Aye, dwarves eat 2 units of food a season. So 300 food supports 150 dwarves for only 3 months (roughly). Now, a bookkeeper will only tell you your total amount of food NOT how much you are producing/consuming. With the civ alert/remodeling your production chains were disrupted... and they may not have been strong enough in the first place. Only 300 food for 150 dwarves is really skimping it. Its quite possible you've been oustripping your food production for some time as your migrants built up rapidly. The waves are small... its easy to account for 7-15 more dwarves. Then you geta wave of 30 followed by two waves of 50. BOOM your farms are overwhelmed.
I'd suggest creating a few immediate food sources (butchering animals, fishers, plant gatherers) while expanding your farms. If you must, flood a small area of stone to create the mud needed. Just remember that if you are trying to fish in the caverns, your dwarves can be pulled in by cave crocs and pond grabbers. Its best to have them fish out of a grate that is in a secured part of your fort. Hilariously, you can also (safely) fish out of a tile filled by an aquifer. Plant gathering is actually very strong right now too. Its the least safe (unless you have an underground herb/tree farm), but herbalists can carry 100+ plants at a time with multi-hauling. You can easily create a (secured) herb farm by leveling some miners through soil/stone (that you flood). I am not sure what the upper limit is... the most I've see is 135 on an herbalist still making his way to more plants. I had to cancel the rest of his jobs because he was already moving at a crawl.
If you have 1000+ plus seeds, you can enable them as food stuff to be cooked. You only need 50 max of each seed. Also keep in mind that some of your farm production is being used up to create booze. That 300+ plants needs to be both booze and food.