Meaning of three words

Meaning of three words

The passage below is from Fathoms: The World in the Whale by Rebecca Giggs.

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I asked him, ‘Is it you, who makes the decision?’ I suspected he could get a gun instead, and use that. I had heard he was empowered by certain regulations to fire on a suffering whale, as though it were a chassis on chocks in a paddock.

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Here I am a volunteer who wants to help save a stranded whale. It seems that as the possibility of the stranded whale is becoming meager, it is discussed how to euthanize the whale.

Here’s my question.

However hard I may try, I cannot find out the meaning of the underlined part, specifically three words, chassis, chocks, and paddock.

Any meaning dictionaries present does not fit with these three words within this context.

Can you help me?

Thanks in advance.