What is M74 Syndrome? M74 Syndrome is a serious reproductive disorder affecting Baltic salmon, leading to high mortality rates among their yolk-sac fry. This condition is primarily caused by a deficiency in thiamine (vitamin B1) in salmon eggs. The deficiency stems from the salmon's diet, which is rich in fatty prey fish like young sprat and herring but low in thiamine. Affected fry exhibit symptoms such as low motor activity, skin discoloration, and exophthalmia (popeye). The syndrome has had a significant impact on salmon populations in the Baltic Sea, with mortality rates reaching up to 95% in some hatcheries during peak years. Understanding and addressing M74 Syndrome is crucial for the conservation of Baltic salmon.
Key Takeaways:
- M74 Syndrome is a reproductive disorder affecting Baltic salmon, causing high mortality rates among yolk-sac fry due to thiamine deficiency from their diet of fatty prey fish.
- Efforts to prevent M74 Syndrome focus on addressing thiamine deficiency in salmon eggs, including thiamine treatments and dietary adjustments to improve salmon recruitment.
What is M74 Syndrome?
M74 Syndrome is a reproductive disorder affecting Baltic salmon. It leads to high mortality rates among yolk-sac fry, the early stages of salmon development after hatching. This condition has been a significant concern for researchers and conservationists.
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Definition and Manifestation: M74 syndrome is a reproductive disorder of Baltic salmon characterized by high mortality rates among yolk-sac fry. Affected fry show symptoms like low motor activity, skin discoloration, and exophthalmia (popeye).
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Causes of M74 Syndrome: The primary cause is thiamine (vitamin B1) deficiency in salmon eggs. This deficiency is linked to the salmon's diet, particularly their consumption of young sprat and herring, which are high in fat but low in thiamine.
Symptoms and Effects
Understanding the symptoms and effects of M74 Syndrome is crucial for identifying and addressing the issue.
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Thiamine Deficiency Mechanism: Thiamine deficiency results from an unbalanced diet rich in fatty prey fish like young sprat. This imbalance leads to thiamine deficiency in the eggs, causing mortality in yolk-sac fry.
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Symptoms of M74 Syndrome: Affected fry exhibit low motor activity, skin discoloration, exophthalmia, hemorrhages near the heart, and liver changes with more vacuoles and lower glycogen content.
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Mortality Rate: Death usually occurs within three weeks after hatching. Mortality rates can be as high as 95% in some hatcheries, especially during the early 1990s.
Geographical and Dietary Factors
The syndrome primarily affects Baltic salmon populations in the Baltic Sea. The diet of these salmon plays a crucial role in the development of M74 Syndrome.
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Geographical Distribution: M74 syndrome primarily affects Baltic salmon populations in the Baltic Sea. The syndrome is linked to specific feeding grounds and the diet composition of the salmon.
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Dietary Factors: The main prey species of Baltic salmon, such as sprat and herring, play a crucial role in the development of M74 syndrome. Young sprat have the highest fat content and the lowest thiamine concentration.
Prevention and Treatment
Efforts to prevent and treat M74 Syndrome focus on addressing thiamine deficiency in salmon eggs.
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Thiamine Levels in Eggs: Thiamine levels in unfertilized eggs from M74-monitored salmon have been analyzed since 1994. Studies show significant thiamine deficiency in eggs from affected females.
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Prevention and Treatment: Thiamine treatments can prevent M74 syndrome. Thiamine bathing of eggs and newly hatched fry is now used in hatcheries to improve fry recruitment and reduce mortality rates.
Historical Context and Biological Variability
The history and biological variability of M74 Syndrome provide insights into its development and impact.
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Correlation with Female Behavior: A correlation has been found between a "wiggling" behavior in females and M74 syndrome in their progeny. This behavior is thought to be related to thiamine deficiency in females.
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Historical Context: M74 syndrome was first discovered in 1974 at Swedish hatcheries. The incidence increased successively from 1974 and peaked in the early 1990s.
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Biological Variability: The biological variability between clinical stages in the early (E) group may be higher than in the late (L) group due to the rapid progression of M74 and the severe condition of the fry.
Gene Expression and Nutritional Deficiencies
Research into gene expression and nutritional deficiencies helps to understand the underlying mechanisms of M74 Syndrome.
