Staying Alive: Long-Term Success of Bottlenose Dolphin Interventions in Southwest Florida

Abstract

Small cetaceans face persistent threats from fisheries interactions, making effective mitigation a priority for conservation. In southwest Florida, interactions come primarily from small-scale recreational hook and line and trap/pot fisheries, with regional stranding network partners working with federal agency managers to assess and intervene as possible in cases of live animal entanglement. Evaluating success of intervention cases is difficult due to financial and logistical constraints which may preclude detailed follow-up monitoring. Survival over the initial 6 weeks post-release has been used as a marker of short-term success for small-cetacean rescue and/or rehabilitation cases. Early intervention prior to stranding, especially via remote disentanglement or rescue and immediate re-release onsite, can save entangled free-ranging dolphins facing life-threatening anthropogenic injuries. However, given the costs associated with interventions, it is important to understand the benefits of these endeavors not only to save individuals, but also to establish if and how saved individuals contribute to social functioning, survival and reproduction within small, resident populations facing multiple concurrent threats. Here we provide evidence from 27 well-documented common bottlenose dolphin (Tursiops truncatus) intervention cases during 1985–2019 where follow-up monitoring over multiple years sheds light on the longer-term success of these efforts and potential benefits to local populations. Nearly all rescued individuals (92%) survived longer than 6 weeks post-release (mean minimum survival period = 5 years, range 0–35 years), with 13 still observed frequently within their prior resident communities, in good physical health, and engaging in normal behavior. Survivorship rates did not decline substantially between 1 and 5 years post-rescue, meaning survival beyond 1 year may be a useful benchmark of long-term success. Rescued females that reached reproductive maturity (n = 4) have produced 12 post-intervention offspring to date. Social network analysis and demographic modeling applied to cases from the long-term resident community in Sarasota Bay confirmed that animals maintain social connections post-intervention and that interventions result in higher population growth rates. While not every intervention succeeds, this study demonstrates the conservation value of pre-stranding interventions which allow individuals that otherwise would be lost to remain viable and productive members of local populations when prevention of anthropogenic injury is not possible.

Assessing the Viability of the Sarasota Bay Community of Bottlenose Dolphins

“…Thus, with an estimated K = 200, the loss of an additional one dolphin per year (0.5%) would cause significant harm. Conversely, a reduction in the number of deaths due to human interactions will yield comparable improvements in population growth (McHugh et al, 2021).…”

“…Beyond the assessment of the Sarasota Bay population, the analyses can inform estimates of Minimum Viable Population (MVP) size for isolated, small populations and Potential Biological Removal (PBR) for directly or indirectly harvested populations. We used the PVA for some initial assessment of the resistance and resilience to threats affecting survival or breeding, and it can provide a basis for further investigations of the population-level impacts of causes of mortality that might be partitioned and quantified (e.g., McHugh et al, 2021) or of projected improvements in reproduction or survival resulting from management actions. Williams et al (2020) noted that as the multiple effects of chronic ocean noise on cetaceans are measured, PVA can be used to quantify the population level impacts.…”

Demographics and viability of an estuarine community of Indo‐Pacific bottlenose dolphins

“…No adverse impacts from commercial fishing have been recognized for this community; however, five individuals and an additional three coastal dolphins who regularly use the Dawesville Channel, have been observed entangled in recreational fishing gear between 2016 and 2021. Discarded fishing gear has also been identified as a major threat to marine megafauna globally (Moore et al, 2009;Stelfox et al, 2016) with recreational fishing gear, mainly fishing line, affecting estuarine and coastal bottlenose dolphins (Mann et al, 1995;Marks et al, 2020;McHugh et al, 2021;Miketa et al, 2017;Wells et al, 2008Wells et al, , 2013. Educating the public on impacts of discarded fishing gear on wildlife is imperative to deal with the source of the problem while building capacity in local dolphin incident response to ensure quick disentanglement of individuals (Wells et al, 2008(Wells et al, , 2013.…”

Estimating Population Abundance of Atlantic Bottlenose Dolphins (Tursiops truncatus) in the Coastal Waters of Palm Beach County, Southeastern Florida

“…Atlantic bottlenose dolphins frequent the coastal waters of Palm Beach County (PBC), Florida, in the United States, but little is known about the abundance, distribution, and natural history of this species in the region (Read et al, 2003). Lack of information on the population dynamics, population structure, distribution, genetic diversity, health, and habitat use of dolphins in this particular study area hampers the assessment of impacts of numerous direct and indirect anthropogenic threats, including persistent organic pollutants (POPs) and subsequent bioaccumulation of contaminants, reduction in prey availability due to overfishing, bycatch, exposure and ingestion of microplastics, toxic algal blooms, and habitat degradation (Balmer et al, 2011;Nery & Simão, 2012;Vollmer & Rosel, 2013;Wells et al, 2019;McHugh et al, 2021).…”