Theme in Hatch
The theme of the book Hatch by Kenneth Oppel is that you need to take matters into your own hands.
For example; in chapter 11 Dr. Ritter Tries to remove the alien transmitter from Seth’s brain in part of a cruel experiment. Because of this, the rest of the Hybrids are extremely angered at Ritter, not just about the transmitter but the hybrids are really angered that Ritter wanted to do such a cruel experiment on Seth so they took matters into their own hands and killed him which would stop him from doing anything else because they considered him to be “Too dangerous to live”. If Dr Ritter had been able to remove the transmitter then it would have taken away Seth’s ability of telepathy and sound weapon which was supposed to make him “Normal” and would have been able to carry out even more cruel experiments after that.
Another example is at the very start of the book when Seth, Anaya, and Petra retrieve soil samples from Cordova island and save Anaya’s father With those samples and with Anaya’s father saved they were able to create the herbicide that helped to clear important areas of the cryptogenic gardens.
Without this Herbicide, the entire world would be covered in these plants and the world would have been overrun by the plants.
The adults with them (Dr. Weber & Mr. Riggs) had encouraged them to not go after the Main pit plant that sourced the vines of the entire island. The pit plant was extremely big and by killing it it would kill all the vines on the island. But it was extremely dangerous even though Seth, Petra, and Anaya were all immune to its acid. In the end, they went through with the plan without the parent's permission and ended up killing the pit plant and all of the vines on the island because they took matters into their own hands.
With this, we can confirm that the theme of Hatch is that you need to take matters into your own hands.
Recurring Motifs
A common motif in the book Hatch by Kenneth Oppel is the main characters taking matters into their own hands, this can be seen in the first part of the series where the kids take matters into their own hands at the end of the last book, they retrieve samples and completely killed all the cryptogenic plants on Cordova island with planning and bravery when the supervisors wouldn't let them go by themselves but after presenting their smart plan, they were successful. Another time they took matters into their own hands was in chapter 20 when on Deadman's Island the Giant Water Striders attacked the military base, the soldiers had told them to stay inside because they were only children but the hybrid kids decided to join the fight, together they were able to kill all the water striders and the only casualties were from the non-hybrid soldiers, without the help of the hybrids, the military base would have been overrun and the soldiers massacred.
Setting (early book)
Early in the book it's happy, everyone is cheering that the Herbicide worked and that an apocalypse would soon end.
Setting (middle book)
Inside the bunker it’s a stressful and sad environment, the facility is mainly nice but the stress of not knowing what’s on the surface, and it’s sad in there because you’re separated from your parents and your parents might have been arrested because of you.
Setting (late book)
It's mainly gloomy with all of the deadly plants and bugs, and the thought that a building might collapse because of metal-eating worms, or that you might be impaled by giant water striders haunts your everyday life.