Reproductive biology of endemic sailfin silversides fish Telmatherina antoniae Kottelat, 1991 in Matano Lake, South Sulawesi

Sailfin silversides (Telmatherina antoniae) is an endemic species from Matano Lake, South Sulawesi. This study aims to learn about sailfin silversides’ reproductive aspects including sex ratio, length at first maturity, gonad maturity stage, gonad maturity index, fecundity, and egg diameter. Fish samples were taken using multifilament nets from March 2018 until February 2019. A total of 1,792 individuals fish comprises 922 males and 870 females were captured during the study period. The sex ratio was found in a balanced condition with 1.06:1. Length at first maturity for males and females were 48.08-57.73 mm and 56.78-68.55 mm, respectively. Based on sailfin silversides’ gonad maturity stage and maturity index are thought to be year-round spawning with a peak spawning season around March. The reproductive potential of sailfin silversides fish is estimated to be 179–1,690 eggs. Based on the egg diameter distribution, the sailfin silversides fish is classified as a partial spawner.


Introduction
Sulawesi is one of the largest islands in Indonesia that is included in the Wallacea area, which is a transitional area between the Oriental and Australian regions, so it has a fairly high level of biodiversity and endemicity [1].All fish native to Sulawesi are brackish water that can adapt to fresh water.Some fish species live confined to lakes, while others migrate between land and sea waters [2].Based on the amount of endemic fish species biodiversity in Sulawesi, around 27 species live in the Malili Lake system, South Sulawesi [3].
Matano Lake is one of the oldest lakes in the Malili Lake system, South Sulawesi Province.Matano Lake is located at 382 meters above the sea level, with a length of 31 km and a width of 6.5 km.Matano Lake has an area of up to 164 km 2 with a maximum depth of 590 m [4].The water mass of Matano Lake is isothermal with a water brightness level of 25 meters and this lake belongs to the crypto depression lake, which means the bottom of the lake is deeper than the sea level [1].
Sailfin silversides (Telmatherina antoniae) is a species of fish from family Telmatherinidae that lives in the waters of Matano Lake [5].It has two morphotypes: slender body shapes less than 55 mm and larger body shapes more than 60 mm.Male individuals have brighter body colors than female individuals, and male individuals have various colors between blue, yellow, and blue-yellow (bluish 1260 (2023) 012003 IOP Publishing doi:10.1088/1755-1315/1260/1/012003 2 body and yellow fin) [6].Sailfin silversides is listed as a species that is Near Threatened by the IUCN in 2019 [7].
Sailfin silversides currently is not used for fish consumption or marketed.However, common problems that affect the existence of sailfin silversides are human activities around the lake such as the construction, expansion of settlements, land conversion, and aquaculture activities that affect the lake ecosystem indirectly.In addition, there were also introductions of alien species such as hybrid flowerhorn cichlids [8], nile tilapia (Oreochromis niloticus), common carp (Cyprinus carpio), broomsticks (Hypostomus plecostomus), and others, that are thought to endanger the population of this endemic fish [1].
This study aims to learn about sailfin silversides' reproductive aspects including sex ratio, length at first maturity, gonad maturity stage, gonad maturity index, fecundity, and egg diameter.

Data collection
The fish collection was held monthly from March 2018 until February 2019, and the fishing time was in the morning from 07.00 to 10.00 Central Indonesia Time.Fish was caught using multifilament nets 30 m in length, 2 m wide, and 0.5 inches in mesh size.Sample analysis was performed at the Laboratory of Macrobiological I, Department of Aquatic Resources Management, Faculty of Fisheries and Marine Science, IPB University during April-June 2021.
The laboratory works started with fish length measurement using a Vernier caliper at 0.01 mm accuracy and weight the total body mass using digital scales PW254 type digital ADAM at 0.0001 g accuracy.In the following, the fish was dissected using a dissection kit and observed for the testis or ovarium, depending on the fish's sex.Each gonad was weighted using the same scales before conducting gonad maturity stages, referring to [5].The gonads with gonadal maturity stage (GMS) IV were separated and placed in a petri dish for observation in the Nikon SMZ745T stereo microscope.Then the gonads are teased out, and eggs are spread over the petri dish before the egg number is calculated using a census method assisted by a hand-tally counter.The diameter of the egg was observed by taking 50 eggs each from anterior, middle, and posterior parts of the females' gonad with GMS IV.Measurement of egg diameter was made under the microscope objective of Nikon with 4x10 magnitudes.

