Length-weight relationship of the most landed pelagic fish species European pilchard (Sardina pilchardus Walbaum, 1792) and European anchovy (Engraulis encrasicolus Linnaeus, 1758) in the Izmir Bay (Aegean Sea, Turkey) purse seine fishery

Length-weight relationships (LWR) of the most landed pelagic fish species Sardina pilchardus Walbaum, 1792 and Engraulis encrasicolus Linnaeus, 1758 in the Izmir Bay purse seine fishery were determined to reveal latest situation. Purse seine is a non-selective fishing gear compare to the other fishing gear such as gillnet or trammel net. For this reason, sampling all size individuals is very important to calculate mean length and other LWR parameters. In this study, seasonal LWR coefficient and minimum-maximum lengths were established as monthly basis. LWR of S. pilchardus and E. encrasicolus were W = 0.0059L2.7930 (r2 = 0.94) and W = 0.0019L3.4207, (r2 = 0.87), respectively. Growth type of the S. pilchardus was found negative allometric whereas E. encrasicolus was positive allometric. A decrease of the mean total length of S. pilchardus has been considerable variable from 1994 to 2014 in Izmir Bay but with this study, it is observed that mean length of the sardine found near of 2006 value related to seasonal fishing pressure. Please cite this paper as follows: Şenbahar, A. M., Güleç, Ö., Tosunoğlu, Z., Özaydın, O. (2020). Length-weight relationship of the most landed pelagic fish species European pilchard (Sardina pilchardus Walbaum, 1792) and European anchovy (Engraulis encrasicolus Linnaeus, 1758) in the Izmir Bay (Aegean Sea, Turkey) purse seine fishery. Marine Science and Technology Bulletin, 9(1): 32-37. * Corresponding author E-mail address: a.mertsenbahar@gmail.com (A. M. Şenbahar) Şenbahar et al. (2020) Marine Science and Technology Bulletin 9(1): 32-37


Introduction
The length-weight relationship (LWR) is an important tool in fish biology, physiology, ecology and fisheries assessment (Oscoz et al., 2005) and also, provide invaluable information on stock assessment studies (Moutopoulos and Stergiou, 2002;Gonzalez Acosta et al., 2004) for conversion of length observations into weight estimates to provide some measurements of biomass (Froese, 1998;Gonzalez Acosta et al., 2004).
Purse seine fishery is especially important for the Turkish fishery since it is the most important gear that targets small pelagic species especially anchovy and sardines as well as big pelagic species such as tunas. Once a fish school has been detected and surrounded by the purse seine net, there is no selectivity for individual size, species or catch quantity (Handegard et al., 2017). The catch quantity of a purse seiner is too much to compare with other fishing gears (e.g. trawls, seines). However, scientific studies on this fishing gear and method are quite limited in Turkey (Özbilgin et al., 2015).
Landing coming from purse seine accounts for about 30% of the world's total catch (Watson et al., 2006). Vast majority marine fish landing (approximately 60-70%) achieved by purse seine in 2018 fishing season (TurkStat, 2019). According to the official catch records, anchovy is the most landed fish species in Turkey with 96452 tons (43%). Although sardine landing is only 8.5% in Turkey, this value is substantially higher for the Aegean Sea (67%). Anchovy (12969 tons) and sardine (12654 tons) are the most landed pelagic fish species in the Aegean Sea (TurkStat, 2019). However, anchovy landing was the first time recorded higher than the sardine's in 2018 in the Aegean Sea. So far, a few studies conducted to determine the LWR of S.
pilchardus and E. encrasicolus with 10-year intervals (Hoşsucu et al., 1994;Özaydin and Taskavak, 2006;Acarli et al., 2014). For this reason, the purpose of the study is to reveal the current LWR parameters and compare it with the previous studies.

Material and Methods
During the study, a total of 567 of S. pilchardus and 212 of E. encrasicolus individual sampled in seven months. All the materials obtained from the monthly purse seine operations between September 28, 2017 and March 21, 2018 from Izmir Bay ( Fig. 1) in depths between 26 and 60 m. The purse seine net used by the commercial purse seiner Afala 24 m LOA is overall 750 m in length, 164 m net in height and 14 mm mesh size. Purse seine is a non-selective fishing gear compare to the other fishing gear such as gillnet or trammel net. For this reason, sampling all size individuals is very important to calculate mean length and other LWR parameters.
In this study sampling was made only for seven months (three seasons) due to the 4/1 notification regulates commercial fishery by the Ministry of Agriculture and Forestry of Turkey. According to the regulation, there was a closed season for purse seine fisheries between 15th April and 31st August in Turkish waters. In the analysis of LWR, monthly data was converted to seasons and seasons converted to the total value. Final estimations made on the total values.
Total length (TL) of all individuals were measured to the nearest centimeter (cm), and wet weight (W) was recorded to the nearest gram (g). The functional relationship between the size and weight of the samples were fitted to the equation: W= aL b , where W is the wet weight in grams, L the size in centimeters, a and b are the parameters to be estimated, with b being the coefficient of allometry (Ricker, 1975). The basic statistical data of the measured values were calculated and the relationships between them were determined (Sokal and Rohlf, 1973). Additionally, t-test was used for carried out to determine if the b coefficient was different from "3" (Sokal and Rohlf, 1969).

