Hey, great find with those field logs! A few things keep the hiss at bay. Store the reels in a cool, dry place – 15–20 °C and 30–50 % humidity is ideal. Use a sturdy case, preferably with a metal or plastic lid to block drafts. Make sure the tape isn’t touching the metal of the case; a little plastic sleeve or a padded spacer works wonders. Keep them out of direct light and away from strong magnets. Flip the reels every couple of weeks, so the tape never stays in the same position for too long. And if you’re not using them, try to play them back slowly at least once a month; a gentle play keeps the binder fibers from drying out. That should help the audio stay as clear as when you first caught it.