Age Structure, Growth Parameters, and Otolith Traits of Two Species of the Genus Trachurus in the Central Mediterranean

Abstract

The Atlantic horse mackerel (Trachurus trachurus) and the Mediterranean horse mackerel (T. mediterraneus) are two commercially important species whose biological traits remain insufficiently studied in the Central Mediterranean Sea. This study examines their age, growth pattern, and, for the first time, otolith morphology in both species in the Eastern Ionian Sea. The intercept of the weight–length relationship was a = 0.00599 (95% CI = 0.0050–0.0072) for T. trachurus and a = 0.00801 (95% CI = 0.0072–0.0089) for T. mediterraneus, and the slope was b = 3.121 (95% CI: 3.058–3.183) and b = 2.994 (95% CI: 2.957–3.031), respectively. Age was estimated by counting annual growth increments, visible as alternating opaque and clear bands along the axis of the left sagittal otolith from the core to the posterior margin. Von Bertalanffy growth parameters were estimated as L_∞ = 34.65 cm, k = 0.31 year⁻¹, and t₀= −1.11 years for T. trachurus and L_∞ = 35.98 cm, k = 0.23 year⁻¹, and t_o = −1.60 years for T. mediterraneus. Otolith morphometrics showed significantly higher values (ANOVA, p-value < 0.05) in T. mediterraneus for all morphometric variables, except one, indicating larger and wider otoliths than those of T. trachurus, which can be a tool to distinguish the two species. A strong correlation was observed between the total length of the body and otolith metrics in both species. This study enhanced our scientific knowledge on the studied species’ life history traits and provides information for further ecological and stock assessment studies.

1. Introduction

Knowledge of fish age, growth patterns, and length–weight relationship is fundamental in fisheries biology, population dynamics, and for effective stock assessment [1,2,3]. These parameters not only support the development of robust models for estimating biomass and evaluating fish condition [4] but also facilitate the monitoring of seasonal growth fluctuations and the comparison of morphological and life-history traits across species and geographic regions [5]. Such data are critical for the management of both targeted and discarded species, as the absence of biological information can lead to inaccurate stock evaluations and suboptimal fisheries policies [6].

Knowledge on otolith morphometrics enhances the biological resolution of stock assessments by enabling species and stock discrimination, age validation, and the detection of growth patterns that may be obscured in traditional analyses [7,8]. By capturing subtle shape variations influenced by genetic and environmental factors, otolith morphometric analysis is non-invasive to the otolith, as it does not require destructive techniques in the processing of their integrity, and it is a cost-effective method for identifying population structure and assessing phenotypic plasticity across spatial and temporal scales [9,10]. This approach is particularly valuable in heterogeneous stock fisheries and data-limited contexts, where standard methods may fail to resolve biologically significant differences [11].

The Atlantic horse mackerel Trachurus trachurus [12] and the Mediterranean horse mackerel Trachurus mediterraneus [13] are semi-pelagic species that belong to the family Carangidae with commercial interest worldwide [14]. T. trachurus occurs in the Eastern Atlantic (from Norway to South Africa) and the Mediterranean Sea. It inhabits mainly coastal areas with sandy substrate ranging from a few meters to 600 m maximum depth, described as a benthopelagic, opportunistic predator feeding mainly on small pelagic fishes, crustaceans, and other nekto-benthic invertebrates over soft-sediment continental shelf habitats [15]. The minimum and maximum total lengths of the species are recorded at 8.5 and 70.0 cm [16], respectively, while sexual maturity is reached at approximately 22.9 cm (L50) [16]. T. mediterraneus occurs in the Eastern Atlantic (from the Bay of Biscay to Mauritania), Mediterranean, Marmara, and Black Sea, as well as the western part of the Azov Sea [16]. It inhabits mainly waters from a few meters to 100 m with maximum depth distribution at 500 m [16], characterized as a flexible, opportunistic predator that adjusts its diet to locally abundant prey, including small fishes, crustaceans, and zooplankton [17]. Minimum and maximum recorded lengths are 4.0 cm total length [18] and 60 cm fork length [16], with sexual maturity occurring at 20.0 cm total length (L50) [16], although specimens of both species with smaller sizes have been collected during scientific surveys (as in the present study). Adults of T. mediterraneus are rarely found near the seafloor, indicating a more active pelagic presence [16]. Both species form schools [16] and constitute commercially important small pelagic resources in the Eastern Ionian Sea (E. Ionian Sea), where they are mainly caught with pelagic trawls and purse seines and their landings reach several thousand tons annually [18,19,20,21,22].

