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Source: NIWA – National Institute of Water and Atmospheric Research

Is this tuna a male or female?

When tuna arrive in the freshwater as glass eels they are neither male nor female. They turn into male or female depending on the environment they live in and other circumstances.

As an eel grows, the sex organs (testes in males, ovaries in females) can be seen as thin lines of tissue running along the backbone inside the body cavity. The male testis looks like a string of beads, while the female ovary looks like a folded net curtain.

Generally, eels growing in high densities (i.e. with many other eels present) tend to become males, whereas eels growing in areas where there are few other eels present tend to become females. 

In all species of freshwater eels, the females grow to a much larger size than males. Thus, any eel above the maximum size to which males grow must be a female. However, anythingsmaller than this could be either male or female. 

In shortfins, males do not usually grow larger than 550mm (about 350g), so any eel larger than that is a female. In longfins, males grow to a maximum of 750mm (about 1.25kg), so larger eels are females.

Protect the big girls!

Female tuna grow a lot bigger than males. Anything you catch over about 750mm long is probably a female.

The larger the female, the more eggs she has. Female tuna can have more than 20 million eggs! However, female longfin eels are long-lived and take several decades to reach reproductive maturity. 

Until they reach maturity and migrate to sea, tuna (particularly the females) are susceptible for a long period of time to fishing activities as well as mortality due to non-fishing activities. 

There are concerns about a decline in the number of large longfin female eels in the fishery and what impact this might have.

MIL OSI