Jwill, Since Newburyport/ Plum Island are so much further north it those first bright fish a little longer to make the trip. Last season there were guys catching fish in late April up there but the first waves of fish really started to hit the Plum beaches the first week in May. By the second week we were catching 50-100 fish in a few hours off the front beaches. I would fish herring in the Canal in the am and hit Plum Island in the PM (sunset is the best time due to the warmer water). Talk about doing some driving!
Of course if you really had the itch like some of us you can find fish in Rhody in March but you asked about Newburyport.
By the last week in May there will be more reports of bigger fish showing up up there and the first two weeks in June usually are teh hottest times for fish over 20lbs up there. Most of these fish will be taken in the river on bait. I think it has something to do with the herring scent in the river because I have tried all sorts of plugs and the big, wise fish don't fall for them as much. If you just want to plug you can do very well though. THis is the only place that I will chunk bait since thats what 95 percent of the big ones are taken on at that time of the year up there. Good luck