Although several studies have investigated the weight–length relationship (WLR), age, and growth of T. trachurus and T. mediterraneus in various areas of the Mediterranean and Black Sea [19,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40], similar studies for these species in the Ionian Sea remain scarce, with [30] providing the only available work to date on the age and growth of T. trachurus in the area of the Ionian Sea. Studies on T. mediterraneus are limited to the Black Sea, where seven otolith characters were examined [36], and the relationship of three otolith characters with body length was reported [37,38]. Moreover, four otolith characters for both species were studied in the Western Mediterranean and the Eastern Atlantic [39]. No comparable data exist for the species in the study area.

Both species often co-occur in Mediterranean fisheries. Their external morphology is highly similar, particularly during early life stages, which makes visual identification difficult and prone to error [40]. Otoliths, however, provide species-specific morphological features and growth marks that can be used as reliable taxonomic characters. For this reason, otolith morphology has been widely applied to distinguish between cryptic or closely related fish species, offering a robust alternative to external traits [36,37,38,39,41]. Clarifying these differences is essential for accurate age and growth studies, stock assessment, and fisheries management.

The E. Ionian Sea presents strong vertical stratification, where warm surface waters overlay cooler intermediate layers [42]. Seasonal thermoclines regulate temperature and oxygen, directly influencing metabolic rates and growth patterns of fish populations [43]. At greater depths, stable low temperatures and high hydrostatic pressure create habitats that slow growth and extend lifespan, typical of deep-sea species [44]. Limited nutrient inputs and circulation of Levantine Intermediate Water further constrain productivity, shaping food availability and thus the biological performance of fish communities [45].

This study presents updated data (samples collected during 2021) on the age and growth pattern of T. trachurus after the only existing study by [30] in the E. Ionian Sea while offering—for the first time in the area—data on the age and growth pattern of T. mediterraneus and a detailed otolith morphometric analysis for both species to enhance stock assessment, support stock identification, and contribute to sustainable fisheries management in the study area.

2. Materials and Methods

2.1. Study Area and Sampling Procedures

Specimens of T. trachurus (N = 200) and T. mediterraneus (N = 310) were sampled in the E. Ionian Sea (Figure 1) during 2021 in the framework of the Greek National Fisheries Data Collection Program of the Hellenic Centre for Marine Research and Institute of Fisheries Research within the framework of the EU Data Collection Regulation (EU 2017/1004). The program, co-financed by the EU and the Greek government, provides standardized biological and fisheries data (e.g., length, weight, age, otoliths) for stock assessment and management purposes for all European countries. All samples examined for the present study were collected as dead individuals in the fishing market due to fishing practices. They were kept frozen for a period before being analyzed in the laboratory for the collection of biological data.

The total length (TL) and total weight (TW) of each individual were measured with precision to the nearest 0.01 mm with a tape measure and 0.01 g with a scale. Sagittal otoliths were carefully removed, rinsed with water to eliminate residual organic material, and subsequently stored in a dry state. For imaging, each left otolith was placed in a glass Petri dish filled with water for observation with a stereoscope. A digital camera (SONY ExwaveHAD COLOR VIDEO) (Sony Corporation, Tokyo, Japan) was connected to a stereoscope (Wild Heer-brugg M420 Makroskop 1.25×) (Wild Heerbrugg AG, Heerbrugg, Switzerland) and a computer monitor. Each left otolith was observed under reflecting light against a black background. A digital Image-Pro Plus (Version 6.0.0. 260) software (Media Cybernetics, Rockville, MD, USA) system was used to acquire calibrated digital images of the otoliths at 8.0× and 10.0× magnification in T. trachurus and T. mediterraneus, respectively. The same software was used for age reading and otolith measurements as well.

Age estimation was based on counting annual growth increments, identified as a sequence of opaque and clear bands across the axis of the left sagittal otolith, extending from the core to the posterior margin [46]. Three independent readers conducted the initial reading, and in cases of disagreement, a second round of evaluation was performed to reach consensus or to exclude the otolith if no agreement could be achieved. In total, 200 otoliths of T. trachurus and 200 of T. mediterraneus were examined for age determination and morphometric measurements. To standardize age calculations, the birth date for T. trachurus was set to 1 January, while for T. mediterraneus it was set to 1 July, following internationally accepted protocols commonly applied in fish ageing bibliography [46].

