The article states it’s a little over 75,000. It also states that those salmon will then follow the river to the ocean since they are freshly hatched salmon they will be migrating to the ocean, which is what the tanker was transporting them to do anyway
If it were that easy, the hatchery would just do that and save money on trucking.
Don't salmon swim upstream to spawn? I assume the reason they don't release them right there is the risk some would stick around and mess up an existing ecosystem.