SOME REPRODUCTIVE BIOLOGY OF SKIPJACK TUNA (Katsuwonus pelamis LINNAEUS, 1758) IN TOLI-TOLI WATERS, CENTRAL SULAWESI

Skipjack tuna (Katsuwonus pelamis) was one of the export commodities, where the demand and exploitation tend to increase, so sustainable fisheries management is needed based on biological data. The objective of the research was to study the several aspects of biology including of length of weight relationship, sex ratio maturity stage, gonada somatic index (GSI), length at first capture, and length at first maturity. The research was conducted in February December 2015 in Toli-toli, Central Sulawesi. The results showed that the skipjack tuna growth pattern was allometrically positive (b= 3,318 for male and b = 3.3049 for female), where growth weight was faster than the increased length. The length of the first time capture skipjack tuna of pole and line (43.49 cmFL) was bigger than length at first maturity (41,007 cmFL). It means the mostly skipjack tuna caught have already spawned. The spawning season occurred throughout the year with spawning peaks are in April and September, with fecundity ranging from 450,570 to 1,707,390 eggs.

Toli-Toli is one of the important fish landing bases in contributing to the skipjack tuna production of Indonesia. Based on Toli-Toli Regency Statistics 2014, total catch of this fish was 60,673 ton, comprising 69,81% tuna fish group and 30.19% of other fish species (pelagic small, demersal, and shrimp) (Anonimous,2014). Global catches on tropical tuna fish dominated by skipjack 47,91%, yellowfin 21,51%, and other tuna 0,40%. Recent data showed that global catch of skipjack in 2016, which was around 2.8 million tons, ranked at the third for sevenconsecutive years (FAO, 2018).
The products of skipjack tuna together with other fish products covered 40% of the world food trade (FAO, 2001 in Suhana et al., 2016) then increased to 52% in 2016 with prices remained low and consumer demand for end-products increased in many markets (FAO, 2018a). Market demand for skipjack tuna products continues to increase, thereby encouraging over-exploitation which is indicated by high fishing mortality above the maximum sustainable yield level in some areas (Miyake et al., 2010).
To fulfill market demand and sustainability of skipjack tuna resources, a management strategy based on biological aspect is needed. This biological aspect is the identity of population characteristics that are very important in applying rational resource use (Matsumoto et al., 1984;Jin et al., 2015) and factors related to migration trajectory which are important in implementing comprehensive management for sustainable exploitation (Kumar et al., 2019).
The purpose of this research is to know the biological aspect of skipjack tuna, including the relationship of length of weight, sex ratio, maturity stage of gonad, average length at first capture, and length of first maturity. The results of this study are tuna fish landed from the Sulawesi Sea. Sampling and observation of biological aspect (length, weight, and gonad maturity) of skipjack tuna samples were conducted at the Tandoleo Fish Landing Center, Toli-Toli, Central Sulawesi. Sample of gonads were preserved using 10% formalin solution and Gilson. Further sample analysis was conducted at the Laboratory of Research Institute Marine Fisheries, Cibinong Bogor. Determination of maturity stage follows the criteria of I to V (Holden & Raitt, 1974), listed in Table 1. Ovary and testis about 1/2 length of body cavity. Ovary pinkish, translucent.
Testis whitish, more or less symmetrical. Ovary not visible to naked eye.

III (Ripening)
Ovary and testis is about 2/3rds length of body cavity. Ovary pinkish-yellow colour with granular appearance.
Testis whitish to creamy. No trans-parent or translucent ova visible.

IV (Ripe)
Ovary and testis from 2/3rds to full length of body cavity. Ovary orange-pink in colour with conspicuous superficial blood vessels. Large transparent, ripe ova visible.

V (Spent)
Ovary and testis shrunken to about 1/2 length of body cavity. Walls loose. Ovary may contain remnants of disintegrating opaque and ripe ova, darkened or translucent.
Testis bloodshot and flabby.

