Corresponding author: Habib Bal ( habib.bal@tarim.gov.tr ) Academic editor: Alexei Orlov
© 2021 Habib Bal, Telat Yanik, Dilek Türker.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Bal H, Yanik T, Türker D (2021) Assessment of morphological variation between stocks of bluefish, Pomatomus saltatrix (Actinopterygii, Perciformes, Pomatomidae), in the Aegean Sea, Black Sea, and Sea of Marmara. Acta Ichthyologica et Piscatoria 51(1): 85-94. https://doi.org/10.3897/aiep.51.63319
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The population structure of the bluefish, Pomatomus saltatrix (Linnaeus, 1766), in Turkish waters is scarcely described in the literature. To identify any distinct population units of bluefish, and reaffirm the findings of a previous study, four areas were selected: the Aegean Sea, western Black Sea, eastern Black Sea, and the Sea of Marmara. In this study, truss network morphometrics, meristics, and otolith shape analyses were successfully applied for different population identification of the bluefish. Multivariate analysis of variance (MANOVA) revealed no differences for truss network morphometrics, meristic, and otolith shape characters between males and females. Hence, both sexes were combined for the discriminant function (DFA) and the Principal Component Analysis (PCA). Using univariate ANOVA based on the stepwise method revealed a highly significant difference among different locations for each truss-morphometrics and otolith shape characters. Furthermore, six out of seven meristic characters also showed significant differences between different areas. Based on PCA, 25 out of 27 truss-morphometric characters had a loading value above 0.70, which was considered significant in this study. The results of DFA show clear patterns of truss-morphometric character variations, forming four distinct clusters that were well separated from each other, indicating the existence of four morphologically differentiated populations of the bluefish. The proportion of the correctly classified Aegean Sea, western Black Sea, and eastern Black Sea bluefish samples to their original groups were 100%, demonstrating clear separation of these stocks from each other. Whereas up to 5% of the total samples of the Sea of Marmara were incorrectly classified, assigning to the eastern Black Sea. These findings were supported by meristic and otolith shape characters that also indicated four morphologically differentiated populations of the bluefish. However, their overall proportion of correct classification was relatively lower than the truss-morphometric traits method. The findings suggest the requirement of strategic assessment and management of each bluefish stock separately to use them sustainably in the future.
Climate change, factor analysis, Pomatomidae, stock structure, truss network system
The bluefish, Pomatomus saltatrix (Linnaeus, 1766), is a highly migratory pelagic streamlined predatory species with a wide geographical distribution that occurs in the majority of major ocean basins throughout the world except for the eastern Pacific (
Bluefish is subjected to over-exploitation threats and has been considered a globally vulnerable species (
A previous study by
Samples of bluefish were collected from four commercial fish landing centers: Aegean Sea (Gulf of İzmir), western Black Sea (Şile İstanbul), eastern Black Sea (Trabzon: Akçaabat), and the Sea of Marmara (Erdek Balıkesir) (Fig.
Descriptive data of bluefish, Pomatomus saltatrix, collected from the Aegean Sea, Western Black Sea, Eastern Black Sea, and Sea of Marmara.
Sea | Location | Coordinates | n | Sex ratio (♀:♂) | Date of capture | Sampling gear |
---|---|---|---|---|---|---|
Aegean Sea | Gulf of İzmir | 38°36′32.8″N, 26°38′53.9″E | 31 | 1.0:1.1 | 05 Apr. 2014 | Fishhook |
Western Black Sea | Şile İstanbul | 41°13′39.4″N, 29°43′09.1″E | 36 | 1.0:1.1 | 29 Sep. 2014 | Gillnet |
Eastern Black Sea | Akçaabat Trabzon | 41°02′45.3″N, 39°36′18.5″E | 33 | 1.0:0.9 | 14 Nov. 2014 | Fishhook |
Sea of Marmara | Erdek Balıkesir | 40°28′59.1″N, 27°33′37.9″E | 31 | 1.0:0.8 | 29 Feb. 2014 | Purse-seine |
Before taking the measurements, the frozen samples of bluefish were thawed for 1 hour under running water, placed on their right side on a water-resistant graph. Body posture and fins were forced into a natural position. Each fish was examined for physical damage, and a sample with any physical damage was removed from the analysis. Furthermore, their sexes were determined by reviewing their gonads under a dissecting microscope.
