Introduction

Trachurus trachurus (Linnaeus, 1758; Pisces; Perciformes; Teleostei; Carangidae), commonly known as horse mackerel, is a semi-pelagic fish widely distributed throughout the Northeast Atlantic Ocean, where it represents a species of interest to fisheries, being subject to intense exploitation and globally classified as vulnerable (Smith-Vaniz et al. 2015). Horse mackerel usually inhabits the continental shelf from the Norwegian Sea to Senegal, including the North Sea, and it is also distributed in the Mediterranean and the Black Sea (Smith-Vaniz et al. 1986). The bathymetric distribution of this species ranges from coastal waters to the continental slope up to 350 m depth. The maximum size reported for T. trachurus is 60 cm and in the NW Mediterranean Sea its spawning period occurs from April to August, with a peak in July (Carrillo 1978; Casaponsa 1993; Lloris and Moreno 1995). The diet composition of this species varies depending on the zone and length of the specimens, although crustaceans, cephalopods and teleost fish are often part of it (Ferreri et al. 2019; Rahmani et al. 2020; Ahmed et al. 2021).

In the Spanish Mediterranean, except for the Balearic Islands, FAO (2022) statistics for T. trachurus are incomplete, probably because they are grouped with other species of the same genus and catalogued as Trachurus sp. Most catches of this species in the Mediterranean come from Turkey, the Sea of Marmara, and the western Black Sea, however, the different stocks are currently declining possibly due to fishing pressure and poor recruitment (Smith-Vaniz et al. 2015). In the Mediterranean, T. trachurus populations from the Alboran and Black Sea have severely reduced, being considered fully exploited, while populations in the northeastern Atlantic and Turkey are deemed to be overexploited, thus, the species is globally listed as “vulnerable” in the IUCN Red List of Threatened Species (Smith-Vaniz et al. 2015).

Recent investigations conducted in the Northwestern (NW) Mediterranean Sea revealed a decline in some life history traits of relevant small and medium pelagic fishes, i.e., Sardina pilchardus, Engraulis encrasicolus and Trachurus mediterraneus (Albo-Puigserver et al. 2021; Rodríguez-Castañeda et al. 2022). Their results suggest the existence of phenotypic adaptive responses with changes in the size at first maturity, fecundity, condition factor and age structure possibly promoted by the current exploitation of stocks and the influence of environmental conditions. Nonetheless, the current regulation (CE) Nº 1967/2006 states a minimum catch size for T. trachurus at 15 cm of total length, which is lower than the size at first maturity of this species (Abaunza et al. 2008). Therefore, the exploitation of juveniles throughout time is probably influencing the life history of T. trachurus (Roff 1992).

The size at first maturity (L50), which measures the mean length at which 50% of the individuals in a population attain sexual maturity, is one of the most important metrics for fisheries management. (Fontoura et al. 2009; Hashiguti et al. 2018). It provides information on the maturity and reproductive cycle of a population as a complement to variable life growth patterns (Lorenzen 2016), leading to the creation of fishery restrictions, as it allows the estimation of the minimum catch size of a species (Shephard and Jackson 2005; Schill et al. 2010; Stark 2012).

On the other hand, applications of length–weight relationships in fisheries science and population dynamics are varied, including the conversion of equations from growth in length to growth in weight, the creation of models for stock assessment, the estimation of biomass values from length observations and the determination of fish condition status (Erzini 1994; Froese 2006; İşmen et al. 2009). Condition indexes are commonly applied in studies of fish biology as well, since they offer significant information on their physiological state, indicating that individuals of a specific length with greater weight have a better condition status than the rest (Lima-Junior et al. 2002; Craig et al. 2005; Tzikas et al. 2007).

