M/S Stording, the fishing vessel that was supporting this year’s cruise by catching herring.

New mark-recapture program of Norwegian spring-spawning herring launched

Over the course of three weeks in the last months of 2016, scientists from the Institute of Marine Research (IMR) including ConEvolHer member Fabian Zimmermann marked several thousand Atlantic herring (Clupea harengus) in Kaldfjord near Tromsø, Norway. This represents the launch of a new mark-recapture program for Norwegian spring-spawning herring after the previous one was ceased in 2007, and will provide helpful information about stock size and natural mortality that can help to improve future stock assessments.

Norwegian spring-spawning herring (NSSH) was marked with steel tags since its collapse during the 1970ies, however the program was stopped in 2007 after recaptures became too unreliable due to the large increase in stock size. To close this current gap in data collection and following a successfully implemented tagging program for Atlantic mackerel (Scomber scombrus), a new program to mark and recapture NSSH was initiated in 2016. With the support of fishing vessel M/S Stording’s crew, five members of the pelagic fish group at IMR (Jostein Røttingen, Adam Custer, Justine Diaz, Florian Eggers and Fabian Zimmermann) set up a floating work station in the Kaldfjord in Troms, where NSSH overwinters in large densities, and tagged the first fish within the new tagging program.

In total, over 6400 fish were tagged and released. To do so, fish were caught, measured, tagged with a RFID chip and released again. Scanners in the factories where herring is processed will then automatically register tagged individuals within the catches. In the overwintering areas, the entire stock of NSSH aggregates to build large schools within the fjords and remains there until they move further out to their spawning grounds in early spring. The large densities and known locations make it particularly efficient to find, catch and process herring in sufficient numbers, which was the reason why the area and time of the year was selected for the tagging program. For the same reasons, this period is traditionally of high importance for the fishing industry too, in particular the coastal purse seiners. This also applies for other species that feed on herring: hundreds of humpback whales and orcas follow the migrating herring each year into the fjords to feed on it.

Mark-recapture data provides useful indications on the stock dynamics and may contribute to better estimates of the stock size. Based on the number of tagged fish within the stock and the ones recaptured in the commercial catches over time, the total number of fish in the stock can be estimated. Subsequently, the number of fish dying of natural causes can be estimated. This so-called natural mortality, i.e. the mortality caused by everything else than fishing, is a crucial component in stock assessment models, yet very little is known about its exact value and how it may change over time in response to stock size, environmental factors or fishing. ConEvolHer aims to shed some light into these questions for NSSH by analysing the data of the previous tagging program. Once sufficient number of years has passed, the now initiated program will provide an additional data source and allow to compare past periods with the current development of the stock.

Injection of internal RFID tag into a herring.

Orcas visiting the work station in Kaldfjord, north of Tromsø.