Fluit, fluit – Namibiese dolfyne laat van hulle hoor

27 August 2020 | Environment

'n Internasionale span wetenskaplikes het daarin geslaag om met behulp van die eiesoortige fluitgeluide van bottelneusdolfyne, die grootte van hul bevolking te bepaal én hul beweging langs die Namibiese kuslyn na te spoor.
Met hierdie navorsing wat deur die Universiteit Stellenbosch (RSA) en die Universiteit van Plymouth (UK) gelei is, is akoestiese monitering vir die eerste keer ingespan in die generering van getalleskattings van dolfynbevolkings, in plaas van die gebruik van fotos.
In hul artikel in die Journal of Mammalogy, sê die navorsers dat hulle baie opgewonde is oor die positiewe resultate wat hierdie metode opgelewer het. Die geskatte getal dolfyne (deur middel van die gebruik van klank) was byna presies dieselfde as die geskatte getal wat bepaal is deur middel van die meer tradisionele metode waar foto’s gebruik word vir die uitkenning en tel van die diere.
Hulle werk nou aan die verfyning van die tegniek met die hoop dat dit ook gebruik kan word om ander spesies na te spoor – met ’n fokus tans op bedreigde spesies soos die boggelrugdolfyne.
Met die vinniger prossessering van inligting en vooruitgang op die gebied van statistiese ontleding, sal die outomatiese opsporing van eiesoortige roepe van individue dalk in die toekoms moontlik kan wees. Dit kan belangrike inligting oor individuele diere genereer en sal veral nuttig wees vir klein, bedreigde bevolkings waarin elke individu belangrik is.
“Die waarneming-herwaarneming (capture-recapture) van individueel eiesoortige fluitgeluide is nog nooit vantevore aangepak nie,” sê die senior outeur van die artikel, dr Tess Gridley, mededirekteur van Sea Search en die Namibiese Dolfynprojek en ’n nadoktorale genoot in die Departement Plant- en Dierkunde by die US.
“Die dolfyne gebruik hierdie geluide regdeur hulle lewe en elkeen het sy of haar unieke fluit. Deur die opneem van eiesoortige fluite oor ’n tydperk en in verskillende plekke, kon ons uitwerk hoe diere rondbeweeg en hoeveel daar van hulle is.”

Bioakoestiek
Die Namibiese Dolfynprojek doen al die afgelope 12 jaar navorsing onder Namibië se plaaslike bottelneusdolfyne, in samewerking met dr. Simon Elwen van die Universiteit Stellenbosch. Sedert 2009 het hulle 'n katalogus met meer as 55 eiesoortige fluitgeluide opgebou.
Hierdie spesifieke studie is deur Emma Longden gelei, wat die projek tydens haar Honneurs BSc-studies in Mariene Biologie aan die Universiteit van Plymouth begin het. As voorgraadse student het Emma in 2016 ’n maandlange internskap by die Namibiese Dolfynprojek voltooi en in 2018 teruggekeer ten einde haar werk met die waarneming-herwaarnemingsprojek te voltooi.
Sy het meer as 400 uur se akoestiese data ontleed wat verkry is van vier hidrofone wat gedurende die eerste ses maande van 2016 langs die kus, noord en suid van Walvisbaai geposisioneer is.
Hulle het altesaam 204 akoestiese gebeurtenisse geïdentifiseer, waarvan 50 eiesoortige fluittipes bevat het. Vanuit hierdie gebeurtenisse, is 53 eiesoortige fluittipes geïdentifiseer; 40 was reeds in ’n bestaande katalogus wat in 2014 vir die Walvisbaaise bottelneusdolfynpopulasie opgestel is, en 13 nuwes is geïdentifiseer.
Van die 53 eiesoortige fluittipes wat geïdentifiseer is, is 43% slegs een maal teëgekom, terwyl die meerderheid (57%) wéér twee of meer keer teëgekom is. “Een van die wonderlike dinge oor bioakoestiek is dat jy ’n hidrofoon vir weke lank op ’n slag in die water kan los en so baie data kan insamel sonder om met die lewe van die diere wat jy bestudeer, in te meng,” sê Emma, wat haar projek onder die studieleiding van dr Clare Embling, medeprofessor in mariene biologie by die Universiteit van Plymouth, gedoen het.
Toekomstige navorsing sluit die werk van PhD-student Sasha Dines van die Universiteit Stellenbosch in, wat die tegniek verder sal verfyn ten einde die populasie bedreigde boggelrugdolfyne in Suid-Afrika beter te verstaan. Nog ’n PhD-student, Jack Fearey van die Universiteit Kaapstad sal voortgaan om navorsing langs die Namibiese kus te doen.

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