Increasing numbers and speeds of vessels in areas with populations of cetaceans may have the cumulative effect of reducing habitat quality by increasing the underwater noise level. Here, we first use digital acoustic tags to demonstrate that free-ranging delphinids in a coastal deepwater habitat are subjected to varying and occasionally intense levels of vessel noise. Vessel noise and sound propagation measurements from a shallow-water habitat are then used to model the potential impact of high sound levels from small vessels on delphinid communication in both shallow and deep habitats, with bottlenose dolphins Tursiops sp. and short-finned pilot whales Globicephala macrorhynchus as model organisms. We find that small vessels travelling at 5 knots in shallow water can reduce the communication range of bottlenose dolphins within 50 m by 26%. Pilot whales in a quieter deep-water habitat could suffer a reduction in their communication range of 58% caused by a vessel at similar range and speed. Increased cavitation noise at higher speeds drastically increases the impact on the communication range. Gear shifts generate high-level transient sounds (peakpeak source levels of up to 200 dB re 1 碌Pa) that may be audible over many kilometres and may disturb close-range animals. We conclude that noise from small vessels can significantly mask acoustically mediated communication in delphinids and contribute to the documented negative impacts on animal fitness.
Summary. The device for splitting a plankton sample into two approximately equal parts consists of a hollow cylindrical drum mounted to turn on a horizontal axis, and a vertical semi-circular septum cemented into place midway between the end walls of the drum. About a quart wilt fill the drum up to the axis. After rotating the drum until the septum is above, the sample is poured in. Then the drum is rotated until the septum splits the sample. Lifting the drum ahd rotating a little more, the two separated samples are drained into containers of convenient shape for emptying and filling. Smaller samples are obtained by emptying one container into the drum. Thus aliquot portions o F approximately 1/2 , 5/4, 1/s , etc. of the original are obtained, and the process is continued until the sample is small enough for counting. In general the sample consists of a number of different kinds of plankters each of which is counted separately. Multiplying each count in the ruth fraction by 2m gives an estimate of the number in the original sample. Each step in the splitting process is subject to random errors and possibly biased errors. Studies of these errors are based upon a series of test runs involving both volume measurements and plankton counts. The samples split were specially chosen to represent different types of plankton communities.The several types tested proved not to influence the results materially.Statistical tests involving distribution of Z 2 and departure from the mean, of left and right portions gave convincing evidence that the errors .were random.Thus, averaging separate counts would be expected to reduce the error. Researches on the magnitude of the error were based upon the law of propagation of errors of a product. Furthermore a study of the difference between the left and right samples as well as other investigations on the different sources of error have been made.
Analyses in the gulf of Maine and bay of Fundy show the zooplankton population to be dominated by a relatively few species of boreal endemic crustaceans. Calanus finmarchicus, the most abundant form, averaged 39.9 per cent by number in the total region during the period, April to September in 1932, and 35.5 per cent for the year in the bay of Fundy. Fluctuations in the volume of zooplankton reflect to a large extent numerical changes in the stock of this species. The vernal rise in 1932 occurred following propagation of Calanus, and the rapid downward trend in June coincided with the critical period of maturation and subsequent mortality of adults after spawning. Due to differences in the time of spawning in different parts of the region, two, and in some cases three, breeding stocks of boreal plankton animals can usually be distinguished. The distinct spawning periods are continued in subsequent generations that year no matter where distributed. Productivity was found to be closely correlated with temperature and stability of the water mass, and dispersal with the nontidal circulation in the region. The vernal crop of boreal plankton species appears to be derived largely from adults maturing in the western or outer gulf. With the advance of the season the centre of production moves to the eastern basin. The turbulent New Brunswick-eastern Maine coastal zone as far west as Mount Desert is relatively unproductive, and characterized by small zooplankton volumes.
Some marine animals produce sounds which, under certain conditions, completely dominate the ambient noise in the sea. The snapping shrimp (not to be confused with the edible shrimp) are the most widely distributed of these animals; they are frequently less than 3 cm in length and produces the sound by snapping of the claw. Shrimp noise is likely to be found around the world in tropical and subtropical waters less than 55 meters deep wherever rock, coral, or other material on the bottom provides interstices in which shrimp thrive. This paper describing the acoustic output of the snapping shrimp (Crangon and Synalpheus) is based upon measurements made off the Southwestern and Southeastern coasts of the United States, the Hawaiian Islands, and several islands in the Southwest Pacific. Over a shrimp bed, the noise spectrum is found to be roughly independent of frequency from 2 to 24 k陆 (the upper limit of measurement), whereas the ambient noise normally present in the deep sea decreases with frequency. There is a broad peak in the shrimp spectrum somewhere between 2 and 15 kc. Over a shrimp bed the noise at 20 kc is about 25 db above the ambient noise which accompanies a sea state of 2 (waves less than a meter high, not including swell). For low sea states, shrimp noise is appreciable a mile or more from the boundary of the bed. The maximum diurnal variation of shrimp noise is from 3 to 6 db, the noise being greater at night. Single snaps from isolated specimens have been subjected to a Fourier integral analysis which indicates a spectrum comparable to that measured over shrimp beds where the underwater sound consists of a multitude of snaps. A typical peak sound pressure at the distance of a meter from a single shrimp is of the order of 200 dynes/cmL the U.S. Navy Radio and Sound Laboratory (now called the U.S. Navy
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