Age, Growth, and Utility of Otolith Morphometrics as Predictors of Age in the European Barracuda, Sphyraena sphyraena (Sphyraenidae): Preliminary Data

Abstract

Age and growth of the European barracuda, Sphyraena sphyraena, were determined by examining sagittal otoliths belonging to fish sampled in the eastern Adriatic Sea, as the northernmost region of the Mediterranean. A total of 113 specimens (59 females, 53 males, and one individual of indeterminate sex), ranging from 23.4 to 42.5 cm in total length, were analyzed. The maximum observed age was 5 years for both females and males, and this barracuda population was dominated by 3 year old fish. Growth was described by the von Bertalanffy growth curve (L_∞ = 55.58 cm, K = 0.12 year⁻¹, t₀ = −4.29 year, R² = 0.580), and the growth performance index (Φ’) was 2.57. Otolith length, width, and mass were measured, and the utility of these morphometrics as predictors of age in S. sphyraena was evaluated. The results showed that counting otolith annuli produced a better estimation of age than proposed linear models based on relationships between observed fish age and otolith morphometrics. In comparison with age and growth data available in the literature for S. sphyraena, our results from the Adriatic Sea provide more insights into the life-history traits of this species and can be used in the future effective management and conservation.

1. Introduction

The family Sphyraenidae (barracudas), which is typically represented in shelf waters of tropical and temperate seas in many regions of the world, comprises only one genus, Sphyraena, and 28 species [1,2,3]. They are all active predatory and voracious fishes that can adapt to diverse ecological conditions [4,5]. Large adult barracudas are often found to be solitary, whereas juveniles and small adults are gregarious and aggregate to form schools [6]. In the Mediterranean, six Sphyraenidae species have been recorded so far: three Lessepsian (S. chrysotaenia Klunzinger, 1884; S. flavicauda Rüppell, 1838; S. obtusata Cuvier, 1829), two Atlanto-Mediterranean (S. sphyraena (Linnaeus, 1758); S. viridensis Cuvier, 1829), and one new species identified from the Gulf of Taranto (S. intermedia Pastore, 2009) [7,8].

The European barracuda, S. sphyraena, is a widespread species found in the eastern Atlantic (Bay of Biscay to Angola), including the Mediterranean and Black Sea, Canary Islands, and Azores [9]. It is a pelagic species living close to the surface [10], although individuals can be observed in a midwater, down to depths of 100 m [11]. In the Mediterranean, S. sphyraena is a specialized piscivore fish reaching 165 cm of the total length, while its size is usually between 30 and 50 cm [12]. This barracuda is a temperate species and generally shares habitats and resources with the thermophilic S. viridensis [5,12]. In comparison to other Sphyraenidae, only a few investigations have been carried out on the life-history traits of S. sphyraena. The growth [4,13], otolith shape analysis [8], reproductive biology [14], and feeding ecology [12] have been investigated in the southeastern Mediterranean. Otolith shape analysis has been also undertaken in the southwestern Mediterranean [7], and some patterns of reproductive biology have been reported in the northwestern part of the basin [5]. However, information regarding this barracuda species in the northernmost region of the Mediterranean (Adriatic Sea) has not yet been described in detail.

This study is the first attempt to investigate the age and growth of the eastern Adriatic population of the European barracuda; therefore, the aim is to provide better insight into the life-history of this species. Fish age was determined by counting annuli in sagittal otoliths; however, because this is one of the most time-consuming aging methods, we also evaluated the utility of otolith morphometrics (length, width, and mass) as an alternative.

2. Materials and Methods

2.1. Sampling

Samples of the European barracuda were obtained monthly from the local fish market between November 2020 and February 2021 and belonged to catches taken by purse and boat seiners in the central eastern Adriatic Sea during that period. Each fish was measured to the nearest 0.1 cm total length (TL) and weighed (W) to the nearest 0.1 g. Sex was determined by macroscopic analysis of the gonads, and sagittal otolith pairs were removed, cleaned, and stored dry in labeled Eppendorf tubes for later analyses. Sex ratio was tested for equality (female to male = 1:1) using the chi-square goodness-of-fit test at the 0.05 significance level. Length frequency distributions of females and males were compared using the Kolmogorov–Smirnov two-sample test.

