Among the main problems faced during the study of populations obtained from a series of samples is the determination of the magnitude and sources of the expected variability that arises from characteristics associated with sampling techniques (Clyde 1953). Therefore, a thorough study of the target species, based on the collection of data on their life-history traits, is essential for quantitative assessment in fisheries management and conservation programs, to reduce the vulnerability of populations under fishing pressure (González-Acosta et al. 2024).

Fishing activity in coastal areas is largely conducted by small-scale fisheries that use both selective gears (e.g., gill nets, longlines, traps) and non-selective gears (e.g., trawls), collectively accounting for a substantial share of global (75%) fish catches. Although these fisheries target a range of commercially valuable species, they can directly and indirectly affect the sustainability of fishery resources and marine biodiversity (Reis-Filho et al. 2019; Adamidou et al. 2023; Peixoto et al. 2025). Fishing pressure can contribute to overexploitation and associated changes in biological communities and trophic structure (Alverson et al. 1996; Hall 1996; Aguirre-Villaseñor et al. 2025). However, assessments based solely on fishery data from target species may be insufficient to quantitatively detect fishing impacts on coastal fish populations and ecosystems because such analyses provide only a partial view of exploited stocks and represent only a fraction of the total catch (Cook and Heath 2018).

The Northwestern Mexican Pacific, including Baja California Sur, is recognized as one of the most productive fishing regions worldwide (Finkbeiner and Basurto 2015) and represents the principal fishing zone in the state (Ojeda-Ruiz et al. 2018; González-Acosta et al. 2024). Within this region, artisanal fisheries operate under multispecies and multigear modalities, and the collection of biological variables for exploited species—such as length and weight—is fundamental to fisheries assessment and management (Mendoza-Portillo et al. 2020). This information allows us to understand the specific condition of each population, as well as the influence of seasonality, the reproductive cycle, and the condition factor (Wooton 1998). Since teleost fish represent a significant percentage of total global catches, the scarcity of biological and historical data covering the entire fishing period has hindered the effective application of quantitative population assessments. Thus, we present a comprehensive analysis based on biological data (e.g., total and standard length, and biomass) for four commercially valuable fish species captured within Mexican Priority Marine Region 4 (PMR 4: Bahía Magdalena) in the northwestern Pacific. Specifically, we evaluate size structure, length–weight relationships, condition factors, and catch volume for species exploited by small-scale fisheries, including shrimp trawling and multigear artisanal fishing, to characterize how multispecies, multigear fishing modalities may influence the populations of these exploited species.

The Mexican Prioritary Marine Regions (PMRs) comprise 76 polygons of coastal and oceanic zones included in the exclusive economic zone of Mexico, which were established for the conservation and sustainable management of the sites of greatest biodiversity and those of current or potential use in Mexico (CONABIO 2026).

Study area. Priority Marine Region 4 (PMR 4: Bahía Magdalena) is located on the western coast of the Baja California Peninsula, Mexico (25°42.0333'–23°43.8'N, 112°55.8'–111°21.6'W), within a transition zone influenced by the California Current and the North Equatorial Current. These oceanographic conditions promote cold–temperate waters that characterize the region (Arriaga-Cabrera et al. 1998). PMR4 encompasses the Bahía Magdalena–Almejas lagoon system (BMA; 24°21'–24°46'N, 110°30'–112°15'W) and adjacent marine areas of the northwestern Mexican Pacific (Fig. 1). This coastal ecosystem includes extensive mangrove habitats that support a high diversity of fish species and function as key areas for breeding, recruitment, and feeding (González-Acosta et al. 2024; Rábago-Quiroz et al. 2024). The region is also among the most productive coastal systems in Mexico, where artisanal fisheries—alongside industrial sardine fishing—target a wide range of mollusks, crustaceans, and finfish species (García-Martínez and Chávez-Ortiz 2007).

