Overfishing has caused cod to halve in body size since 1990s, study finds – The Guardian

Report on the Evolutionary Impact of Overfishing on Eastern Baltic Cod and Its Relation to Sustainable Development Goals

Introduction

Overfishing has caused a significant collapse in the eastern Baltic cod population. Over the past three decades, not only has the population declined, but the average size of the fish has dramatically and mysteriously decreased. This report highlights recent scientific findings that demonstrate the genetic and evolutionary consequences of intensive fishing on cod, emphasizing the importance of Sustainable Development Goals (SDGs), particularly SDG 14 (Life Below Water) and SDG 15 (Life on Land).

Genomic Evidence of Evolutionary Changes

Scientists have uncovered genomic evidence indicating that intensive fishing has driven rapid evolutionary changes in eastern Baltic cod. Since the 1990s, the average body length of cod has roughly halved, from a median mature length of 40 cm in 1996 to 20 cm in 2019. This shrinking has a genetic basis, showing that human activities have profoundly impacted the population’s DNA.

Mechanism of Evolutionary Pressure

• Consistent removal of the largest individuals over many years gives smaller, faster-maturing fish an evolutionary advantage.
• This selective pressure favors fish that can escape fishing nets due to their smaller size.
• Legal fishing practices, such as size-selective trawling with minimal mesh sizes, intended to protect smaller fish, may unintentionally accelerate this evolutionary trend.

Research Methodology

The study analyzed 152 cod specimens caught in the Bornholm Basin from 1996 to 2019 using otoliths (ear bones) which record annual growth similarly to tree rings. Combining growth data, body size metrics, and genetic analysis, researchers assessed genetic shifts under fishing pressure over 25 years.

Key Findings

1. The median length of mature cod decreased from 40 cm to 20 cm between 1996 and 2019.
2. The median weight dropped from 1,356 grams to 272 grams in the same period.
3. Gene variants associated with larger body size have become less common, indicating evolutionary pressure favoring smaller fish.
4. Environmental factors such as hypoxia, pollution, and temperature changes may also contribute but genetic evolution is a significant driver.

Ecological and Conservation Implications

• The evolutionary changes complicate efforts to restore cod populations despite the 2019 fishing ban on eastern Baltic cod.
• Maintaining genetic diversity is crucial to ensure resilience and adaptability of fish populations, aligning with SDG 14 targets on conserving marine biodiversity.
• Monitoring genetic diversity alongside population numbers is essential for sustainable fisheries management.

Relevance to Sustainable Development Goals (SDGs)

This study underscores the critical role of sustainable fishing practices in achieving the following SDGs:

• SDG 14: Life Below Water – Promoting sustainable use of marine resources to prevent overfishing and protect aquatic ecosystems.
• SDG 15: Life on Land – Protecting genetic diversity and ecosystems impacted by human activity.
• SDG 12: Responsible Consumption and Production – Encouraging responsible fishing practices to reduce ecological damage.

Conclusion

The research published in Science Advances reveals that human-induced fishing pressure has accelerated evolutionary changes in eastern Baltic cod, resulting in smaller fish sizes and altered genetic composition. This finding highlights the urgent need for integrated conservation strategies that consider genetic diversity and evolutionary impacts to support sustainable fisheries and marine biodiversity conservation, in line with global Sustainable Development Goals.

1. Which SDGs are addressed or connected to the issues highlighted in the article?

• SDG 14: Life Below Water – The article focuses on the collapse of the eastern Baltic cod population due to overfishing and its ecological consequences, directly relating to the conservation and sustainable use of oceans, seas, and marine resources.
• SDG 12: Responsible Consumption and Production – The discussion on intensive fishing practices and their evolutionary impact on fish populations relates to sustainable management and efficient use of natural resources.
• SDG 15: Life on Land (indirectly) – While the article primarily focuses on marine life, the genetic diversity and ecosystem health aspects can be linked to broader biodiversity conservation goals.

2. What specific targets under those SDGs can be identified based on the article’s content?

1. SDG 14 Targets:
   • 14.4: By 2020, effectively regulate harvesting and end overfishing, illegal, unreported and unregulated fishing and destructive fishing practices to restore fish stocks in the shortest time feasible.
   • 14.2: Sustainably manage and protect marine and coastal ecosystems to avoid significant adverse impacts, including by strengthening their resilience, and take action for their restoration to achieve healthy and productive oceans.
2. SDG 12 Targets:
   • 12.2: By 2030, achieve the sustainable management and efficient use of natural resources.
3. SDG 15 Targets:
   • 15.5: Take urgent and significant action to reduce the degradation of natural habitats, halt the loss of biodiversity and, by 2020, protect and prevent the extinction of threatened species.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

• Fish Population Size and Biomass: The article provides data on median body length and weight of cod over time (from 40cm and 1,356 grams in 1996 to 20cm and 272 grams in 2019), which can serve as biological indicators of fish stock health and recovery.
• Genetic Diversity Indicators: The study’s genomic evidence showing changes in gene variants related to body size implies the use of genetic markers to monitor evolutionary changes and biodiversity within fish populations.
• Fishing Pressure and Regulation Compliance: The article mentions legally binding minimal mesh sizes and fishing bans, implying indicators related to enforcement and adherence to sustainable fishing practices.
• Environmental Condition Indicators: Although not quantified in the article, factors such as hypoxic conditions, algal blooms, pollution, and temperature changes are mentioned as environmental stressors affecting fish populations, suggesting the use of environmental monitoring indicators.

4. Table: SDGs, Targets and Indicators

Source: theguardian.com