Patterns of fish calling in a nearshore environment in the Great Barrier Reef.

Long-term sea-noise statistics have been obtained from a region of the central section of the Great Barrier Reef. Fish calling was a major contributor to sea-noise levels. Calling was either in choruses, where groups of fishes called en masse, or as isolated calls repeated ad nauseam. Four calling types predominated, with each displaying unique call characteristics and calling patterns through time and space. Analysis of call types offered information on the fish's calling physiology, behaviour and, through the call's interaction with the local environment, on the location of the caller. Call types ranged from less than 10 ms to several seconds long, and were comprised from one to nearly 40 pulses. The structure of each pulse was related to swim-bladder mechanics; normally swim-bladders were lightly damped. Fish calling was most common during the Australian summer with one call type also displaying lunar trends. All calls had daily patterns of sound production with highest activity levels generally at night. There was some spatial separation of zones of highest call rates, but sources avoided competition for the 'sound space' primarily by offsetting the time of chorus or maximum call rate. On some occasions, a call type attributed to nocturnal planktivorous fishes may have ensonified much of the Great Barrier Reef.

Simple methods of estimating source levels and locations of marine animal sounds.

This paper describes three relatively simple methods of estimating source levels of marine animal sounds by estimating the source distance acoustically, using one or two hydrophones. The methods are logistically simpler than using arrays of hydrophones of known positions but are accurate over a smaller range of distances. One method makes use of the differences in the arrival times and levels of the signals received at two hydrophones from the same sound emission. No knowledge of the positions of the hydrophones is required, however, if these are known the position of the source can be determined, with left-right ambiguity. The second method uses the difference in received levels only, but requires the hydrophone spacing to be known. Adequate accuracy requires the source to be significantly closer to one hydrophone than to the other. Third, if the direct and surface reflected arrivals can be separated, the source level can be determined with a single hydrophone. The methods require accurately calibrated hydrophones.