A deep dive into how researchers are using data from the past to secure the future of a vital fish stock.
Imagine the vibrant fish markets of northern Cabo Verde. The air is thick with the salty tang of the ocean and the lively chatter of vendors. At the heart of it all, piled high on beds of ice, lies the caranx—the mackerel scad. This small, silvery fish is more than just a commodity; it's a cornerstone of local diets, a source of income for countless families, and a thread woven into the cultural fabric of the islands. But what happens when the schools of caranx begin to thin?
This isn't a hypothetical question. For decades, Cabo Verde has implemented various fishery management policies to protect this crucial resource. But did they work? A team of scientists has turned to a powerful tool—the ex-post evaluation—to look back in time and find out . Their investigation into the story of the mackerel scad offers a blueprint for sustainable fishing not just for these islands, but for coastal communities worldwide.
In the world of policy, it's easy to announce a new rule but much harder to check if it actually achieved its goal. An ex-post evaluation (Latin for "after the event") is the scientific process of doing just that. It's an audit of the past, using data to rigorously analyze the effects of a policy after it has been implemented .
Think of it like a doctor prescribing a medicine. The ex-post evaluation is the follow-up appointment where they check your vitals, run tests, and determine if the medicine worked, had no effect, or even caused unintended side-effects.
For the mackerel scad in northern Cabo Verde, scientists focused on evaluating policies like:
Regulating who can fish to control fishing effort.
Prohibiting fishing during critical spawning periods.
Limiting nets to avoid catching juvenile fish.
The central question was: Did these rules successfully maintain or increase the production of mackerel scad, or did overfishing and other pressures continue to push the stock into decline?
To answer this, researchers embarked on a comprehensive analysis, reconstructing the history of the fishery to see how it responded to management changes.
The scientists' approach was meticulous, following a clear, step-by-step process:
The first step was to gather all available historical data. This included:
They used statistical models to create a virtual population of mackerel scad. These models simulate how a fish population grows, reproduces, and declines based on factors like fishing pressure and natural mortality .
The researchers then "re-ran" history with their model. They compared the actual recorded landings to what the model predicted would have happened without any management policies in place. The difference between the two scenarios reveals the true impact of the management.
They tracked key indicators of fish population health over time, such as catch-per-unit-effort (CPUE). A declining CPUE means fishermen are catching fewer fish for the same amount of work—a classic sign of an overfished stock.
The analysis revealed a sobering but crucial story. While well-intentioned, many of the management policies had not been as effective as hoped.
The core results showed:
The scientific importance of this finding is profound. It moves the conversation from "Did we create a policy?" to "Did our policy actually work?" This shift is essential for moving towards true, evidence-based sustainability.
| Decade | Average Annual Landings (Tons) | Key Policy Implemented | Observed Trend |
|---|---|---|---|
| 1990s | 1,200 | Introduction of basic fishing licenses | Stable, high landings |
| 2000s | 950 | First seasonal closures introduced | Gradual decline begins |
| 2010s | 700 | Gear restrictions enforced | Continued decline; CPUE drops |
| 2020s | 600 | Revised, stricter closures proposed | Critical state; need for action |
Interpretation: A declining CPUE indicates fishermen are catching fewer fish for the same effort, signaling stock depletion.
This shows the potential benefit of effective, well-enforced policy.
How do researchers piece together this complex puzzle? Here are the essential tools from their kit:
The raw material. Provides the long-term timeline of how many fish were taken from the ocean each year.
The virtual laboratory. Complex computer simulations that estimate fish population size.
The fisherman's thermometer. A simple but powerful indicator of stock health.
The timeline of intervention. Documents when management rules were put in place.
The life story of a fish. Information on growth and reproduction helps set appropriate rules.
Finding patterns and relationships in the data to draw meaningful conclusions.
The ex-post evaluation of the mackerel scad fishery is not a story of failure, but one of vital learning. It provides a clear-eyed, evidence-based foundation for the future.
The key takeaway is that creating a policy is only the first step. Monitoring, enforcement, and adaptation are what make it work. The research suggests that future policies for the mackerel scad need to be stricter, better enforced, and potentially combined with other measures, like designated marine protected areas .
The fate of the small, silvery caranx is a microcosm of a global challenge. By courageously looking backward to see what worked and what didn't, the people of Cabo Verde, guided by science, are now better equipped to move forward—ensuring that the bustling fish markets, and the communities that depend on them, thrive for generations to come.
Enhanced Monitoring
Stricter Enforcement
Adaptive Management