This May, I spent a few days at Flødevigen, near Arendal, on the south coast of Norway.

Since 1882, Flødevigen has hosted a research station associated with the Institute of Marine Research (IMR), where some very important research in the field of fish biology and fisheries is taking place. At the moment, for example, one of the ongoing projects is on the effects of marine protected areas on cod. This topic is of particular interest for my own PhD project within NorMER, and in accordance with my supervisors, I will include a study based on this system within my PhD.

I was invited to the research station by Esben Olsen, a researcher at Flødevigen and at CEES, University of Oslo. Esben manages a project using telemetry tagging of cod in Tvedestrand, one of the fjords nearby Flødevigen. In this fjord, a Marine Protected Area (MPA) will soon be established (or may already be enforced), so this is an excellent chance to organize a before-after-control-impact experimental study on the effects of an MPA on cod. Such experimental setups are extremely rare in the field of MPAs, so this is a unique chance to gather very interesting data.

The purpose of the telemetry tagging study is to:

  1. Investigate the effect of the closure in this area on the life history traits of cod, and in particular if there is any reversal of the trends thought to be caused by fishing pressure (reduced size, reduced size at maturity, truncated age distribution).
  2. Understand the habitat use, movement patterns, behavior, and other elements of cod dynamics and behavior, and observe if the MPA has any effect on these elements.

The project started in 2011, with a certain number of telemetry receivers deployed on the sea bottom both in Tvedestrand and in another area, near to Flødevigen, used as control area. The fish are equipped with a transmitter, so that their movement is registered and patterns can be identified. This work is coupled with a traditional mark-recapture study with T-bar tags, which provides information on the population size, on fishing mortality, and also on movement (although of course not as detailed as that of telemetry).

One important part of the work is, of course, the process of catching and tagging the cod. I was invited to join in this work by Esben. I generally really enjoy working on the field, especially when it comes to being on the sea and handling the fish. My PhD project does not involve any field work, so I was enthusiastic for this opportunity.

I arrived late on Sunday evening at the Research Station. I was, however, in time to see the sunset on the bay of Flødevigen.  The sea was incredibly flat, there was not a cloud in the sky, and the bay was completely deserted. The only sound in the air was that of songbirds. What a wonderful place! There were all the ingredients for a great experience.

The next day, early in the morning, I met Esben, together with his colleague Dr. Even Moland and Ida, a Master student. We met directly on the pier in front of the station. After a quick welcome, we jumped on board the small boat waiting for us, and we left straight away to start with the sampling. The trip to Tvedestrand took about 40 minutes, sailing among the labyrinth of channels, islands and skerries that surround Flødevigen and the bay of Arendal. During this time, Esben explained to me the details of the work, as well as some more information about this and other projects.

The fish were captured with Fyke nets, deployed at depths of 2-5 meters. Sampling through these nets at moderate depths guarantees that the fish do not suffer excessive physical stress when caught. In fact, we needed them to be alive and in good health for the tagging operation. The nets were hauled onto the boat one by one. For every net, the cod were retained for special treatment, and other species were counted, measured and released. Cod were measured and kept in a tank: if their size and condition were appropriate, they were retained, if not, they immediately received a T-bar tag and were released. The size selection was an important process: Esben wanted to tag fish representatives of all size classes. The desired sample populations of the most common size classes, between approximately 35 and 40 cm, had already been attained during the previous weeks of work. We therefore focused on smaller and larger individuals.

Once our tank was full of good-sized, healthy cod that we landed for the tagging operations. The fish being tagged were anesthetized with a bath in clove oil (which gave the whole operation a Christmas scent) as prep for surgery. Esben and Even first made a small incision in the belly of each fish, where the transmitter was placed. Transmitters are cylinders about 3 cm long and 0.8 cm wide. Quite large objects for the smaller fish. The cut was then sewed closed by the skilled Esben, and the fish was returned to its tank. The whole operation lasted about 5-8 minutes per fish. My main duty at this time was to repeatedly fill buckets of water from the fjord to put into the tank. In fact, the fish suffer from the warm temperature of the water in the tank, and need a continuous supply of cold water. The larger fish suffered the most, and we had a few casualties during the week. 

When all the fish were tagged, and showed signs of recovery, they were returned to the same location where we captured them. The smaller fish swam away healthy and happily, while for the larger individuals it was sometimes a bit more traumatic.

During the week I was there, we sampled both Tvedestrand and the control area twice. By our final day of work: all of the planned 70 fish in Tvedestrand and 80 fish in the control area were tagged, one week in advance of what Esben had forecast. We also managed to catch and tag some very large individuals: two measuring 79 and 80 cms, a relatively small size in other areas, but a very big cod for the Skagerrak populations. Esben and Even were happy with these results. I was happy for the great experience, the fun time, the useful information and the ideas I’ve had for my own projects, and the stimulating discussions on cod, and on research in general, with Esben and Even.

In those four days, I learned more through hands-on experience about telemetry tagging and other techniques, about this unique coastal system, and about cod biology and ecology than what I could have learned in weeks of work inside my office. Plus, I’ve never worked with a team that so effectively combines both professional work and fun in such a relaxed atmosphere. Esben proposed that I return next year for more field work, and I’m already looking forward to it!

By NorMER PhD student, Giovanni Romagnoni
Published June 12, 2012 8:53 AM - Last modified Aug. 27, 2012 2:23 PM