This is probably the work of aphids. Did you notice white spots on the leaves or anywhere else?
I don't think they are aphids Eric, more like grubs of some sort.
Gary don't you see any other evidence? What about the underside of the leaves?
A Neem spray should take care of most infestations.
Eric, aphids will usually (always) just suck the sap causing mottled leaves, wilting, etc... From Gary's foto it looks like something was actually chomping on the leaves. I've had decent success with using homemade chili pepper and garlic spray's, for a lot of leaf chomping insects (in a non-AP setting), but you'll have to see about the fish safe-ness of such sprays. Or take care not to get it into the system water.
My guess is worms...... if you see nothing during the day light hours. A flash light in the middle of the night as worms bury into the soil during the day. Carey's advice per Neem. Coffee Grounds or Diatomaceous Earth spread around the base of the plant and back to the Flashlight and night hunting. snails would leave tracks or be visible....Worms
Aphids are green or black and they do NOT hide just multiply
Looks like probable caterpillar damage to me.
Leaf eating caterpillars can be safely controlled in Aquaponics using a Bacillus Thurgensis product designed to work against leaf eating caterpillars. Dipel Dust or Thuricide are the two I've used around my aquaponics. Basically it is a bacteria spore that you dust or spray on the leaves, when the caterpillar eats some of the bacteria it gets a bad belly ach and will quit eating and die in a few days.
Actually here in Trinidad & Tobago we have something which looks like a larger version of an ant referred to locally as a bachac which does that type of damage. i don't know if you have them out there but it sure looks like their handiwork. You can look at this link http://ttfnc.org/livworld/Papers/Bachacs/Bachacs.html
If it is an ant type creature, dusting with Diatomaceous Earth might be a good choice (which would also work against most insects to an extent if it stays dry.)
My guess is slug/snail. Aphids are out for two simple reasons. First, aphids don't actually consume the leaf but suck out it's nutrients. And two, they woulds still be there. You would see them.
I don't think it's a caterpillar. Caterpillar eating patterns are typically from the outer edge inwards leaving very neat semi-circular cuts. They also don't typically leave anything behind. If they are feeding (and they always are), they consume everything they can come into contact with.
These random "blobs" of damage are indicative of the snails method of ingestion, which is to scrape away at their food with their rasp like "tongues".
I get very similar leaf damage to my kale which comes from 3 sources. The first is slugs. I use DE and small plates of Sluggo on top of my media. The second is Japanese beetles. They hide in the media during the day and come out at night to dine. I simply use solar lights in areas that the beetles like to eat. When they come out of the ground at night and find light they will move over to the neighbors house where it is dark. The third is cut worms. They too hide in the media during the day and come out at night. Dipel or any BT containing pesticide will work on them. Water beads off kale leaves so I add 1 drop of dish soap to help the spray to stick to the leaves. I usually spray everything else with Dipel before adding the soap to spray the kale. This limits soap from entering the system.
Kale is in the Brassica family and when I see this kind of damage I think one of two;Japanese beetle or the larval stage of white/sulfur moths (Small green catipillars underneath the leafs aka inch worms). Have you noticed any white or yellow small butterflies flying about your grow?
Go with the flashlight advice. It's not necessary to buy anything or apply anything. You can easily kill whatever is doing the damage. At our place it is Cutworm which eats Kale and causes that type of damage and sometimes slugs/snails too. Control is the key - you can't kill em all. Good luck and happy gardening.
Thanks for all the thoughts and ideas. I found a slug on my pak choy (wicking bed near by), so I put slug bait down at the base of the grow bed stand. Went out tonight and found a green little guy on the Kale. I'm 99% sure these are the guys eating my Kale (Kale looks bad). So what are these and what should I do? Searching every night with a flashlight seems like a battle I'll lose. Thanks for your help, I'm a really novice gardener. :)