sounds (major frequency bands and levels) of Aframax class oil tankers (245 m length)

Have you measured the acoustic characteristics of the Aframax class oil tankers that Kinder Morgan would use in its proposal to quadruple the monthly number of these vessels in the Salish Sea? You state that container vessels are loudest but the max. size of those vessels seems to be well below that of the Aframax tankers. I have studied Pacific herring responses to vessel noise, not responses by marine mammals. Pacific herring is a key species in the coastal marine food web of the Pacific NW (Canada's SW).

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Good question, Abby. I've consulted with Val and we offer an answer below. Unfortunately, I'm not able to embed images in this answer, so I'm providing you with plots via hypertext links. (Please note that these are quick plots that have not been peer-reviewed, so the data underlying them may not match exactly the data set presented in our paper.)

In short, the answer to your question is: "Yes, we measured Aframax class tankers (if you define them as tankers with lengths between 245 and 285 meters)." This length range comes from this GeoTrans web site.

First, here is a histogram of tanker lengths --

Histogram of measured tanker lengths

You can see that in the 245-250 meter bin there are 8 tankers. So, the sample size for these Aframax class tankers is small -- only 5% of the total measurements we made of tankers -- but we did measure their acoustic characteristics.

To address the rest of your question we should examine this histogram of container ships lengths --

Histogram of measured container ship lengths

It shows that in our data set the longest container ships are about 350 meters long, 100 meters longer than the longest tankers. We are still trying to understand what makes certain ships louder than others, but our best explanation for why container ships are the most intense at almost all frequencies is that they are some of the largest and fastest-moving ships in the Salish Sea. Notice in the paper's Table 2 that the mean speed of container ships is 5 knots greater than the mean tanker speed.

Your question title suggests you're interested in how tanker noise spectra vary with tanker length. Here's a plot of tanker noise spectra in three length bins: <200m, 200-230m, and >230m --

Source spectrum levels of tankers in three size classes

Counter-intuitively, it appears that the largest tankers have lower source levels (by ~5 dB from 500-5,000 Hz) than shorter tankers! Perhaps the bigger tankers are moving more slowly on average?

Thank you for your question. We look forward to reading your papers as we consider the potential impacts of ship noise on Salish Sea species of concern.

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