Otolith measurements were taken from the left sagittal otoliths of both species, focusing on the following five morphometric variables, as shown in Figure 2: otolith area (OA, mm²), otolith length (OL, mm), otolith width (OW, mm), radius (RA, mm), and perimeter (PE, mm). Five shape descriptors were also studied: (i) roundness (RD), which represents the ratio between the actual otolith area and that of a circle with the same diameter, with higher values indicating a more circular shape [47], taking a minimum value of 1 [48]; (ii) circularity (CI), which reflects the complexity of the otolith’s outline [49], with a minimum value of 4π [48]; (iii) form factor (FF), defined as 4 × OA/PE², which ranges from 0 to 1, where 1 indicates a perfectly circular outline; (iv) rectangularity (RC), calculated as OA/(OL × OW), which assesses how closely the otolith resembles close to a rectangular outline, with 1 representing a perfect square; and (v) ellipticity (EL), given by (OL − OW)/(OL + OW), which indicates how elliptical the shape is, with values near 0 suggesting a more circular form [49].

2.2. Data Analysis

2.2.1. Growth

The total length frequency distribution for each species was based on 1 cm interval classes. The relationship between weight and length (WLR) was determined using the formula TW = αTL^b, where TW represents total weight in grams, TL is the total length in centimeters, α is the regression intercept, and b is the slope derived by log-transforming the data and applying ordinary least squares regression. The confidence intervals (CI) of the parameters a and b are also shown. To test whether growth was isometric, positive, or negative allometric (H₀: b = 3, b > 3, or b < 3, respectively), a Student’s t-test was applied for each species.

An age–length key was developed for each species using age data obtained from otolith readings. The von Bertalanffy growth function [50] was applied to estimate growth parameters, expressed by the equation Lt = L_∞ (1 − e^−k(t−to)), where Lt is the expected length at age t (in cm), L_∞ represents the theoretical maximum length (in cm), k is the growth coefficient, and t_o is the hypothetical age at which the fish’s length would be zero. For the evaluation and comparison of growth parameters between the two examined species and with published data, the growth performance index Φ′ was computed using the following formula: Φ′ (Φ′ = logk + 2logL_∞) [51].

2.2.2. Otolith Morphometry

Mean values (±standard error), with minimum and maximum measurements of the otolith morphometric traits (RA, OL, OW, OA, PE, RD, CI, FF, RC, and EL) were calculated by species. The statistical package Statgraphics was applied for all the statistical analyses. Analysis of variance (ANOVA) was conducted with species as the single fixed factor to test for differences in otolith morphometric variables between the two species, using a significance level of α = 0.05.

Exponential regression was applied to assess the relationship between otolith morphometric variables and TL in each species: y = Ax^B, where y represents the otolith morphometric variable, x is the total length in millimeters, A is the intercept, and B is the slope of the regression. For the variables with a statistically significant relationship with length, the slopes (B) of the two species were compared using ANCOVA analysis (with the level of statistical significance set at α = 0.05).

3. Results

3.1. Growth

3.1.1. Length Distribution

The length frequency distribution of both species is presented in Figure 3. The size of T. trachurus ranged from 6.1 to 34.2 cm TL; that of T. mediterraneus ranged from 6.6 to 34.4 cm TL (Figure 3).

3.1.2. Weight–Length Relationship

The total weight (TW) values varied from 1.5 to 317.5 g in T. trachurus; in T. mediterraneus, these values ranged from 2.6 to 392.5 g. For the sample size examined, WLR for T. trachurus was TW = 0.00599 × TL^3.121 (R² = 98%) (95% a: CI = 0.0050–0.0072, b: 3.058–3.183); for T. mediterraneus WLR was TW = 0.00801 × TL^2.994 (R² = 99%) (95% a: CI = 0.0072–0.0089, b: 2.957–3.031). Results of the t-test demonstrated that b exceeded the value of 3 at a statistically significant level in T. trachurus (t-test = 3.80, p-value < 0.01), while no statistically significant difference was observed for T. mediterraneus (t-test = −0.31, p-value = 0.76).