2.2. Otolith Morphometrics

Prior to polishing, otoliths were photographed using an Olympus DP-25 digital camera (Olympus Corporation, Hamburg, Germany) attached to an Olympus SZX10 stereo microscope. Otolith length (OL) and width (OW) were measured to the nearest 0.01 mm using Olympus Cell^A Imaging Software, version 2.6 (Build 1210). Otolith length was defined as the longest axis between anterior and posterior otolith edge, and width was defined as the distance from dorsal to ventral edge taken perpendicular to the length throughout the otolith focus. Otolith mass (OM) was weighed to the nearest 0.0001 g. Differences between left and right otoliths were tested by paired t-test, while ANCOVA was used to test for differences in otolith measures between females and males, considering fish length as a covariate. The statistical assumptions of these tests were tested using the Kolmogorov–Smirnov test of normality, and, when necessary, data were log-transformed. Relationships between fish total length and otolith morphometrics (length, width, and mass) were constructed using the linear model.

2.3. Age and Growth Estimation

The medial face of a single, randomly selected otolith from each pair was polished using a wet abrasive paper and photographed under reflected light against a dark background, using a stereo microscope coupled with an Olympus DP-25 digital camera. Growth rings were visible as alternating opaque and translucent zones, and ages were assigned to fish specimens on the basis of their counts. Therefore, one opaque zone combined with one subsequent translucent zone was interpreted as one annulus, as already observed in Sphyraena species [2,15]. Each otolith was read by one reader, without auxiliary data on the fish size. A second reading was carried out 1 week later, and, when readings differed by one or more years, a third reading was made. We used images for the age estimation because their quality and ability to adjust contrast and brightness made it easier to interpret zoning patterns than direct observations under the stereo microscope.

The von Bertalanffy growth model was fitted to the estimated age–length dataset using a nonlinear least-square procedure of a Gauss–Newton algorithm: TL = L_∞ (1 − exp(−K(t − t₀))), where TL is the total length of fish at age t, L_∞ is the estimated asymptotic length, K is a constant that determines the rate at which TL approaches L_∞, and t₀ is the hypothetical age at zero length. To compare the fish within the genus Sphyraena, the index Φ’ was calculated as follows: Φ’ = 2 log L_∞ + log K, where Φ’ is the growth performance of fish, L_∞ is the asymptotic length derived from the von Bertalanffy growth model, and K is the growth constant also derived from the von Bertalanffy growth model [16]. Once the age was estimated by counting annual growth rings, relationships between observed fish age and otolith morphometrics (length, width, and mass) were constructed using the linear model.

3. Results

Of the 113 individuals of the European barracuda sampled, 59 (52.2%) were females, 53 (46.9%) were males, and one (0.9%) individual was of indeterminate sex. The sex ratio was not significantly skewed toward females or males (p = 0.57). The total length of females and males ranged from 28.5 to 42.5 cm and from 23.4 to 38.6 cm, respectively (

Table 1. Range of total length and weight and mean total length and weight with standard deviation (SD) of the European barracuda, Sphyraena sphyraena, in the eastern Adriatic.

	Total Length (cm)		Weight (g)
	Range	Mean ± SD	Range	Mean ± SD
Females	28.5–42.5	34.6 ± 3.75	91.5–270.0	159.9 ± 47.09
Males	23.4–38.6	32.0 ± 3.56	47.3–224.2	131.7 ± 37.27
All specimens	23.4–42.5	33.4 ± 3.89	47.3–270.0	146.1 ± 44.95

). A higher proportion of males were smaller than females. Males dominated in the ≤31.0 cm length classes, whereas females were more abundant in the ≥36.0 cm length classes (Figure 1). The Kolmogorov–Smirnov two-sample test showed a significant difference between length frequency distributions of females and males (p < 0.01). The mean length of females (34.6 ± 3.75 cm) was significantly greater than that of males (32.0 ± 3.56 cm) (t-test, df = 1, p < 0.01).