Results
The overall mean length of the S. pilchardus was found 12.1 cm. However, vast majority of the sardine individuals (91%) accumulated between 11.0 and 14.0 cm (Fig. 2). It was found that there was no significant allometry coefficients of LWR among seasons (Table 1) and also, the LWR curve of the S. pilchardus has shown in Fig. 3. The estimated total value of b coefficient indicating negative allometric growth (b=2.79; ttest, t<t 0.05 , n>500 =1.65) (Table 1). Furthermore, the r 2 values of S. pilchardus indicated a strong relationship between length and weight as 0.94.  Note: SE is the standard error.  Weight (g)

Total length (cm)
According to length-frequency distribution, mean length of the E. encrasicoulus was found as 11.4 cm and vast majority (87%) accumulated between 10.5 and 14.0 cm (Fig. 4). Allometry coefficient of the seasonal LWR parameters estimated and have been found for every season (Table 2). In detail, b value of the E. encrasicoulus was found for months as 3.1460, 3.2584, 3.4370 and total as 3.4207, respectively (Table 2) and these values are indicating positive allometric growth (b=3.42; t-test, t>t 0.05 , n>200 =1.65) (Fig. 5). Also, r 2 values of E.
encrasicoulus shown a strong relationship between length and weight as 0.87.

Discussion
Overall results of S. pilchardus and E. encrasicolus showed dissimilarities in total length (TL) and mean length based on sampling sites (Table 3 and Table 4). So far, TL of S. pilchardus has been shown a great variety in the Aegean Sea. However, maximum total length value of S. pilchardus reported from Izmir Bay by Hoşsucu et al. (1994) as 17.0 cm and it is still maintaining validity. In Izmir Bay, prior records indicating that the mean length of European pilchard has been reported as 14.2 cm by Hoşsucu et al. (1994), 11.82 cm by Özaydin and Taskavak (2006) and 9.39 cm by Acarli et al. (2014). In the results of this study, total length distribution of S. pilchardus between 9.5-15.3 cm. For the estimation of the mean length values of E. encrasicolus distribution range has been reported as 9.95 cm by Acarli et al. (2014) and 12.09 cm by Özaydin and Taskavak (2006). In this study, the mean length found as 11.4 cm and it has been shown similarity and also, it has been found as a medium value of these results.
Furthermore, the reported results of the b coefficient, which show different types of growth, such as isometric and allometric growth depending on different sampling areas are notable. As a short note, the b value is useful in explaining the body shape (growth type) according to the conditions in which the fish is present. If this value is equal to "3" it is called isometric but if it is a different value than "3", then it is called allometric growth (Ricker, 1975;Sparre et al., 1989;Sparre and Venema, 1992;Avsar, 2016). So far, many studies of S. pilchardus indicating allometric growth and only one study reported as isometric growth such as Torres et al. (2012). As it seems in Table 3, there were differences between allometric growth. So that, Mendes et al. (2004), Petrakis and Stergiou (1995) and this present study results has been shown negative allometric growth. Otherwise, the rest of them has been indicated positive allometric growth.
Comparison of the reported values of E. encrasicolus shown that all researchers have been agreed on the allometric growth of this species. However, growth type of depending on b value have a variety among conducted studies. Such that, Sinovčić et al. (2004), Karachle et al. (2008), Veiga et al. (2009), Torres et al. (2012 and this present studies b value indicating positive allometric growth. On the contrary, other studies has been shown negative allometric growth (Table 4). Length-frequency distributions and b value is directly associated to the fishing gear and method. While gillnets/trammel nets are shown higher selectivity for sardine related to mesh size and mesh shape, selectivity of the purse seine bunt is so poor that even very small sizes of juveniles are not selected. For instance, Torres et al. (2012) reported an unusually isometric growth of S. pilchardus with bottom trawl and this shows us the importance of sampling method. Also, except for the method, there are so many contributing variables (feeding, reproduction and temperature of the habitat that fish population live, etc.) to the effect of change of b value. Izmir Bay is also known as an important spawning and nursery ground for several fish species, mainly because of lagoons which serve as sheltered habitats and the input of nutrients from the Gediz River (Özaydın and Taskavak, 2006). So that, sampling sites that fish caught is also an important variable to establish the b value, even in the Izmir Bay.

Conclusion
S. pilchardus and E. encrasicolus are highly demanding and invaluable fish species for human consumption as well as fish meal and oil industry in worldwide and also in Turkey. We believe that this study will contribute to understanding the changing of the populations of S. pilchardus and E. encrasicolus in Izmir Bay. A decrease of the mean total length of S. pilchardus has been considerable variable from 1994 to 2014 in Izmir Bay but with this study, it is observed that mean length of the sardine found near of 2006 value related to seasonal fishing pressure.