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Biological reference points for data-limited artisanal fisheries: A CMSY application to black skipjack (Euthynnus lineatus) in the Mexican Tropical Pacific

This study applies the CMSY method to historical catch data (2005–2023) of black skipjack in the Mexican South Pacific to reveal that unregulated effort has caused severe biomass depletion and overfishing, prompting a recommendation for an immediate precautionary Total Allowable Catch of 800 tons per year to restore the stock.

Original authors: Yuliesky Garcés-Rodríguez, Romina Alzugaray-Martínez, Abel Betanzos-Vega, Arturo Tripp-Quezada, Rocío Rivera-Campos, Amilcar Mitjáns-Sánchez

Published 2026-07-17

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Yuliesky Garcés-Rodríguez, Romina Alzugaray-Martínez, Abel Betanzos-Vega, Arturo Tripp-Quezada, Rocío Rivera-Campos, Amilcar Mitjáns-Sánchez

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the ocean as a giant, underwater bank account. For centuries, humans have been making withdrawals from this account, taking out fish to eat and sell. But for a long time, we didn't really know how much money was actually in the vault. We just kept spending, hoping we weren't going bankrupt. This is especially true for "small-scale fisheries," where local fishermen use small boats and simple gear to catch fish right off their coast. These fishermen are the backbone of food security for millions of people, but because they don't always keep detailed spreadsheets, scientists often can't tell if the fish population is healthy or if it's running a deficit.

To fix this, scientists use special tools called "stock assessments." Think of these like a financial audit for the ocean. They try to figure out two main things: how fast the fish population can grow back (its "intrinsic growth rate") and how many fish can be caught without the account ever running dry (the "Maximum Sustainable Yield," or MSY). If we catch more than the MSY, we are essentially eating into the principal, and the population will crash. The big question is: how do we do this audit when we don't have perfect records? That's where a clever method called CMSY comes in. It's like a detective who can solve a mystery just by looking at the receipts (catch data) and knowing a little bit about how the suspect usually behaves, even without seeing the suspect directly.

The Mystery of the Vanishing Black Skipjack

In the warm, blue waters off the coast of Oaxaca, Mexico, there lives a fish called the black skipjack (Euthynnus lineatus). It's not the biggest tuna in the ocean, but for the local small-scale fishermen, it's a goldmine. These fish swim in schools right near the shore, making them a vital source of food and income for coastal communities. However, for a long time, nobody knew exactly how many of these fish were left or if the fishermen were catching too many. The data was missing, like a bank account with no statements for the last twenty years.

This paper is the story of a team of scientists who decided to solve this mystery using a digital detective tool called CMSY (Catch-Maximum Sustainable Yield). They didn't have fancy underwater cameras or perfect logs of every fish caught. Instead, they had a historical record of how many tons of fish were landed between 2005 and 2023. They fed this data into a computer model that simulates how fish populations grow and shrink, using the catch numbers as clues to figure out the hidden truth.

The Plot Twist: The Boom and the Bust

The story the data told was dramatic. For the first part of the timeline (2005–2013), things were calm. The fishermen caught about 511 tons a year, and the fish population seemed happy and healthy. But then, around 2014, the plot thickened. The number of fishing boats exploded. More permits were handed out, and suddenly, the fleet was much bigger.

Between 2014 and 2019, the catch numbers skyrocketed. In 2015 alone, they pulled in a massive 2,097 tons—more than double what they usually caught. It looked like a golden age for the fishery. But here is the twist: the fish didn't like the party. The computer model revealed that this sudden surge in fishing was like a hungry crowd raiding a buffet. The fishermen were catching fish faster than the fish could have babies.

The Verdict: We Are in the Red Zone

By running their simulations, the scientists found that the black skipjack population is currently in trouble. They estimated that the ocean can naturally support a total of about 8,779 tons of these fish (this is called the "carrying capacity"). To keep the fishery sustainable forever, the "safe" amount to catch every year is about 1,000.5 tons. This is the Maximum Sustainable Yield (MSY).

However, the reality is much grimmer. The model shows that by 2023, the fishermen were catching too much. The fishing pressure was 1.35 times higher than the safe limit. Even worse, the actual amount of fish left in the ocean (the biomass) had dropped to just 57.5% of what it should be to stay healthy. In the language of the scientists, the fishery has moved from the "green zone" (safe) to the "red zone" (danger). The fish are being overfished, and the population is shrinking.

The Solution: Hitting the Pause Button

The paper argues that the only way to save the black skipjack is to stop the "race to fish." The scientists suggest that the government and the fishermen need to set a strict limit on how many fish can be caught. They propose a "Total Allowable Catch" (TAC) set at 800 tons per year. This is 80% of the maximum sustainable amount, acting as a safety buffer to let the fish population recover.

They also suggest capping the number of active boats at 200. If the fishermen stick to these rules, the model suggests the fish population has a good chance of bouncing back because black skipjack are actually pretty good at reproducing. But if they keep catching at the current rate, the fishery could collapse, leaving the local communities without their main source of food and money.

The Takeaway

This study is the first time anyone has put a number on the health of this specific fishery. It proves that even without perfect data, we can use smart math to see the warning signs. The black skipjack isn't gone yet, but it's on life support. The paper doesn't claim to have fixed the problem, but it has handed the fishermen and the government a clear map: catch less, limit the boats, and let the ocean heal. If they listen, the fish will come back; if they don't, the account might just go bankrupt.

