PhD project within ReCod provides new insights into the early life stages of cod
This spring, Maddi Garate Olaizola, PhD student at Uppsala University and researcher in BalticWaters’ project ReCod – release of small cod in the Baltic Sea – presented her doctoral thesis, Early life performance in Eastern Baltic cod: responses to environmental challenges. The thesis provides new and long-awaited insights into how the early life stages of Baltic Sea cod are affected by current and future environmental conditions – and what this means for the prospects of strengthening the severely depleted stock.
The Baltic Sea cod population has declined dramatically in recent decades. At the same time, the stock has undergone biological changes: today’s cod reach sexual maturity at roughly half the size they used to. Combined with a warmer, more oxygen-depleted sea, this has created a situation in which the cod struggle to recover on their own.
The early life stages of cod are crucial to its survival; yet knowledge of these phases has been very limited. As part of the ReCod project, Maddi Garate Olaizola has had a unique opportunity to study the very earliest stages of cod development – from eggs to the larvae of the first few weeks – and how they are affected by the environmental challenges facing the Baltic Sea.
Five studies that deepen our understanding of the survival of young cod
The thesis comprises five sub-studies which, taken together, provide a more detailed picture of how cod, during their early life stages, are affected by salinity, temperature, the characteristics of their parents and local environmental conditions.
1. The movement pattern of cod larvae is determined by their buoyancy
The study shows that the position of the larvae within the water column is closely linked to their neutral buoyancy. The larvae seek out the levels at which they can remain suspended whilst expending the least possible energy.
2. Yolk sack larvae can adapt when released
The study shows that newly hatched larvae cope better than expected with a rapid transition to the low salinity of the Baltic Sea. This indicates that very early life stages can be used in restocking programmes without being stressed by the change in salinity.
3. The Åland cod – locally adapted but not genetically distinct
The large and vigorous cod in the Sea of Åland do not differ genetically from the rest of the eastern stock, but are locally adapted to the region’s low salinity. This means that the stock’s positive characteristics – size, condition and reproductive capacity – may be the result of local environmental conditions rather than genetics. The results also show that cod eggs can develop on sediment, something that has previously been a matter of debate.
4. Small parent fish produce large, buoyant eggs
Surprisingly, the smallest parent fish produced the most buoyant eggs. These eggs had sufficient buoyancy to avoid the oxygen-depleted bottom waters of the Baltic Sea, which may be crucial for survival. This means that the small, early-maturing cod that dominate today’s stocks can still produce viable offspring.
5. Future climate conditions may hamper the recovery of cod stocks
The results show that higher temperatures and lower salinity – two clear trends in the Baltic Sea – have a direct negative impact on the early development of cod. The results highlight that climate change risks exacerbating the situation for an already vulnerable stock..
Maddi with a small, meagre cod from the southern Baltic Sea. Photo: Olof Lövgren
Maddi with a large Åland cod. Photo: Yvette Heimbrand
An important contribution to ReCod and future management
Maddis’s findings provide new insights that can be applied directly within ReCod whilst also contributing to the long-term management of Baltic Sea cod. By combining behavioural studies, physiology, genetics and climate perspectives, the thesis offers an in-depth understanding of the factors that govern survival during the cod’s most vulnerable life stages. This knowledge is directly relevant to ReCod’s ongoing work on developing methods for the release of cod larvae and fry as a means of strengthening the threatened stock, as well as to understanding what environmental improvements are required for the cod stock to recover in the long term.
Read the thesis
Maddis’s thesis is available in full via Uppsala University’s DiVA portal: Early life performance in Eastern Baltic cod: responses to environmental challenges.