Learn Dutch: Ik denk de bevolking krimpt zonder migratie. - I think the population shrinks without migration.

In informal and spoken Dutch, the conjunction dat is frequently dropped after verbs like denken (to think), vinden (to find/feel), and zeggen (to say). This works exactly like English, where you can naturally say "I think [that] the population shrinks."

Why is the verb krimpt not at the end of the second clause?

Because dat was omitted, the second half of the sentence (de bevolking krimpt zonder migratie) acts as an independent main clause, meaning it keeps the standard verb-second word order. If you included dat, it would become a subordinate clause and push the verb to the end: Ik denk dat de bevolking zonder migratie krimpt.

Could I use neemt af instead of krimpt here?

Yes, afnemen (to decrease) is a very natural alternative and highly common when discussing statistics or populations. Krimpen (to shrink) gives a slightly more visual sense of contraction, but both are excellent and interchangeable in this context.

Why is there no article before migratie?

Just like in English, uncountable abstract nouns such as migratie often don't take a definite or indefinite article, especially after prepositions like zonder (without).