Analysis data 2.3.1. Sex ratio.
The sex ratio is the comparison between total amount of male to female fish.The sex ratio was calculated using the following formula [9]: The amount of male fish (ind) F = The amount of female fish (ind).
The sex ratio was tested using the chi-square test to determine the relationship between male and female fish from a population under study.Here is the chi-square formula [10]: = value for a random variable whose sample distribution is close to the chi-square o i = The number of observed frequencies of male and female fish e i = the number of expected frequencies of male and female fish.

Length at first maturity.
The length at first maturity is used to estimate the average size of fish that undergo mature gonads.The method used is the Spearman-Karber method with the following formula [11]: The length range of the fish's first maturing gonads can be calculated by a 95% confidence interval, with the following formula:

Gonad maturity index.
The gonadal maturity index was calculated using the following formula [12]: Description: GMI = Gonad Maturity Index (%) Wg = Gonadal weight (g) Wt = Total weight of fish (g) 2. 3.4.Fecundity.The reproductive potential was analyzed based on the frequency or number of eggs that the female fish can produce in a single spawning cycle.Sailfin silversides fish fecundity was calculated by the census method [13].An analysis of the fecundity relationship between the total length and body weight of fish was presented using the following equation [14]: Description: F = Fecundity (eggs) X = Total length of fish (mm) and body weight of fish (mm) a and b = Constants.

Egg diameter.
The spawning type can be estimated based on the number of modes obtained in the egg diameter distribution [15].The egg diameter data of GMS IV is converted into millimeters (mm) by multiplying the data by 0.025.The converted data are grouped into class hoses each class has a frequency of egg count [16].

Sex ratio
A total amount of 1,792 individuals fish comprises of 922 males and 870 females were captured during the study period.The sailfin silversides fish has a 1.06:1 sex ratio respectively.Based on a temporal chart, the value does not change significantly every month.However, in August 2018, the number of fish increased and decreased until October 2018.The sex ratio graph displayed in Figure 2 based on length-frequency distribution, shows that the highest-value sex ratio is present in the interval 59.77-64.93mm.In intervals, 75.28-80.44mm; 80.45-85.61mm; and 95.96-101.12mm have a zero value because there are only male fish.Meanwhile, the interval 85.62-90.78mm has a value of 0 (zero) because there are no male or female.An increase in the value of the sex ratio indicates that the presence of male sailfin silversides fish is dominant over female, while a decrease in the sex ratio indicates otherwise.The sex ratio of sailfin silversides was 1.06:1 with 51.45% and 48.55%, respectively.The Chi-square test with 0.05 probability level showed that the sex ratio of sailfin silversides was balanced (χ 2 count < χ 2 .This is in contrast to [5], which states that the chi-square test in sailfin silversides fish sex ratio (T.antoniae) indicates an unbalanced condition.The sex ratio of T. sarasinorum [17] also showed an unbalanced state.T. Prognatha also shows an unbalanced state [18].The difference in sex ratio can be caused by several factors, including abundant food availability that is used by the female fish, the difference in growth rate [19], and the mortality between male and female fish [20].Balanced conditions generally occur in more stable environments with minimal human influence [21].The sex ratio is said to be balanced at a 1:1 comparison value, as the balanced sex ratio condition indicates the stability of the fish population in nature.When fish is not balanced, population development will experience obstacles leading to a shrinking number of fish recruitment [22].

Length at first maturity
Length at first maturity (Lm) for male sailfin silversides is 52.69 mm with Lm (95%) between 48.08-57.73mm.For the females, Lm was at 62.39 mm with Lm (95%) between 56.78-68.55mm.This indicated that the mature males were smaller than the females.This is in contrast to the study of [5], that female sailfin silversides (total length = 70 mm) undergo an earlier gonad mature phase than male fish (total length = 77 mm).The length at first maturity is thought to be influenced by the physiological properties of fish, age, behavior, distribution, and environmental conditions [21].Even in the same species, the length at first maturity can be different because of environments and geographical conditions [22].

Gonad maturity stage
The gonads maturity level is based on the percentage of GMS I to GMS V.The value is distributed evenly almost every month.The relative percentage value of male fish with GMS IV was more dominant in December 2018 at 38.67%.According to the percentage of female fish, GMS IV was also the most dominant found in December 2018 with a percentage of 45.78%.The maturity of the gonads is the aspect that is used to analyze the biology of reproduction.GMS is the stage of the development of the gonads going from before to after spawning [23].Based on gonads maturity, sailfin silversides with both male and female GMS IV are found almost year-round with a dominant percentage found in December 2018.Sailfin silversides with GMS V are almost found throughout the year, except for male silversides in March 2018.The percentage of GMS V found indicates that sailfin silversides fish spawn these months [24].This is similar to the Marosatherina ladigesi, which has GMS V every month and indicates that the fish has spawn every month [25].The mature condition of gonads in fish is influenced by the availability of food and supportive conditions will trigger spawning in fish [26].Changes in environmental conditions such as changes in temperature conditions to dissolved oxygen level can affect fish's response to the environment and affect fish metabolism.Environmental conditions can also affect the somatic and reproductive growth in fish and can promote the maturation of gonads based on [27] and [28].

