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To do a task you need to have a skill.
To complete a task you need to have a passion for work because some tasks take a lot of time and effort to finish.
A fishman trains his mind to learn the fishing skill. A skill that helps him to know when and where the fish is. However, if he wants to catch a lot of fish he must have a passion for fishing. Passion for work is your love of doing something. Without it you are unable to overcome problems in achieving your work.

@Thai Tran: the word "fishman" is not in any dictionary. It's a lexical lacuna/ or a lexical gap.

I use Yahoo.com to search for "define fishman", it returns some definitions in the dictionary. But I could not find it in any dictionary neither.

http://www.merriam-webster.com/

Sorry for all mistakes I made. Fisherman is the word I wanted to use in my comment. Thanks for your suggestion.