Life history traits regulate populations, community interactions and species responses to fluctuations in the environment (Moore et al. 2014; Valladares et al. 2014; Zimmermann et al. 2018), thus understanding the mechanisms causing variation in these traits is important for the management of economically relevant species and to guarantee their sustainable exploitation (Şahin et al. 2009). Vulnerability assessment criteria are varied and generally comprise a set of parameters associated with population dynamics, mainly the geographical distribution and abundance of a species, and fish life history traits, such as body length, growth rate, life span, age at maturity, trophic level, among others (Parent and Schriml 1995; Strona et al. 2013; Strona 2014). Thus, these approaches should be considered accordingly for studies dwelling on fish conservation (Strona 2014).

The parameters of abundance and distribution are fundamental for the management of fishery resources, as they represent the basic units on which population dynamics models are applied (Abaunza et al. 2008). Assessing the status of a population can be achieved through the monitoring over time of analysis of the spatial patterns of the population abundance, which allows ensuring its long-term sustainability (Mustafa 1999). Population dynamics are heavily influenced by these patterns and, thus, information on them is essential to partially anticipate species vulnerability to overexploitation and to accordingly implement management regulations for their protection (Taylor et al. 2014). The inclusion of the spatial component facilitates defining areas of preferential distribution of different stages of the population, establishing essential habitats, defining spawning areas, and source-sink zones and tracing the geographical limits of the stocks, which is essential for the proper management of populations.

This study aims to fill the gap in knowledge with updated information, on the structure and state of conservation of T. trachurus population in the Western Mediterranean continental shelf, describing the evolution of its biological traits throughout time. Data from a long-time series collected in the MEDiterranean International Acoustic Survey (MEDIAS), conducted in the NW Mediterranean Sea during July, was analyzed with the purpose of determining the distribution pattern of T. trachurus and analyzing the interannual variations of its main biological parameters (Length Frequency Distribution, length–weight relationship, condition factor, size at first maturity and growth). Two Geographical Sub-Areas (GSA) are subject to monitoring, which were designated by the General Fisheries Commission for the Mediterranean (GFCM) in 2009 (GFCM 2009).

Material and methods

Study area and sampling

MEDIAS survey examines the continental shelf, from 30 to 200 m depth, between the French border and the Strait of Gibraltar (Fig. 1), collecting data during the summer on an annual basis from GSA06 (Northern Spain) and GSA01 (Northern Alboran Sea). The acoustic survey design is based on parallel transects, perpendicular to the coastline/bathymetry, with an inter-transect distance of 8 nautical miles (NM) in GSA06 (from the French border to Palos Cape) and 4 NM in GSA01 (from Palos Cape to the Strait of Gibraltar) (MEDIAS 2021) (Fig. 1).

Fig. 1
figure 1

Map of the study area: NW Mediterranean Sea, showing the GFCM subareas GSA06 (Northern Spain) and GSA01 (Northern Alboran Sea), the systematic sampling survey design based on parallel transects and fishing haul locations for the analyzed time series (2010–2020). Data from the MEDiterranean International Acoustic Survey

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Acoustic data were collected through an EK-60 scientific echosounder (SIMRAD), equipped with five frequencies, 18, 38, 70, 120 and 200 kHz. The elementary distance sampling unit (EDSU) was one nautical mile (1852 m). To (i) identify the fish echotraces, (ii) determine the species composition and proportion and (iii) perform biological measurements, fish samples were collected by a pelagic trawl, mainly during the daytime (MEDIAS 2021). The position of each trawl was identified by geographical coordinates (i.e., latitude and longitude).

For each pelagic trawl, the total catch was classified by species and the total weight and number of individuals was determined. T. trachurus individuals were randomly selected and measured to the nearest 0.5 cm (random sampling) for the determination of the length frequency distribution (LFD). A length-stratified subsampling was employed to determine the length–weight relationship, measuring the total length in mm and the total wet weight in grams of five individuals per centimeter. In addition, from 2012 onwards the sex and maturity stage were estimated according to the recommendations of ICES (2008).