3.1.3. Age Structure and Growth Analysis

Otolith readings revealed eight age groups (ranging from 0+ to 7) in T. trachurus, and ten age groups (from 0+ to 9) in T. mediterraneus. In both species, sometimes false rings were observed surrounding the first annual ring of the otolith, which appears as a clear translucent band after a central, mostly opaque area, including thin opaque and translucent rings (see Figure 4 and Figure 5). Age–length keys are provided in

Table 1. Age–length key of Trachurus trachurus and Trachurus mediterraneus from E. Ionian Sea [TL: total length (cm); N: total number of individuals].

	Trachurus trachurus									Trachurus mediterraneus
Length Classes (TL, cm)	Age Classes
	0+	1	2	3	4	5	6	7	0+	1	2	3	4	5	6	7	8	9
6.0–6.9	2								1
7.0–7.9
8.0–8.9									1
9.0–9.9	7
10.0–10.9	10								2
11.0–11.9	11								7
12.0–12.9	18	1							6	5
13.0–13.9	18	2								6
14.0–14.9	4	7								6
15.0–15.9		4								7
16.0–16.9		10								9
17.0–17.9		12								6
18.0–18.9		35								6	4
19.0–19.9		12	2							8	10
20.0–20.9			1								14
21.0–21.9			1	1							15
22.0–22.9				2							3	8
23.0–23.9				4								13
24.0–24.9				3	1							16	1
25.0–25.9				4	3							3	10	1
26.0–26.9				1	3								4	1
27.0–27.9				2	3									6	1
28.0–28.9					7	3								2	3
29.0–29.9						2	1								1
30.0–30.9						1									2	1
31.0–31.9							1									1
32.0–32.9																1	3
33.0–33.9
34.0–34.9								1										1
N	70	83	4	17	17	6	3	1	17	53	51	40	15	10	7	3	3	1

for each species. Most of the individuals, 41.5% of T. trachurus and 26.5% of T. mediterranmeus, were classified in age group one.

Table 2. Von Bertalanffy growth parameters of Trachurus trachurus and Trachurus mediterraneus in the E. Ionian Sea (N: total number of individuals; L_∞: the mean theoretical asymptotic length in cm; k: a growth rate parameter in year⁻¹; t₀: the theoretical age at zero length in years; S.E: standard error; Φ′: the growth performance index).

Species	N	L∞ ± S.E (cm)	k ± S.E (Year−1)	t0 ± S.E (Year)	Φ′
Trachurus trachurus	200	34.65 ± 1.23	0.31 ± 0.04	−1.11 ± 0.14	2.57
Trachurus mediterraneus	200	35.98 ± 0.88	0.23 ± 0.02	−1.60 ± 0.17	2.47

outlines the von Bertalanffy growth parameters and the growth performance index Φ′ for each species, with corresponding growth curves shown in Figure 6. Results indicate that T. mediterraneus has more age groups than T. trachurus in the same length range and lower k.

3.2. Otolith Morphometrics

The mean (±standard error), minimum, and maximum values of the morphometric measurements RA, OL, OW, OA, PE, RD, CI, FF, RC, and EL based on measurements of the left otolith of each species are presented in

Table 3. Mean (±standard error), minimum, and maximum (in parenthesis) values of the otolith morphometric variables: RA (radius, mm), OL (otolith length, mm), OW (otolith width, mm), OA (otolith area, mm²), PE (perimeter, mm), RD (roundness), CI (circularity), FF (form factor), RC (rectangularity), and EL (ellipticity) of Trachurus trachurus and Trachurus mediterraneus for the E. Ionian Sea. The p-values (ANOVA) for comparisons of otolith morphometric variables between species are presented, too.

Morphometric Variable	Trachurus trachurus	Trachurus mediterraneus	ANOVA (p-Value)
RA (mm)	2.80 ± 1.09 (0.97–5.05)	3.42 ± 0.94 (1.07–5.08)	<0.05 *
OL (mm)	5.92 ± 0.11 (2.01–10.18)	6.83 ± 1.70 (2.11–9.72)	<0.05 *
OW (mm)	2.77 ± 0.04 (1.11–4.91)	3.49 ± 0.82 (1.25–4.51)	<0.05 *
OA (mm2)	12.50 ± 6.23 (1.64–34.06)	16.93 ± 7.51 (2.00–28.80)	<0.05 *
PE (mm)	15.20 ± 0.27 (5.29–25.70)	17.71 ± 4.80 (5.86–24.69)	<0.05 *
RD	1.55 ± 0.15 (1.27–3.43)	1.47 ± 0.13 (1.29–2.97)	<0.05 *
CI	19.46 ± 1.86 (15.93–43.10)	18.45 ± 0.16 (16.17–37.36)	<0.05 *
FF	0.21 ± 0.02 (0.09–0.25)	0.22 ± 0.02 (0.11–0.25)	<0.05 *
RC	0.71 ± 0.03 (0.31–0.88)	0.71 ± 0.04 (0.40–0.83)	0.17
EL	0.36 ± 0.04 (0.23–0.44)	0.32 ± 0.04 (0.20–0.44)	<0.05 *

* Significance level α = 0.05.

