Dolphin population specialized in foraging with artisanal fishers requires zero‐bycatch management to persist

Abstract

1. The small population paradigm assumes that populations with low numbers of individuals intrinsically have a high probability of extinction. The small population of Lahille's bottlenose dolphins Tursiops truncatus gephyreus that specializes in foraging with artisanal fishers in Laguna, southern Brazil, faces human pressures including bycatch in fishing gear. The viability of this population was modelled over 30 and 100 years under different levels of bycatch, including the current scenario of two bycatches every year, two scenarios with higher incidence of bycatches and three management scenarios. The sensitivity of predicted growth rates to fixed-proportion and observed-variation changes in life history parameters was explored.2. The current scenario predicted a declining population (r = À0.014; λ = 0.986) with a high probability of extinction in the long term (PE = 0.71). A small increase in bycatches would result in a marked increase in the probability of extinction.Management scenarios seem promising, but only the zero-bycatch management scenario would make the difference between a declining and an increasing population.3. As expected for slow-growing species, population growth rate was most sensitive to proportional changes in adult female and juvenile survival. However, considering observed variation in vital rates, population dynamics were most influenced by variation in reproductive rates. 4. To determine the highest priority for management action, another simulation was made of how additional threat scenarios of recognized human activities (i.e. bycatch influencing adult survival and increased underwater noise or pollution influencing calf survival) would affect population dynamics. Population growth rate was very sensitive to changes in adult bycatch (especially females), as expected, and only subtly sensitive to a reduction in calf survival.5. The current level of bycatch is unsustainable. Bycatch needs to be eliminated to maximize the probability of long-term persistence of this dolphin population.However, this population's persistence could be threatened by natural variation in reproductive rates.

Safeguarding human–wildlife cooperation

“…We summarize the threats faced by each of these components (numbered according to sections in main text), whether these are driven by environmental and/or cultural change, and the causes of decline and loss for active and inactive forms of human-wildlife cooperation, respectively. See Table S3 for references (Bezamat et al, 2018(Bezamat et al, , 2021. Cooperation is also associated with stronger social relationships (Machado, Cantor, et al, 2019), and could broadly confer similar benefits to "play" behavior seen in many dolphin populations, by contributing to the dolphins' physical, social, and emotional development and well-being (Hill et al, 2017).…”

“…Although the benefits to dolphins remain inconclusive due to the logistical difficulties of following them in turbid water, the fact that the dolphins often actively initiate the interaction suggests that doing so is beneficial (Neil, 2002; Simões‐Lopes et al., 1998; Smith et al., 2009; Tun, 2004). In one population of Lahille's bottlenose dolphins, cooperating with fishers is associated with increased survival, arising from reduced risk of bycatch for those that cooperate (Bezamat et al., 2018, 2021). Cooperation is also associated with stronger social relationships (Machado, Cantor, et al., 2019), and could broadly confer similar benefits to “play” behavior seen in many dolphin populations, by contributing to the dolphins’ physical, social, and emotional development and well‐being (Hill et al., 2017).…”

“…There are numerous anecdotal reports of honeyguide populations that no longer regularly guide humans in locations where humans ceased to respond to them multiple human generations ago (Dean et al., 1990; Friedmann, 1955; Isack, 1999). Increased commercial and illegal fishing activity (e.g., electric fishing and gillnets in Myanmar [Smith et al., 2009; Tun, 2014] and gill and trammel nets in Brazil [Peterson et al., 2008]) leads to more dolphin bycatch (Bezamat et al., 2021; Tun, 2014), and boat engine noise and chemical pollution can alter dolphins’ behavior (Pellegrini et al., 2021; Tun, 2014), health (Righetti et al., 2019), and survival (Bezamat et al., 2020). This not only decreases the subpopulation of cooperating dolphins, but often endangers the species (e.g., Bezamat et al., 2021; Tun, 2014).…”

“…Increased commercial and illegal fishing activity (e.g., electric fishing and gillnets in Myanmar [Smith et al., 2009; Tun, 2014] and gill and trammel nets in Brazil [Peterson et al., 2008]) leads to more dolphin bycatch (Bezamat et al., 2021; Tun, 2014), and boat engine noise and chemical pollution can alter dolphins’ behavior (Pellegrini et al., 2021; Tun, 2014), health (Righetti et al., 2019), and survival (Bezamat et al., 2020). This not only decreases the subpopulation of cooperating dolphins, but often endangers the species (e.g., Bezamat et al., 2021; Tun, 2014). Both dolphin species involved in human–dolphin cooperation are threatened, with the Lahille's bottlenose dolphin listed as Vulnerable, and the Ayeyarwady River subpopulation of Irrawaddy dolphin in Myanmar considered Critically Endangered (IUCN, 2021).…”

“…Wildlife populations need a relatively intact habitat without human‐induced risk and mortality, so enforcement of legal protection is required. In southern Brazil, current legislation bans gill and trammel nets near cooperative sites, which, if combined with enforcement operations, could successfully reduce dolphin bycatch (Bezamat et al., 2021). In Myanmar, increased patrolling will help battle illegal fishing, by enforcing the 2018 legislation protecting Irrawaddy dolphins and limiting net deployment at cooperative sites.…”