Eerste skatting van Namibië se dolfynbevolkings

01 March 2021 | Environment

Die Namibiese eilande se mariene bewaringsgebied (bekend as NIMPA), wat 25% van die algehele kuslyn van Namibië beslaan, en een van die grootste mariene bewaringsgebiede in Afrika is, is die tuiste van ’n groot bevolking van ongeveer 1 600 Heaviside se dolfyne en 3 500 Afrika-vaaldolfyne.
Dit is die bevindings van die eerste getalleskatting van die aantal dolfyne wat van hierdie area gebruik maak, sedert NIMPA se totstandkoming in 2009. Hierdie bevindings is onlangs in die vaktydskrifte Frontiers in Marine Science en die African Journal of Marine Science gepubliseer.
Die Namibiese eilande se mariene bewaringsgebied is geleë in die noorderlike Benguela-stroom se opwellingsgebied, een van die mees produktiewe mariene ekosisteme ter wêreld. Mariene bioloë van die Namibiese Dolfynprojek en die Namibiese ministerie van visserye en mariene hulpbronne, het die navorsing vanaf 2012 tot 2014 onderneem.
Die span het van nuwe opname- en statistiese metodes gebruik gemaak om getalleskattings (bevolkingsgrootte) uit skeepsopnamedata te genereer. Die dolfyne is gevind deur ’n kombinasie van visuele waarnemings wat van die skip se dek af gemaak is, en deur middel van onderwater akoestiese opnames wat gemaak is van ’n stel hidrofone (onderwatermikrofone) wat 400m agter die vaartuig gesleep is.
Aangesien die meeste dolfyne baie op klank staatmaak – hulle maak bedags, sowel as snags gebruik van klank om hulself te oriënteer, kos te soek en vir kommunikasie – is hulle ideale kandidate vir akoestiese monitering. Hierdie hidrofoonopstelling was in staat om dolfynklanke waar te neem soos wat die navorsingskip deur hulle habitat beweeg het.

Opname metodes
Dr Tess Gridley, mededirekteur van Sea Search en die Namibiese Dolpfynprojek en ’n mede-outeur, sê die vermoë om die teenwoordigheid van diere te identifiseer onderwyl hulle onderwater is, het die opsporingskoers verbeter en help om te verseker dat diere nie gemis word nie.
“Alhoewel die visuele opnamespan bedags baie effektief was, was die gesleepte hidrofoonopstelling ook in staat om diere waar te neem in swak weersomstandighede en snags wanneer visuele waarneming nie moontlik was nie,” sê sy.
Dr Morgan Martin, wat die studie onderneem het as deel van haar navorsing vir haar PhD-proefskrif by die Universiteit van Pretoria, sê die studie verskaf belangrike basislyninligting oor dolfyngetalle vir Namibië se enigste mariene bewaringsgebied, en dien as model vir die oorblywende Benguela-stroomekosisteem.
Dr Simon Elwin, mede-direkteur van die Sea Search en Namibiese Dolfynprojek, sê die uitvoer van hierdie tipe van projekte is fisies uitdagend: “Ons het 24 uur per dag die hidrofone en navigasie gemonitor, en dit op ’n kleinerige boot in redelike rowwe seewaters. Ons is egter baie tevrede dat ons nou uiteindelik die resultate kon publiseer.
Terwyl Namibië se dolfyne nie so erg deur byvangste of jagaktiwiteite benadeel word nie, was daar in die verlede groot veranderings in die ekosisteem, waaronder die ineenstorting van die sardienbevolking, en die toename in myn- en prospekteringsaktiwiteite.
“Terwyl die aantal dolfyne in NIMPA goed vergelyk met soorgelyke bevolkings in ander areas, het ons nie toegang tot historiese data nie. Dit is dus moeilik om te sê of die Namibiese bevolking verminder of vermeerder het,” sluit die navorsers af.

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