Baltic Sea garfish are in trouble

12 November 2019, by Dr. Katharina Alter

Photo: UHH/CEN

The garfish has a long, pointed nose and grass-green bones. It is a predatory fish in the Baltic Sea, where it has almost no natural predators itself. If the size of its population changes, it affects other fish and organisms in the food web, e.g. the herring. The ties between predator and prey have always been complex in the ocean. In addition, climatic changes become apparent. What effects will climate change have on garfish?

Together with my team at Universität Hamburg’s Center for Earth System Research and Sustainability (CEN), I’m currently investigating how garfish will cope with both increasing temperatures and increasing CO₂ content in the water.

Climate change effects are often investigated using larval or juvenile fish. But for me it was important to study the influences from the very beginning. So, my plan was to start from fertilization – which was not easy, because little is known about the biology of garfish. We know that it migrates from the Atlantic Ocean into the Baltic Sea to spawn in spring. Some do so in the Western Baltic Sea near the island Fehmarn, while others migrate further east into the Baltic Sea to spawn.

This spring we received several adult specimen that were ready to spawn from fishers in the Western Baltic Sea close to Fehmarn. According to a study conducted in the 1970s, the garfish develops best at temperatures of 17 degrees or warmer. So we combined eggs and sperm in 17-degree-warm water, but 80 percent of the embryos did not survive the first three days. Clearly, the temperature was not optimal.  

Because garfish only spawn for a few weeks every year, the clock was ticking. If we could not find any new ready-to-spawn adult garfish, we would have to wait an entire year. We ultimately found a fisher further east, who was able to catch new fish for us in the Bay of Greifswald. At the same time, we modified the experiment: now the fertilization would take place at 13 degrees, the local water temperature at that time. This time it worked much better!

We started the tests. Several thousand embryos were divided between four tanks, so that we could test present and future climate factors independently. We simulated two temperature scenarios: for “today’s” water temperature, we selected a warming rate of 0.1 degrees Celsius per day, which is roughly equivalent to the natural springtime warming in the Bay. For the future temperature under climate change, we warmed the water by 0.3 degrees per day. In addition, we combined both temperature treatments with CO₂-poor and CO₂-rich water, in order to simulate the present and future CO₂ conditions. In this regard, we drew on values provided in the United Nations’ climate report, which reflect a scenario in which CO₂ emissions are not reduced. According to the report, the CO₂ content could climb from approx. 400 parts per million to 1300 parts per million in the year 2100.

One of our findings was particularly extreme: in the CO₂-rich water, once again 80 percent of the embryos died in the first three days; in the water with today’s CO₂ level, the number was only 20 percent. The different temperature treatments made no difference during this phase; instead, the influence of CO₂ level became apparent.

We subsequently tested the freshly hatched garfish from the experiment to determine their fitness. What we found: the few survivors subjected to CO₂-rich water were just as fit as their peers from the other tanks. In terms of their build, swimming performance and metabolism, they scored just as high as the others. Thus, the garfish has the potential to adapt to future climate conditions.

Yet, the number of surviving embryos remains dramatically low. One important take home message: we also need to study the embryos! The drastic effect of high CO₂ concentrations cannot be seen any more in the larval fish – those fish that manage to survive the initial shock are just as robust as their counterparts.