Data Analysis
Length-weight data analysis is a standard method in fisheries stock assessment to describe fish growth (Kuriakose, 2014). The relationship between length and weight was described by Effendie (1979 The length-weight relationship that has been obtained was tested by t-test, that is testing the hypothesis: If t-count is smaller than t-table then H0 is accepted. If t-count is greater than t-table then H0 is rejected. Length-at-first-capture are among vital scientific information needed for the management and sustainability of a fishery (Udoh & Ukpatu, 2017). Length at first capture (Lc=50%) was fraction retained from fishing gear. The value of Lc is obtained from the length distribution examined by the formula (Sparre & Venema, 1992): where: SL est : logistics curve S1 and S2: constants Sex ratio can influence ecological and population conditions (Fryxell et al., 2015). The sex ratio was analyzed by comparing the number of male fish with female fish: where: X = sex ratio M = number of male fish observed F = number of female fish observed Sex ratio was tested by chi-square according to Sugiyono (2004) as follows: ..…………………………… (5) where: X 2 = Chi -Square Fo = observed frequency Fn = expected frequency With the hypothesis (H0): there is no significant difference between the number of male and female fish. If, ÷2 calculated < ÷2 tables, H0 is accepted, and conversely.
The figuring of length at first maturity was examined according to Spearman-Karber method with the following equation (Udupa, 1986 Where : M = logarithm length of the first gonadal maturity Xk = logarithm of mid length class where the fish was 100% mature. X = average logarithm of difference length class pi = proportion of mature fish in the-i class Gonad maturity is useful for estimating reproductive potential, spawning stock biomass, and recruitment potential (Yaragina, 2010). Visual stage maturity determination refers to the classification of Gonado Somatic Index (GSI) using the formula according to Effendie (1979): (7) where: GSI = gonado somatic index (%) Wg = weight of gonad (gram) Wt = total weight of fish (gram).
Fecundity is a m easure of reproductive performance, related to population density and environmental stochastic influences (McMahon & Bradshaw, 2008). The calculation of skipjack tuna fecundity is done by taking adult fish gonads that have reached TKG III and IV. Measurement of diameter size and number of eggs was done by using a magnification microscope 4 x 10. Observation of the number and distribution of egg size by using egg samples as much as 0.5 grams. Fecundity is calculated gravimetrically with the formula by Holden & Raitt (1974) 8) where: F = fecundity n = number of eggs in subsample g = gonad weight g = weight of sub sample gonad (0.5 gram).

Sex Ratio and Maturity Stages of Gonads
The number of female skipjack tuna measured during this study was 249 while the the male was 196 specimens. Based on the chi-square test, the female and male tuna ratio is statistically in an unbalanced state. Understanding sex ratio is related to maintaining the sustainability of fish populations (Budd et al., 2015;Fryxell et al., 2015;Madenjian et al., 2016;Benevenuto et al., 2017;Maskill et al., 2017;Provoost et al., 2017). Therefore, it is expected that the ratio of male and female fish is balanced. The balance of male and female condition allows the fertilization of eggs by spermatozoa to become new individuals have bigger chance (Effendie, 2002, Chan et al., 2012. This would also describe the status of its demographic structure within population. The results showed that the development of gonad maturity stage of skipjack tuna varied on monthly basis. Figure 3 showed the composition of gonad maturity stage. It shows phases classified into 5 stages of both male and female. Stage I to IV spread almost in every month. The gonad maturity of female skipjack tuna that had stage I was dominant in May (46.4%), stage II in June (58.33%), stage III in February (50%), and stage IV in August (53.8%). The gonad maturity of male skipjack tuna that had stage I was dominant in March (75%), stage II in October (62%), stage III in August (41.67%), and stage IV in July (50%). Most of the male and female tuna are caught in immature gonads. The highest of the gonad maturity stage IV of male skipjack tuna was found in July, while the female one was found in August.

Gonado Somatic Index (GSI)
Gonado Somatic Index (GSI) of female skipjack tuna ranged from 0.16% to 3.33% with an average of 1.31%. The highest index occurred in March and August then decreasing in each following months (Figure 4), suspected the spawning occurred in April and September.

Fecundity and Egg Diameter
Fecundity is the number of female fish eggs before they are released at the time of spawning. Fecundity in the ovaries can be morphologically known from eggs that have matured gonad (level IV). The fecundity of skipjack tuna in Toli-toli ranged from 450,570 -1,707,390 eggs in the fish length-range of 49-55.5 cmFL.
Based on the results of the analysis, the diameter of skipjack tuna (Katsuwonus pelamis) from Toli-toli waters obtained the distribution of eggs as presented in Figure 5. The size of skipjack tuna (Katsuwonus pelamis) in GSI IV was 0.22 -0.67 mm, at mode on the diameter of 0.42 mm. Figure 5. Eggs diameter of skipjack tuna (Katsuwonus pelamis) in Toli-toli.

