Saving the Baltic Sea cod

In a new project, researchers at U ppsala U niversity are studying whether it is possible to help t he B altic S ea cod by releasing l arvae hatched in t he nursery at t he Ar Research St ation on t he island of Gotland.

text LINDA KOFFMAR photo DANIEL OLSSON

THE BALTIC SEA COD

The Baltic Sea cod is in trouble. They are few in number and small in size. The cod situation affects not only the fishing industry but also the entire ecosystem of the semi-enclosed sea.

The cod is an apex predator and when its numbers decline and less herring and sprat are eaten, the ecological cycle is thrown off balance. After several stages, the result is an excess of phytoplankton that, as they sink, contribute to the oxygen deficiency that already plagues the Baltic seabed.

Oxygen deficiency is one of the reasons why cod eggs have trouble surviving.

“While this project will not save the entire Baltic Sea population, if it works one might conceivably do the same thing in multiple locations in the Baltic Sea. That said, other measures will also need to be taken to save the Baltic Sea and the cod from environmental toxins, overfishing and several other issues, all of which must be addressed in parallel,” says Gunilla Rosenqvist, Director of the Ar Research Station, Professor of Behavioural Ecology and project manager for Blue Centre Gotland.

THE PROJECT IS STILL in its first year and has demanded some adaptations to the Ar Research Station on northern Gotland, between the Baltic Sea and Lake Bästeträsk. Once caught, the cod initially swam in large pools. During the summer, they were transferred to other pools with saltier water, more like the breeding grounds they

The ReCod project has attracted a great deal of public interest. "Cod are important, both to the ecosystem and to livelihoods and culturally," says Gunilla Rosenqvist, Director of the Ar Research Station.

would have sought out in the wild.

Once the cod have mated, the eggs float to the surface and can be fished out and placed in the nursery. There, they hatch into larvae, which are released into the Baltic Sea before they are large enough to need to eat. Releases have taken place several times over the summer.

“Quite simply, the main research question the project is seeking to answer is whether it is possible to reinforce the cod population by restocking of this kind,” says Anssi Laurila, Professor of Population Biology, the scientific director of the project.

Laurila explains that the larvae are released into fjord-like bays where there are currently no cod. The goal is to recreate local populations in these bays.

Neele Schmidt is one of the doctoral students working on the project. One of her jobs is to figure out how to tag the larvae so they can be recognised as adult fish.

“We are keen to try various methods and then we’ll see what works. The method mustn’t harm the larvae, it needs to be readable after several years and to be time-efficient as millions of larvae will have to be tagged,” says Schmidt.

THE LARVAE WILL be tagged using the otolith. Both humans and fish have otoliths in the inner ear but these are relatively much larger in bony fish, where they are built up with new rings as the fish grows, much like the growth rings of a tree. By moving larvae between water at different temperatures, research-

Neele Schmidt and Maddi Garate Olaizola are doctoral students working on the project. One of their jobs is to figure out how to tag larvae so they can be recognised as adult fish.

ers can create a unique pattern in the otolith that can be recognised later when the project is evaluated by exploratory fishing after a couple of years.

The project presents other challenges in addition to recognising the fish, such as adapting the larvae to the salinity they will encounter when released into the Baltic Sea, and releasing them in a manner that ensures the highest possible survival rate.

One prerequisite for the project is that, once caught, the breeding fish deliver. Fertilisation should take place naturally, with the cod choosing a partner themselves. Hopefully, however, this will not be a problem. Even before they are allowed into the specially adapted breeding pools they have begun to show a special interest in one another.

“It looks promising and we can already see that they are flirting. It’s actually quite cute. They swim up and down and show off their bellies to each other, so they seem to be getting in the mood. They usually make a very specific sound as well, but I haven’t heard that yet,” says Schmidt. ●

One prerequisite for the project is that, once caught, the breeding fish deliver. Fertilisation should take place naturally, with the cod choosing a partner themselves.

Five-year project

The project ReCod: Release of small cod into the Baltic Sea is funded and implemented by BalticWaters2030 and Uppsala University.

Other partners contributing in various ways are: Leader Gute, Region Gotland, the Swedish University of Agricultural Sciences (SLU), SportFiskarna and the Ulla and Curt Nicolin Foundation.

The project will last for five years and has a budget of just over SEK 50 million.

A visitor centre where the public can follow the project will be built at the Ar Research Station.