A total of 13 anatomical landmarks were chosen for the study, and by inter-connecting these landmarks, the box-truss network was produced, representing a truss network of 27 lines (Fig.
Description of morphometric measurements made for each sample of bluefish, Pomatomus saltatrix, collected from the Aegean Sea, western Black Sea, eastern Black Sea, and the Sea of Marmara between February 2014 and November 2014.
Measurement No. | Distance code | Distance | Landmarks |
---|---|---|---|
1 | HL1 | Head length 1 | 1–2 |
2 | BL1 | Body length 1 (Pre-dorsal length) | 2–3 |
3 | DFBL1 | First dorsal fin base Length | 3–4 |
4 | MDL | Mid dorsal length | 4–5 |
5 | DFBL2 | Second dorsal fin base length | 5–6 |
6 | PDL | Post-dorsal length | 6–7 |
7 | CL1 | Caudal length 1 | 7–8 |
8 | CL2 | Caudal length 2 | 8–9 |
9 | BL2 | Body length 2 | 9–10 |
10 | AFBL | Anal fin base length | 10–11 |
11 | BL3 | Body length 3 | 11–12 |
12 | BL4 | Body length 4 | 12–13 |
13 | HL2 | Head length 2 | 1–13 |
14 | BD1 | Head diagonal 1 | 2–13 |
15 | BH1 | Body height 1 | 3–12 |
16 | BD2 | Body diagonal 2 | 4–11 |
17 | BH2 | Body height 2 | 5–11 |
18 | BD3 | Body diagonal 3 | 6–11 |
19 | BD4 | Body diagonal 4 | 7–10 |
20 | BD5 | Head diagonal2 | 1–12 |
21 | BD5 | Body diagonal 5 | 2–12 |
22 | BD6 | Body diagonal 6 | 3–11 |
23 | BD7 | Body diagonal 7 | 5–10 |
24 | BH2 | Body height 2 | 6–10 |
25 | BH3 | Body height 3 | 7–9 |
26 | BD8 | Body diagonal 8 | 2–11 |
27 | BD9 | Body diagonal 9 | 6–9 |
Using a binocular microscope, the number of branched and un-branched rays in dorsal fin spines, dorsal fin rays, ventral fin rays, pectoral fin rays, and anal fin rays as well as right and left gill rakers were obtained (
The sagittal otoliths were removed from all individuals. Each otolith was carefully wiped, clean, and stored dry in U-plates (
Truss-morphometric and otolith variables were standardized separately for each region to eliminate the effect of fish size on these variables. The meristic characters were not standardized as they did not show a significant correlation with the bluefish body size (
V trans = log V – â(log SL – log SLmean)
where Vtrans is the transformed morphometric variable, V is the non-transformed variable, SL is the standard length of each fish, SLmean is the overall mean standard length of all the fish from each group (region), and β is the slope of the relation between log V and log SL.
The modified morphometric variables were tested for normality check, and outliers, if any, were excluded before subsequent analysis. Multivariate analysis of variance (MANOVA) was performed to check significant variation between different sex groups as well as sampling locations based on morphometrics, meristic, and otolith characters. The univariate ANOVA for each variable was then used to test significant differences among different sampling areas. The differences were considered statistically significant at P-values below 0.05. Principal component analysis (PCA) was used to uncover the morphometric variables with a highly influential role in distinguishing between the four populations. Discriminant function analysis (DFA) was used to demonstrate the variations among different bluefish stocks by classifying them to their respective groups based on morphometrics, meristic, or otolith characters. Dendrogram based Euclidean distance method was used to depict similarities between different locations. All statistical analyses were carried out with IBM SPSS Statistics software ver. 25.0.
