Length-weight relationship and condition factor of longnose trevally (Platycaranx chrysophrys) of the coast of Qeshm Island, Iran

Document Type : Research Paper

Authors

Persian Gulf and Oman Sea Ecological Research Center, Iranian Fisheries Science Research Institute, Agricultural Research Education and Extension Organization, Bandar Abbas, Iran

10.22124/janb.2026.33357.1304

Abstract

Introduction: Reports from the Iranian Fisheries Research Organization (IFRO) suggest that fish stocks in the Persian Gulf are largely fully exploited or overexploited. Over the past decade, fishing effort in the region has increased, while management measures have focused mainly on limiting trawler numbers, regulating fishing gear, and imposing spatial and seasonal closures. Despite these measures, biological information on industrial and small-scale fisheries, particularly along the northern coast of the Persian Gulf, remains limited. Consequently, catch-based data combined with biological analyses can provide valuable inputs for fisheries assessment and management. The family Carangidae is among the most species-rich and economically important groups of pelagic fishes in the Persian Gulf. The longnose trevally, Platycaranx chrysophrys (formerly Carangoides chrysophrys), is widely distributed across the Indian Ocean, Red Sea, Persian Gulf, and Western Pacific. Length-weight relationships are widely used as indicators of growth pattern, condition, and stock status, while the condition factor provides insight into fish well-being and habitat quality. Therefore, this study aimed to estimate the length-weight relationship and condition factor of P. chrysophrys in Hormozgan waters, providing baseline information for fisheries management.
Materials and methods: A total of 42 specimens of P. chrysophrys were collected in December 2025 from Basaidu region, located in the northwestern waters of Qeshm Island, Persian Gulf. Sampling was conducted using a commercial bottom trawler. For biometric measurements, the total length (TL) of each individual was recorded to the nearest 1 mm using a graduated measuring board, and the total body weight (W) was determined to the nearest 0.01 g using a high-precision digital scale. The relationship between TL and W was estimated using the power curve equation: W=aLb, where “W” is the total weight (g), “L” is the total length (cm), “a” is the intercept (regression constant), and “b” is the slope (growth exponent). The parameters a and b were estimated via linear regression analysis after logarithmic transformation of the data: ln(W) = ln(a) + bln(L). Standard errors of the slope (b) and the coefficient of determination (R2) were calculated to evaluate the fit of the model. Fulton’s condition factor (K) was calculated to assess the overall health and well-being of the fish using the following formula: K=100 × W/L3, where “W” is the body weight (g) and “L” is the total length (cm). In addition, length frequency distribution was analyzed. The optimal number of length classes (k) was determined using Sturges’ rule: k = 1 + 3.322log10(n), where “n” is the total number of sampled fish. All statistical analyses, parameter estimations, and visualizations were performed using R software (version 4.4.2) and the stats package.
Results: The examined specimens of P. chrysophrys (n = 42) exhibited a mean (± SD) total length of 37.60 ± 4.82 cm (range: 29.5-46.2 cm) and a mean (± SD) total weight of 613.10±195.23 g (range: 350-1100 g). The linear regression of the log-transformed data yielded the following length-weight relationship equation: lnW = ln (0.2622) + 2.13ln (L), which corresponds to the non-linear power equation: W = 0.2622L^2.1374. The coefficient of determination (R2 = 0.81) indicated a strong correlation between length and weight. The slope value (b = 2.13) was significantly lower than the isometric value of 3 (Student’s t-test: t = -5.087, p < 0.001), demonstrating a negative allometric growth pattern for P. chrysophrys in the study area. The mean Fulton’s condition factor for all sampled individuals was estimated 1.19 ± 0.24 (range: 0.87- 1.82). The length-frequency histogram showed that the length data followed an approximately normal distribution. The highest frequency of caught fish was observed in the 34-36 cm length class, representing approximately 24% (n = 10) of the total sampled population.
Discussion and Conclusion: The present study showed that the sampled individuals of P. chrysophrys in Hormozgan waters were generally smaller than those reported for this species in previous studies from the Persian Gulf, Oman Sea, and Arabian Sea. The maximum observed total length in the present study was lower than previously reported values, and the dominant length class was 34-36 cm, whereas larger size classes were more abundant in earlier reports from neighboring regions. These patterns may suggest a temporal shift toward smaller individuals in the exploited stock. Several factors may contribute to such differences in size structure, including habitat conditions, food availability, environmental variability, and fishing pressure. However, given the relatively acceptable Fulton’s condition factor observed in the present study (mean K = 1.19), the reduced size of the sampled fish is less likely to reflect poor nutritional or environmental conditions alone. Instead, the observed pattern is more likely associated with increased fishing pressure and harvesting intensity in recent years, which may selectively remove larger individuals from the population. In addition, if the reported length at first maturity is applicable to this species, a considerable proportion of the sampled fish may have been below maturity size; however, this inference should be confirmed with direct maturity assessment. If confirmed by broader sampling, this pattern could have important implications for stock sustainability, because persistent removal of immature fish may reduce reproductive output and limit stock rebuilding capacity. The negative allometric growth pattern observed in this study (b < 3) further indicates that weight increased at a lower rate than length in the sampled individuals. Although this pattern may reflect species-specific growth characteristics, it may also be influenced by environmental conditions, season, sample size, and fishing selectivity. Therefore, caution is needed when comparing the present estimates directly with those of other regions or time periods. Overall, the results indicate that P. chrysophrys in Hormozgan waters is characterized by relatively small body size, a dominance of intermediate-to-small length classes, and acceptable somatic condition. These findings provide preliminary evidence of possible size reduction in the exploited stock and highlight the need for continued monitoring of size structure, maturity composition, and fishing pressure. Future studies based on larger sample sizes, multiple seasons, and broader spatial coverage are recommended to confirm these patterns and support effective management of this commercially valuable species in the Persian Gulf.

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References

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