Acoustic data processing and abundance estimation

Echoview Software Pty Ltd (formerly Myriax Pty Ltd) was used for acoustic data analysis. The result of the echo integration, the nautical-area scattering coefficient (m2 mile−2; sA; (MacLennan et al. 2002), was divided into species using the species composition in the pelagic trawl catches and applying the Nakken and Dommasnes (1975) method for multiple species but using backscattering cross section by length class rather than mean backscattering cross section (see Tugores et al. 2010). Finally, the target strength equation and length–weight relationship were applied to estimate T. trachurus abundance and biomass, respectively. The estimation abundance in a number of individuals (millions) and biomass in metric tons (t) for T. trachurus from 2009 to 2020 was calculated based on the acoustic records detected at 38 kHz frequency (Iglesias 2022), by nautical mile and year. A simple linear model was built to compare the total abundance, biomass, and density values per year between GSAs, including an interaction term between the year and GSA factors. Then, a two-way analysis of variance (ANOVA II) was applied to detect significant differences in the model. Data per year and GSA were examined beforehand to validate the assumptions of a normal distribution and homogeneity of variances between groups, using the Shapiro–Wilk (n < 50) and Levene tests, respectively.

In addition, the spatial distribution of T. trachurus biomass in kilogram (kg) per NM2 was mapped every year using QGIS 3.10 software. Identification of T. trachurus persistent areas was achieved by calculating the average biomass map for the time series (2010–2020). To transform the point shapefile to raster, an interpolation tool (ordinary kriging) was employed using a grid cell (2 × 2 NM), and the average biomass was calculated with the “cell statistics” tool. The mean length of T. trachurus in each pelagic trawl was also mapped to assess its spatial distribution considering the whole data set.

A simple linear model was formulated to assess the influence of depth on T. trachurus total length, considering the GSA and its interaction with depth. Subsequently, an ANOVA II was performed to evaluate the significance of this relationship, for which the normality and homoscedasticity between GSAs were previously confirmed for the data with the Kolmogorov–Smirnov (n > 50) and Levene tests, respectively.

Estimation of biological parameters and data analysis

The length–weight relationship of T. trachurus in the NW Mediterranean Sea was determined per year for the 2010–2020 time series per GSA applying a power regression and the values of \(a\) and \(b\) were estimated using the following model:

$$W=a\times {L}^{b}$$
(1)

where W corresponds to weight in grams, \(L\) length in cm, \(a\) the intercept, and \(b\) the value of the slope.

Length–weight relationships estimated per GSAs were tested for possible differences through an ANOVA, which required the verification of normality and homogeneity of variances among groups through a graphical representation of the model residuals.

A t-Student test was employed for detecting the growth type of T. trachurus, using the allometry coefficient to confirm whether the estimated growth corresponded to the isometric type (\(b=3\)). If \(b>3\), the fish of larger size gain weight in a greater proportion compared to their length, showing a positive allometric growth. As for \(b<3\), the individuals increment their relative length more than their weight (Froese 2006).

Furthermore, a Pearson’s correlation analysis was done to illustrate the association between \(b\) values and years of study.

The methodology presented by Le Cren (1951) was implemented to evaluate the state or condition of T. trachurus from 2010 to 2020 based on the condition factor (Kn) and assess the wellness of the population in relation to its nutritional state (Piedra et al. 2012). Values greater than 1 for the condition factor (Kn) suggest a better condition. The Kn parameter was determined with data from 2010 to 2020, using the following equation:

$$Kn=\frac{W}{a\times {L}^{b}}\times 100$$
(2)

where \(W\) corresponds to weight in grams, \(L\) length in cm, \(a\) the intercept, and \(b\) the value of the slope in the length–weight relationship.

As the data lacked homogeneity of variances between groups, previously verified with a Levene test, a generalized least squares (GLS) model was used to assess the existence of differences in Kn between GSAs and years. Furthermore, a Pearson’s correlation analysis was done to illustrate the association between Kn values and years of study.

