The study, published in the journal Acta Ichthyologica et Piscatoria, focused on Mexican Priority Marine Region 4, which encompasses the Bahía Magdalena–Almejas lagoon system. This coastal ecosystem sits in a transition zone influenced by the California Current and the North Equatorial Current, producing cold-temperate waters and among the most productive fishing grounds in the country. Extensive mangrove habitats make the lagoon system a critical breeding, recruitment, and feeding area for a high diversity of fish, which is precisely why researchers consider it a natural laboratory for understanding how multispecies, multigear fishing shapes fish populations.

Between 2014 and 2022, the research team measured 3,215 individual fish belonging to four species: California halibut (Paralichthys californicus), finescale triggerfish (Balistes polylepis), bullseye puffer (Sphoeroides annulatus), and spotted sand bass (Paralabrax maculatofasciatus). Specimens came from two very different sources. The first was shrimp-trawl bycatch, collected monthly from pangas towing trawl nets with a 35 mm mesh size, sampled both during the open shrimp season from September to February and, as a complementary effort, during the closed season from March to August. The second was the artisanal commercial fleet itself, sampled at fish-processing sites in Puerto San Carlos, where fishers use gillnets, traps, hook-and-line, and demersal trawls depending on the target species. Every fish was identified taxonomically, measured for total length to the nearest 0.1 centimeter, and weighed to the nearest 0.1 gram at the regional research laboratory CRIAP La Paz of IMIPAS in Baja California Sur.

The species composition of the two catch streams could hardly have been more different. In the bycatch alone, spotted sand bass overwhelmingly dominated, making up 81.6 percent of individuals, with 1,930 specimens recorded. In the commercial catches, by contrast, California halibut reigned supreme at 61.1 percent of the 851 fish sampled from the artisanal fleet. Overall, bycatch contributed 2,364 fish versus 851 from commercial landings, a ratio that alone speaks to the sheer volume of non-target life stages moving through the shrimp fishery.

Statistical comparison of size-frequency distributions using two-sample Kolmogorov–Smirnov tests confirmed significant differences between the two fishing sources for every one of the four species, with p-values below 0.001 in all cases. Bycatch was consistently dominated by smaller size classes: total lengths in the trawl bycatch ranged from 6 to 39 centimeters, while commercial catches stretched from 17.3 to 81.5 centimeters. The California halibut showed the widest overall span, from 14.0 to 81.5 centimeters, ranging from a 24.6-gram juvenile to a 5.6-kilogram adult. According to the researchers, this pattern reflects a combination of ontogenetic habitat use—younger fish occupying nursery habitats where trawls operate—and the selectivity of the different gears, since commercial gillnets with 13 to 20 centimeter mesh preferentially retain larger, marketable individuals.

To quantify growth patterns, the team fitted length–weight relationships of the classic power form W = aLᵇ, where the allometric coefficient b indicates whether weight increases proportionally with length. All regressions were strong, with coefficients of determination above 0.93. For the pooled data, b ranged from 2.70 in the bullseye puffer to 2.94 in the spotted sand bass, and Student’s t tests showed significant deviation from the isometric value of 3.0, indicating negative allometric growth across the combined datasets—fish were gaining length proportionally faster than biomass. Interestingly, the pattern reversed in some subsets: in bycatch alone, the California halibut showed a b value of 3.112, roughly isometric to slightly positive allometric, while the spotted sand bass registered 3.013. The team attributes these dataset-specific differences to the predominance of juveniles in trawl samples and adults in commercial catches, suggesting ontogenetic shifts in growth whereby adults gain proportionally more weight per unit length than juveniles. The researchers caution that because parameters were estimated across multiple fishing events over the full eight-year span, the values should not be extrapolated beyond the length ranges examined.

