Pardon my information if it's wrong, but I remember will and surv combined to give you fishing options visible when using the casting rod.
Right now, the casting rod being able to see lower catch chance fish seems to be an attempt at helping fishing credo players find that elusive eel they're looking for. I'm making an assumption about intentions, but rolling with that there's still an issue. Most players who do fishing credo will do so relatively early on in their development. This means low will*surv score, which usually means <3 visible options. With river fish, there are so many options in any given fishing node that seeing the bottom 2 catch chance fish doesn't help you much more than the top 2. Except now you don't necessarily want the best lure combo, because it can push the fish you want up the catch chance list and out of vision. I ended up doing a weird node edging thing where I'd look for the very edge of a fish node to find the one I wanted, and then suffer with the abysmal 2-5% catch rate it took to show up at the bottom of the list. Basically, if we're looking at the casting rod to make the fishing credo more palatable it doesn't quite fill that role right now. People still avoid it and go looking to the net and/or bushcraft fishing because of this issue.
Due to the low level of early utility combined with the system's complexity and relatively unusual stat requirement, the casting rod seems to see very little use. During a player's experimental phase, the system is just a bit too useless and arcane, which seems to cause most players to ignore it. Fully empowered though, it can produce huge amounts of very useful fish so I understand approaching this balance needs to be done with the possible mass extinction of local fish populations in mind.
If we wanted casting rod to be useful during the fisher credo, fish visibility can't be how will*surv limits the system. The first three credos always seem to be done with less than 50 skill, 50 attribute which is 2 options iirc. My thought was perhaps we could look at bite chance (tackle combos cover landing chance well, don't want to touch that) as a possible angle to approach the casting rod. Make low will*surv act as division based penalty to bite chance. Low will*surv the rod would be very bad at mass producing fish, but you would see what fish can be caught and make the system easier to get into. It would also produce lots of low % catch rate options for fisher, giving you a painful but viable way to catch rare fish without needing to fully understand the intricacies of casting rod system. After say 100 will 100 surv in midgame character development most fish start to lose a bit of competitiveness compared to baked and farmed goods. Dried is still good, but no where near as powerful as early game barring a great node. At 100*100 I think it'd be fair to return to baseline catch chance, with further investment providing a boost in catch chance to make it continue to be useful leveling.