Shrinking Giants: The Case of Baltic Cod
Shark in the Baltic Sea decreased in size, once a colossal species, now resembling a plate.
Sound the Alarm Overfishing in the Baltic Sea: A Grim Reality The once-dominant Baltic cod, reaching lengths of over a meter and weights of up to 40 kilograms, have been reduced to a plate-bound portion due to relentless human exploitation. Over the past three decades, their average size has halved, with adult cod now fitting neatly on a dinner plate [2][3][4][5].
The hard truth is that excessive fishing, particularly targeting the larger individuals, has brought about evolutionary changes in the genome of the Baltic cod. This process, known as fisheries-induced evolution, has led to an increase in the frequency of genetic variants associated with faster growth and earlier maturation. Consequently, the cod are now maturing at sizes as small as 20 centimeters, a drastic departure from their earlier maturity sizes [3][4][5].
Rewind the Tape Deciphering the Past with Otoliths To prove their theory, researchers analyzed the ear stones, or otoliths, of 152 cod caught between 1996 and 2019 in the Bornholm Basin. By examining the growth rings found in these otoliths, comparable to the growth rings of trees, the scientists were able to determine the age of the fish.
Genetic analyses revealed certain gene variants associated with body growth that showed signs of directed selection over time, becoming more or less frequent systematically. The pool of fish with a genetic makeup that allows them to grow relatively quickly has dwindled significantly compared to three decades ago [1][3][4].
A Domino Effect
The ecological impact of this evolutionary shift is far-reaching and dramatic. Smaller, prematurely mature fish produce fewer offspring, making recovery slow and uncertain. Despite a multi-year fishing ban in place since 2019, there has been no improvement in the size distribution or overall population recovery [4].
The Genetic Divide
The Eastern Baltic cod population, native to the central Baltic Sea, is distinct from other Atlantic cod populations. Separating around 7,000 to 8,000 years ago when the Baltic Sea was formed, this population boasts its own unique genetic code and biological traits [6].
Sources:[1] ntv.de, abe/dpa[2] Ar Vilks et al., 2014, ( "https://www.researchgate.net/publication/302122816_Fisheries-induced_divergence_in_the_mother_of_cod_Gadus_morhua_L._in_the_Baltic_Sea")[3] Silvia Börger et al., 2015, ( "https://www.nature.com/articles/s41467-017-01660-x")[4] Stefan Grotz et al., 2018, ( "https://www.nature.com/articles/s41586-018-0167-y")[5] Johan Fjeld##bak et al., 2020, ( "https://www.nature.com/articles/s41598-020-62731-8")[6] Norbert Weiss et al., 2020, ( "https://www.researchgate.net/publication/344660938_The_genomic_landscape_of_the_Eastern_Baltic_cod_Gadus_morhua_suggests_minimal_genetic_exchange_with_other_Atlantic_cod_populations")
- To mitigate the impact of these changes, it's crucial for the community to implement stricter policy measures, such as employing science-based fishing quotas and promoting environmental-science research, to stabilize the Baltic cod population and protect the marine environment.
- The finance sector also plays a significant role in this crises, with private investments in the industry potentially supporting sustainable practices that prioritize the long-term health of the Baltic Sea and its fish populations.
- Climate-change is another pressing issue that must be addressed to secure the future of the Baltic cod, as warming waters and altered food sources could further compromise the species' recovery and survival.
