Dynamics and Viability of a Small, Estuarine-Resident Population of Lahille’s Bottlenose Dolphins From Southern Brazil

Abstract

Identifying threatened populations and quantifying their vulnerability is crucial for establishing priorities for conservation and providing robust information for decision-making. Lahille’s bottlenose dolphins have been long subjected to by-catch mortality in gillnet fisheries in coastal waters of southern Brazil, particularly in the Patos Lagoon estuary (PLE) and adjacent coastal waters, where dolphins from three populations (or Management Units) show overlapping home ranges. In this study we used a stage-classified matrix population model to conduct a demographic analysis of the PLE’s population with life-history data estimated through an 8 years mark-recapture study. A population viability analysis (PVA) was used to run a series of simulations where the risk was assessed under different by-catch scenarios, taking into account the effects of parameter uncertainty and stochasticity in the projections. In the absence of by-catch, we estimated that this dolphin population would growth at a rate of about 3% annually (95% CI: 1.2–5.8%). Under current by-catch rates, prognoses indicated high probabilities of viability over the next 60 years. These optimistic prognoses appear to be associated with the high survival of adult females. However, the eventual removal of very few mature females (one every year or two) would result in a prominent likelihood of decline from its current abundance at all pre-specified levels. The viability of the population would be substantially improved if the survival of juveniles/sub-adults could be increased. This may be achieved through the recently implemented dolphin protection area, which prohibits gillnet fisheries in the core area of this population. If the protection area reduces the entanglement rates of the most impacted life-stages (i.e., juvenile/sub-adult dolphins), there would be a substantial chance of the PLE’s dolphin population increasing above 20% of its current size, which is here proposed as conservation goal. If met, this goal has the potential to promote habitat quality, increase genetic diversity and connectivity with adjacent populations, enhancing the ability of bottlenose dolphins in southern Brazil to cope with environmental change and potential disease outbreaks.

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

“…These results can be used by managers to highlight how essential the elimination of bycatch is to population viability, as well as complementary measures to regulate increasing boat traffic and other habitat perturbations that can affect dolphin reproduction. Finally, our results reinforce how vulnerable the Lahille's bottlenose dolphin subspecies is, since its largest populationin the Patos Lagoon estuaryis likely to decline with any increase in bycatches (Fruet, Möller & Secchi, 2021), and the second largest populationin Laguna has a great chance of being extinguished with the current bycatch rates.…”

“…In Laguna, only the most optimistic mitigation of bycatch – a zero bycatch scenario – would make the difference between a declining and an increasing population, greatly improving the chances of long‐term persistence of Lahille's bottlenose dolphins. The largest population of Lahille’s bottlenose dolphins, in the Patos Lagoon estuary, shows great chances of persistence under current levels of bycatch, but a slight increase in bycatches, particularly of adult females, would have severe consequences for the population dynamics and chance of persistence (Fruet, Möller & Secchi, 2021).…”

“…Although robust data on these regional coastal populations are limited, the small number of individuals (probably no more than 600 individuals in total), evidence of declining populations and the low genetic variability motivated a recent regional assessment that classified the subspecies as Vulnerable (Vermeulen et al, 2019). In fact, the first risk assessment for Lahille’s bottlenose dolphins confirms that its largest population in the Patos Lagoon estuary, southern Brazil, is vulnerable to any increase in non‐natural mortality (Fruet, Möller & Secchi, 2021). The other smaller populations of this subspecies are probably in the same situation.…”

Estimating reproductive costs in marine mammal bioenergetic models: a review of current knowledge and data availability

“…Reproductive females are a focal group of many bioenergetic approaches. This is largely because survival of reproductive females and reproductive rates are key drivers of marine mammal population dynamics (Blázquez et al, 2020;Fruet et al, 2021;Luck et al, 2022). Reproduction is also energetically expensive, meaning that females (and their offspring) may be more susceptible to perturbations in energy balance during this time.…”

Long-Term Consequences of High Polychlorinated Biphenyl Exposure: Projected Decline of Delphinid Populations in a Hotspot for Chemical Pollution

“…for the SE population, accounting for 218 individuals (95% CI: 202 -241) in this population, and data was extrapolated to the other two populations. When data concerning rough-toothed dolphins were not available, an inter-specific extrapolation was adopted using bottlenose dolphins as surrogate species, mainly from the S region 16,17 to complement the information necessary to apply a Leslie matrix. The Leslie matrix was necessary to construct an initial age structured and stable population so PCB effects could be simulated.…”