I am sure some of you know about this, so save those kind of comments of "old news", please.
You can NOT alch on the trawler. Sucks doesn't it? Well, you can use humidify for the same, cheaper exp. But, since you can't alch people believe why should you be able to cast this spell? is this acceptable? is it a bug in your guy's opinion? should jagex update the trawler to exclude this spell. It does nothing to help us get more fish or anything.
The fishing trawler runs on world 116 almost constantly between 9am EST and Midnight EST it runs pretty regularly and rarely sinks. You can get a considerable amount of fish from it.
Humidify is apparently faster, however I am not sure of this for a fact. But it seems so. The runes are cheaper and even with a steam staff its automatically just astrals.
So anyway, by combining the two, after 76 fishing you are most likely going to profit off of this method of humidifying while doing the trawler. This is frowned upon as it is "freeloading" which some consider bug abuse and against the spirit of the game its self.
Now the real tip, with the hotkeys (F4 & F7) this spell can be casted quickly, you have to empty your vessel however. With the boat running usually 1k catch each trip i typically get 14 manta ray, its no 91 RC or anything but, it still makes $. And with the PVP update coming manta rays have been going up in price and will probably be in demand when the update is then released. If you were to do humidify at the trawler until the PVP update and you plan on doing PVP your self, now you have a considerably higher magic level and some nice fish.
also, if your going to do this, please go to the top of the boat and bring ONE single rope just in case everyone runs out of rope. That way we ALL get more fish, and ropes are only like 20GP ea.
and again this is a discussion involved on if it is OKAY OR NOT that is why i decided to explain it some people aren't questioning the details. But of course I wont deny anyone an answer.