. Analysis of variance revealed statistically significant differences across the majority of variables between the two species (p-value < 0.05), except for RC (p-value > 0.05) (Table 3).

For both species, statistically significant correlations (p-value < 0.05) were observed between TL and almost all otolith variables, with the exception of RC (

Table 4. Parameters of the exponential regression of the total length (TL) of Trachurus trachurus and Trachurus mediterraneus with the otolith morphometric variables: RA, radius (mm); OL, otolith length (mm); OW, otolith width (mm); OA, otolith area (mm²); PE, perimeter (mm); RD, roundness; CI, circularity; FF, form factor; RC, rectangularity; EL, ellipticity. A, intercept; B, slope; R², coefficient of determination; r, correlation coefficient. The p-values of the regressions and the p-values of the comparison of the slope B of the regression lines between the two species (ANCOVA) are also shown.

Variables	Species	A	B	R2	r	Regression p-Value	ANCOVA p-Value for B
TL/RA	T. trachurus	0.04	0.83	0.92	0.96	<0.01 *	0.51
	T. mediterraneus	0.04	0.84	0.93	0.96	<0.01 *
TL/OL	T. trachurus	0.10	0.80	0.88	0.94	<0.01 *	0.16
	T. mediterraneus	0.12	0.75	0.89	0.94	<0.01 *
TL/OW	T. trachurus	0.08	0.68	0.90	0.95	<0.01 *	0.14
	T. mediterraneus	0.11	0.65	0.88	0.94	<0.01 *
TL/OA	T. trachurus	0.01	1.46	0.92	0.95	<0.01 *	0.13
	T. mediterraneus	0.01	1.38	0.89	0.94	<0.01 *
TL/PE	T. trachurus	0.29	0.77	0.89	0.95	<0.01 *	0.54
	T. mediterraneus	0.31	0.75	0.91	0.95	<0.01 *
TL/RD	T. trachurus	1.12	0.07	0.05	0.22	<0.01 *	0.09
	T. mediterraneus	0.78	0.12	0.12	0.35	<0.01 *
TL/CI	T. trachurus	13.15	0.08	0.10	0.31	<0.01 *	0.15
	T. mediterraneus	9.83	0.12	0.12	0.35	<0.01 *
TL/FF	T. trachurus	0.30	−0.08	0.10	−0.31	<0.01 *	0.15
	T. mediterraneus	0.41	−0.12	0.12	−0.35	<0.01 *
TL/RC	T. trachurus	0.78	−0.02	0.10	−0.10	0.18	-
	T. mediterraneus	0.77	−0.02	0.05	−0.07	0.33
TL/EL	T. trachurus	0.18	0.14	0.24	0.49	<0.01 *	0.81
	T. mediterraneus	0.15	0.14	0.15	0.39	<0.01 *

* Significance level α = 0.05.

). ANCOVA revealed no significant differences between species in the slope B of the regressions (p-value > 0.05) (Table 4).

4. Discussion

T. trachurus and T. mediterraneus are commercially important and widely distributed species. Limited research on their biology has been published for these species in the central Mediterranean in recent years. This work provides updated information on WLR, age, and growth for both species in the E. Ionian Sea and a detailed study on their otolith morphology conducted for the first time for T. trachurus in the central Mediterranean.

