The decent catch size of mackerel tuna (Euthynnus affinis Cantor, 1849) and its implication to fisheries management in the border area of Indonesia and Timor Leste

A study of mackerel tuna (Euthynnus affinis) has been conducted in Ombai Strait in the border area of Indonesia and Timor Leste. The objective of this study is to determine the decent catch size of mackerel tuna in Ombai Strait, based on the length at first caught (Lc), the length of first mature (Lm), and prediction of spawning season. Monthly data collection conducted in May 2018 – June 2019 from fish landing sites in Belu, East Nusa Tenggara, Indonesia. There were 1516 fish collected to measure the total length and gonadal maturity level. The results showed that the size of the mackerel tuna population in the Ombai Strait was between 131 - 450 mmTL. The length at first matured is 210,9 mmTL and the average length at first caught (Lc50) is 152,1 mmTL. Length of first matured was larger than length at first caught (Lm > Lc). The decent catch size is 211 mmTL and the prediction of spawning season in May – June. These results indicate that it is necessary to regulate selective fishing gears to ensure the sustainability of mackerel tuna resources in Ombai Strait.


Introduction
The Ombai Strait is a portion of Indonesia and Timor Leste territorial seas.The waters of the Belu Regency are included within the seas of the Ombai Strait.Atapupu is the location of the catch fisheries center or potential fishing base in Belu Regency.Small and large pelagic fish are landed at Atapupu fish landing site.The tuna, skipjack, and mackerel tuna (TCT) resources in the Indonesia and Timor Leste border region are exceptional resources [1].Mackerel tuna or kawakawa (Euthynus affinis) is a type of huge pelagic fish that is nearly caught year-round in border seas.Mackerel tuna are schooling, swift swimmers, and carnivorous (carnivores).Mackerel tuna is also a large pelagic fish that plays an important function in aquatic environments as a carnivore since it has a significant impact on the ecological equilibrium [2].
Mackerel Tuna has a high animal protein content and is frequently consumed by the public, mackerel tuna has significant economic importance in the region between Indonesia and Timor Leste.Payang, 2 mini purse seine, drift gillnet, and hand line are a few of the popular types of fishing gears used by fishermen to catch mackerel tuna.Data from the Belu Regency's Fisheries and Marine Statistics show a rise in tuna mackerel fishing since 2012 of 11.7 tons to 175 tons in 2017 [3].Continuous mackerel tuna fishing operations have the potential to influence the state of tuna mackerel fish resources in the border area of Indonesia and Timor Leste in the Ombai Strait as well as their availability.Therefore, it is important to make a management effort in order to guarantee the sustainability of mackerel tuna resources.Information on fish biology is beneficial to the management of mackerel tuna resources in order to realize the sustainable use of mackerel tuna resources.The objective of this study is to determine the decent catch size of mackerel tuna in Ombai Strait (Euthynus affinis) in the border area of Indonesia and Timor Leste in the Ombai Strait.

Study area
The research was conducted from May 2018 -April 2019.The locations of data collection of fish caught by fishermen are fish landings at Jenilu, Dualaus, Silawan and Kenebibi Village in Belu Regency, East Nusa Tenggara Indonesia (Figure 1).

Sampling procedure
During the study, the collecting of mackerel tuna samples totals 1516 individuals from any proportion catch of mini purse seines (lampara), drift gillnets, and hand lines fisheries in each month.Sampling was taken continuously every month from May 2018 to April 2019 with measuring the total length of samples and observing of fish gonad to determine of sex and Gonad Maturity Level (GML).Sex is known by dissecting and observing the morphology of the fish gonads.Determination of GML was carried out based on observations of the morphological characteristics of gonad maturity based on Cassie (1956) in Effendie [4].

II
The size of the ovaries is greater, the color of the ovaries is yellowish, and the eggs are obscured.
The of the testicles has increased, and their coloring resembles milk.

III
The ovaries are yellow and eggs are evident morphologically.
The testes' surface seems jagged, their color is becoming whiter, and their size is increasing.

IV
The ovaries are expanding, the eggs are yellow and easy to separate, the oil grains are no longer visible, and the abdominal cavity is half to two-thirds full.
It is fragile in a preserved form, and the testes are becoming stiffer.

V
The ovary is wrinkly, its walls are thick, and the remaining eggs are close to being released.
The testes in the rear are deflated, however the portion near the release remains swollen.

Data analysis
The sex ratio was determined using the following formula discovered by Effendie [4] as equation ( 1): where p is the proportion of male or female fish (%), n is the number of male or female species, and N is the total number of male and female fish present.Analysis of the sex ratio of fish using the test reveals the relationship between males and females in a community Chi-square ( 2 ) in Steel and Torrie [5] as equation ( 2): where oi is the total number of observed frequencies of male and female fish, ei is the sum of anticipated frequencies of male and female fish, and 2 is the value for a random variable whose sampling distribution follows a Chi-square distribution.
Estimation of the average length of the first matured (Lm) using the equation with the formula [6]: where, L∞ is the theoretical maximum length (asymptotic length) from von Bertalanffy growth formula.
The value of L50 is used to calculate the average length at first captured (LC) estimate.The 50% point was chosen due to the fact that the logistic curve linked with the ogive selection curve has an inflection point at 50%.According to Sparre and Venema [7], to compute the SLest value with the following equation ( 4) in order to create the curve.
In addition, to estimate the average size of the first fish caught (LC) with the equation (5) (6) (7) (8): where, SLest is the estimated value; S1, S2 is a constant, S1 is the value of the intercept (a) and S2 is the value slope (-b).The procedure of estimating the value of LC is carried out with the program software FISAT II (FAO-ICLARM Stock Assessment)-ELEFAN [8].