Otoliths of S. sphyraena are spindle-shaped, with a short rostrum and poorly defined antirostrum. Their anterior region is peaked, while the posterior is oblique. No significant differences in morphometric measures (length, width, and mass) were found between left and right otoliths (paired t-test, p > 0.05 for all measures) and between females and males (ANCOVA, p > 0.05 for all measures); thus, data were pooled for both sexes, and mean values for each otolith pair were used in further analyses (

Table 2. Range of otolith measures (OL = otolith length, OW = otolith width, OM = otolith mass) and their mean values with standard deviation (SD) of the European barracuda, Sphyraena sphyraena, in the eastern Adriatic.

Otolith Measures	Range	Mean ± SD
OL (mm)	7.56–12.96	10.20 ± 1.10
OW (mm)	2.43–4.09	3.29 ± 0.34
OM (g)	0.0157–0.0769	0.0390 ± 0.01

). A linear model described relationships between fish total length and otolith length, width, and mass, with high coefficients of determination (≥0.805) (

Table 3. Parameters of the linear regression between fish total length (TL) and otolith morphometrics (OL = otolith length, OW = otolith width, OM = otolith mass) of the European barracuda, Sphyraena sphyraena, in the eastern Adriatic (a = intercept value; b = regression slope; R² = coefficient of determination).

Relationships	a	b	R2
TL vs. OL	1.683	0.256	0.865
TL vs. OW	0.690	0.078	0.805
TL vs. OM	−0.073	0.003	0.851

).

Sagittal otoliths displayed alternating opaque and translucent zones under reflected light (Figure 2). The maximum age of the sampled European barracuda was 5 years for both females and males (Figure 3). Age distribution was similar for both sexes and was skewed to the right. The population was dominated by 3 year old fish, which represented almost 45% of the fish sampled. Fish younger than 3 years were poorly represented (we found only one 1 year old individual). Mean lengths (±standard deviation) of 1, 2, 3, 4, and 5 year old fish were 23.4 cm, 29.0 ± 0.41 cm, 32.4 ± 3.27 cm, 33.9 ± 3.29 cm, and 37.5 ± 3.15 cm, respectively. Females were larger than males at all ages. The von Bertalanffy growth curve was fitted to the age–length dataset estimated for all specimens (Figure 4). Estimated parameters were L_∞ = 55.58 cm, K = 0.12 year⁻¹, and t₀ = −4.29 year (R² = 0.580). The growth performance index (Φ’) was 2.57 for the sampled fish. Relationships between observed fish age and otolith morphometrics are presented in

Table 4. Relationships between observed fish age and otolith morphometrics (OL = otolith length, OW = otolith width, OM = otolith mass) of the European barracuda, Sphyraena sphyraena, in the eastern Adriatic, described using the linear model (age = a (otolith morphometric) + b); R² = coefficient of determination.

Linear Model	R2
Age = 0.425 OL − 0.761	0.277
Age = 1.294 OW − 0.694	0.255
Age = 28.912 OM + 2.445	0.236

. The linear model explained between 23.6% and 27.7% of the variation in age.