Sampling. Sampling was conducted to obtain length and weight data for four selected fish species: California halibut, Paralichthys californicus (Ayres, 1859); finescale triggerfish, Balistes polylepis Steindachner, 1876; bullseye puffer, Sphoeroides annulatus (Jenyns, 1842); and the spotted sand bass, Paralabrax maculatofasciatus (Steindachner, 1868). Specimens were obtained from shrimp-trawl bycatch collected monthly during both the open (September–February) and closed (March–August) shrimp fishing seasons from 2014 to 2022. The catch data corresponding to the closed season were carried out complementary to the open season to cover a longer sampling period (e.g., González-Acosta et al. 2024, 2026). Bycatch samples were taken from small fiberglass boats (pangas) operating shrimp trawl nets with 16–17 m headline (top rope) and 35 mm mesh size, towed at approximately 2 knots (González-Acosta et al. 2024; Rábago-Quiroz et al. 2024).

After capture, specimens were frozen (−4°C) and transported to the laboratory of the Centro Regional de Investigacion Acuicola y Pesquera–La Paz, Baja California Sur, Mexico (CRIAP La Paz–IMIPAS), where they were taxonomically identified using specialized keys and catalogues (Fischer et al. 1995; Robertson and Allen 2015; Love and Passarelli 2020). In the laboratory, individuals were counted, measured for total length (TL; from the tip of the snout to the end of the caudal fin), and weighed for total weight (TW) using a measuring tape [0.1 cm] and a digital balance [0.1 g], respectively.

In addition, specimens of the same species captured by the artisanal commercial fleet were sampled from 2014 to 2022 at fish-processing sites in Puerto San Carlos, Baja California Sur (Fig. 1). The artisanal fleet operates pangas and uses multiple gear types to capture the focal species, including gillnets with 13–20 cm mesh size for California halibut; gillnets, traps, and hook-and-line of various sizes for finescale triggerfish and spotted sand bass; and traps and demersal trawls for bullseye puffer. All specimens were identified using the visual guides cited above and were measured for TL [cm] and TW [g].

Data analysis. The selected species were analyzed separately using data from artisanal commercial catches (Comm-Esc), shrimp-trawl bycatch (bycatch), and the combined dataset. Because annual sample sizes were limited for the focal species, all analyses were conducted using pooled data from 2014–2022 period.

Sizes data. Total length (TL) data for each species were grouped into 3-cm class intervals to compare size-frequency distributions. Relative size frequencies were calculated as proportions to facilitate comparisons among datasets. Differences in size-frequency distributions between fishing datasets were evaluated using a two-sample Kolmogorov–Smirnov test (Gotelli and Ellison 2004).

Length–weight relationships. Length–weight relationship (LWR) was calculated for each species using pooled sexes because sex could not be reliably determined during the sampling event. The relationship between length and weight was fitted using the power function

W = a L b

where W is calculated total weight [g], L is total length [cm], a is the intercept, and b is the allometric coefficient (Ricker 1973). To identify and remove outliers, length and weight values were transformed to logarithm (ln) (Froese 2006). The coefficient of determination (R²) was used to evaluate the strength of the association. The 95% confidence intervals for parameters a and b were calculated in Statistica 10.0 (StatSoft Inc., Tulsa, OK, USA). Significant deviations of b from 3.0 (α ≤ 0.05) were assessed using Student’s t test (Zar 2010), and growth was classified as isometric when b = 3.0, positively allometric when b > 3.0, and negatively allometric when b < 3.0 (Jiang et al. 2017; Hong et al. 2020).

Condition factor. Le Cren’s relative weight condition factor (K_n) was estimated for each individual using the formula:

K n = W o ⋅ W c − 1

Where W_o is the observed total weight and W_c is the expected (calculated) total weight derived from the length–weight relationship, W_c = aL^b (Le Cren 1951). Values of K_n > 1.0 were interpreted as an indicator of good somatic condition. Data processing was conducted in Microsoft Excel, and statistical tests (Student’s t test) were performed in Statistica 10.0 (StatSoft Inc., Tulsa, OK, USA).

