Am unable to process what would cause the straight-line demarcation between the slugs and the smooth ice.
The straight line you see is part of the typical crack process when a crack is spread apart, the open water between freezes again, then it spreads again, and the open water freezes again. You end up with stairsteps of ice on either side of the crack. So the line is the original crack. The seals are laying on snow that is covering the remaining "stairsteps", and then there is a main crack running through the middle (which is why there are seals hauled out). On the far side, which is hard to see, the smooth ice begins again along a similar parallel line.Good observation!