Captain Dana Willis of Maine Maritime Academy led the search that turned up the lost bale of compressed plastic waste in Searsport on December 21. The bale was recovered two days later by divers hired by Sprague Operating Resources.

It was myself and Captain Derek Chase, the master of the tug Pentagoet. He’s very good technically and was really my preferred person to have on the boat in that role.

Sprague had sent us a rough outline of the grid that I believe NOAA had provided after doing a current profile, and we set up our grid pattern over that. We ran across the target on the first session across the grid, finished the grid just to make sure there were no additional targets we want to go back to. And then typically — and what we did — you set up at 90 degrees to your track line and narrow the pattern down to get a picture of it from a different angle to get a better determination of the shape. And then after that we closed the range in tight on the sonar and did a couple of passes directly over it.

You’re looking at a picture of the bottom as it reflects sound waves issued by the sending unit on the sonar. It measures the time they take to return and projects an image from that. That’s very oversimplified.

The bale was almost directly east of the dock between their largest pier and the Sears Island Causeway in that cove. Average water depth was probably 30 feet, I would say the [search area] was probably 800 by 1,000 feet, give or take. Our original search pattern was probably about four hours to complete. You have to go pretty slow.

It was midway through the four-hour period that we identified that target. And we continued our grid just to make sure we got everything covered in that area and then spent another hour and a half probably getting better images and also as close as we can get on our latitude and longitude. We also dropped a buoy as close as we could on top of it, so the divers would have something to reference. And they said our buoy was within feet of the bale.

At that point, we still didn’t know it was the bale we just knew it was a target similar to the shape and size.

We were told that one of the bales had broken up on impact with either the pier or the water, and they saw the other one, pretty much intact, slowly floating away from a dock and starting to sink. So we were hoping to find something intact. Just a mound or scrap plastic on the bottom would have been tougher to identify. You can get a pretty good picture, but a big square object like that was a nice thing to be looking for.

The sonar images didn’t turn up anything else of note — in addition to the lost bale, Willis recalled a tire, a mooring chain and a few objects that were probably abandoned lobster traps. Maine Maritime Academy students use side-scan sonar in senior research projects and lower-level marine science classes. Past student projects have included several scans of the large field of pockmarks on the bay floor several miles south of Mack Point.