Learn Spanish (Latin America): Al enganchar el atún, el capitán acababa de parar el barco. - Upon hooking the tuna, the captain had just stopped the boat.

The structure al + infinitive is a shortcut for saying "when [something] happens" or "upon [doing something]". In this sentence, because no explicit subject is placed next to the verb, we naturally assume the person doing the hooking is the subject of the main clause: el capitán.

What if someone else hooked the tuna? How would we show that using the al + infinitive structure?

You can add a different subject right after the infinitive. For example, if a sailor hooked the fish, you would say: Al enganchar el marinero el atún, el capitán acababa de parar el barco (Upon the sailor hooking the tuna, the captain...).

Could I say cuando enganchó instead of al enganchar?

Yes, cuando enganchó el atún (when he hooked the tuna) means the exact same thing. However, al + infinitive is a very common, elegant shortcut that native speakers use frequently to compress a "when" clause without having to conjugate a second verb.

Why is acabar in the imperfect tense (acababa) here?