The size distribution of the bluefish based on total length is presented in Fig.
There was no statistical difference observed between truss-morphometric characteristics for females and males (one-way MANOVA; F (27, 32) = 26.4, Wilk’s λ = 0.456, P = 0.172); hence, sexes were combined for further analysis. While there were highly significant differences among the stocks of bluefish from different locations using all data (one-way MANOVA; F (81, 108) = 26.4, Wilk’s λ = 0.0001, P < 0.0001). Also, the univariate ANOVA based on the stepwise method further revealed a highly significant difference among different locations for each truss-morphometric trait (Table
Descriptive statistics of univariate ANOVA based on morphometric characters of bluefish, Pomatomus saltatrix, collected from the Aegean Sea, western Black Sea, eastern Black Sea, and the Sea of Marmara.
Morphometric characters | Univariate ANOVA | ||||||
---|---|---|---|---|---|---|---|
Wilks’ λ | F | Sign. | Morphometric characters | Wilks’ λ | F | Sign. | |
1–2 | 0.276 | 54.241 | <0.0001 | 3–12 | 0.068 | 285.212 | <0.0001 |
2–3 | 0.085 | 221.971 | <0.0001 | 4–11 | 0.056 | 349.181 | <0.0001 |
3–4 | 0.163 | 106.011 | <0.0001 | 5–11 | 0.070 | 274.448 | <0.0001 |
4–5 | 0.315 | 44.959 | <0.0001 | 6–11 | 0.035 | 575.511 | <0.0001 |
5–6 | 0.036 | 557.940 | <0.0001 | 7–10 | 0.093 | 201.551 | <0.0001 |
6–7 | 0.176 | 96.828 | <0.0001 | 1–12 | 0.067 | 289.156 | <0.0001 |
7–8 | 0.215 | 75.242 | <0.0001 | 2–12 | 0.067 | 289.493 | <0.0001 |
8–9 | 0.174 | 97.954 | <0.0001 | 3–11 | 0.038 | 526.114 | <0.0001 |
9–10 | 0.228 | 70.132 | <0.0001 | 5–10 | 0.025 | 807.113 | <0.0001 |
10–11 | 0.033 | 602.729 | <0.0001 | 6–10 | 0.072 | 265.311 | <0.0001 |
11–12 | 0.055 | 351.826 | <0.0001 | 7–9 | 0.132 | 135.666 | <0.0001 |
12–13 | 0.113 | 162.162 | <0.0001 | 2–11 | 0.022 | 925.575 | <0.0001 |
1–13 | 0.308 | 46.441 | <0.0001 | 6–9 | 0.157 | 110.827 | <0.0001 |
2–13 | 0.201 | 82.171 | <0.0001 |
The first two component-loading scores of principal components based on morphometric characters of bluefish, Pomatomus saltatrix, sampled from the Aegean Sea, western Black Sea, eastern Black Sea, and the Sea of Marmara.
Morphometric characters | Principal component | ||||
---|---|---|---|---|---|
PC1 (84.81%) | PC2 (5.27%) | Morphometric characters | PC1 (84.81%) | PC2 (5.27%) | |
1–2 | 0.757 | 0.479 | 3–12 | 0.970 | –0.092 |
2–3 | 0.936 | –0.049 | 4–11 | 0.981 | –0.035 |
3–4 | 0.859 | –0.402 | 5–11 | 0.977 | –0.093 |
4–5 | 0.615 | 0.616 | 6–11 | 0.983 | –0.067 |
5–6 | 0.976 | 0.025 | 7–10 | 0.964 | –0.104 |
6–7 | 0.917 | 0.077 | 1–12 | 0.969 | 0.028 |
7–8 | 0.894 | 0.007 | 2–12 | 0.967 | –0.113 |
8–9 | 0.894 | 0.144 | 3–11 | 0.984 | –0.079 |
9–10 | 0.888 | –0.200 | 5–10 | 0.986 | 0.020 |
10–11 | 0.973 | 0.016 | 6–10 | 0.974 | –0.052 |
11–12 | 0.968 | –0.010 | 7–9 | 0.954 | –0.021 |
12–13 | 0.863 | –0.373 | 2–11 | 0.984 | –0.041 |
1–13 | 0.676 | 0.570 | 6–9 | 0.943 | –0.058 |
2–13 | 0.887 | 0.242 |
DFA results show clear patterns of truss-morphometric trait variations, forming four distinct clusters that are well separated from each other (Fig.
