I've been dreaming about building a squid catcher for quite sometime. I think it would need to be:
1. Cheap (why, because I am and you can bet you are going to lose 'em).
2. Able to sense the squid (or other large object)
3. Able to capture some evidence (Camera - still or video)
4. Able to record depth, temperature, etc...
5. Able to surface and send out a beacon of some sort.
I'd love to be able to build 100 of these and drop them off a boat and let them do their polling. After a set amount of time or when their batteries would run out, they would release some gas and float to the surface for collection.<br />
To save money, no tethers would be used. They would be expensive and also would get tangled. Of course, without a tether they would need to get their energy from someplace internal...batteries which would be heavy and a pain to charge.
Re Sensing something, I haven't a clue what would be reasonable. Some sort of sonar, but that would take some significant energy as well I would assume.
The beauty of the Japanese success in filming Giant Squids is the incredible low-tech approach - a hook and line with a camera. The downside is it took 3 years to get the first picture...wow, that is patience.