It rather depends on many factors, the strength of the tidal stream between the harbour and the fishing ground, the method of fishing, ie drift netting, seining, trawling, dredging, line fishing or potting. The animal being fished for, Scampi can only be caught at night because during the day they hide in their burrows and some fish only come near to the surface at night such as herring because they follow the zooplankton upon which they feed. The wind strength and direction is also a factor as it will determine the sea state along with the tidal currents. So if they can leave harbour whenever they like and and don't have to fish at night they would leave harbour in the early morning so that they can work in daylight( which is much easier and safer) and then return in time for a meal at home and a pint at the local pub. Any further questions don't hesitate to ask.