Evaluation of the large pelagic fishes (Scomberidae family) for optimum exploitation level in the Persian Gulf

Reproductive biology, Diet and population dynamics parameters of narrow-barred Spanish mackerel, Scomberomorus commerson, in the Persian Gulf were studied during 2011-2012. Fork length frequencies were collected from the commercial catch and from the current fishery nets (gill nets with 9 and 14cm mesh size) at the landing places. During 5 periods in October, December, March, April and July 20-40 specimens were also purchased from the fishermen at several landing sites to investigate and recording of data for feeding and reproductive conditions. Fork length (FL) of 2742 fishes was recorded to the nearest cm and weight of the specimens were measured to the nearest 0.01kg. FiSAT program was used to assess growth and mortality parameters. Reproductive maturity stages were assessed macroscopically using a five element scheme based on gonad size and appearance. The numbers of ovaries were fixed and preserved in 10% formalin. Central sections were embedded in wax, sectioned Hematoxylin–Eosin stains. For this subset, gonad maturation was categorized histological using a simplified five-stage description scheme and compared with the macroscopic assessments for validation. A Gonosomatic index (GSI) was also calculated for each fish. Feeding condition results showed that, sardines are the major prey of S. commerson. Pony fishes, Haltbeak and Indian mackerel were observed in the stomach content. These preys can be assumed as a secondary or accidental food items. Liver Somatic Index (HSI) was maximum in April and minimum in July. The annual instantaneous rate of fishing mortality (F= 0.42 year^−1) was considerably greater than the target (Fopt= 0.28) and limit (Flimit= 0.37) biological reference points, suggesting that the stock is heavily overexploited. Feeding and nutrition results showed that sardines are the major items prey of king S. commerson. Ponyfish, haltbeak and Indian mackerel fishes were observed in diet that can be considered as secondary or accidental foods. Maximum and minimum of Kn index was in October (9.3) and in March respectively. Liver Somatic Index (HSI) was the highest rate in February (1.70) and the lowest in July (0.85) that indicated on the worst and the best condition of fish in the mentioned periods respectively. No difference was observed in the weight stomach index and weight index (SI) in two sexes and it was about 0.09. The smallest of fish was observed 17cm FL and the biggest was 152cm FL. The mean size of fork length frequency was observed from maximum108cm (SD= 24) in February to minimum 29.4cm (SD= 5.5) in September. Based on growth curve analysis, growth parameters, K and L∞ were estimated 0.23year^−1 and 156.45 cm respectively. Instantaneous total mortality (Z), was 1.13 year^−1 . The estimate of M was 0.43 year^−1 and thus, the estimate of F was0.7 year^−1 . This translates to an exploitation rate (F/Z) of 0.69 year^−1 . Target (Fopt) and limit (Flimit) biological reference points were calculated 0.17 year^−1 and 0.22 year^−1 respectively. The size at capture at a probability of 0.25 (L25), 0.5 (L.5) and 0.75 (L.75) was35.84 cm, 40.28cm and 61.98 cm respectively. Tmax was calculated as 3/K equal to 6 years. Matured females (Stages III, IV) were observed between mostly between April to July. By July, most fishes were in ripped and spent stages (StagesIV, V) indicating the end of the spawning season. The result of GSI avtivity in 153 male and female fish indicated the higest reproductive activity from April to July with the peak of July. The present study results shows that the highest of catch rate were done in 1 and 2 years olds of fish.Although exploitation rate have not indicated over fishing but Fopt and Flimit rates are less than fishing mortality that shows overexploitation was happened. Changes of the mesh size in thegillnet of 9cm and the size of the nets should be considered. Banning of the king mackerel catch in the June and July that is the spawning peak period can be helped to the brood stocks. Iranian Fisheries Science Research Institute Published