The Mackerel is a gear made by ROBLOX on February 18, 2011. It is a melee weapon and it has the genre of "Funny". This fish does a small amount of damage and pushes the opponent back, but has a nice sound and motion. These same attributes are later used in Captain Steelshanks Recruiting Staff.
Speculation
Many users have found the gear as a reference to the popular 2007 first person shooter Team Fortress 2, more specifically to an in-game class called the Scout. This is because the class can use a similar melee weapon called the Holy Mackerel, although the difference is that the lower part of the fish is covered with faded newspaper and tied with a rubber band.