Mackerel, a sea fish rich in omega-3 and protein, contains more omega-3 than salmon. The freshness of mackerel can be determined by observing its eyes, gills, belly, and the firmness of its flesh. However, signs of a poisoned fish include rough or dark pink gills, body deformities, unusual smells, sliminess, and changes in the color of the skin, cornea, and scales. A fish soaked in Urea will have meat that is not firm and lacks elasticity.