Spatial analyses of bottlenose dolphin–fisheries interactions reveal human avoidance off a productive lagoon in the western Gulf of Mexico

Abstract

Studying interactions between cetaceans and humans is fundamental to assess their ecological significance and the impact of human activities on marine wildlife. Delphinids have historically been associated with human maritime activities, and while evidence suggests that such interactions are becoming more frequent worldwide, these remain poorly studied. Areas of potential interaction and differences in dolphin affinity to interact with humans were used to test hypotheses about the spatial distribution and temporal variation in dolphin–fisheries interactions off the highly productive Alvarado lagoon, in the western Gulf of Mexico. Line-transect surveys yielded 928 dolphin, 980 vessel, and 320 fishing gear target records, the latter involving mostly the shrimp fishery. No temporal differences were found in daily relative abundance of dolphins ( = 8.1 h−1, SD = 9.7), vessels ( = 7.8 h−1, SD = 5.9) or fishing gear ( = 2.4 h−1, SD = 2.6) between two consecutive years. Non-random spatial distributions indicated higher target concentrations at the lagoon entrance; however, dolphins and fishers were found to evade each other, possibly to prevent competition; dolphins only interacted with gillnets (28.6% of vessels and 22.6% of fishing gear). We observed small areas of potentially intentional and random encounters outside the entrance of the lagoon and low or null potential for interactions elsewhere. Only 18.9% of dolphin schools (44.8% of the 172 photo-identified animals) interacted with fisheries mostly by chance. Resident individuals (N = 23) tended to avoid humans, likely in response to negative reinforcement caused by aggressions from fishers. Hence, the potential intentionality of a few individuals to interact with fisheries, show they bare higher risks while attempting to benefit from gillnetted prey. This research unveils the chronic and acute exposure of the dolphin community to artisanal fisheries within the area, having important reciprocal consequences on their distributions and activities.

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

“…Resident individuals face the inherent risk of being harassed, entangled or even killed, thus individuals rather than groups are more likely to evade detection by fishermen due to the extremely low visibility below the sea surface (Morteo et al 2012). The latter also supports the argument that individuals in general gain larger benefits by aggregating in small groups (Figure 2) within this heavily fished area, and in extreme cases points to a "selfish strategy" as a mean for auto-preservation.…”

“…Morteo et al (2012) pointed out the importance of boat traffic and fishing activities in the area, which seem to influence the presence and distribution of dolphins on a daily basis or even instantaneously (Constantine et al 2004;Lusseau et al 2006;Hernandez-Candelario 2009). Although many cetaceans seem to have habituated to certain levels of marine traffic, other evidence suggest that it may cause severe alterations in their behavior and in this case may promote the separation of group members, probably altering the social bonds within the community (Constantine et al 2004;Morteo et al 2012;Morteo et al 2014).…”

“…The regional climate is tropical with three seasons: a Dry season (March-June) with a significant reduction in average precipitation; a Rainy season (July-October), in which runoff from rivers into the adjacent lagoon and mangrove forest causes high organic matter and nutrient input into coastal waters; and a Windy season (November-February, locally known as "Nortes"), featuring strong winds (up to 80 km h -1 ) associated with the incursion of northern cold fronts, which may last several days. Fisheries are relevant to coastal dolphin populations due to the frequent adverse interactions between dolphins and local fisheries, causing incidental mortality of dolphins in nets (Morteo 2011;Morteo et al 2012). Around 2000 fishermen were active in the area, most of which (75 -85 %) operate in the lagoon and the rest operate in open waters.…”

“…Individual interactions are influenced by intrinsic factors such as the presence of recurrent associations (Connor et al 2001), which in turn may be determined by extrinsic factors such as habitat variability (e. g. food abundance and availability, as well as natural and human-related threats (Lusseau et al 2006;Quintana-Rizo 2006;Morteo and Hernández 2007;Morteo et al 2012). Social and ecological pressures may dictate the temporality of the associations at different scales (McDonald and Carr 1989;Quintana-Rizzo 2006;Morteo et al 2014).…”

Phenotypic variation in dorsal fin morphology of coastal bottlenose dolphins (Tursiops truncatus) off Mexico

“…We 85 Distances among adjacent locations should allow for individual exchange considering the 86 dispersal capabilities of the species, and 4) Photo-identification catalogs of coastal bottlenose 87 dolphin populations must be available. Detailed descriptions on the ecology of the study areas 88 and the biology of dolphin populations in those areas are provided elsewhere (see Espinosa, 1986, 89 Ballance, 1987Salinas and Bourillón, 1988;Acevedo, 1989; Ballance, 1990 Ballance, , 1992 Delgado, 90 1996 Delgado, 90 , 2002 Caldwell, 1992;Heckel, 1992;Schramm, 1993;Silber et al, 1994; Silber and Fertl, 91 1995; López, 1997López, , 2002Defran et al, 1999;Díaz, 2001;Orozco, 2001; Guzón, 92 2002;Morteo, 2002;Rodríguez et al, 2003;Ladrón de Guevara and Heckel, 2004; Morteo et al, 93 2004;Ramírez et al, 2005;Mellink-Bijtel and Orozco-Meyer, 2006; Pérez-Cortés, 2006; 94 Rodríguez-Vázquez, 2008;Morteo et al, 2012, 2015 Ruiz-Hernández, 2014; 95 Zepeda-Borja, In prep.). Study areas were grouped by region into 1) Pacific Ocean, 2) Gulf of 96 California and 3) Gulf of Mexico (Fig.…”