The examination of the size frequency distribution showed that the dominant size for T. trachurus was at 18 cm TL, while in T. mediterraneus, the dominant size was at 21 cm TL. The WLR analysis indicated that the value of b was significantly higher than 3 in T. trachurus, indicating that the species has growth that tends toward positive allometry, whereas no significant difference from 3 was observed in T. mediterraneus, based on the sampled population and the size range examined. The value of slope b of the present study was consistent with the values reported in previous studies for various regions, including the Mediterranean Coast of Morocco, the Black Sea, and the Aegean Sea. This indicates a broadly similar growth in weight pattern among populations (Table 5). In T. mediterraneus, the b of the WLR was found to fall within the range of values mentioned in the published literature. Higher b values have been reported in the Black Sea and the Aegean Sea (Table 6), suggesting regional variability in growth patterns. However, the higher b values in these cases seemed to be associated with a sample of individuals of quite lower body size (Table 6). Variability in WLR parameters is commonly attributed to a range of biological and methodological factors, including genetic differences, habitat conditions, length distribution of specimens, sample size, preservation techniques, ambient temperature, feeding habits, sex, maturity stage, sampling season, growth rates, and age [52,53,54].

In the current study, individuals of the first age class of T. trachurus ranged in length from 12.0 to 19.9 cm, which aligns well with several previous findings for the Mediterranean and Black Sea [23,29,30]. Similarly, the first age class of T. mediterraneus ranged from 12.0 to 19.9 cm in length in our work. Our results are quite close to those mentioned by [25,31,33,34] for the Black Sea. Any variation in first age–length between studies is most plausibly explained by environmental or methodological differences, since fish growth and age–length variation are shaped by temperature-driven metabolism [55], nutrient availability [56], oxygen and stratification [42], hydrostatic pressure at depth [43], and the combined impacts of fishing pressure and climate change [57,58]. Accurate identification of the first age is critically important, as it forms the basis for reliable age estimation and precise interpretation of otolith growth patterns. In the two studied species, however, this process is particularly challenging due to the presence of false juvenile rings, which seem like true annuli, often leading to misinterpretation. This problem is more evident in older specimens, where otolith thickening reduces ring visibility and complicates age determination [22,23,59]. Otolith microstructure and daily ring analysis may clarify age identification inconsistencies in the species studied.

In this study, T. mediterraneus showed a slightly lower k than T. trachurus, suggesting that it attains its asymptotic size with a slower growth rate than the latter species. Regarding T. trachurus, the highest k values were reported in the Aegean and Ionian Seas than in other Mediterranean and Black Sea areas (Table 5), possibly due to different environmental or ecological conditions. It is, however, worth mentioning that with the exception of a few cases, most areas showed similar Φ′ value, ranging between 2.4 and 2.6. Regarding T. mediterraneus, quite a high L_∞ and low k accompanied by quite a high Φ value were reported for the Western Mediterranean; the opposite was observed for the Black Sea, with some exceptionally low values of k in some cases, indicating high variability in this region (Table 6). Based on the recent literature, L_∞, k, and Φ′ were quite close in the Aegean and the Ionian Seas, their values ranging between those of the Western Mediterranean and those of the Black Sea (Table 6). The higher Φ′ values reflect areas where the species exhibits relatively strong growth performance, possibly linked to regional habitat factors or population-specific traits. This variability may be influenced by environmental factors such as temperature, food availability, and fishing pressure, as well as differences in sample size ranges, life history traits, and analytical methods [60,61]. Regional discrepancies in growth dynamics and age structure have been widely documented in fish population studies [62].

Table 5. Growth parameters of Trachurus trachurus from different study areas (TL Range: minimum–maximum total length in cm; α: intercept and b: slope of the weight–length relationship; N: number of age groups; L_∞: mean theoretical asymptotic length in cm; k: growth rate in year⁻¹; t₀: theoretical age at zero length in years; Φ′: growth performance index).

Area	Reference	TL Range (cm)	a	b	L∞ (cm)	k (Year−1)	t0 (Year)	Φ′
Mediterranean Coast of Morocco	[29]	10.0–30.4	0.007	3.065	43.90	0.10	0.32	2.28
	[40]	7.8–33.8	-	-	38.39 *I	0.22	−0.95	2.51
			-	-	38.56 *II	0.23	−0.90	2.53
Central Mediterranean and Adriatic regions	[63]	10.3–37.3	-	-	37.68	0.23	0.30	2.51
	[64]	4.0–35.0	-	-	30.9	0.24	−1.82	2.36
Black Sea	[28]	6.9–19.0	0.005	3.170	20.5	0.23	−3.00	4.58
Aegean Sea	[23]	6.5–33.9	0.006	3.070	30.27	0.37	−0.94	2.53
	[30]	3.5–35.7	0.007	3.055	32.0	0.29	−0.08	2.47
E. Ionian Sea		5.7–37.4	0.008	3.015	33.0	0.28	0.09	2.48
	This study	6.1–34.2	0.006	3.121	34.6	0.31	−1.11	2.57

*^I: Males; *^II: Females.

