These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The baby Kaluga at the end of "Russian Killer". If there is a baby, it's gigantic mothers must be out there.
Fridge Logic: The little girl who got killed by a Goliath Tigerfish as a result of her amulet belt has this, which just makes it more tragic. Yes, good and bad magic, spirits, all that is Serious Business out in Africa — however, surely people who have been surviving by fishing in the waters for centuries would have worked out that "shiny flashy thing in water" means "good bait for predatory fish and crocodiles"? Good fishers and hunters get to be good by studying, observing and understanding their quarry. So how could they not think "belt of shiny bottlecaps" is counter-intuitive to repel tigerfish, which are some of the most common predatory fish in that region?
Goddamned Bats: Kind of a Real Life example when trying to catch a short-tailed river stingray, Jeremy constantly catches piranha, or has his bait stolen by (he presumes) piranha. He caught a couple of catfish, too, but the piranha were the main problem.
It seems that whenever he goes to South America, he catches/gets his bait stolen by piranha before he actually catches the intended fish.
It's either this or Demonic Spiders when Jeremy keeps trying to catch a Sawfish, but no matter where he goes, all he seems to pull out of the water are bull sharks or crabs.
Iron Woobie | Stoic Woobie: The Sawfish. Not only is it really just a gentle giant (to us, not to its prey), but it's been nearly fished to extinction (and almost always just for its badass titular Natural Weapon). As a result, it's nearly extinct from everywhere in the world but Australia. The one Jeremy caught was covered with crocodile and shark attack wounds, because this juvenile was farther downstream towards the harsh, often brackish environment of the lower river, because humans have forced it there by blocking its way upstream thanks to climate change. Humans Are Bastards, indeed.
This applies to almost all the river monsters Jeremy goes after, in fact. They are driven more and more into remote locations by human pressure.
Tear Jerker: The Goliath Tigerfish episode. One of the attacks was a little girl, whose shiny belt attracted the Tigerfish which killed her. The belt was supposed to protect her from bad spirits in the river.
Although not as the little girl, try not to feel bad for Jeremy after the third time he tries to land the Tigerfish and doesn't. They way he voices his disappointment almost sounds like a Despair Event Horizon.
The death of the tigerfish in the same episode. Jeremy nearly cried when he found out the fish had fatally injured itself and would probably fall victim to another tigerfish if released.
The last few minutes of the 'Russian Killer' episode are largely about how screwed the featured fish, the kaluga sturgeon, is, due to the caviar market. Even an 2500-pound apex predator that is effectively immortal (sturgeons don't age) cannot last against the human onslaught.
In a similar vein, the near extinction of the alligator gar at the hands of humans based on misunderstandings and hogwash. Nice work, Jeremy, for clearing its reputation and saving it from certain oblivion in the nick of time.
In one episode, Jeremy travels to a Matis village in South America only to learn that the village is in the midst of a vigil for the chief's mother whom is near death from an unknown illness. The sight and sounds of the whole village mourning her imminent passing is incredibly sobering.