The sex ratio of T. trachurus was determined for the entire time series (2012–2020), per year and GSA. Additionally, a Chi-squared test (\({X}^{2}\)) was used to evaluate if there were significant differences in the sex ratio (Zar 1996), based on Fisher's hypothesis, which states that there is one female for every male in a population (1F:1M).

Maturity stages, macroscopically designated for each specimen, were considered to estimate the size at first maturity (L50) of the time series (2012–2020) and GSA (ICES 2008). The calculation consisted of obtaining the percentage of immature (1–2) or mature (3–4–5–6) individuals (Ferreri et al. 2019). Generalized linear models (GLM) with a binomial error distribution were applied to estimate the L50 for each year and GSA, as well as to predict the percentage of immature specimens caught at the currently permitted length (i.e., 15 cm of total length).

To determine if the fitted logistic regressions of GSA01 and GSA06 differed significantly from each other, a generalized linear model with binomial distribution was used. The slopes of the models were tested for differences with the Chi-square contrast test, presented as the interaction between GSA and total length. Furthermore, a Pearson’s correlation analysis was done to illustrate the association between the L50 values and years of study.

All statistical analyses were executed using the R package, version 4.0.3 (R Core Team 2022).

Results

Abundance and biomass

The abundance of Trachurus trachurus in the NW Mediterranean Sea during the summer, fluctuated throughout the study period (2009–2020) (Fig. 2a). GSA01 evidenced the highest abundance of individuals in 2012 with 240.8 million, while GSA06 showed one of the lowest levels in 2017 with 0.2 million. Furthermore, the average abundance per year was significantly higher in GSA01 than in GSA06 (F-value = 5.34, p < 0.05, Table SI1), with values of 69 ± 69 million and 19.6 ± 17 million, respectively. The highest biomass values of T. trachurus were also determined in GSA01 with 5727 t in 2017, influenced by the presence of large individuals, and, in the same year, the lowest biomass level was recorded in GSA06 with 4 t (Fig. 2b). Biomass values showed interannual oscillations, with average values of 1832 ± 1710 t in GSA01 and 318 ± 290 t in GSA06, that differed significantly from each other (F-value = 8.28, p < 0.05, Table SI2) (Fig. 7).

Fig. 2
figure 2

Interannual variation of theabundance (a millions of individuals), biomass (b metric tons) and density (c t/NM2) of Trachurus trachurus in the GSA06 and GSA01 zones, during the 2009–2020 period. Data from the MEDiterranean International Acoustic Survey

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Given that the extension of GSA06 (6222 Nm2) is several times greater than GSA01 (900 Nm2), the differences in the biomass density between GSAs increased by a factor of seven (F-value = 40.52, p < 0.05; Fig. 2c, Table SI3).

Length frequency distribution (LFD)

The horse mackerel presented a size range between 3.5 and 35.5 cm in the Western Mediterranean (Table SI4). For the time series studied, the species presented a wider range of sizes in GSA06 (from 3.5 to 35.5 cm) than in GSA01 (from 3.5 to 27.5 cm; Table SI4). However, the LFD (Fig. 3) suggested a lower number of modal classes in GSA06 compared to GSA01, with individuals greater than 20 cm in total length almost absent in GSA06. For both GSAs, the LFD was multimodal, with the lowest modal value corresponding to the strength of recruitment. Juveniles, with a modal size between 8 and 10 cm, were found annually in GSA06, while in GSA01 they were only found in 2012, 2015, 2018 and 2020.

