[repooc]

Hmm I dont think there is an aura or anything while fishing to track. You can use the events to track when you are channeling a spell and stop channeling it. You can then break it down to see if the spell is fishing.
Below is a quick script I threw together in WoWLua and you can see how it shows when I am channeling a spell and the data it dumps. This isnt made to track just fishing but can be very easily adapted to. I am sure there are other ways but this is something I thought up real fast that may help you get started.

local f =  CreateFrame("Frame", nil, UIParent)
f:RegisterEvent('UNIT_SPELLCAST_CHANNEL_START')
f:RegisterEvent('UNIT_SPELLCAST_CHANNEL_STOP')
f:SetScript('OnEvent', function(self, event, ...)
      if event == 'UNIT_SPELLCAST_CHANNEL_START' then
         print(...)
         print("Cast started")
      elseif event == 'UNIT_SPELLCAST_CHANNEL_STOP' then
         print("Cast stopped")
      end
      
end)

Event Documentation:
UNIT_SPELLCAST_CHANNEL_START
UNIT_SPELLCAST_CHANNEL_STOP