Effects of Low Frequency Tones on Sea Lion Predation Near the Bristol Locks
Abstract
The purpose of this study is to determine the effects of low frequency tones on sea lion behavior near the Bristol Locks in Alaska. High frequency tones have been shown to have an effect on sea lion activity but only for a short time period. During this study we observed the predatory habits of sea lions over four weeks during which we changed the frequency of tones being emitted. We found that the tones do deter the sea lions from preying upon salmon near the locks and we recommend further research be done to determine if low frequency tones are a viable long term solution to the sea lion problem.
Introduction
For years the Alaska Department of Fish and Game has viewed the sea lions around the Bristol Locks as a problem because the sea lions prey upon the salmon that are trying to enter the locks. The Bristol Locks are one of the last places the salmon must pass through before the fish head upstream to spawn. The salmon tend to gather in large schools around the Locks providing an easy meal for the sea lions. Each salmon that is eaten is capable of producing hundreds of fry if the salmon is able to spawn. Therefore each salmon eaten by a sea lion means the death of any potential offspring as well. With salmon populations already struggling due to overfishing in certain areas, the Alaska Department of Fish and Game is looking for a way to slow the salmon predation around the Bristol Locks in an effort to allow more salmon to get upstream to spawn.
A previous study by Dr. Maria L. Wharton demonstrated that high frequency tones emitted underwater decrease sea lion activity for a short period of time. However the sea lions grew accustomed to the tones and continued their predation on the salmon. We were interested to see if low frequency tones might be a better deterrent than the high frequency tones. The purpose of this study is to evaluate the effectiveness of low frequency tones on the sea lion activity. This report will discuss the results of a four week study conducted in an effort to assess the effect of low frequency tones on sea lion predation.
In this study we also took the weather into account. A previous study by Dr. Daniel W. Jones showed that sea lions are not as active on sunny days. For the purpose of this study a sunny day is defined as a day during which the sun is not obscured by clouds for at least half of the daylight hours. In contrast a cloudy day is defined as a day during which the sun is obscured by clouds for at least half of the daylight hours.
Materials
In this test our underwater camcorder recorded the underwater activity on mini DVD disks. Each disk had enough memory to record three hours of continuous footage. We needed to change the disks every three hours in order to continue recording. Since we filmed for fifteen hours each day we used five DVDs each day and over the four week test we used a total of 140 mini DVDs.
Apparatus
In order to conduct the experiment we used an underwater camera to observe the sea lion activity. The camera we used was a Canon GL2/XM2 Camcorder with a Gates Video Housing. This particular camera gave a clear recording even in the dark water. Made from machined aluminum, sealed with a military-grade dichromate process, anodized, and then finished in silver metallic urethane the Gates Video Housing provided the best protection available for the camera. We actually had two of these cameras so that when we needed to change the film in one we could put the next one in the water so that we always had a view of what was taking place underwater.
We also placed a low frequency transmitter capable of transmitting a tone 150 yards in every direction. The transmitter we used was a LEX 100. This particular transmitter emitted the frequencies we wanted to test and was powerful enough to broadcast 150 yards in every direction. It was also able to