Fig. 3
figure 3

Length frequency distributions, probability density function of Trachurus trachurus by GSA and year. Data from the MEDiterranean International Acoustic Survey

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Length–weight relationship

The length–weight relationship and condition factor of T. trachurus were estimated from the measurement of 6481 specimens. For the time series studied, T. trachurus exhibited an isometric growth (b = 3.017, p > 0.05), however, variations were observed in the growth type depending on the year and GSA (Table 1). In GSA01, T. trachurus presented a negative allometric growth, except for 2010, 2011, 2013 and 2018, in which it had an isometric growth, the years with positive allometric growth were, 2012 and 2016. In contrast, the T. trachurus growth in GSA06 was positive allometric, except for 2011, 2019 and 2020, in which it resulted isometric. The growth coefficient showed a decreasing pattern in both GSAs, but it was more marked in GSA01 (r = − 0.64, p < 0.05, Fig. 5e) compared with GSA06, in which the negative correlation was not significant (r = − 0.27, p > 0.05, Fig. 5f).

Table 1 Parameters \(a\) and \(b\), coefficient of determination, coefficient of determination (R2), and associated p values from the t-Student test for the length–weight relationship (length in cm and weight in g) of Trachurus trachurus by GSAs units during the 2010–2020 period
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As previously described for the length comparison in the spatial distribution, the linear model suggested that in summer, T. trachurus individuals show significantly greater lengths in GSA01 compared to GSA06 (F-value = 1078.6, p < 0.05).

Condition factor

The average condition factor of T. trachurus in the NW Mediterranean Sea during summer, was Kn = 1.01 ± 0.09, demonstrating a good weight in relation to its length. However, the generalized least squares (GLS) model detected significant variations between years (F-value = 228.1, p < 0.05) and GSAs (F-value = 39.9, p < 0.05; Table SI5), suggesting a better condition status in GSA01 compared to GSA06 (Fig. 4). These differences also varied depending on the year (F-value = 11.3, p < 0.05), as the horse mackerel had a better condition in GSA06 only in 2011 and 2013.

Fig. 4
figure 4

Condition factor of Trachurus trachurus per year and GSA. Data from the MEDiterranean International Acoustic Survey

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In general, a decreasing pattern was determined in the Kn over the years (r = − 0.44, p < 0.05). The decline of the Kn was more pronounced in GSA06 (r = − 0.56, p < 0.05, Fig. 5b) in comparison to GSA01 (r = − 0.30, p < 0.05, Fig. 5a).

Fig. 5
figure 5

Pearson’s correlation analysis of the condition factor (a, b), size a first maturity(c, d), and coefficient allometric (e, f), of Trachurus trachurus vs. the years of study. Data from the MEDiterranean International Acoustic Survey

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Sex ratio

Considering the total set of sampled specimens (5702) of T. trachurus, the sexual ratio during summer was generally one male per female (Table 2). Nevertheless, some variations were visible in GSA06 during 2013 and 2014, when at least three females were present per every two males, while males dominated in 2015 and 2020. In GSA01, males were also dominant over females during 2014, 2016 and 2020.

Table 2 Variation in the sex ratio of Trachurus trachurusduring the study period and GSAs from data obtained in the MEDiterranean International Acoustic Survey
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Size at first maturity (L 50)

To estimate the size at first maturity, 5702 individuals of T. trachurus were examined. For the study period (2012–2020), the smallest size of a mature T. trachurus specimen was 14.6 cm in GSA01, while the largest corresponded to 27.5 cm. Similarly, in GSA06, 14.0 cm was the smallest size for a mature individual of this species and 31.7 cm was the largest.

Considering the general data set for the period analyzed, the L50 of T. trachurus was 18.71 (Fig. SI1) (Chisq = 0.5, p > 0.05). In GSA06, the model that explained the greatest deviation (D2 = 99.9%) was obtained for 2017, suggesting a L50 of 18.49 cm for this species, while the least explanatory (D2 = 46.3%) corresponded to 2012 with a L50 of 18.9 cm. For GSA01, the best model was determined for 2012 (D2 = 86.5%), suggesting a length of 19.4 cm, while the lowest deviance (D2 = 36.3%) was reported for 2015, with a length of 16.8 cm (Table 3). In addition, significant differences were detected in the L50 between GSAs, being greater in GSA01 only in 2013 and 2014 (Chisq = 34.0, p < 0.05 and Chisq = 21.5, p < 0.05), while GSA06 showed a higher L50 in 2015 and 2017 (Chisq = 23.6, p < 0.05 and Chisq = 4.74, p < 0.05) (Table SI6).

