what's the usage of fish trawler's reward?

I have soloed a fish trawler game for completing the elite Ardougne Tasks, and got one manta ray, two turtle and some shark for reward. The things I can't understand is that how can I instantly sell turtle and manta ray for more than 3k each, while the highest level fish, aka rocktail, is only worth like 1.5k~1.7k.

 

I can't imagine the trade volume for these fish is very high since there is only one world that operates the trawler. It could be that people are able to cook manta and sea turtle, but not able to cook rocktail?

 

No, that's not the reason. Sharks would still be better anyway.

Sea turtles and manta rays just are pretty rare, and some people probably collect them.