DOI: 10.21335/nmdc-2114050995
title: PIT-tag time series for use in stock assessment and migration studies of North East Atlantic mackerel (Scomber Scombrus) and Norwegian spring spawning herring (Clupea harengus)

description: This publication reports data from large scale PIT-tagging programs on NEA mackerel and Norwegian spring spawning herring for use in stock assessment and migration studies. Radio Frequency Identification (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 and herring 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 is ISO FDX-B 134,3 kHz, 3.85x23mm glass tags. All PIT- tagging experiments are approved by national Animal Research Authorities. 
Since 2011 Institute of Marine Research in Bergen, Norway (IMR) has PIT-tagged mackerel annually during a month-long survey at spawning grounds off Ireland-Hebrides-Shetland in April-May. Iceland (HAFRO) conducted experiments on feeding mackerel at their west coast in August 2015-2019. In 2023 UK (CEFAS) initiated experiments in the North Sea during September. The mackerel are 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 directly released to the sea again through pipes with running water. Daily experiments runs for up to 16h starting 6 o’clock in the morning.
Since 2016, IMR has also tagged NSS herring annually over a 20-days survey in the wintering areas in fjords of northern Norway during November-January. The herring is fished with purse seine in the darkness around 4-5 o’clock in the morning and carefully pumped live onboard to Refrigerated Sea Water (RSW) storage tanks though pipes. After anchoring the vessel the tagging process on herring stored in the tanks runs for maximum 12 hours every day. The actual tagging process is the same as for mackerel. However, tagged herring is kept in a larger storage tank until 200 individuals are tagged, after which they are released to the sea as a school rather than single fish as the mackerel.
During tagging of both mackerel and herring there is a combined PC-reader system recording the unique tag IDs together with the body length and other details relevant for the experiment. These data are synchronized with the IMR database over internet several times per day. Later on the monitoring systems with RFID antennas specially designed for pipes or conveyor belt systems are detecting the tagged fish at factories producing mackerel for human consumption. Such systems have over the years been established in Norway, Iceland, Faroes, Scotland and Ireland, scanning a large proportion of total landings, where recaptures are updated real time to the data base at IMR over internet. 
The PIT-tag time series are openly available through a series of APIs, JSON links described in this publication. This includes data on released and recaptured fish, as well as detailed data on the catches scanned and biological data of both released fish and scanned fish. All these data are required to estimate numbers of mackerel and herring released per year class, as well as the corresponding numbers scanned and recaptured per year class in the years after release, which can be used as input to the stock assessment.