Table 3 Size at first maturity (L50) of Trachurus trachurus in the NW Mediterranean Sea by year and GSA zones, with data obtained in the MEDiterranean International Acoustic Survey
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The L50 showed some annual variations and an overall negative trend over time (@@r = − 0.48, p < 0.05; Fig. 5), which indicates a reduction in the L50 of T. trachurus with the years in both GSAs, being more prominent in GSA06 (r = − 0.54, p < 0.05, Fig. 5d) than in GSA01 (r = − 0.44, p < 0.05, Fig. 5c).

Spatial distribution

T. trachurus was mainly distributed in GSA01, particularly on the coasts of the Alboran Sea (Fig. 6), and its higher biomass (Kg) was located off Fuengirola, followed by Motril and, to a lesser extent, the Almería Bay; meanwhile, the presence of the species was very scarce in GSA06. The highest occurrence of individuals was between 40 and 120 m depth.

Fig. 6
figure 6

Average time series (2010–2020) distribution map of the biomass in kg of Trachurus trachurus in the NW Mediterranean Sea. The light coloration reflects the absence of fish and the red color the maximum biomass. Data from the MEDiterranean International Acoustic Survey

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On the other hand, the spatial distribution of the mean length showed differences between GSAs (Fig. 7a), with larger organisms found in GSA01 of total lengths mainly between 12 and 24 cm, while specimens smaller than 12 cm were present in GSA06, except in some areas of the northeast zone. In GSA06, T. trachurus was distributed almost exclusively in the northern part where the continental shelf is narrow and very similar to GSA01.

Fig. 7
figure 7

Average time series (2010–2020) distribution map of the length in cm of Trachurus trachurus (HOM) on the NW Mediterranean Sea. Light coloration reflects the minimum length and red represents the maximum length. Data from the MEDiterranean International Acoustic Survey

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The tested linear model suggested that T. Trachurus specimens were significantly larger in GSA01 than in GSA06 (F-value = 92.89, p < 0.05; Fig. 7b; Table SI7). Similarly, fish length varied depending on the bathymetric strata, indicating a bathymetric segregation by length (F-value = 7.67, p < 0.05; Fig. 7b). In this case, length and depth were positively related in both zones; hence, larger specimens congregated at greater depths (F-value = 0.4964, p > 0.05).

Discussion

The study of fish life history traits is essential to understand their biology, ecology, and behavior. For commercial species, the estimation of these aspects and their temporal analysis is crucial to detect inter-annual trends and contribute to the sustainable management of populations. Less studies species, such as Trachurus trachurus in the Western Mediterranean, suffer from a lack of precise fisheries-dependent data, so the more reliable data come from scientific surveys such as MEDIAS. MEDIAS survey data have allowed the estimation, analysis, and comparison of the main life history traits of T. trachurus in summer throughout a long time series (12 years) in two GFCM subareas: Northern Spain (GSA06) and Northern Alboran Sea (GSA01). The main strengths of the data set are that (i) the sampling design covers almost all the T. trachurus distribution area, (ii) it is carried out during its spawning season, (iii) there is no minimum catch size and (iv) a standardized, exhaustive, and big sample sized biological analysis is annually executed. In contrast, a limitation would be that the data come from a single month of the year (July). Another limitation is that the survey does not cover the entire bathymetric range of horse mackerel distribution, but it does cover where the majority of the population is distributed.

