Estimates of Tag Extrusion and Initial Tagging Mortality in Atlantic Herring (Clupea harengus harengus) Released with Abdominally Inserted Magnetic Tags

Returns of Atlantic herring (Clupea harengus harengus) tagged internally with magnetic tags along southwest Newfoundland and in the southern Gulf of St. Lawrence in 1970–71 were analyzed for biases due to initial tagging mortality and to tag extrusion during the reproductive act. Initial tagging mortalities calculated by two methods ranged from 4 to 49% and increased with the duration of impoundment of the fish prior to tagging. Estimates of tag loss during the spawning season indicated that most of the tags inserted in ripening herring were extruded with the reproductive products, whereas those applied to recently spent or recovering herring had sufficient time to become attached to internal organs and hence had a high retention rate. The results indicate that the effectiveness of tagging herring internally will be greatly improved if conducted on recovering or spent fish impounded for only short periods of time before tagging.