequencies for socio-sexual and play were low, but they showed significantly higher local rates during the operational interaction (Figure 4A and B, respectively). This may be explained considering that dolphins that are interacting with the fisheries are not categorized as residents, as shown by the occurrence records from the long-term photo-identification programme for this dolphin population since 2002 (Morteo, 2011; Morteo et al ., 2012, 2014, 2017, Garcia-Vital et al ., 2015). Thus intense social and playful contact between dolphins may be indicative of recognition among the individuals involved (Dudzinski, 1998).…”

Opportunistic gillnet depredation by common bottlenose dolphins in the southwestern Gulf of Mexico: Testing the relationship with ecological, trophic, and nutritional characteristics of their prey

“…Hurtado-Mejorada (2021) already found that bottlenose dolphin echolocation trains were significantly more frequent during fishing manouvers in the Alvarado area. Also, Garcıá-Vital et al (2015) found lower group sizes as a possible adaptative response to lower the risk of human retaliation during interactions with fisheris (Morales-Rincon et al, 2019). Therefore, hearing interference due to high marine traffic (e.g.…”

“…In addition, as noted by Morteo et al (2017) and Bolaños-Jimeńez et al (2021), individual dolphins that are "new" to the area (the majority of which were presumed to be male; Morales-Rincon et al, 2019), have been found to have a higher tendency to interact with local artisanal fisheries than local residents. Furthermore, dolphins in the region reduce their group sizes and reduce their surface behaviors when interacting with gillnet fisheries (Garcıá-Vital et al, 2015;Morales-Rincon et al, 2019), presumably to lower the risk of being spotted, since they often face retaliation while interacting with local artisanal fisheries (Morteo et al, 2017;Rechimont et al, 2018). In addition, dolphin's groups may reduce their size possibly to lower intraspecific competition, and also because obtaining the resource in fishing nets is easier, so they do not need to employ cooperative hunting strategies, which also reduces interindividual competition (Methion and Dıáz Loṕez, 2020).…”