The Franciscana Flight Count in Uruguay - a Challenge
The Franciscana(Pontoporia blainvillei) is a small dolphin native to the coastal waters of Brazil, Uruguay and Argentina. Franciscanas live mainly in coastal waters beyond the surf zone to a depth of 50 m, although they also occur in some bays and estuaries. The species is considered the most threatened dolphin species in South America, due to incidental killing in artisanal and industrial fisheries and increasing habitat destruction. The Franciscana is currently classified as "endangered" on the IUCN Red List of Threatened Species, and Uruguay's National System of Protected Natural Areas(SNAP) considers it a priority species for conservation. Four Franciscana Management Areas (FMAs) have been proposed, of which FMA III in Rio Grande do Sul (southern Brazil) and the Uruguayan coast have the highest bycatch estimates. Historically, franciscana have been killed in relatively large numbers in Uruguay (nearly 4000 animals between 1974 and 1993) and more recently in both Uruguay and Brazil (annual mortality of about 1000-2000 animals). Estimating stock size in this region is therefore important to assess the potential impact of this high fishing-related mortality on the stock. To date, the stock size has only been estimated in the Brazilian portion of FMA III and no reliable estimates are available for Uruguayan waters.
From 01 March to 04 April 2023, aerial surveys, supported by YAQU PACHA, were conducted in Uruguayan waters to estimate the population of Franciscana dolphins and obtain records of the species' abundance. The search for Franciscana groups was conducted from a twin-engine Aerocommander 500B high-wing aircraft at an approximately constant altitude of 150 m (500 ft) and at a speed of 170-200 km/h (~90-110 knots).
Three survey regions were proposed: i) Uruguayan offshore waters (30-50 m) (UY-Offshore stratum), ii) Uruguayan coastal waters (0-30 m) (UY-Inshore stratum), and iii) Río de la Plata estuary in Uruguay (UY-Río de la Plata stratum). A total of 2629 km of transects were surveyed covering a total area of 49,483 km2 and a total of 53 franciscana groups were recorded with an average group size of 1.8 individuals. The total density was estimated at 0.606 individuals/km2 and the abundance for the whole area was 30,011 individuals (95% CI = 15,304 - 58,852). On the other hand, aerial surveys of the southern coast of Brazil (sector that is part of FMA III) covered a total area of 30,859 km2, and total density was estimated at 0.426 individuals/km2 and abundance at 13,137 individuals (95% CI = 7,037 - 24,526). Although the high abundance estimated for FMA III in this study may indicate a healthy status for this stock, the bycatch estimates for FMA III were the highest among all FMAs. This is the first study to conduct a full aerial survey of FMA III and obtain an estimate of abundance (43,148 individuals with 95% CI = 23,786 - 78,271) and density (0.537 individuals/km2) for the entire stock, as well as previously unknown density and abundance values for Uruguayan waters. Continued population monitoring through aerial surveys is critical to better understand the effects of bycatch and other sources of unrecorded mortality on the population dynamics of Franciscana dolphins living in Uruguayan waters.
Uruguayan research team: Valentina Franco-Trecu, Cecilia Passadore, Caterina Dimitriadis.
Brazilian research team: Federico Sucunza, Daniel Danilewicz, Emanuel Ferreira, Martin S. Perez, Alexandre N. Zerbini.