For numbers of fish in the 30-80#+ range I would say April through October.
For larger fish over 100# I would say November through April.
I took a trip with Capt Eddie Burger Sept of 2007 and we caught around 60 fish, mostly Blackfin and Skipjacks to about 20# on Poppers and heavy spinning gear but we also ended up with several nice Yellowfin between 50-70#.
Took another trip with him in November and fished the "Mullet Run" and caught two Yellowfin(135# and 115#) and a Blue Marlin about 250-300#. We "only" caught the three fish but we got to see several viscious topwater strikes on large live Mullet.
Here's a link to Capt Eddie's website.
World-class Venice Louisiana offshore fishing charters specializing in Yellowfin Tuna and Swordfish!