November 2102 Expedition - Counting fish with sound
The ocean in the Chagos is exceptionally clear and on calm and sunny days when we are in the shallows, it is easy to see the seabed below the Pacific Marlin. The clear water is crucial for the camera work on this expedition, but even in perfect conditions light fades quickly into a dim blue glow as one goes deeper.
Just off the atolls and islands the ocean floor rapidly drops to depths of 1000 metres and more making the MPA a vast blue expanse. Luckily, sound travels much further through water than light, and it too can be used to discover marine life. Martin and I are using scientific echo sounders for just this purpose. An echo sounder is a combined underwater loudspeaker and microphone that transmits short pulses of sound – “pings” – into the water and detects returning echoes. A computer then calculates the range of any detected echo and displays the result as an acoustic picture of the water column.
Echoes are created by any object that has a density different from that of seawater. Rocks for example are much denser than seawater and so the seabed gives a strong echo. Gas-bubbles also strongly reflect sound and exactly this makes it possible for us to detect fish.
Many fish have a gas-filled swim bladder which they use to control their buoyancy. The swim bladder creates a strong echo which tends to increase with the size of a fish. By counting echoes we can estimate the number of fish in the water and also get a rough idea of their size.
The sound speed in seawater is about 1500 meters per second, meaning our pings run from the surface to the seabed and back in less than two seconds, giving a nearly instant snapshot of the number of fish between the surface and the deep seabed. We deploy our sounders from one of the Marlin’s work boats and operate them while driving along on the surface. This way we can survey several nautical miles every day covering much more ground than we could with the cameras alone.
Acoustics are not quite perfect, however, and one limitation is their bias towards swim bladdered fish. Marine animals without swim bladders also create echoes, but they tend to be much weaker. This is particularly limiting as sharks, one of the main target groups on our trip, do not have a swim bladder. The biggest limitation is probably that species identification from echoes alone is very difficult. By using two frequencies of sound we get a rough idea of animal class and size, for example we can distinguish between tuna and their prey but telling a small tuna from a large mackerel can be impossible without additional information.
By combining echo sounders with the camera work taking place, we hope to combine their strengths and explore this vast mid-water ecosystem in a comprehensive yet non-invasive way.
Philippe
Creole Translation:
Kont pwason an servi son
Losean dan Chagos i eksepsyonelman kler e lo en lazournen soleye e kalm ler nou pa dan fon, i tre fasil pou vwar fon lanmer anba Pacific Marlin. Sa delo kler it tre krisyal pou bann travay kameralo sa lekspedisyon, me menm The clear water is crucial for the camera work on this expedition, me menm dan bann bon kondisyon lalimyer i disparet vitman dan sa manmer ble pli fon nou ale.
Zis obor sa atol ek bann zil fon lanmer i vitman desann dan en fonder 1000m ki fer sa park maren en gran ble. Erezman son i al pli lwen dan delo ki lalimyer, e i osi kapab gany servi pou dekouver bann lavi marin. Martin ek mwan pe servi bann lekipman siantifik ki servi bann son leko pou sa rezon. En leko son i en konbinezon servi bann spiker anba lanmer ek en mikrofon ki transmet bann batman son kourt- ‘pings’- dan delo e detekte ban leko ki retournen. En konpiter i apre gany servi pou kalkil sa bann diferan leko ki gany detekte e montre bann rezilta koman en portre akoustik dan delo
Leko i gany kre par nenport lobze ki annan dansite plis ki delosale. Par ekzanp roz i pli ansanm ki delo alors fon lanmer i donn en pli for leko. Bann pti boul gas i osi reflekte son e sa i fer li posib pou nou detekte pwason.
Enta pwason anndan zot i annan bann gas ki zot servi pou zot kapab naze. Sa i kre en leko for ki vin pli for tan pwason i pli gro. Par kont bann leko nou kapab estim lakantite pwason dan delo e gany en lide ki groser zot ete.
Sa son i al en vites 1500 met par segond, ki vedir nou pings i sorti lo sirfas pou fon lanmer e retournen dan mwens ki 2 segond, ki donn pres ki en portre lo lakantitepwason ant sirfas avek fon lanmer. Nou larg nou bann son sorti lo en bato sorti lo Marlin e servi nou lekipman pandan kit nou pe bouze lo sirfas. Sa fason nou kapab sirvey plizyer de mil toulezour ki plis ki si nou ti pe servi zis bann kamera.
Akoustik i pa totalman parfe, me i annan so limitasyon avek bann blad pwason. Bann zannimo maren san en blad naze i osi kre leko, me zot pli feb. Sa i partikilyerman limite koman reken, en sa lespes ki nou pe target dan sa lekspedisyon ki zot napa sa blad. Sa pli gran limitasyon i probableman sa idantifikasyon bann lespes par leko tousel i tre difisil. Par servi 2 frekans son nou gany en lide sa kalite zannimo ek groser, par ekzanp nou kapab disteng ant ton ek bann pwason ki zot manze me pou dir ant en pti ton ek en gro makro i kapab enposib san lenformasyon adisyonel.
Par azout leko son avek kamera, nou espere pou kapab eksplor sa dan lekosistenm milye-delo dan en fason pou gany lenformasyon me san en metod envasiv.
Philippe