Summary output of stepwise canonical discriminant analysis based on morphometric characters bluefish, Pomatomus saltatrix, samples from the Aegean Sea, western Black Sea, eastern Black Sea, and the Sea of Marmara collected between February 2014 to November 2014; Overall, 99.0% of original grouped cases correctly classified.
Populations | Predicted group membership | Total | |||
---|---|---|---|---|---|
Aegean Sea | Western Black Sea | Eastern Black Sea | Sea of Marmara | ||
Aegean Sea | 100% | 100% | |||
Western Black Sea | 100% | 100% | |||
Eastern Black Sea | 100% | 100% | |||
Sea of Marmara | 5.26% | 94.74% | 100% |
Dendrogram, based on the Euclidean distance method, formed three main clusters (Fig.
The range of the bluefish meristic counts from the Aegean Sea, western Black Sea, eastern Black Sea, and the Sea of Marmara are given in Table
Descriptive data of the meristic counts of bluefish, Pomatomus saltatrix, collected from the Aegean Sea, western Black Sea, eastern Black Sea, and the Sea of Marmara.
Meristic characters | Aegean Sea | Western Black Sea | Eastern Black Sea | Sea of Marmara | ||||
---|---|---|---|---|---|---|---|---|
Mean ± SD | Range | Mean ± SD | Range | Mean ± SD | Range | Mean ± SD | Range | |
Right gill rakers | 10.97 ± 0.84 | 9–12 | 11.17 ± 0.97 | 9–13 | 11.00 ± 0.00 | 11–11 | 10.39 ± 1.15 | 9–12 |
Left gill rakers | 10.87 ± 0.81 | 9–12 | 11.25 ± 1.00 | 9–13 | 10.91 ± 0.52 | 10–12 | 10.39 ± 1.05 | 9–12 |
Dorsal fin spines | 7.19 ± 0.70 | 6–10 | 7.72 ± 0.45 | 7–8 | 7.55 ± 0.51 | 7–8 | 6.65 ± 0.49 | 6–7 |
Dorsal fin rays | 24.39 ± 1.17 | 22–27 | 24.44 ± 0.88 | 23–26 | 24.15 ± 0.94 | 22–26 | 22.68 ± 0.79 | 22–24 |
Ventral fin rays | 11.77 ± 0.72 | 10–13 | 12.00 ± 0.00 | 12–12 | 12.00 ± 0.00 | 12–12 | 12.10 ± 0.30 | 12–13 |
Pectoral fin rays | 15.13 ± 0.85 | 12–16 | 14.97 ± 0.38 | 14–16 | 15.18 ± 0.46 | 14–16 | 15.03 ± 0.66 | 14–16 |
Anal fin rays | 25.48 ± 1.09 | 24–28 | 25.03 ± 1.03 | 23–27 | 24.70 ± 0.88 | 24–27 | 24.10 ± 1.14 | 23–28 |
Descriptive statistics of univariate ANOVA based on meristic characters of the bluefish, Pomatomus saltatrix, sampled from the Aegean Sea, western Black Sea, eastern Black Sea, and the Sea of Marmara collected between February 2014 and November 2014.