Result
Based on the result of the research, the length distribution of mackerel tuna captured in the Ombai Strait ranges from 131 to 450 millimeters total length (mmTL), with a mean length of 219 mmTL.Sex ratio is the percentage of male to female fish.Figure 2 depicts the predominance of females in the population of mackerel tuna in the border area of Indonesia and Timor Leste in the Ombai Strait waters.The research done between May 2018 until April 2019 observed and determined totals 1516 mackerel tuna, including 494 males (32,6%) and 1022 females (67,4%).According to the findings of the statistical test analysis, there were more female mackerel tuna fish in the population than male ones (1 : 2.1) The sex ratio of mackerel tuna can be regarded to be out of balance as a result of this evidence of a sizable difference.During the study, mackerel tuna caught were dominated by GML I and II, which were immature gonads, about 70% of male and 88% of female.Only a small portion of the catch is in GML III and IV or is mature gonads.In May and June, the catch was dominated by GML II -IV, there was no GML I (Figure 3 and Figure 4).Length of first mature gonad (Lm) of mackerel tuna captured in the Ombai Strait were 210.9 mmTL and length at first caught (Lc) were 152.1 mmTL.These results show that length of first mature gonad larger than length at first caught (Lm > Lc).Based on this result analysis, the decent of catch size of mackerel tuna is 211 mmTL.

Discussion
The size of fish captured in the Ombai Strait is smaller when compared to research conducted in other areas.Kaymaram et al. [9] reported that the length of mackerel tuna captured in Iranian waters ranged from 280 mm to 880 mm, while the length ranged from 320 mm to 660 mm in North Andhra Pradesh, India.According to Motlagh et al. [10], this disparity in length is likely attributable to variations in fishing gear, ambient circumstances, and fishing effort.Mini purse seines, drift gill nets, and hand lines are utilized to catch mackerel tuna in the Ombai Strait.
Male and female mackerel tuna are equally distributed, although females predominate.The results of this study are comparable to the sex ratio between male and female mackerel tuna (1.2 : 1) along the West Coast of Sumatra in the Indian Ocean [11] and male and female tuna mackerel along the East Coast of Peninsular in Malaysia 1.6 : 1 [12].There are both internal and extrinsic causes of variations in sex proportions.External influences include the availability of food, population density, and the balance of the food chain, whilst internal factors might take the shape of fish behavior, variations in death rates, and growth [4].
Changes or phases of gonad maturity level (GML) will reveal when fish spawn, have just spawned, or have already spawned [13].The distribution of GML III and IV in male and female mackerel tuna (Figures 3 and 4) indicates that they can be discovered between April and June.It appears that the mackerel tuna breeding season occurs during the months of May and June.According to research conducted by Abdussamad et al. [14] in Indian seas, the peak of mackerel tuna spawning occurs between May and June.Due to differences in size and age of spawning fish in a body of water, the length of gonadal mature in each body of water varies.
The mackerel tuna seen during the research were smaller than the length of the first mature gonad and were dominated by fish with GML I and II.Therefore, it is likely that mackerel tuna collected in the Ombai Strait waters were subjected to growth overfishing, i.e., the fish were taken before they had the opportunity to grow and mature.According to a study conducted by Arnenda et al. [15], the Lm value in WPP 572 is 42.03 cmFL and the Lc value is 16 cmFL.In contrast, in a number of other locations, the Lc value was greater than the Lm value, such as the results of research by Rohit et al. [16] in Indian waters, where the fish captured for the first time were 41.43 cm longer than the Lm value or the length at first maturity of the gonads, which was 377 mm for male fish and 480 mm for female fish.The Lm value of Southeast Sulawesi's waters is 50 cmTL, which is higher than the first caught.According to Lagler et al. [17], the kind, age, size, and ability to adapt to the environment are the characteristics that impact the first-time gonadal maturity.If the average size of tuna mackerel gathered is smaller than the size of the first gonad maturity, then tuna mackerel management in the Ombai Strait will not be viable from a biological standpoint.
The high level of mackerel tuna fishing in the seas of the Ombai Strait will reduce the availability of mackerel tuna.Management plan that can be implemented include: Indonesia-Timor Leste cooperation for fisheries management in the Ombai Strait, determining the appropriate size of mackerel tuna capture in the Ombai Strait, regulating the selectivity of fishing gear by increasing the size of drift gill nets, and switching fishing gear to capture mackerel tuna during the spawning season in May and June.

Conclusion
The catch of mackerel tuna revealed that the fish measured between 131 and 450 mmTL.Young fish dominate the fish population, which has a mean size of 219 mm.The average length of caught was smaller than the length of the gonads at initial maturity (Lc < Lm).The decent of catch size is 211 mmTL to allow the fish to reproduce.In order to prevent the usage of drift gillnets and mini purse seines when fishing in the waters of the Ombai Strait, the spawning season, which is predicted to occur in May-June, may be prohibited.

Figure 3 .
Figure 3. Distribution of male mackerel tuna gonad maturity level during May 2018 to April 2019.

Figure 4 .
Figure 4. Distribution of female mackerel tuna gonad maturity level during May 2018 to April 2019.