Analysis of the state of conservation of Trachurus trachurus in the Western Mediterranean Sea based on the interannual (2009-2020) changes in their life history traits.

Horse mackerel is a semi-pelagic �sh widely distributed in the eastern Atlantic, where it represents a species of interest for �sheries and is subject to intense exploitation, leading to its global classi�cation as a vulnerable species by IUCN. In the Northwestern (NW) Mediterranean Sea, the knowledge of their distribution, abundance and main biological parameters is fundamental to achieve their sustainable exploitation and contribute to the ecosystem approach to �sheries management. The time series (2009–2020) data set analyzed in this study came from two geographical subareas (GSAs) de�ned by the General Fisheries Commission for the Mediterranean (GFCM), Northern Spain (GSA06) and Northern Alboran Sea (GSA01) sampled in July during the MEDiterranean International Acoustic Survey (MEDIAS). Our results suggest that in summer, a higher concentration and aggregation of �sh exists in GSA01, with a density ten times greater than in GSA06. Possible life history changes can be inferred through the analysis of the biological parameters of Trachurus trachurus in the NW Mediterranean Sea. In both GSAs, the annual decline of the size at �rst maturity (L 50 ) and condition factor (Kn) was prominent. Moreover, the population structure lacked adult modes that could guarantee a successful recuperation of future stocks. Nonetheless, �sh inhabiting GSA01 seem to have a better adaptive response to the events responsible for these alterations. In comparison, GSA01 is under less �shing pressure and the upwelling occurring in this area contributes to a higher availability of food, in addition to reducing the increases in sea surface temperature due to climate change during the study period.


INTRODUCTION
Trachurus trachurus (Linnaeus, 1758; Pisces; Perciformes; Teleostei; Carangidae), commonly known as horse mackerel, is a semi-pelagic sh widely distributed throughout the Northeast Atlantic Ocean, where it represents a species of interest to sheries, being subject to intense exploitation and globally classi ed 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 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).Diet composition of this species varies depending on the zone and length of the specimens, although crustaceans, cephalopods and teleost sh 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, Sea of Marmara, and western Black Sea, however, the different stocks are currently declining possibly due to shing 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 shes, 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 rst maturity, fecundity, condition factor and age structure possibly promoted by the current exploitation of stocks and the in uence 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 rst maturity of this species (Abaunza et al. 2008).Therefore, the exploitation of juveniles throughout time is probably in uencing the life history of T. trachurus (Roff 1992).
The size at rst maturity (L 50 ), 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 sheries management.(Fontoura et al. 2009;Hashiguti et al. 2018).It provides information on the maturity and reproductive cycle of a population as complement to variable life growth patterns (Lorenzen 2016), leading to the creation of shery 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 sheries 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 sh condition status (Erzini 1994;Froese 2006; İşmen et al. 2009).Condition indexes are commonly applied in studies of sh biology as well, since they offer signi cant information on their physiological state, indicating that individuals of a speci c length with greater weight have a better condition status than the rest (Lima ).Thus, these approaches should be considered accordingly for studies dwelling on sh conservation (Strona 2014).
The parameters of abundance and distribution are fundamental for the management of shery 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 longterm sustainability (Mustafa 1999).Population dynamics are heavily in uenced 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 de ning areas of preferential distribution of different stages of the population, establishing essential habitats, de ning spawning areas, source-sink zones and tracing the geographical limits of the stocks, which is essential for the proper management of populations.
This study aims to ll 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 rst 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).

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, 2022) (Fig. 1).Acoustic data were collected through an EK-60 scienti c echosounder (SIMRAD), equipped with ve frequencies, 18, 38, 70, 120 and 200 kHz.The elementary distance sampling unit (EDSU) was one nautical mile (1852 m).To (i) identify the sh echotraces, (ii) determine the species composition and proportion and (iii) perform biological measurements, sh samples were collected by a pelagic trawl, mainly during the daytime (MEDIAS 2022).The position of each trawl was identi ed by geographical coordinates (i.e., latitude and longitude).
For each pelagic trawl, the total catch was classi ed 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 strati ed subsampling was employed to determine the length-weight relationship, measuring the total length in mm and the total wet weight in grams of ve 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 coe cient (m 2 mile − 2 ; s A ; (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 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 signi cant differences in the model.Data per year and GSA was 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 NM 2 was mapped for every year using QGIS 3.10 software.Identi cation of T. trachurus persistent areas was achieved by calculating the average biomass map for the time series (2010-2020).To transform the point shape le to raster, an interpolation tool (ordinary kriging) was employed using a grid cell (2 x 2 NM), and the average biomass was calculated with the "cell statistics" tool.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 in uence of depth on T. trachurus total length, considering the GSA and its interaction with depth.Subsequently, an ANOVA II was performed to evaluate the signi cance of this relationship, for which the normality and homoscedasticity between GSAs was previously con rmed 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 and were estimated using the following model: Where: W corresponds to weight in grams, length in cm, the intercept, and the value of the slope.
Length-weight relationships estimated per GSAs were tested for possible differences through an ANOVA, which required the veri cation of normality and homogeneity of variances among groups through graphical representation of the model residuals.
A t-Student test was employed for detecting the growth type of T. trachurus, using the allometry coe cient to con rm whether the estimated growth corresponded to the isometric type ( ).If , the sh of larger size gain weight in a greater proportion compared to their length, showing a positive allometric growth.As for , 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 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: Where: corresponds to weight in grams, length in cm, the intercept, and the value of the slope in the length-weight relationship.
As the data lacked homogeneity of variances between groups, previously veri ed 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 Chisquared test ( ) was used to evaluate if there were signi cant 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 rst maturity (L 50 ) 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 L 50 for each year and GSA, as well as to predict the percentage of immature specimens catched at the current permitted length (i.e., 15 cm of total length).
To determine if the tted logistic regressions of GSA01 and GSA06 differed signi cantly 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 L 50 values and years of study.
All statistical analyses were executed using the R package, version 4.0.3.(R Core Team 2022).The abundance of Trachurus trachurus in the NW Mediterranean Sea during the summer, uctuated 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 signi cantly 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, in uenced 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 signi cantly from each other (F-value = 8.28, p < 0.05, Table SI2) (Fig. 7).

