PIT-tag time series for studying migrations and use in stock assessment of North East Atlantic mackerel (Scomber Scombrus)

The PIT-tagging project on NEA mackerel was initiated in 2011 by the Institute of Marine Research (IMR) in Bergen, Norway, with the main purpose to use the data as a basis for stock assessment and exploring migratory behaviour. RFID is a technology that uses radio waves to transfer data from an electronic tag through a reader for the purpose of identifying and tracking the object. The RFID tags used for tagging mackerel are passive, commonly called PIT-tags (Passive Integrated Transponders), specifically developed for tagging fish and animals. They are made of biocompatible glass, the specific type used for mackerel is ISO FDX-B 134,3 kHz, 3.85x23mm glass tags. The mackerel have been PIT-tagged during a month-long survey at spawning grounds off Ireland-Hebrides in May on annual basis from 2011 onwards, and data from experiments off Iceland and Norway are also available. The mackerel are typically captured by jigging from rented purse seine vessels, kept for up to a maximum of 30 minutes in 1 m diameter round tanks with running water, before they are measured (length and weight), tag injected into the abdomen and released through pipes with running water. All PIT-tagging experiments are approved by the Norwegian Animal Research Authority. During tagging, there is a combined PC-reader system, where each unique tag ID is recorded together with the body length and other details, which are synchronized with the IMR database over internet. IMR has developed monitoring systems where antennas are specially designed for pipes (round antennas) or conveyor belt systems (flat antennas) to detect the tagged fish as they are pumped from fishing vessels or during processing inside factories producing mackerel for human consumption. Such systems have over the years been established in Norway, Iceland, Faroes and Scotland, scanning a large proportion of total landings. These antennas are connected with readers and PCs, and detections of tagged fish are automatically updated in real-time to the database at IMR over internet. The PIT-tag time series is available through a series of APIs, JSON links, to both data of the fish released and recaptured, as well as detailed data on the catches scanned and biological data of both released fish and scanned fish. All these data are needed to estimate numbers of mackerel released per year class, and numbers scanned and recaptured per year class in the years after release, which serve as the basis for use in the stock assessment.