Fishing in Jail means to use a string to fish for things from jail cell to jail cell.
Inmates commonly use a string to fish for contraband and share contraband from one cell to another.
The term fish in jail also refers to new inmates.
Fish has been recorded as prison slang for new inmates since the 1870s.
Fish for new inmates shouldn't be confused with another prison slang term, fishing.
This refers to using a string to pass contraband items between cells in a manner similar to casting a fishing line.
'Fish' is the term that inmates use to refer to the newest cons to be incarcerated.
Another possible meaning may refer to the phrase, "A small fish in a big ocean", simply lost among the world around them or "A big fish in a small pond", where the individual usually, for good or bad, stands out from the rest.
Basically when inmates want to share things through the cells they will use a fishing line such as a string to fish for that contraband that they are not supposed to be sharing in jail.
Sharing illegal contraband or fishing in jail can get you into more trouble and get you more jail time as well.