No, I am saying that this was broken in an older version of Simutrans or pak64, and never acknowledged as a error.
However, what you write about weight does not appear to be entirely correct. Fish is measured in crates, not tonnes. The weight of one crate of fish was set to 600 kg in 2014. Before that, it was undefined, which means some default value was used. I don't know what that default value is or was.