Table 5. Growth parameters of Trachurus trachurus from different study areas (TL Range: minimum–maximum total length in cm; α: intercept and b: slope of the weight–length relationship; N: number of age groups; L_∞: mean theoretical asymptotic length in cm; k: growth rate in year⁻¹; t₀: theoretical age at zero length in years; Φ′: growth performance index).

Area	Reference	TL Range (cm)	a	b	L∞ (cm)	k (Year−1)	t0 (Year)	Φ′
Mediterranean Coast of Morocco	[29]	10.0–30.4	0.007	3.065	43.90	0.10	0.32	2.28
	[40]	7.8–33.8	-	-	38.39 *I	0.22	−0.95	2.51
			-	-	38.56 *II	0.23	−0.90	2.53
Central Mediterranean and Adriatic regions	[63]	10.3–37.3	-	-	37.68	0.23	0.30	2.51
	[64]	4.0–35.0	-	-	30.9	0.24	−1.82	2.36
Black Sea	[28]	6.9–19.0	0.005	3.170	20.5	0.23	−3.00	4.58
Aegean Sea	[23]	6.5–33.9	0.006	3.070	30.27	0.37	−0.94	2.53
	[30]	3.5–35.7	0.007	3.055	32.0	0.29	−0.08	2.47
E. Ionian Sea		5.7–37.4	0.008	3.015	33.0	0.28	0.09	2.48
	This study	6.1–34.2	0.006	3.121	34.6	0.31	−1.11	2.57

*^I: Males; *^II: Females.

Table 6. Growth parameters of Trachurus meditarraneus from different study areas (TL Range: minimum–maximum total length in cm; α: intercept and b: slope of the weight–length relationship; N: number of age groups; L∞: mean theoretical asymptotic length in cm; k: growth rate in year⁻¹; t₀: theoretical age at zero length in years; Φ′: growth performance index).

Area	Reference	TL Range (cm)	a	b	L∞ (cm)	k (Year−1)	t0 (Year)	Φ′
Western Mediterranean Sea	[65]	4.0–42.5	-	-	44.45	0.18	−0.98	2.55
Black Sea	[25]	N.A.	-	-	19.90	0.29	−0.64	2.06
	[26]	9.2–19.0	0.009	2.955	26.09	0.13	−4.002	1.95
	[27]	10.5–17.0	0.004	3.305	19.73	0.30	−0.8305	2.07
	[66]	8.0–19.0	-	-	19.95 *IV	0.64	−0.55	2.41
	[31]	6.2–19.5	0.005	3.138	24.26	0.22	−2.04	2.11
	[33]	7.4–22.5	0.003	3.360	19.00	0.44	–0.36	2.21
	[34]	8.9–20.5	0.0055	3.150	18.80 *V	0.43	−0.223	2.18
			0.0048	3.180	19.50 *VI	0.44	−0.400	2.22
			0.0040	3.20	20.80 *VII	0.48	−0.360	2.32
	[35]	7.7–21.5	0.0057	3.127	25.02	0.13	−3.63	1.92
Aegean Sea	[67]	13.2–26.1 *III	0.000	2.804	-	-	-	-
	[24]	N.A.	-	-	37.24	0.33	−0.84	2.66
	[68]	14.0–30.0	0.008	3.002	-	-	-	-
	[69]	7.0–25.8 *I	0.004	3.271	-	-	-	-
		7.9–22.8 *II	0.004	3.285	-	-	-	-
	[32]	10.0–33.3	0.009	2.977	32.43	0.30	−1.40	2.49
E. Ionian Sea	This study	6.6–34.4	0.008	2.994	35.98	0.23	−1.60	2.47

N.A.: Not Answered; *^I: Males; *^II: Females; *^III: Fork length; *^IV: ELEFAN I; *^V: 2000–2007; *^VI: 2008–2013; *^VII: 2014–2020.

Table 6. Growth parameters of Trachurus meditarraneus from different study areas (TL Range: minimum–maximum total length in cm; α: intercept and b: slope of the weight–length relationship; N: number of age groups; L∞: mean theoretical asymptotic length in cm; k: growth rate in year⁻¹; t₀: theoretical age at zero length in years; Φ′: growth performance index).