Technical Summary: Biological Reference Points for Data-Limited Artisanal Fisheries

Problem Statement
Small-scale fisheries (SSF) are critical for global food security and coastal livelihoods, yet they are frequently excluded from formal stock assessment frameworks due to chronic data limitations. This lack of scientific information hinders sustainable management and masks overexploitation, particularly in tropical regions. The black skipjack (Euthynnus lineatus) in the Mexican South Pacific, specifically along the coast of Oaxaca, exemplifies this issue. Despite being a primary resource for local artisanal fleets and a key component of regional food security, the stock has historically lacked formal assessment. The fishery operates under a de facto open-access framework with no legal catch limits, making it highly vulnerable to synergistic impacts of intense fishing pressure and climate-driven ecosystem shifts. Previous assessments have been absent, leaving a regulatory void where the stock's status remains unknown despite increasing extraction volumes and a subsequent decline in catch trends.

Methodology
To address the scarcity of biomass and effort data, this study applied the Catch-Maximum Sustainable Yield (CMSY) method, a Bayesian state-space framework designed for data-poor fisheries. The analysis utilized a historical time series of annual commercial landings from 2005 to 2023, sourced from official National Fisheries Statistics Yearbooks (CONAPESCA).

Data Inputs: The model relied exclusively on the catch time series and informative uniform prior distributions. The intrinsic rate of population increase ( $r$ ) was constrained to a "Medium resilience" range (0.2–0.8) derived from FishBase. Prior distributions for relative biomass ( $B/k$ ) were set for the start, middle, and end of the time series.
Effort Proxy: Due to the lack of systematic fishing effort data, the annual number of active vessels (derived from commercial fishing permits) was used as a proxy for nominal fishing effort, following standardized data-poor approaches.
Statistical Analysis: The study employed non-parametric Kruskal-Wallis tests to identify significant differences in production phases and Ordinary Least Squares (OLS) regression to analyze the relationship between vessel numbers and landings.
Assessment Framework: The CMSY model estimated population parameters ( $k$ , $r$ ) and derived biological reference points (MSY, $B_{MSY}$ , $F_{MSY}$ ). Stock status was visualized using a four-quadrant Kobe plot to classify the fishery based on relative biomass ( $B/B_{MSY}$ ) and fishing mortality ( $F/F_{MSY}$ ).

Key Results
The CMSY framework estimated a population carrying capacity ( $k$ ) of 8,779 tons and an intrinsic growth rate ( $r$ ) of 0.464. The Maximum Sustainable Yield (MSY) was calculated at 1,000.5 tons/year, corresponding to a target biomass ( $B_{MSY}$ ) of 4,389.5 tons.

The analysis revealed three distinct operational phases:

2005–2013: Fluctuating landings below 850 tons/year with moderate effort (~103 vessels).
2014–2019: An exponential surge in catch, peaking at 2,097 tons in 2015, driven by an unregulated expansion of the fleet to ~200 active vessels.
2020–2023: A drastic collapse in landings to ~700 tons/year, despite nominal effort remaining stable at ~200 vessels.

The stock assessment indicates a critical status for the fishery:

Overfishing: The fishery entered a state of active overfishing ( $F/F_{MSY} > 1$ ) in 2015. By 2023, the relative fishing mortality was estimated at 1.352.
Depletion: The relative biomass ( $B/B_{MSY}$ ) declined from a sustainable level to 0.575 by 2023, approaching the critical biological limit of 0.5.
Kobe Plot Trajectory: The stock trajectory moved from a sustainable "green" quadrant (2005–2014) to a transitional "orange" quadrant (2015–2016), and finally into a critical "red" quadrant (post-2016), characterized by simultaneous overfishing and severe biomass depletion.

Key Contributions
This study provides the first quantitative, science-based baseline for the black skipjack fishery in the Mexican South Pacific. By successfully applying the CMSY method to a data-limited context, the authors established:

Specific biological reference points (MSY, $B_{MSY}$ , $F_{MSY}$ ) for a species previously unassessed in this region.
A diagnostic history of the fishery's trajectory, identifying the specific period (2014–2019) where unregulated fleet expansion triggered an unsustainable trajectory.
A methodological framework that demonstrates how catch time-series can be interpreted as biological yield production to derive management targets even in the absence of direct biomass surveys.

Significance and Recommendations
The paper asserts that the current regulatory void and unregulated expansion have pushed the stock into a state of depletion that threatens its long-term resilience. While the species possesses moderate biological resilience ( $r = 0.464$ ), immediate intervention is required to prevent recruitment impairment.

The authors recommend the immediate implementation of a precautionary output-control framework. Specifically, they propose:

Total Allowable Catch (TAC): Setting a conservative TAC at 80% of MSY (800 tons/year) to facilitate stock rebuilding.
Effort Control: Capping nominal effort at a maximum of 200 active vessels.
Management Integration: Integrating these quotas into a Catch Share System and complementing them with input regulations, such as minimum landing sizes and Fish Replenishment Zones.

The study concludes that these data-limited biological benchmarks provide a robust scientific foundation to update the National Fisheries Charter (CNP) and guide evidence-based governance, balancing the ecological resilience of the resource with the economic viability of coastal communities in the Mexican South Pacific.

Technical Summary: Biological Reference Points for Data-Limited Artisanal Fisheries

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