Size at first capture (L₅₀). Because the selectivity–length relationship is typically described by a logistic curve, size at first capture (L₅₀) was estimated by fitting a logistic function using nonlinear least-squares procedures following Gulland (1983):

S ( L 50 ) = 1 1 + e S 1 − S 2 ( L t )

where S is selectivity, L_t is the length of the individual (or length class), and S₁ and S₂ are constants in the linear relationship between log-transformed net size and the size of captured organisms. The resulting model represents the probability that an organism will be retained by the gear as a function of its size (Sparre and Venema 1997). Size at first capture (L₅₀) was defined as the length corresponding to 50% retention on the fitted logistic curve. Based on this, L₅₀ was calculated for each species using pooled sexes and separately using data from artisanal commercial catches (Comm-Esc), and shrimp-trawl bycatch (bycatch), respectively. Estimates of L₅₀ were compared with published values of size at sexual maturity for each species to evaluate potential differences between the onset of vulnerability to capture and reproductive maturity.

Official landings. To assess the relative importance of each selected species in Baja California Sur, Mexico, in terms of catch volume and landed value, we compiled official annual landings statistics for 2014–2022 from the National Commission of Aquaculture and Fisheries (CONAPESCA) production database available at https://historico.datos.gob.mx/busca/dataset/produccion-pesquera. Using these records, we also estimated the proportional contribution of each species to total catches in the Bahía Magdalena–Almejas (BMA) lagoon system and across Baja California Sur.

The presently reported study analyzed 3215 specimens of four selected species (Paralichthys californicus, Balistes polylepis, Sphoeroides annulatus, and Paralabrax maculatofasciatus) collected from artisanal commercial catches (Comm-Esc; n = 851) and shrimp-trawl bycatch (n = 2364) in the Bahía Magdalena–Almejas (BMA) lagoon system (PMR4), Baja California Sur, Mexico, during 2014–2022 (Table 1). Species composition differed among datasets. In the combined dataset and in bycatch alone, spotted sand bass (P. maculatofasciatus) was the most abundant species, representing 62.0% and 81.6% of total catch, respectively. In contrast, in the Comm-Esc dataset, California halibut (P. californicus) was the most abundant species, accounting for 61.1% of the total catch (Table 1).

Size frequency. Total length (TL) of the four selected species ranged from 6 to 39 cm in shrimp–trawl bycatch and from 17.3 to 81.5 cm in artisanal commercial catches. The widest size range in the pooled dataset was observed for Paralichthys californicus (14.0–81.5 cm TL). Length–frequency distributions indicated that bycatch was dominated by smaller size classes across all species. Two-sample Kolmogorov–Smirnov tests confirmed significant differences in size distributions for each species between datasets (Table 1; Fig. 2).

Length–weight relationship. Regression analyses for the four selected species indicated strong and statistically significant relationships between total length (TL) and total weight (TW) (R² > 0.93, p < 0.005). For pooled data, the length–weight parameter b ranged from 2.70 in Sphoeroides annulatus to 2.94 in Paralabrax maculatofasciatus. Student’s t tests indicated significant deviations from isometry, consistent with negative allometric growth (b < 3, P < 0.001) for these four species (Table 2). In the Comm-Esc dataset, b ranged from 2.60 in Balistes polylepis to 2.93 in P. maculatofasciatus, indicating negative allometric growth in the former and approximately isometric growth in the latter. In the bycatch dataset, b ranged from 2.80 in B. polylepis to 3.10 in Paralichthys californicus, corresponding to negative allometric growth and approximately isometric to slightly positive allometric growth, respectively (Table 2). Species-specific estimates of b for shrimp bycatch and artisanal commercial catches are summarized in Table 2, and the fitted length–weight regressions for each species are shown in Fig. 3.

