Hydrobiology has recently conducted groundbreaking tests to determine the viability of hydroacoustics to identify elasmobranchs (sharks and rays), without requiring netting.
Hydrobiology was commissioned by Rio Tinto Alcan (RTA) to undertake a survey of elasmobranchs (cartilaginous fish such as sharks and rays) near Weipa. Traditionally, surveys of this type are undertaken using methods which involve netting. However, this can cause undue stress and mortality of fish that are caught. Further, handling sharks can sometimes be hazardous to scientists.
Hydrobiology investigated the use of hydroacoustics—using a scientific digital echo sounder—as a non ‐ invasive means to determine the presence and number of elasmobranchs in the survey area. Although this technique has been used to quantify teleosts (bony fishes) populations and is a widely accepted method, little work has been done on its suitability for elasmobranch surveys. Prior to this study the anatomical and physiological differences between elasmobranchs and teleosts fish had never been accounted for in hydroacoustic surveys. Teleosts have bones and a swim bladder whereas elasmobranchs are cartilaginous and do not possess a swim bladder, making them much weaker at reflecting acoustic pulses.
This difference required other indicators to be developed to distinguish the echoes from elasmobranchs from those of teleosts during sampling. Once separated, the weaker signal from elasmobranchs could be used to more accurately estimate the size of the fish.
In order to be able to do this effectively, test data were collected from sharks and teleosts in the controlled environment of the SeaWorld (Gold Coast) and UnderWater World (Mooloolaba) aquariums. The species, fish length and their associated target strength were recorded and these data were used to formulate a mathematical algorithm which could distinguish between elasmobranchs and teleosts. Following extensive successful quality testing and development of the algorithm in this controlled environment, the algorithm was then tested on the dataset collected from the survey sites near Weipa. The results of the hydroacoustics were reported in conjunction with a netting survey and also with side scan sonar imaging. It was concluded that each of the methods, which yielded different types of data, complemented each other but overall the results of the hydroacoustics survey were deemed to be representative of the elasmobranch activity and numbers in the areas surveyed, verifying this as a new, non‐invasive survey method.
