And also, what interventions can be done to increase the amount of human-digestible calories (as well as various nutrients) in the ocean that would be available after some global catastrophe?

Actually, similar questions also apply for other calorie sources. For example, maybe eating insects is good on utilitarian grounds because it encourages the insect industry which can more easily continue to thrive even if the Sun gets blocked.

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You’ve also suggested that we eat bacteria. How would that work?

There are two main sources of bacteria that we looked at. There is a methane-digesting bacteria that you basically grow on natural gas. And then we can either eat that directly or process it or say, feed it to rats and then eat the rats. Then there’s the bacteria that we can grow directly on wood. Or on leftover mushroom waste. And so this would be taking down a tree, pulverizing it, turning it into a slurry, and then letting the bacteria go at it.

So for instance, there are bacteria that secrete sugars they then use to feed themselves. You can pull out the sugars, and eat those ourselves and leave the bacteria and the partially decaying wood pulp. And we can feed that stuff to other things. So for instance, rats digest wood to some degree, particularly after it is partially broken down that way. This makes a fairly good solution. We could feed something similar to chickens. And chicken is something maybe people would maybe be happier to eat than bacteria milkshakes.

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(source: http://nautil.us/issue/101/in-our-nature/what-to-eat-after-the-apocalypse )

I agree it might be an important moral harm to create all those insects, but if the above premise is true, then the extinction resilience aspect seems more important (because if we go extinct, wild insects will likely continue to be created for the next ~5 billion years).