Body condition told an encouraging story. Using Le Cren’s relative condition factor Kn, which compares each fish’s observed weight to the weight expected from the fitted length–weight relationship, the team found values at or above 1.0 across virtually all species and datasets, differing significantly from unity in most comparisons. In the pooled data, mean Kn ranged from 0.954 for the bullseye puffer to 1.049 for the finescale triggerfish, whose bycatch mean reached 1.085. Values exceeding 1.0 indicate that fish are in good somatic condition and capable of assimilating available energy over extended periods, which the authors interpret as evidence of favorable environmental conditions within the lagoon system—a rare bright spot in an otherwise cautionary assessment.

The most management-relevant metric was size at first capture, or L50, the length at which a fish has a 50 percent probability of being retained by the gear, estimated by fitting logistic selectivity curves separately for each fishing source. The contrasts were dramatic. For the California halibut, L50 was 24.5 centimeters in commercial catches but 46.9 centimeters in bycatch; for the finescale triggerfish, 14.7 versus 37.8 centimeters; for the bullseye puffer, 16.4 versus 28.7 centimeters; and for the spotted sand bass, 13.8 versus 32.3 centimeters. When these estimates were compared against published sizes at sexual maturity, the implications became stark: approximately 95 percent of the California halibut, finescale triggerfish, and bullseye puffer taken in shrimp-trawl bycatch were juveniles that had never spawned, while roughly half of the individuals captured by the artisanal commercial fishery were adults. The spotted sand bass stood apart—based on a reported maturity size of about 15 centimeters, all commercial-catch individuals and more than 60 percent of bycatch individuals were mature adults.

Official landing statistics compiled from CONAPESCA added an economic dimension. During the study period, the bullseye puffer and California halibut were the two heavyweight contributors in the lagoon system, with mean catches of 394 and 393 tonnes respectively, while the spotted sand bass contributed around 105 tonnes annually. Remarkably, bullseye puffer landings from Bahía Magdalena–Almejas accounted for roughly 74 to 91 percent of the total reported catch of that species for the entire state of Baja California Sur, meaning the lagoon system is effectively the state’s puffer fishery. The California halibut, meanwhile, provides a seasonal source of employment for local communities for four to six months each year.

The broader lesson, the authors argue, is methodological as much as biological. Assessments built solely on commercial catch data of target species provide only a partial view of exploited stocks and can underestimate the true fishing impact on coastal ecosystems, because they ignore the enormous throughput of juvenile fish in bycatch. Combining data from multiple fisheries and gear types reduces bias and improves the accuracy of growth and selectivity estimates, which are essential inputs for quantitative stock assessments. The high proportion of juveniles and subadults in the shrimp-trawl bycatch underscores the ecological importance of the Bahía Magdalena–Almejas lagoon system as a nursery, but it also raises concerns about long-term sustainability and points to an urgent need for research on bycatch mortality. For a region where small-scale fisheries account for a substantial share of the catch and operate under multispecies, multigear conditions, the study provides exactly the kind of life-history baseline—size structure, growth parameters, condition factors, and selectivity estimates—needed to reduce the vulnerability of populations that have been, until now, largely flying under the assessment radar.

Subject of Research: Population parameters of four commercially valuable fish species caught by small-scale fisheries in a Mexican Priority Marine Region

Article Title: Assessing population parameters of four commercially valuable fish species ( Actinopterygii ) caught by small-scale fisheries in a Mexican Priority Marine Region from the northwestern Pacific

Article References: Assessing population parameters of four commercially valuable fish species ( Actinopterygii ) caught by small-scale fisheries in a Mexican Priority Marine Region from the northwestern Pacific. (n.d.). https://doi.org/10.3897/aiep.56.187971

Image Credits: AI Generated

DOI: 10.3897/aiep.56.187971

Keywords: fisheries science, bycatch, length-weight relationship, condition factor, size at first capture, Bahía Magdalena, California halibut, finescale triggerfish, bullseye puffer, spotted sand bass, small-scale fisheries, Mexico