-80 ºC

That is the safest way.

@Aurora Silva can I store the freshly cut tissue samples directly in -80. Or do I have to process the tissue before storing?

To do the proximate analysis I will keep the samples at -80 ( the entire fish). but if you are interested in muscle, You can clean and cut before storing at -80.

Under, nitrogen gas

Thank you so much for you valuable suggestions

For proximate composition analysis you can dry the tissue in the incubator at 45-47 degree. Repeatedly weighing the tissue after 12-14 hours when the weight become constant (indicates 0 moisture) you can gently store it in the refrigerator at (4 degree) in a simple zipper bag upto 1-2 months.