You have to keep this growth at bay. Can't let up on it, or it'll just keep spreading. You try to shoo the fish away as you scrub, but it makes a beeline straight for your foot. It strikes you that the tendrils of this fungus thing probably look a hell of a lot like worms to this fish. So when the rainbow trout bites down, you can't blame it all that much.
Unfortunately, the fungus thinks fast on its feet. The moment that fish makes contact, it takes advantage of the situation. Fungal fibers pour into the mouth and out through the gills, trapping the poor thing in place as it panics and thrashes, knocking the scrub brush from your hand. Oh god. In seconds the tendrils have covered the fish entirely, and rush up your leg with renewed strength. You try to grab at them, but now they're angry. One of the tendrils sharpens into a needle-like point, jamming straight through the leather glove before tearing it off and burrowing beneath the skin of your hand, detaching from your leg in the process. Shit. It's inside you now, and spreading fast. You look down at your fish friend, trapped alongside you, but what you see is even worse. The fungus seems to be melding it into the flesh of your leg, scales and skin flowing together as nerves and blood vessels hook together. You can feel its presence in the back of your mind, and it seems to slowly realize you're in there too. It's utterly terrified, as it should be. As the fungus spreads over your body and locks you in place, you try to reach out to the fish, calm it, and with some effort you manage to make contact.
It's scared, wordlessly calling out for help, asking what's happening.
You start to answer, and feel your answer spring to being instantly, wordless and unfiltered. Thoughts rush outward, reaching the simple animal mind, filling it with answers and knowledge and language, lifting it up to a new level of awareness. Now, in the darkness of your conjoined mind, you can see them clearly. You know what they are, who they are, everything they've experienced. They are, she is, a female rainbow trout, nine years old. Their short life flashes vividly within your mind, filling you with knowledge and memories and sensations. Being born upstream in this very forest, breaking free from their egg, swimming downstream to their riverbed homes, so many of their, your siblings and friends lost to the predators above and below the surface. Growing up, flourishing in the forest rivers and streams, becoming full grown and ready to start the cycle anew with another generation. Then capture, absorption, and a bizarre enlightenment.
The process links your minds, the fish becoming another aspect of you, part of a new being. You remember everything of your old life, far longer and more vivid and more powerful in your mind, but the new life and memories are inseparable from you. Joined together, both of you feel a new calm and strength wash over you. And with that, your mind surfaces within your body, mere seconds after diving below.