Because, a "portion" of a sea land container is about 6'x6' or so. Two ways you can harvest your fish. 1) when you put them in, you put them into rotational net cages allowing you to polyculture multiple species and maintain a symbiotic relationship resulting faster growth and less disease, etc.
or 2) when your ready to harvest pump it out. This is practical if you're running a once a year crop. But costs more due to fuel, water, etc.

For me, option one and two are not really practical just yet. My plan was to set up auto feeder at opening, then when fish are used to the regular feedings, wait till it kicks on and throw the cast net. No sense in making this harder than it has to be. Or spending money you don't have to.

For me, this isn't a commercial operation yet. I don't have to harvest all the fish, just the amount I wish to eat. IF I am worried about size grading or stunting growth due to over population a grading rack will resolve OR a handfull of large mouth bass or blue cats should resolve the problems of smaller fish over populating. This process of using predators has worked well for me in my existing ponds. Fish only eat other to a size that the spines etc motivate them to leave that fish alone and look for a smaller one.
Kinda using feeding instinct to grade my fish for me. This also increases urgancy at feeding and helps to have more of the food consumed regardless if hungry or not, my fish seem to try and eat it just to keep the other one from getting it. This forces faster growth.