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Gene Expression Analysis: Cluster analysis of gene expression data places all terminal and preclinical stages in separate clusters. Genes related to oxidative stress and apoptosis are upregulated at all M74 terminal stages.
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Astaxanthin Deficiency: Astaxanthin deficiency has been linked to M74 syndrome. Astaxanthin is a carotenoid responsible for the pink color of salmon flesh. A decrease in astaxanthin levels has been observed in eggs and fry of Baltic salmon affected by M74.
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Fatty Acid Composition: M74-affected eggs show differences in fatty acid profiles, particularly in the levels of unsaturated fatty acids, which may contribute to thiamine deficiency and overall health issues.
Impact on Salmon Populations
The syndrome has had a significant impact on Baltic salmon populations, affecting their sustainability.
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Impact on Salmon Populations: M74 syndrome has led to reduced recruitment and population decline, affecting the overall sustainability of salmon stocks in the Baltic Sea.
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Relationship with Atlantic Cod Stock: The syndrome is connected to changes in fish stock dynamics in the Baltic Sea. The collapse of the Atlantic cod stock and the strong year classes of sprat have contributed to the development of M74 syndrome.
Feeding Patterns and Biochemical Changes
Feeding patterns and biochemical changes in salmon provide further insights into M74 Syndrome.
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Feeding Migration Patterns: Salmon from northeastern GoB rivers that feed on lipid-rich prey fish like young sprat are more likely to produce eggs with thiamine deficiency, leading to M74 syndrome.
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Thiaminase Activity: Thiaminase activity in alewife has been negatively related to lipid content. This suggests that the lipid-rich diet of Baltic salmon may contribute to thiamine deficiency through thiaminase activity.
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Biochemical and Ultrastructural Changes: Biochemical and ultrastructural changes in the liver of Baltic salmon sac fry suffering from M74 syndrome include lower glycogen content and more vacuoles, indicating severe metabolic disturbances.
Histopathological Studies and Thiamine Levels
Histopathological studies and thiamine levels in muscle tissue provide additional insights into M74 Syndrome.
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Histopathological Studies: Studies of yolk-sac fry of Baltic salmon with M74 syndrome have revealed several pathological findings, including ruptures of blood vessels and white precipitates in the yolk sac.
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Thiamine Levels in Muscle: M74-affected salmon show significantly lower thiamine levels in their muscle compared to healthy salmon, indicating widespread thiamine deficiency.
Astaxanthin Mobilization and Prey Biomass
Astaxanthin mobilization and the energy content of prey biomass play roles in the development of M74 Syndrome.
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Astaxanthin Mobilization: Mobilization of astaxanthin from the female muscle and liver during egg formation is crucial for normal development. A deficiency in astaxanthin can contribute to the overall health issues observed in M74-affected salmon.
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Prey Biomass and Energy Content: The incidence of M74 syndrome is most clearly related to the energy content of the prey biomass in the southernmost Baltic Proper. Salmon feeding on lipid-rich prey fish like young sprat have higher fat content and lower thiamine levels.
Research and Management Efforts
Ongoing research and management efforts aim to mitigate the effects of M74 Syndrome and improve salmon recruitment.
- Research and Management Efforts: Research efforts focus on understanding the mechanisms leading to thiamine deficiency in salmon eggs and the subsequent mortality of yolk-sac fry. Management strategies, including thiamine treatments and dietary adjustments, are being implemented to mitigate the effects of M74 syndrome and improve salmon recruitment.
The Impact of M74 Syndrome on Baltic Salmon
M74 syndrome is a serious reproductive disorder affecting Baltic salmon, causing high mortality rates among yolk-sac fry. This condition stems from thiamine deficiency in salmon eggs, often due to a diet rich in fatty prey fish like young sprat and herring. Symptoms include low motor activity, skin discoloration, and exophthalmia. The syndrome has led to significant declines in salmon populations, with mortality rates reaching up to 95% in some hatcheries.
Preventive measures like thiamine treatments have shown promise in reducing mortality rates. Understanding the link between salmon diet and thiamine levels is crucial for developing effective management strategies. The collapse of Atlantic cod stocks and changes in fish stock dynamics have also contributed to the spread of M74 syndrome. Continued research and targeted interventions are essential to protect and conserve Baltic salmon populations for future generations.
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