Environmental and behavioral factors influencing individual variation in spatial use by Guiana dolphins (Sotalia guianensis)

Abstract

Marine apex predators, such as coastal dolphins, are key components of ecosystem function and dynamics; understanding their spatial use therefore is important for conservation planning. Here, we describe spatial use of the threatened Guiana dolphin, Sotalia guianensis, within a mosaic of protected areas at their southern range in the Paranaguá estuarine complex (PEC), Brazil. Based on environmental characteristics and S. guianensis population densities, individual photo-identification surveys were focused on two sectors of the northern mouth of the PEC between 2013 and 2015. We photo-identified 107 individuals and recorded 351 groups of Guiana dolphins; nearly 50% of which contained calves. Group sizes were small (mean ± SD: Sector 1 = 3.4 ± 2.7; Sector 2 = 3.6 ± 2.0) and ranged from 1 to 30 individuals. Clustering algorithms were applied to resighted, photo-identified Guiana dolphins to classify individuals as “residents” (sighted in 87% of surveys, n = 11) and “transients” (two clusters; 62%, n = 22; 95%, n = 72). All individuals predominantly were observed foraging (90% in Sector 1 and 76% in Sector 2), but spatial use by transient individuals differed from the residents in terms of environmental (slope, water depth, and rainfall) and behavioral factors (presence of calves and foraging behavior). Compared to transients, resident dolphins ranged over smaller areas, typically more suitable for feeding, calf caring, and more protected from predators and vessels. Because resident Guiana dolphins exhibit small home ranges, they are particularly vulnerable to local anthropogenic activities and habitat loss. These characteristics imply a strong need for regionalized management plans to reduce negative impacts and to maintain the ecological functionality of the species.

Health conditions of Guiana dolphins facing cumulative anthropogenic impacts

“…Most dolphins in the Paranaguá Estuarine Complex showed poor body condition ("emaciated" and "thin") and presented several types of cutaneous conditions (in 139 of 204 individuals), despite their different frequency of use of the port and the conservation unit areas. The lack of difference in the health conditions between dolphins using these two sampled areas may be due to the movement of dolphins throughout the entire estuary (Moura et al 2021) and their fluid social dynamics with frequent social associations amongst individuals (Cantor et al 2012; see also Powell et al 2020). Guiana dolphin populations are typically composed by a combination of mostly transient and fewer resident individuals (e.g.…”

“…Guiana dolphin populations are typically composed by a combination of mostly transient and fewer resident individuals (e.g. Cantor et al 2012;Moura et al 2021), and most of the individual Guiana dolphins were considered transient II during this study (85% in the island and 78% in the port). In the Paranaguá Estuarine Complex, individual variation in spatial use may be related to intraspecific competition and the environmental characteristic that influence prey dynamics and quality (e.g.…”

“…In the Paranaguá Estuarine Complex, individual variation in spatial use may be related to intraspecific competition and the environmental characteristic that influence prey dynamics and quality (e.g. Santos et al 2010;Possatto et al 2016;Moura et al 2021), but it can also be related to behavioural tactics and health conditions (Van Bressem et al 2009a;Domit 2010). Recent studies suggest that dolphins more exposed to anthropogenic impacts have a greater chance of being severely affected by a viral epidemic than those less exposed (Collier et al 2020;Manhães et al 2021), possibly because of a depressed immune response and their poorer health.…”

“…The frequency of use of each sector was evaluated by the proportion between the number of months each individual was sighted in each sector in relation to the total of months sampled (Rossi- Santos et al 2007). From this frequency, individuals were classified as resident (> 50%), transient I (25-50%) and transient II (< 25%) in each sector (adapted from Moura et al 2021).…”

“…In the Paranaguá Estuarine Complex, Guiana dolphins are found year-round, feeding and breeding (Domit 2010;Santos et al 2010;Bonin et al 2017;Moura et al 2021), even in noisy and degraded areas. The exposure to human activities might be counterbalanced by high prey availability, as Guiana dolphins are often observed using the side of anchored ships in ports as a barrier to corral and capture fish (Domit 2010).…”