There's really only the following options:
A: Lots of recalling
B: Fish with a necro-mage, effectively doubling your capacity, and gating back home/to boat with a horde minion. Even if the minion dies, all you have to do is cast another and transfer the contents.
C: Use pack animals. Even a bonded pack horse or pack llama (one slot) is an option, if you can fish at a dungeon spot where you can recall to/from that has no hostile spawn. The Beetle that you don't want to use, is just the most convenient option.
D. Bag of Sending, but costs a lot, and transmits much less to your bank each use than A or B, let alone C.
Notes:
If the dungeon doesn't allow you to recall in, you can have the pets (but not summons) stay at home, log out at the destination, then log in, to get bonded pets past the spawn to the fishing spot on the way in. A really hard-core way to do it would be to have 4 pack horses bonded, leave them outside the Underworld, use an ethy to run through the slimes, worms and undead to get to the teleporter to the large fishing area below the undead, then log-out and recall; recall out after all of them are fully loaded. That will let you haul out over 6500 stone worth of fish per trip.
You can always load up multiple bags on the ground, sorting fish as you go, and recall in with a different character (such as one that already has a beetle) to pick up the containers.
Under some conditions, the fish that look like the old-style one (Lurker Fish, Orc Bass, Snaggletooth Bass - as well as the other 8 that fall into the shallow & deep categories), once in a container then moved to a different one, will go to 1 stone weight each. This doesn't affect their usefulness. When you recall, any in your personal pack will return to the standard 10-per weight, but those in a pack animal won't do so until added to another stack, or you put into your backpack and recall. I once hauled what should have been nearly 2200 stone of fish from our guild fish sharing storage to my boat, in a beetle, almost 600 more than what the beetle should have held.
Dungeon areas also allow using traps from the shore. When I go to my favorite dungeon spot (a single tile surrounded by water in the underworld, with no spawn), I haul 30 or more lobster traps with me in my beetle, and place them around my standing spot (I can fit 10 into the water around me, if I'm careful). As crustacean fishing and pole fishing don't interfere with each other, in terms of harvest amounts, I kill two birds with one stone, and trap storage doesn't interfere with my backpack capacity (and slowly goes away on the beetle, as I typically lose 1-3 traps per cycle, as I try to get at least 2 bobs per trap).