Characters | Wilks’ λ | F | Significance |
---|---|---|---|
Right gill rakers | 0.889 | 5.311 | 0.002 |
Left gill rakers | 0.884 | 5.562 | 0.001 |
Dorsal fin spines | 0.618 | 26.214 | 0.000 |
Dorsal fin rays | 0.630 | 24.873 | 0.000 |
Ventral fin rays | 0.910 | 4.195 | 0.007 |
Pectoral fin rays | 0.982 | 0.786 | 0.504 |
Anal fin rays | 0.808 | 10.083 | 0.000 |
The component-loading scores of principal components based on meristic characters of bluefish, Pomatomus saltatrix, sampled from the Aegean Sea, western Black Sea, eastern Black Sea, and the Sea of Marmara.
Character | Principal component | |
---|---|---|
PC1 (37.45%) | PC2 (20.94%) | |
Right gill rakers | 0.769 | 0.466 |
Left gill rakers | 0.786 | 0.440 |
Dorsal fin spines | 0.626 | –0.157 |
Dorsal fin rays | 0.612 | –0.574 |
Ventral fin rays | 0.020 | 0.482 |
Anal fin rays | 0.521 | –0.508 |
In DFA, the first DF accounted for 83.0%, and the second corresponded to 16.8% of the between-group variability. Overall, 64.1% of original grouped cases were correctly classified, and the bluefish correct classification into their original population/location ranged from 50.0% to 90.3% by canonical discriminant analysis (Table
The summary output of stepwise canonical discriminant analysis based on meristic characters of bluefish, Pomatomus saltatrix, collected between February 2014 and November 2014; overall, 64.1% of original grouped cases correctly classified.
Populations | Predicted group membership | Total | |||
---|---|---|---|---|---|
Aegean Sea | Western Black Sea | Eastern Black Sea | Sea of Marmara | ||
Aegean Sea | 67.74% | 9.68% | 9.68% | 12.90% | 100% |
Western Black Sea | 16.67% | 50.00% | 27.78% | 5.56% | 100% |
Eastern Black Sea | 15.15% | 24.24% | 51.52% | 9.09% | 100% |
Sea of Marmara | 6.45% | 3.23% | 90.32% | 100% |
The mean, standard deviation, minimum and maximum values for each otolith variable of the bluefish (Pomatomus saltatrix) are given in Table
Descriptive data of otolith variables of bluefish, Pomatomus saltatrix, collected from the Aegean Sea, western Black Sea, eastern Black Sea, and the Sea of Marmara.
Otolith variables | Aegean Sea | Western Black Sea | Eastern Black Sea | *Sea of Marmara | ||||
---|---|---|---|---|---|---|---|---|
Mean ± SD | Range | Mean ± SD | Range | Mean ± SD | Range | Mean ± SD | Range | |
Otolith length | 7.07 ± 0.32 | 6.28–7.55 | 5.38 ± 1.09 | 4.34–7.39 | 5.02 ± 0.38 | 4.37–5.95 | 6.36 ± 0.87 | 4.65–8.98 |
Otolith width | 2.58 ± 0.14 | 2.23–2.81 | 2.16 ± 0.23 | 1.8–2.58 | 2.09 ± 0.1 | 1.91–2.26 | 2.50 ± 0.26 | 1.99–3.16 |
Otolith area | 13.03 ± 0.99 | 10.79–14.94 | 8.46 ± 2.86 | 5.53–13.63 | 7.56 ± 0.97 | 5.82–9.37 | 12.63 ± 3.05 | 7.25–21.63 |
Otolith perimeter | 17.57 ± 1.03 | 15.66–19.79 | 14.3 ± 2.87 | 10.71–19.79 | 12.47 ± 1.03 | 10.52–14.48 | 19.48 ± 3.95 | 14.51–32.09 |
Form factor | 0.53 ± 0.04 | 0.46–0.6 | 0.52 ± 0.06 | 0.39–0.64 | 0.61 ± 0.04 | 0.52–0.72 | 0.43 ± 0.08 | 0.18–0.55 |
Roundness | 0.33 ± 0.02 | 0.3–0.39 | 0.37 ± 0.03 | 0.29–0.42 | 0.38 ± 0.02 | 0.34–0.41 | 0.39 ± 0.03 | 0.33–0.44 |
Aspect ratio | 2.74 ± 0.14 | 2.36–3.03 | 2.47 ± 0.26 | 2.13–3 | 2.4 ± 0.13 | 2.16–2.69 | 2.54 ± 0.17 | 2.28–3.07 |
Descriptive statistics of univariate ANOVA based on otolith variables of bluefish, Pomatomus saltatrix, collected between February 2014 and November 2014 from the Aegean Sea, western Black Sea, eastern Black Sea, and the Sea of Marmara.
