For the larger BG that are unquestionably too large for your largest LMB to swallow, the reason they wait out the night resting on the bottom could be two-fold. First, a remnant behavior leftover from needing that behavior earlier in life too improve prospects of survival. Second, energy conservation. BG feed primarily by sight and the visual reach of their sight at night may be too short to make foraging worth the effort for the tiny things they eat. Conserving energy at night may be a behavior that reduces daily metabolic demands for energy (food) that maximizes the benefits of what they can consume on a daily basis. Just guessing here, I actually don't know. LOL. Mystery is OK in my book, but I do like to try to demystify things I don't understand.
As for LMB sight at night? Probably impaired, but they can approach prey much closer without detection at night. Also, their lateral lines are kind of like eyes that help them identify the location of movements that are relatively close to them. They don't have to see to use this form of determining the location of prey. LMB prey are substantial proportions of their daily energy needs and so foraging at night in stealth mode when the BG defenses are impaired is probably profitable.
BG do not fear a predator that they know has little chance of capturing them. Likewise, LMB have little interest in prey they have little chance of capturing.
So think of it like this. If you have a rabbit nest out back where the offspring have just reached a size they can run about, you will have no trouble capturing them and you could consume them if you want. But as they grow, they will become increasingly difficult to run down and catch, until at some size, you just can't do it anymore often enough to keep you interested, (unless of course the rabbit is sick, dying, or geriatric). When the rabbits are tiny, they don't provide much energy, but as they grow, they provide increasingly larger amounts of energy. If you had knowledge of your success rate for attacking rabbits of various sizes, you could multiply that probability of success by the energy content of various sizes of rabbits and what you would get is the revenue (in energy) of attempting that capture. What you would find is ... that the revenue of trying to capture a rabbit when plotted against the length of a rabbit forms a curve that looks like a bell. So, at very small sizes, the revenue is small but as the length of the rabbit increases, the revenue grows and reaches a maximum at some particular length. This is the optimum length that produces the maximum revenue of energy for a predatory attempt.
It works the same for LMB. No different. As prey grows larger than this optimum length, the revenue of trying decreases and once it falls below the threshold that covers some minimum proportion the energy expended, they just stop trying. Whether a particular size of prey is profitable, that is, can be encountered frequently enough to provide the number of predatory attempts needed to consume more energy than was expended, is more complicated. In part, this is a function of the abundance of that size of prey. So it is conceivable that it is profitable for an LMB to consume large prey that produce only 1/10 the energy of optimum prey per predatory attempt. All that is required, is that the large prey be numerous enough to provide the sufficient number of encounters that culminate in the sufficient number of predatory attempts. In that case, the number of predatory attempts is 10 times higher than the optimum sized prey.
The energy expended is NOT the energy expended in a predatory attempt. This is a very minor portion of metabolic expenditures. Metabolic investment begins at the end of handling the last meal. Handling involves digesting a meal and metabolizing it. When metabolites fall below a minimum threshold, the LMB will resume foraging. It is at the resumption of foraging that the investment of energy for its next meal begins. This investment includes all the energy expended in search for opportunities to launch predatory attempts (including unsuccessful attempts) until it finally consumes another prey item and completes its handling of it. Provided the energy gained by meals exceed the investments of energy expended in searching, subduing, and handling of those meals, the fish is consuming profitably and will gain weight.