Gonad maturity index
The highest males GMI rating was found in March 2018 at 0.99.The lowest gonad maturity index was shown in February 2019 at 0.37.The females sailfin silversides maturity index with the highest GMI score was in September 2018 at 4.79.The value of the maturity index was the lowest in May 2018 with a value of 2.92.The gonad maturity index is a comparison of gonad weights with body weights used in general to analyze the reproductive cycle [29].GMI with a high value indicates a peak of gonad maturity followed by increasing body weight, and the value will decrease when the fish has gone through the spawning process based on [30] and [31].Based on the GMI of fish in the males fish was high in March 2018, June 2018, September 2018, and January 2019.Based on the value of the GMI of females fish it was high in March 2018, September 2018, and November 2018.The fluctuating value of GMI in each month indicates that it has a spawning type that occurs throughout the year [32].According to the presentation of GMS IV with GMI, it is suspected that sailfin silversides have been spawned year-round and the peak of spawning occurs from September, December, to March.This is supported by high fish condition factor values in January and March in male and female fish [33].The high value of condition factors is influenced by the development of gonads and indicates peak spawning [34].The spawning peak is thought to occur during the low levels of the lake's water, as the spawning process is a strategy of sailfin silversides fish in the hope that the eggs will hatch during the rainy season [5].

Fecundity
The fecundity of sailfin silversides fish is taken from 50 females with GMS IV.The number of sailfin silversides eggs ranged from 179-1,690 eggs.The average number of eggs found was 650±270 eggs.Based on the regression results in the relationship of total length to fish fecundity, a low value of determination coefficient (R 2 ) is obtained, resulting in the fact that female sailfin silversides fish fecundity is less closely related to the total length.This is similar to featherback fish with a low coefficient of determination (R 2 = 0.06) in the relationship between fecundity and total body length [35].Based on the results of the regression of body weight relationships with fecundity, a low value of the coefficient of determination (R 2 ) is obtained, but the value is larger than the total length.Low coefficient values due to the increase in fish size tend to have no significant effect on the fecundity, and thus cannot be used as a prediction of the fecundity of the fish based on [27] and [5].In general, fecundity is defined as the number of eggs found in female fish shortly before they are fertilized [36].Variations in fish fecundity are thought due to length and body weight, but fecundity can also be affected by environmental conditions and food supplies [35].Various environmental conditions can affect fish fecundity directly or indirectly, such as the mass amount of fish, an abundance of food, environmental temperature, and fish population density.A year-long calculated frequency can provide an overview of how fish respond to a combination of biological factors with environmental factors [37].

Egg diameter
Based on the analysis using 50 fish samples, the average egg diameter of the sailfin silversides fish was 0.59±0.30mm.Based on the results of frequency distribution analysis, there are two distinct peaks of egg diameter, the first 0.24-0.34mm class with 1,330 eggs, and the second peak 0.87-0.97mm class with 1,622 eggs.The spawning frequencies can be estimated from the distribution of egg diameter that is ready to fertilize.GMS IV fish have developed eggs, larger in diameter and ready to be spawned [38].The size of the egg's diameter will get larger followed by the higher maturity of gonads [39].Based on the distribution of the egg diameter, it shows that fish with GMS IV has two distinct peaks This suggests that sailfin silversides fish have a partial spawning type, which means fish will either fertilize gradually or fertilize more than once in a spawning season [16].Partial spawning is very common for the family Telmatherinidae such as Telmatherina celebensis and Telmatherina ladigesi [40] Partial spawning has advantages for fish, including the availability of fish stocks in the waters and longer spawning times due to different egg diameter sizes so that they are not released at the same time [41].Fish diameters can be affected by the diversity of fish species because they can affect the availability of food in nature [42].
length at first gonad maturity Xk = Logarithm of the middle value of the last long class of fish that has matured gonads X = Logarithmic difference of middle-grade value P i = N b /N i N b = Number of fish ripened gonads on the i-th grade N i = Number of fish in the I-th class Lm = Range of length at first maturity gonads.

Figure 2 .
Figure 2. Sex ratio of sailfin silversides based on (a) temporal and (b) length-frequency distribution.

Figure 4 .
Figure 4. Gonad maturity index of (a) males and (b) females fish.

7 Figure 5 .
Figure 5. Relationship between fecundity of fish with (a) length and (b) weight.