If you happen to be from Sheffield, or anywhere in the vicinity, splodging simply means fishing (or angling, to be more precise).
I couldn't tell you why.
It's not a secret or anything (as far as I am aware), it's just that I don't know. Very little seems to be written about it. It just seems to be one of those South Yorkshire sayings handed down from "fatha" to son.
What I do know is that in the context of this blog, splodging certainly does not mean, "a sexual fetish involving foodstuffs such as custard or baked beans", as defined by the Urban Dictionary!