Basically my take on this is the mother needs to feed and wants to hunt in the harbour mouth on fish coming out of the harbour as the tide drops and surges through the narrows of the harbour mouth. The tide is strong and she cannot look after the youngster in the fast flowing waters and fish at the same time. In all probability the youngster would be swept away and could be lost. She finds a potline/buoy in the slack water/eddy adjacent to the outflow and leaves the baby there where it seems to hang around in the vicinity of the pot bouy/line. Why it should transfer its allegence to a pot buoy or line is not clear to me but they do seem to hang around in the absence of the parent and the mother seems to feel confident in leaving it there for a few minutes at a time. I have also seen this happen with a mother and calf just swimming along in relatively slack water with the mother diving and leaving the youngster which swims in circles until the mother returns, so maybe the bouy just becomes a focus for the temporarily abandoned youngster. I tried to film the situation and got some footage but its not much good due to the poor light etc. Either way its good to know even in the wettest days our porpoises can lift the gloom if we take the trouble to go out and find them!