Area	Reference	TL Range (cm)	a	b	L∞ (cm)	k (Year−1)	t0 (Year)	Φ′
Western Mediterranean Sea	[65]	4.0–42.5	-	-	44.45	0.18	−0.98	2.55
Black Sea	[25]	N.A.	-	-	19.90	0.29	−0.64	2.06
	[26]	9.2–19.0	0.009	2.955	26.09	0.13	−4.002	1.95
	[27]	10.5–17.0	0.004	3.305	19.73	0.30	−0.8305	2.07
	[66]	8.0–19.0	-	-	19.95 *IV	0.64	−0.55	2.41
	[31]	6.2–19.5	0.005	3.138	24.26	0.22	−2.04	2.11
	[33]	7.4–22.5	0.003	3.360	19.00	0.44	–0.36	2.21
	[34]	8.9–20.5	0.0055	3.150	18.80 *V	0.43	−0.223	2.18
			0.0048	3.180	19.50 *VI	0.44	−0.400	2.22
			0.0040	3.20	20.80 *VII	0.48	−0.360	2.32
	[35]	7.7–21.5	0.0057	3.127	25.02	0.13	−3.63	1.92
Aegean Sea	[67]	13.2–26.1 *III	0.000	2.804	-	-	-	-
	[24]	N.A.	-	-	37.24	0.33	−0.84	2.66
	[68]	14.0–30.0	0.008	3.002	-	-	-	-
	[69]	7.0–25.8 *I	0.004	3.271	-	-	-	-
		7.9–22.8 *II	0.004	3.285	-	-	-	-
	[32]	10.0–33.3	0.009	2.977	32.43	0.30	−1.40	2.49
E. Ionian Sea	This study	6.6–34.4	0.008	2.994	35.98	0.23	−1.60	2.47

N.A.: Not Answered; *^I: Males; *^II: Females; *^III: Fork length; *^IV: ELEFAN I; *^V: 2000–2007; *^VI: 2008–2013; *^VII: 2014–2020.

Although previous studies have selectively examined otolith morphometric traits in T. mediterraneus and have provided only limited data for T. trachurus—mostly restricted to specific regions and a single study by [39]—the present work constitutes the first systematic analysis of otolith morphometric and shape characters for both species in the Central Mediterranean Sea. Otolith morphometric variables revealed significant differences (ANOVA, p-value < 0.05) between the two species, with T. mediterraneus presenting higher values than T. trachurus for RA, OL, OW, OA, and PE, indicating larger and wider otoliths for the former species.

Otolith shape characteristics are closely linked to ecological adaptations, feeding behavior, and sensory processing efficiency [14,70]. T. mediterraneus exhibited larger otolith dimensions (OL, OW, OA, PE) and slightly more circular shape (lower EL, lower CI) compared to T. trachurus, which may indicate enhanced auditory sensitivity consistent with functional interpretations of otolith morphology and spatial orientation, potentially reflecting a more active presence in pelagic habitats with greater swimming capabilities and flexible adaptation to varying environmental conditions [70,71]. In contrast, T. trachurus showed smaller otolith dimensions and higher EL, suggesting a morphological structure more consistent with slightly more benthopelagic tendencies [70,71]. This clearly reflects the differences in behavior between the two congeneric species, describing the former as more pelagic than the latter, which exhibits more benthopelagic habits [16].

The relationship between the otolith characters and body length did not present differences between the two species, reflecting similar developmental processes governing their otolith growth. Our study coincides with the findings of [38,39], who also found a high correlation in the relationships between OL and OW with TL in T. mediterraneus.

In summary, the present findings provide important information on age, growth, and otolith morphometrics, necessary in fisheries management and species identification. Furthermore, our results offer valuable information on the behavioral features of the two species, which can support future research concerning their physiological traits. Further investigation on the otolith microstructure may also resolve discrepancies in age identification of the two examined species across various geographic regions and establish links to site-specific environmental factors. Such studies could further elucidate the influence of habitat variability on growth patterns, enhance the accuracy of age estimation techniques, and contribute to more robust stock assessment and management adapted to regional population dynamics.

5. Conclusions

This study focuses on the age, growth, and otolith morphometry of T. trachurus and T. mediterraneus in the E. Ionian Sea. T. trachurus showed a relatively higher growth rate than T. meditarraneus, which exhibits larger and longer otoliths than those of T. trachurus.