Most parameters analyzed, i.e., density, biomass, abundance, total length, and condition status, were higher in GSA01 than in GSA06, corresponding with data reported by Rodríguez-Castañeda et al. (2022) on a species of the same genus, T. mediterraneus, that showed larger congregations off the shores of GSA01. Individuals from GSA01 could be benefiting from the upwelling phenomenon that originates when water from the Atlantic enters the Mediterranean through the Strait of Gibraltar and heads northeast while describing clockwise gyres in the Alboran Sea, which leads to the upwelling of waters rich in nutrients that favor the high primary and secondary productivity evidenced in this area (Gómez 2015). Patterns of increase or decrease in abundance and biomass were not clearly differentiated in the period of study; however, the interannual variations in these medium-sized pelagic fish can be generally attributed to stock exploitation and environmental fluctuations (Cury et al. 2000; Bowler et al. 2017).

According to Casaponsa (1993), the spawning season of T. trachurus in the NW Mediterranean Sea occurs from April to August, therefore, the species is in full spawning during the MEDIAS survey (July) and co-occurs with other species of the same genus in areas that may have reproductive and recruitment potential, such as Fuengirola, Motril and the Gulf of Almeria (all of them located in GSA01), as they have previously shown the highest levels of biomass and largest average sizes of Trachurus spp. in the Iberic Mediterranean Sea (Lloris and Moreno 1995).

Length-frequency distribution values presented correspond to those reported by (Abaunza et al. 2008) for T. trachurus in the Alboran Sea (GSA01) with sizes from 6.1 to 39 cm of total length. Species of the Trachurus genus can exhibit multiple year-classes with a recruitment mode between 8 and 10 cm of total length (Ragonese et al. 2002; Cuscó 2015; Rodríguez-Castañeda et al. 2021). The reduction of large-sized modes was more evident in GSA06, where the population structure of different stocks is seemingly composed of organisms with little reproductive capacity, while individuals in GSA01 showed a recovering trend in 2020, possibly to guarantee a mode with reproductive potential. These results may be due to differences between GSAs in ecological characteristics and the fishing pressure that stocks are subjected to. Usually, individuals living in adverse environments are forced to invest more energy into adapting, which leads species to become vulnerable to external factors (Jørgensen et al. 2007; Ouled-Cheikh et al. 2022).

In general, the sex ratio of T. trachurus in our investigation was one male per female (1 M:1 F), however, this proportion can vary within a population according to size groups, reproduction period (Abaunza et al. 1995), and in some cases when males might be more vulnerable to fishing gears than females (Cuscó 2015). Moreover, T. trachurus tends to aggregate before spawning, consequently, sexual stages are not homogeneously distributed in the catches, and a greater number of samplings is required to ensure representativeness in the data (Abaunza et al. 2003).

The size at first maturity determined for T. trachurus in our study is similar to the values reported by Gherram et al. (2018) for the coasts of Algeria, who suggest an L50 of 18.42 cm for males and 18.28 cm for females, although this measure did not differ significantly by sex. On the other hand, Abaunza et al. (2008) described a lower value for the coasts of GSA01, with 50% of the population reaching maturity at 17.48 cm of total length. In contrast, the current regulation (EC) No. 1967/2006 establishes a minimum catch size of 15 cm of total length for T. trachurus, which is considerably below the value suggested by this investigation. Based on the predictions of the model, this suggests that 94% of the immature specimens of T. trachurus in the area have been allowed to be exploited thus far. This could be an alarming scenario since the uncontrolled overexploitation of immature specimens may have triggered a selection of individuals with early maturation (de Roos et al. 2006; Jørgensen et al. 2007), prompting phenotypic adaptive responses with changes in the size at first maturity and growth (Nash et al. 2000; Ernande et al. 2004) of T. trachurus individuals.