Variable | Wilks’ λ | F | Significance |
---|---|---|---|
Otolith length | 0.102 | 324.325 | 0.000 |
Otolith width | 0.257 | 107.157 | 0.000 |
Otolith area | 0.114 | 288.710 | 0.000 |
Otolith circumference | 0.121 | 269.433 | 0.000 |
Form factor | 0.390 | 57.986 | 0.000 |
Roundness | 0.295 | 88.584 | 0.000 |
Aspect ratio | 0.425 | 50.075 | 0.000 |
The component-loading scores of principal components based on otolith variables of bluefish, Pomatomus saltatrix, sampled from the Aegean Sea, western Black Sea, eastern Black Sea, and the Sea of Marmara.
Variables | Principal component | |
---|---|---|
PC1 (66.07%) | PC2 (21.33%) | |
Otolith length | 0.981 | –0.123 |
Otolith width | 0.893 | 0.181 |
Otolith area | 0.964 | 0.112 |
Otolith circumference | 0.938 | 0.331 |
Form factor | –0.557 | –0.572 |
Roundness | –0.445 | 0.832 |
Aspect ratio | 0.741 | –0.551 |
In DFA, the first DF accounted for 69.1%, and the second corresponded to 28.9% of the between-group variability. Overall, 96.6% of original grouped cases were correctly classified, and the bluefish correct classification into their original population ranged from 91.4% to 100% by canonical discriminant analysis (Table
Summary statistics of stepwise canonical discriminant analysis based on otolith variables of bluefish, Pomatomus saltatrix, collected between February 2014 and November 2014; overall, 96.6% of original grouped cases correctly classified.
Populations | Predicted group membership | Total | |||
---|---|---|---|---|---|
Aegean Sea | Western Black Sea | Eastern Black Sea | Sea of Marmara | ||
Aegean Sea | 100 | 100 | |||
Western Black Sea | 91.43 | 8.57 | 100 | ||
Eastern Black Sea | 3.45 | 96.55 | 100 | ||
Sea of Marmara | 100 | 100 |
The truss-morphometric characteristics analysis provided evidence of the existence of four morphologically differentiated populations of bluefish, with 95% to 100% correct allocation of bluefish individuals into their original stock. These results are in line with the findings of
Several studies suggest that the population structure of highly migratory marine species is strongly regulated by some behavioral traits such as spawning site fidelity, homing behavior (
The use of more than one stock identification approach and comparison between them can enhance the likelihood of extracting differences between classifying for a comprehensive conclusion (
Bluefish stock from the Aegean Sea, western Black Sea, eastern Black Sea, and the Sea of Marmara demonstrated considerable morphometric variations and hence they should be considered as four self-contained stocks that are geographically isolated from each other. Environmental differences between areas probably influence these inter-population morphometric distinctions. This might indicate new environmental consequences hindering the intermingling of bluefish stocks; since the stocks of the Aegean Sea, the Sea of Marmara, and the western Black Sea were observed as a single, morphometrically homogenous stock by
The authors thanks to the General Directorate of Agricultural Research and Policies (TAGEM HAYSÜD/2013/A11/P-02/4).