Depends on where you put in. They are open year round on PA side, you have to put in on PA side but during March and April there is a slot size. Not sure if/when NJ will do the same for those two months North of the C&D. Also, below the C&D canal you can keep them I think Starting in March, will need Delaware license and I know PA does not require them but you should use inline circle's to be safe, Delaware requires them and I am pretty sure NJ does too in the River. I am sure someone will chime in about that and confirm it.
Where in the River were you planning on going?
End of March beginning of April might be a little early if you go by the last two years. If you want a better shot at one of the biggons watch for the Herring run. May was a better month for me than April the last 2 years. Water temp seems to dictate the run. If we get an early warmup you might get lucky in March but I look forward to end of April and the whole month of May.