Impact of maritime traffic and whale‐watching on apparent survival of bottlenose dolphins in the Strait of Gibraltar

Abstract

1. The Strait of Gibraltar is an important habitat for cetaceans due to its high marine productivity. However, it is also the second most navigated channel in the world, subjecting cetaceans to a high level of vessel traffic, including an established whale-watching fleet. Both maritime traffic and whale-watching activities have been shown to impact the behaviour of cetaceans, but little is known about their impact on the demography and dynamics of cetacean populations. 2. The aim of this study was to evaluate the impacts of both ferry traffic and whalewatching vessels on the apparent survival probability of a bottlenose dolphin (Tursiops truncatus) population that occurs in the Strait. 3. A Bayesian hierarchical mark-recapture modelling approach was applied to 8 years of photo-identification data (2002-2009). Apparent annual survival probability was negatively correlated with ferry traffic, which explained >70% of temporal variation in survival, in contrast to whale-watching, the effect of which was almost negligible. Despite these results, other natural and human-related factors are likely to drive apparent survival in the study area. 4. Abundance increased between 2002 and 2005, and then decreased between 2006 and 2009, while local per-capita recruitment decreased from 2004 until the end of the study period. These shifts correlated temporally with the construction of a large harbour on the Moroccan coast, which increased maritime traffic significantly, including a 40% increase in ferry traffic that regularly transited the Strait. 5. These results highlight the need to better understand the impact of maritime traffic on the demography of the dolphin population in the Strait, in order to implement evidence-based conservation regulations in a region of high cetacean occurrence.

Seasonal dynamics of occurrence and group size of three species of cetaceans (Delphinidae and Phocoenidae) on the southwestern coast of the Black Sea

“…Bottlenose dolphins occur in the study area all year around; more often they were recorded in spring and summer, with the number of sightings decreasing in autumn. This decrease might depend on intense marine traffic (Tenan et al 2020) during the simultaneous fishing season on Atlantic bonito in the region. In contrast, in Ukrainian waters, which are much shallower, the highest number of sightings of bottlenose dolphins was recorded in the autumn period (Mikhalev 2005a).…”

Abundance of the bottlenose dolphin (Tursiops truncatus) and proposal for a priority area of conservation in the Southern Gulf of Mexico marine ecoregion

“…These areas frequently coincide with those of dolphin habitat use (Vázquez-Castán et al, 2007;Martínez-Serrano et al, 2011;Hernández-Candelario et al, 2015). Human activities must therefore be considered when studying the environmental pressures that disrupt communication, and thus potentially affect dolphin population dynamics (Marley et al, 2017;Pirotta et al, 2018;Tenan et al, 2020;Avila & Giraldo, 2022). Thus, to lessen the possible impact of human activities on dolphins, activity exclusion zones could be created in those areas that are frequently used by bottlenose dolphins.…”

Resources and population traits modulate the association patterns in the common bottlenose dolphin living nearby the Tiber River estuary (Mediterranean Sea)

“…From a conservation perspective, both aspects (weaker associations between individuals and mothercalf pairs avoidance) may be pivotal since they represent a temporary disruption of adult social bonds due to fishing activity but may also denote that dolphin's social structure may be a complex adaptive system resilient to anthropogenic disturbance (Ansmann et al, 2012;Dıáz Loṕez, 2019;Genov et al, 2019;Frau et al, 2021), as bonds are restored when fishing trawlers are absent. Different anthropogenic activities have been demonstrated to possibly alter population structure in terms of age and sex composition, by influencing the survival rate (Senigaglia et al, 2019;Tenan et al, 2020) or affecting the relationships among individuals (Marley et al, 2017), with the potential of eventually influencing population dynamics (Tenan et al, 2020). Thus, assessing how social structure changes and adapts in response to human activities is essential to investigate the possible consequences of anthropogenic disturbance on a population level.…”