Condition factor. Across all four species and datasets (commercial catch, bycatch, and pooled), mean Le Cren’s condition factor (K_n) values were close to or above 1.0 and differed significantly from 1.0 (Student’s t test, P < 0.01), indicating generally good somatic condition (Table 3). Mean K_n varied among datasets: in the pooled dataset, values ranged from 0.954 for Sphoeroides annulatus to 1.049 for Balistes polylepis; in the Comm-Esc dataset, from 0.987 for Paralichthys californicus to 0.996 for Paralabrax maculatofasciatus; and in the bycatch dataset, from 0.976 for S. annulatus to 1.085 for B. polylepis. Overall, mean K_n values indicated a favorable somatic condition across species and fishing modalities (Fig. 4).

Size at first capture (L₅₀). Size at first capture (L₅₀) differed between the Comm-Esc and bycatch datasets. For Paralichthys californicus, the estimated L₅₀ was 24.5 cm TL (Comm-Esc) and 46.9 cm TL (bycatch). For Balistes polylepis, L₅₀ was 14.7 cm TL (Comm-Esc) and 37.8 cm TL (bycatch); for Sphoeroides annulatus, 16.4 cm TL (Comm-Esc) and 28.7 cm TL (bycatch); and finally, Paralabrax maculatofasciatus, 13.8 cm TL (Comm-Esc) and 32.3 cm TL (bycatch) (Fig. 5). Comparisons between published sizes at sexual maturity (dotted lines) and the L₅₀ estimates obtained here indicated differences for most species (Fig. 5).

Officials catch data. During the study period (2014–2022), the two species with the highest mean catch contributions in the Bahía Magdalena–Almejas (BMA) lagoon system were Sphoeroides annulatus (394 t) and Paralichthys californicus (393 t) (Fig. 6A, C). Notably, landings of S. annulatus from the BMA lagoon system accounted for approximately 74%–91% of the total catch reported for the state of Baja California Sur during this period (Fig. 7C).

A comprehensive assessment of small–scale fisheries targeting multiple species and using different fishing gears can provide valuable insights into the cumulative impacts of fishing pressure on exploited fish populations (Aguirre-Villaseñor et al. 2025; Peixoto et al. 2025), thereby contributing to the reduction of population vulnerability. The scarcity of historical data on bycatch and discard mortality associated with artisanal fisheries remains a major limitation in fisheries assessments and has contributed to overfishing, with cascading effects on biological communities and trophic structure in marine ecosystems (Alverson et al. 1996; Hall 1996). These aspects remain poorly studied in many regions. In this context, the presently reported study evaluated size structure, length–weight relationships, condition factor, and catch patterns of four of the most frequent and commercially valuable fish species exploited by small–scale fisheries in the Bahía Magdalena–Almejas (BMA) lagoon system, located in the southwestern region of Baja California peninsula.

Size-frequency analyses revealed clear differences between shrimp-trawl bycatch and artisanal commercial catches, with bycatch dominated by smaller size classes and commercial fisheries capturing larger individuals (Table 1; Fig. 2). These differences likely reflect ontogenetic habitat use, gear selectivity, mesh size, and fishing effort (Kaller et al. 2022). Shrimp-trawl bycatch typically included younger life stages, whereas commercial fisheries preferentially targeted larger, marketable individuals (Fig. 2). Size ranges observed in bycatch for the four species were consistent with those previously reported for the same ecosystem (González-Acosta et al. 2024), although they were smaller than maximum sizes documented in FishBase (Froese and Pauly 2025). Similar patterns have been reported for bullseye puffer, where size distributions reflect both life-stage diversity and selective fishing practices, with fisheries targeting intermediate sizes and rarely capturing older individuals (Aguirre-Villaseñor et al. 2025).