4. Discussion

Despite its wide distribution, studies concerning the age of S. sphyraena are currently available only for the southeastern Mediterranean populations (Egyptian waters) [4,13]. The maximum age estimated in our study using whole otolith readings (5 years) was lower than the maximum age demonstrated by Allam et al. [13] (8 years) using the same method and a similar length range. However, only 4% of the specimens collected in Egyptian waters were older than 5 years [13]. On the other hand, our age readings were higher than those reported in another available study (3 years) [4]. The most striking difference in the life span of S. sphyraena seems to occur among populations from the same area. As far as growth parameters are concerned, there was good agreement, and parameters estimated in our study were very similar to the only available growth data on this species for Egyptian waters: L_∞ = 55.27 cm and K = 0.123 year⁻¹ [13]. This similarity may be attributable to the dominance of individuals in the same age classes in both populations, in conjunction with the already mentioned small proportion of the oldest individuals (>5 years) in Egyptian waters. In addition, the growth increments for both populations were greatest in the first 2 years. All observed similarities were also reflected in almost the same values for Φ’: 2.57 and 2.58 [13] for the northern and southeastern Mediterranean populations, respectively. The low estimate of t₀ can be explained by the few fish ≤28.0 cm and a complete absence of the fish <23.0 cm in our samples. In fact, if smaller fish were sampled in both areas, t₀ would have been higher and closer to 0. Consequently, the population age structure shows a lack of fish in the first two age classes, principally due to accessibility to the resources on the fish market. Although females and males showed differences in growth, as females were larger than males at all ages sampled, we acknowledge that our study is not conclusive about possible differences in growth between sexes. More data, particularly from smaller and younger fish, would improve estimates and ensure modeling for S. sphyraena females and males separately. However, although the sample size was relatively small for young individuals, the sample covered a wide length and age range; thus, the results of this study are still reliable and valuable. By producing such preliminary growth estimates for S. sphyraena in the studied area, we take an important step toward facilitating future studies and management strategies.

The index of growth performance is a useful tool for comparing the growth curves of different populations of the same species and of different species belonging to the same genus or family [17]. Calculated from published data of L_∞ and K in Mediterranean Sphyraena species, the values of this index were the highest in S. viridensis (2.95 in Egyptian waters [13] and 3.33 in Algerian waters [15]) and the lowest in S. chrysotaenia (2.46 [13] and 2.62 [2] in Egyptian waters). In S. flavicauda from Egyptian waters, recorded values were 2.77 [13] and 2.82 [2]. These variations could be attributed to environmental conditions affecting distribution, population structure, and growth patterns [18]. The obtained Φ’ value in this study was within the range of values for different Sphyraena species in the Mediterranean (2.46 < Φ’ < 3.33). In general, growth studies providing the von Bertalanffy growth parameters for other Sphyraena species are scarce, and the main point of reference is represented by data provided for the southeastern Mediterranean populations.

Interpreting annual growth rings in otoliths, as in this study, is the most frequently used method for estimating individual fish age [19]. On the other hand, it has several disadvantages; it is subjective, laborious, time-consuming, and dependent on the readers’ experience [20]. In that context, several studies have analyzed and demonstrated a close relationship between the size of the otoliths and fish age [20,21,22,23,24,25]. A linear model described all relationships between otolith measures and fish age, and the age of the investigated barracuda could be best predicted from the otolith length. However, we must emphasize that models did not provide precise estimates, and this could be explained by the overlap in lengths between fish of different age (especially in the age classes 3 and 4) and a lack of the fish in the first two age classes. Despite that, the observed otolith morphology of S. sphyraena generally corresponded to descriptions for this species from the eastern Atlantic and Mediterranean [7,8,26], and it can be used in the identification of species (e.g., in feeding studies or species separation within Sphyraena genus).

5. Conclusions

This study provides the first analysis of European barracuda age and growth in the northernmost region of the Mediterranean. We showed that otoliths can be used for aging, and our estimates of growth seems reasonable for a species with a moderate life span. Counting otolith annuli produced better estimation of age than proposed linear models based on relationships between observed fish age and otolith morphometrics. However, these preliminary results should be used with caution, and further studies with larger sample sizes collected throughout the year are required to refine the age and growth information for S. sphyraena in the Adriatic. Similar studies are, therefore, encouraged within the Mediterranean to achieve a more comprehensive understanding of the life-history traits of this temperate fish species.