Based on the results presented, a declining temporal trend is evidenced in two key life history traits of T. trachurus in the Western Mediterranean, i.e., the size at first maturity and condition factor. The decrease in the L50 over time could suggest that the gonadal development of the fish is being accelerated and mature individuals can be encountered in the population earlier than usual. Among the main drivers of these life history changes may be the overfishing of immature individuals, since it is legally permitted to capture T. trachurus fish from 15 cm of total length in the area and the size at first maturity of this species is above this size. Clines in life history traits of fish stocks may be induced by density-dependent factors, such as biomass removal by fishing (Trippel 1995; Law 2000), and density-independent ones, which are often associated with environmental variation and climate change (Morrongiello et al. 2015; Albo-Puigserver et al. 2021).

Several studies confirm that the reduction of the size at first maturity of commercially harvested fish stocks is a widespread fisheries-induced consequence in life history cycles (Hutchings and Baum 2005; Jørgensen et al. 2007; Sharpe and Hendry 2009; Sharpe et al. 2012). High mortality rates due to fishing pressure in age and size ranges at which the maturation starts occurring lean selection towards early maturation (Olsen et al. 2004). A reduction in population density increases food availability, leading to an increase in growth and a decrease in age at maturity (Barot et al. 2004). Moreover, the partial removal of fish that reach sexual maturity at large sizes increases the proportion of genotypes related to reproduction at more reduced sizes (Jørgensen 1990; Rijnsdorp 1993; Barot et al. 2004). Additionally, climate change could play a major role in the alteration of the L50 and fish body condition. According to (Audzijonyte et al. 2016), among the physiological responses of fish towards ocean warming is the earlier energy allocation towards the reproduction process, altering the age and size at maturation of the population.

Modifications of these life history traits appear more prominent in GSA06, possibly related to less food availability in this area. Patterns of energy acquisition and allocation in pelagic fish are heavily influenced by changes in food availability, which is linked to the trade-off between maintenance, reproduction, and growth of individuals (Albo-Puigserver et al. 2021). In general, medium and small pelagic fish populations that have high phenotypic plasticity and are subjected to adverse environmental conditions and high fishing pressure could resort to reducing their L50 and increasing their reproducing effort to maximize their overall fitness (Hunter et al. 2015; Brosset et al. 2016; Basilone et al. 2018). The growth of individuals may also be affected, usually leading to selection for larger body sizes that maximize chances of survival (Stearns 1989; Conover and Munch 2002) and could be compensated in terms of energetic costs (Blanckenhorn 2000). Our data fit into these patterns of life history changes for T. trachurus in the NW Mediterranean Sea, as negative allometric growth was prominent during some years in GSA01, possibly reflecting a mechanism to countermeasure factors pressuring the population, by increasing their growth in length more than weight and possibly guarantee reproduction at larger lengths due to fitness advantages.

Changes observed in the life history traits of T. trachurus in the study areas may not be solely attributed to these factors discussed, yet the interaction between climate change and overfishing is possibly accelerating these processes, which has been previously reported as one of the main causes in the alteration of pelagic fish populations (i.e. Sardina pilchardus and Engraulis encrasicolus) in the NW Mediterranean Sea (GSA06) (Albo-Puigserver et al. 2021; Ramírez et al. 2021).

To determine the state of conservation of a species, the IUCN considers a set of parameters based on the population dynamics of stocks, that the present investigation does not contemplate. Due to a lack of information, T. trachurus is currently catalogued as “least concern” (LC) in the NW Mediterranean Sea. However, (Smith-Vaniz et al. 2015) stated that the species is globally threatened (vulnerable), since several populations in the Mediterranean, including the Alboran Sea (GSA01), have declined drastically. Given that the IUCN system does not relate to any of the life history parameters of fish, our study followed the criteria of FishBase (Strona 2014). Thus, the analysis of the life history parameters of T. trachurus suggests that the species seems to be adapting to external factors, showing a decreasing trend in the size at first maturity and condition factor, simultaneous with a low frequency of modes corresponding to adult individuals with reproductive capacity, that can guarantee future populations of horse mackerel. This behavior is also common on species that are threatened or under intense exploitation. Therefore, the perspective presented could partially contribute to updating the state of conservation of T. trachurus in the NW Mediterranean Sea.