The b parameter of the length–weight relationship for all four species studied here fell within the known biological range of 2.5–3.5 (Froese 2006) and was consistent with values previously reported for these species in this region (González-Acosta et al. 2024) as well as FishBase (Froese and Pauly 2025). Differences in b values among pooled, bycatch, and commercial datasets likely reflect the predominance of juveniles in bycatch samples and adults in commercial catches. Except for Balistes polylepis, larger individuals captured in commercial fisheries generally exhibited higher b values than smaller individuals from bycatch, suggesting ontogenetic shifts in growth, whereby adults gain proportionally more weight per unit length than juveniles. Because length–weight parameters were estimated as mean values across multiple fishing events over an extended period (2014–2022), these results should be interpreted with caution outside the length ranges examined in this study (Table 2; Fig. 3).

Negative allometric growth was predominant in pooled datasets, with isometric growth observed for Paralichthys californicus and Paralabrax maculatofasciatus in some fishing modalities (Table 2). The parameter b reflects multidimensional growth processes that vary throughout ontogeny until sexual maturity is reached (Kaller et al. 2022; González-Acosta et al. 2024). Negative allometric growth, commonly observed in fishes and often associated with females, reflects a greater proportional increase in length relative to biomass (Barros et al. 2023). For the bullseye puffer, the negative allometry reported here contrasts with the positive allometric growth described by Aguirre-Villaseñor et al. (2025), likely reflecting regional differences in population structure, habitat use, or sampling composition.

Relative condition factor values for all species were close to or greater than 1.0 across pooled, bycatch, and commercial datasets, indicating generally good somatic condition and suggesting favorable environmental conditions within the BMA lagoon system (Lagler 1956). These values reflect the capacity of individuals to assimilate available energy over time (Kaller et al. 2022) and may also be influenced by variation in size structure, maturity stage, and fishing pressure (Table 3; Fig. 4). As condition factors were calculated using mean parameters over the study period, interpretations should be limited to the observed weight ranges (Table 3; Fig. 4).

Estimates of size at first capture (L₅₀) differed markedly between bycatch and commercial fisheries, reflecting the contrasting size ranges targeted by each fishing modality (Fig. 5). These findings underscore the importance of incorporating multiple fisheries and gear types in assessments, as analyses based solely on target species or commercial catches provide only a partial view of stock status and may underestimate fishing impacts on coastal fish populations (Cook and Heath 2018). Combining data from multiple fisheries reduces bias and improves the accuracy of growth and selectivity estimates (Wilson et al. 2015; Aguirre-Villaseñor et al. 2025), which are critical inputs for fisheries management (Froese et al. 2011; Curiel-Bernal et al. 2021).

Comparisons between estimated L₅₀ values and published sizes at sexual maturity indicate that approximately 95% of individuals of Paralichthys californicus, Balistes polylepis, and Sphoeroides annulatus captured in shrimp-trawl bycatch were juveniles, whereas roughly half of the individuals captured by artisanal commercial fisheries were adults (Fig. 5A–C). In contrast, based on the reported size at maturity for Paralabrax maculatofasciatus (15 cm TL; Álvarez-Lajonchère and Ibarra-Castro 2012), all individuals from commercial catches and more than 60% from bycatch were mature adults (Fig. 5D).

The high proportion of juveniles and subadults captured as shrimp-trawl bycatch highlights the role of the BMA lagoon system as a nursery area (González-Acosta et al. 2024). While this finding emphasizes the ecological importance of the system, it also raises concerns regarding the long-term sustainability of exploited populations, warranting further research on bycatch mortality and its implications for conservation.

Finally, official landing statistics often exclude bycatch species retained for subsistence or informal sale within the shrimp fishery. Nevertheless, some species contributed substantially to mean annual catches, ranging from approximately 105 t for Paralabrax maculatofasciatus to 394 t for Sphoeroides annulatus during 2014–2022. Notably, S. annulatus accounted for more than 70% of statewide landings in Baja California Sur, while Paralichthys californicus represents an important seasonal source of employment in the region, supporting local fisheries for four to six months each year.