Abundance and biomass
Given that the extension of GSA06 (6222 Nm 2 ) is several times greater than GSA01 (900 Nm 2 ), 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.

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 coe cient 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 signi cant (r=-0.27,p > 0.05, Fig. 5f).As previously described for the length comparison in the spatial distribution, the linear model suggested that in summer, T. trachurus individuals show signi cantly 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 signi cant 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 (Fvalue = 11.3,p < 0.05), as the horse mackerel had a better condition in GSA06 only in 2011 and 2013.

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 a b 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.To estimate the size at rst 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.

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.
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.

DISCUSSION
The study of sh 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 a sustainable management of populations.Less studies species, such as Trachurus trachurus in the Western Mediterranean, suffer from a lack of precise sheries dependent data, so the more reliable data come from scienti c 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 bene ting 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 sh can be generally attributed to stock exploitation and environmental uctuations (Cury et al. 2000;Bowler et al. 2017).
According to Casaponsa (1992), 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  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 shing 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 rst maturity determined for T. trachurus in our study is similar to the values reported byGherram et al. (2018) for the coasts of Algeria, who suggest a L 50 of 18.42 cm for males and 18.28 cm for females, although this measure did 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 rst maturity and condition factor.The decreasing of the L 50 over time could suggest that the gonadal development of the sh 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 over shing of immature individuals, since it is legally permitted to capture T. trachurus sh from 15 cm of total length in the area and the size at rst maturity of this species is above this size.Clines in life history traits of sh stocks may be induced by densitydependent factors, such as biomass removal by shing (Trippel 1995 Modi cations 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 sh are heavily in uenced 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 sh populations, that have high phenotypic plasticity and are subjected to adverse environmental conditions and high shing pressure could resort to reducing their L 50 and increasing their reproducing effort to maximize their overall tness (Hunter et  Conover and Munch 2002) and could be compensated in terms of energetic costs (Blanckenhorn 2000).Our data ts 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 re ecting 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 tness 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 over shing is possibly accelerating these processes, which has been previously reported as one of the main causes in the alteration of pelagic sh populations (i.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, that include the Alboran Sea (GSA01), have declined drastically.Given that the IUCN system does not relate to any of the life history parameters of sh, 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 rst 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 NW Mediterranean Sea.

DECLARATIONS
not differ signi cantly by sex.On the other hand,(Abaunza et al. 2008) describe 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 at 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 on individuals with early maturation (de Roos et al. 2006; Jørgensen et al. 2007), prompting phenotypic adaptive responses with changes in the size at rst maturity and growth(Nash et al. 2000; Ernande et al. 2004) of T. trachurus individuals.

Figure 3 Length
Figure 3

Figure 4 Condition
Figure 4

Figure 7 Average
Figure 7 -Junior et al. 2002; Craig et al. 2005; Tzikas et al. 2007).Life history traits regulate populations, community interactions and species responses to uctuations 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 is varied and generally comprises a set of parameters associated with population dynamics, mainly the geographical distribution and abundance of a species, and sh 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

Table 1
Parameters and , coe cient of determination, coe cient of determination (R 2 ), 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.Data from the MEDiterranean International Acoustic Survey.n: sample size.SE: relative standard error.

Table 2
Variation in the sex ratio of Trachurus trachurusduring the study period and GSAs from data obtained in the MEDiterranean International Acoustic Survey.M: Males; F: Females.X 2 : Chisquare statistic and associated p-value.

Table 3
Size at rst maturity (L 50 ) of Trachurus trachurus in the NW Mediterranean Sea by year and GSA zones, with data obtained in the MEDiterranean International Acoustic Survey.D 2 represents the percentage of deviance explained by the model and SR the sample size range.
(Jørgensen et al. 2007;Ouled-Cheikh et al. 2022GSA01) 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; Rodriguez- 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 shing pressure that stocks are subjected to.Usually, individuals living in adverse environments are forced to invest more energy into adapting, which leads species to becoming vulnerable to external factors(Jørgensen et al. 2007;Ouled-Cheikh et al. 2022).
(Audzijonyte et al. 2016p 1993;Barot et al. 200421)ten associated with environmental variation and climate change(Morrongiello et al. 2015;Albo-Puigserver et al. 2021).Several studies con rm that the reduction of the size at rst maturity of commercially harvested sh stocks is a widespread sheries-induced consequence in life history cycles(Hutchings andBaum 2005; Jørgensen et al. 2007; Sharpe and Hendry 2009; Sharpe et al. 2012).High mortality rates due to shing pressure in age and size ranges at which the maturation starts occurring leans 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 sh 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 be playing a major role in the alteration of the L 50 and sh body condition.According to(Audzijonyte et al. 2016), among the physiological responses of sh towards ocean warming is the earlier energy allocation towards the reproduction process, altering the age and size at maturation of the population.