Discussion
Length size structure of skipjack tuna in Toli-toli water was around 22-62 cmFL, with the mode at 42 cmFL. Anggraeni et al. (2015) found the same mode in Sadeng waters (Indian Ocean of Southern Java Island) with range of fish fork length were 20-68 cm. Tampubolon et al. (2014) stated that the fork length of the skipjack tuna captured by fishing lines and tents in the waters of the East Indian Ocean ranged from 35-68 cmFL with the mode at 50 cmFL. While according to Hidayat et al. (2014) skipjack tuna size caught by troll line at Banda Sea ranged from 20-70 cmFL with the mode 40-50 cmFL. The size of skipjack tuna caught between 20-60 cmFl with 40-50 mode were because these fish were caught around the FAD (fish aggregating device). It is stated by Fonteneau et al. (2000) that the skipjack tuna caught by purse seine in FAD mostly at size of 20-80 cmFL with mode 40-50cmFL, and Aprieto (1990) stated that the skipjack tuna caught by purse seine in FAD in the Philippine waters ranged from 20-60 cmFL with mode 45cmFL. This shows that the size of skipjack tuna caught in Toli-toli water was fish associated with FAD.
Analysis with t-test on parameter b at 95% confidence level (á = 0,05) found that t-count was larger than t-table, which means the length-weight relationship of male and female skipjack tuna were allometric positive, indicates the length increase is slower than the weight gain (Effendi, 2002). The results of several studies of this species were relatively similar, such as Anggraeni et al., (2015) in Sadeng waters (allometric positive); Manik (2007) in Seram Island and Nusa Laut waters (allometric); Nurdin (2017) in Pelabuhanratu waters (allometric positive). However, different result was showed by Jamal et al. (2011) in Bone Bay that the growth of skipjack tuna was isometric.
The variation of the value of b in the length-weight relationship shows that the growth can change over time. Furthermore, this difference can be caused by various factors, such as sea water properties and seasonal conditions (Gokhan et al., 2007in Karna et al., 2011. Gulland (1983) and Sparre & Venema (1992) suggested that the variation of b value can be caused by various factors such as temperature, salinity, food (quantity, quality and size), sexes, gonadal maturity stage, and habitat preservation.
The length at first maturity (Lm) of skipjack tuna caught in Toli-toli (41.01 cmFL) that was different from length at first maturity (Lm) has been documented in other areas. Tampubolon et al. (2015) stated that Lm of skipjack tuna caught in the East Indian Ocean occurred at 42.9 cm in the range of 41.6-44.3 cm. Mallawa et al., (2014) stated that skipjack tuna fish in Flores Sea have matured gonads at more than 51 cm in size and are ready to spawn at sizes greater than 55 cm in females and over 60 cm in males. Ashida et al. (2009) stated the length at first maturity skipjack tuna in the central and western Pacific Ocean was 47.9 cm. Jamal et al. (2011) found that the length at first maturity of this fish in Bone Bay waters was 46.5 1-10 cm.According to Lowe-McConnel (1995) and Moresco & de Bemvenuti (2006), the first maturity size of different gonad ripen is a fish reproduction strategy to restore the population balance due to changes in condition, abiotic factors, and more capture. Furthermore, Nasution (2004) stated the length at first maturitydifferencehas astrongassociationwith fishgrowth, environmental factors, and reproductive strategies.
Most of skipjack tuna caught by hand line and pole-and-line was larger than length at first maturity (Lc>Lm). This indicates that the hand line and poleand-line gear caught the mature fish, so skipjack tuna populations can recruit. Handline and pole-and-line gear can be maintained if there are additional efforts; these types of fishing gear are recommended. While the skipjack tuna caught by purse seine is mostly under length at first maturity or immature fish, so this type of fishing gear is not recommended.
From the development of the monthly GSI, the highest GSI occurred in March and August. According to Widodo (1986) in Mardlijah & Patria (2012), spawning season occurs about one month after the highest percentage of mature fish. So the spawning was estimated in April and September. These results are supported by previous research by Wagiyo & Rahmat (2013) stating that the peak of skipjack tuna spawning in the same waters occurred in April. The results of both studies showed that the peak of spawning occurred in the same season, although more detailed research is needed regarding the spawning season for skipjack tuna in the Sulawesi sea.

CONCLUSION
Skipjack tuna (Katsuwonus pelamis) in Toli-toli waters has a growth of weight was faster than the length increase. The skipjack tuna caught by pole and line and handline was mature fish, while from purse seine was immature. The spawning season is occur throughout the year with peak in April and September as well as fecundity ranging from 450,570 -1,707,390 eggs. For sustainable skipjack tuna resources the caught is better use pole and line and no caught in peak GSI and peak spawning season (March-April).

ACKNOWLEDGEMENT
UC has contributed mainly to this work. TH and KW are supporting contributor to this work. This paper is a contribution of research activities Stock Assessment of tunas -skipjack tuna in Sulawesi Sea, 2015, at the Research Institute Marine Fisheries, Cibinong Bogor. The author would like to thank the Head and staff of the Toli-toli District Fisheries and Marine Service for his assistance in the process of collecting research data.