Determining Striped Marlin Maturity Size in Cabo San Lucas

Overview

Cabo San Lucas, located at the southern tip of Mexico’s Baja Peninsula, is one of the world’s premier destinations for striped marlin sportfishing; there aren't many sure things in the world of fishing, but catching a striped marlin off Cabo is pretty close! However, recent data shows a troubling decline in the average size of these iconic fish — a signal that greater conservation measures may be needed. The IGFA has stepped in to help find answers and ensure that this iconic fishery is healthy for generations to come. 
 
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The Issue

Across large pelagic, highly migratory species, fishery managers require basic life history information on fish biology to manage their populations effectively. These characteristics include estimates of growth rates, longevity, and spawning dynamics. For striped marlin off Cabo and the broader eastern central Pacific, fishery managers require accurate estimates of size at first maturity, a crucial variable that, if accurately characterized, can lead to improved stock assessment efforts and potentially responsible size limits to sustain a robust spawning population.

Currently, anglers in Mexico are allowed to harvest one striped marlin per day, and although local sportfishing charters are increasingly practicing catch-and-release methods, recent data suggest that the overall size of striped marlin caught off Mexico is trending downward. To fill this crucial data gap, the IGFA and partner institutions are working to understand the reproductive biology of striped marlin off Mexico to improve the accuracy of stock assessments and inform more effective conservation strategies. 

The Research

To carry out this important research, the IGFA has partnered with Dr. Sofia Ortega Garcia and Dr. Marcela Selene Zuniga Flores and Dr. Ulianov Jakes Cota from Centro Interdisciplinario de Ciencias Marinas, Instituto Politecino Nacional (CICIMAR-IPN), in La Paz, Mexico. Since 1990, CICIMAR-IPN scientists have been gathering critical data on striped marlin landed by Cabo’s sportfishing fleet, which includes detailed measurements of length, weight, and sex information.

In this study, histological methods will be used to determine the reproductive stages of a subset of sampled fish, where a small section of female ovaries or male testes will be utilized to classify the stages of maturity and reproductive cycle. Then the maturity stages will be related to the reported lengths and weights of the sampled fish to determine reproductive indicators that will produce sex specific sizes at first maturity. These values will be extrapolated across the rest of the striped marlin caught in the long-term sportfishing fleet survey from 1990-2023.

By improving our estimates of L50 — the size at which 50% of striped marlin are sexually mature — stock assessment scientists can accurately characterize the spawning dynamics of striped marlin in the eastern central Pacific, and improve overall assessments of stock status in the region. This information can also be used by policymakers to recommend a biologically appropriate Minimum Legal Size (MLS) that allows more fish to reproduce before harvest.

The Goal

To ensure a healthy, sustainable striped marlin population for future generations, this study will support robust science-based management policies. Improving the assessments for striped marlin is of utmost importance, and a size-based harvest rule informed by improved estimates of maturity and growth information could be the key to improving the stock status of this iconic species.
 
Stay tuned for findings from this long-term study, and explore how IGFA is leading the way in global billfish conservation.