Reproductive biology of Indian mackerel, Rastrelliger kanagurta (Cuvier, 1816) in the southern waters of West Java (FMA 573)

. Indian mackerel is a small pelagic fish with important economic value in Java Island. This study aims to analyze the reproductive characteristics and determine management recommendations for the Indian mackerel in the southern waters of West Java. Fish monthly sampling collection was conducted at three fish landing ports: PPI Ciwaru, PPN Palabuhanratu, and PPI Binuangeun from July to December 2022. A total of 1,038 fish samples, comprised of 582 males and 456 females, were examined. The sex ratio was unbalanced, with a value of 1:0.79. Gonadosomatic indexes ranged from 0.19-0.95 and 0.45-3.41 for males and females, respectively. The fish fecundity ranged from 27,027 to 87,837 eggs. The Indian mackerel is predicted to spawn every month, with a peak season in August, and performs a total spawner pattern. The Indian mackerel reached the size of the first maturity gonad at 198.5 mm and 197 mm for males and females, respectively. Management strategies for Indian mackerel in the southern waters of West Java include setting the catch size above the first mature gonad size (197 mm), the mesh size of gillnets above 1.5 inches, and limiting fishing activities during the spawning season (July, August, and September).


Introduction
Indian Mackerel, Rastrelliger kanagurta (Cuvier, 1816), is a small pelagic fish of the family Scombridae that lives schooling in marine waters at a depth of 20-200 m [1,2].Indian mackerel swim near the surface at night and move to deeper waters during the day, following the daily movement of zooplankton [3].Indian mackerel feeds on zooplankton such as shrimp larvae and fish [4].Indian mackerel have unique characteristics on the head and body that have a greenish color with black spots, while the lower part of the body is white or silvery gray.This fish has a fusiform body shape with a terminal mouth type.The top of the head to the eyes is straight, the dorsal fins are slightly convex, and the pectoral fins are short [5].
Indian mackerel is widely distributed in the Indonesian waters.They are distributed in the Fisheries Management Area of the Republic of Indonesia (FMA 573), located in the Indian Ocean from the south of Java to the south of Nusa Tenggara, the Sawu Sea, and the East Timor Sea.The presence of Indian mackerel stocks in the monsoon season in the Java Sea comes from the South China Sea and the Indian Ocean through the Sunda Strait [6].The high economic value of Indian mackerel has caused fishermen to catch the fish without considering the sustainability of the fish [7].Indian mackerel in FMA 573, classified as a small pelagic fish, has a total allowable catch (TAC) of 437,056 tons.The utilization rate of small pelagic fish in WPP 573 is 0.6, so that is categorized into a fully-exploited condition [8].According to [9], the exploitation rate of Indian mackerel in Palabuhanratu Bay indicates that it has been fully exploited.
Management of fishing activities can be formulated to maintain the Indian mackerel stocks.Sustainability management of fisheries resources does not prohibit commercial fishing activities.Fisheries management is directed to strive for fish stocks in waters above the safe biological limit or the ability to recover fish resources [10].Fisheries resource management needs to be done based on measurable considerations.One of the considerations is to assess biological indicators [11].A fish reproduction approach that examines the biological characteristics of fish resources can provide scientific advice for management to prevent the extinction of fish resources due to fishing activities [12].
Studies on fish reproductive characteristics are necessary and essential for strategic planning in sustainable fisheries resource management [13].Some fish reproductive characteristics that provide an overview of fish reproductive behavior are sex ratio, size at first maturity, gonad maturity stage, gonadosomatic index (GSI), fecundity, and egg diameter [14].Information on the reproductive characteristics of Indian mackerel in the southern waters of West Java is only available in Palabuhanratu Bay [15].Research on the reproductive parts of Indian mackerel in the southern waters of West Java is needed to complete basic information for small pelagic fisheries management efforts in FMA 573.This study would provide information on the reproductive characteristics of Indian mackerel so that it can be the basis for proper and sustainable management of small pelagic fish resources in the southern waters of West Java (FMA 573).

Time and Location
The research was conducted at Binuangeun Fish Landing Base (PPI), Palabuhanratu Fishing Port (PPN), and Ciwaru Fish Landing Base (PPI) from July to December 2022.The fish samples were caught by fishermen in the southern waters of West Java (Figure 1).Laboratory analysis was accomplished at the Macro-biology and Fisheries Biology Laboratories, Department of Aquatic Resources Management, Faculty of Fisheries and Marine Science, IPB University.

Data Collection
Primary data measured in this study are total length, weight, gonad maturity level, gonad maturity index, fecundity, egg diameter, and size at first maturity of Indian mackerel.Secondary data included reproductive characteristics of Indian mackerel in different areas obtained from literature studies.Length measurements were taken using a ruler with an accuracy of 0.5 mm.The total length of the sample fish was measured from the anterior part of the upper jaw to the end of the tail.Total fish weight was measured using Joil digital scales with an accuracy of 0.01 g.
Sex and gonad maturity stage of fish samples are determined morphologically based on the shape, color, size, weight, and development of gonad contents (Table 1).Determination of sex and maturity stage refers to [16].Analysis of fish fecundity was using the gravimetric method.Eggs diameter measurements using a monocular microscope with a magnification of 4×10 equipped with an ocular micrometer.Management plans are based on primary data from the research results, which are supported by the results of literature studies or regulations that have existed before.

Length-frequency distribution
Analysis of length frequency distribution on fish was used to determine population size distribution based on total length data.The data will be substituted into size groups through the issuance of classes.The number of classes can be known by the following formula [17]:

Sex ratio
The sex ratio is determined by comparing the number of male and female fish.Analysis to determine the sex ratio of male and female fish according to [18] using the following formula: The ratio between male and female fish obtained from a fish population was then tested using the chi-square test.This analysis serves to determine the balance in the population.According to [17], the chi-square test is formulated as follows: Note: χ !: Random variable value of chi-square o i : Total frequency of male and female fish e " : Expected frequencies of male and female fishes Hypothesis: H0: The proportion of males and females is 1:1 in the water (sex ratio is balanced) H1: The proportion of males and females is not 1:1 in the water (unbalanced sex ratio).balanced

Gonadosomatic index (GSI)
The gonadosomatic index (GSI) of fish is measured using the following formula [19]:

Fecundity
Fecundity is estimated using the gravimetric method with the following formula [19]: Note: F: Fecundity (eggs) G: Total gonad weight (g) N: Number of eggs in sub gonads (eggs) Q: Sub gonad weight (g)

Size at first maturity (Lm)
The size at first maturity (Lm) in fish was analyzed using the Spearman-Karber method with the following formula [20]: Note: m : Logarithm of fish length at first maturity Xk : Last logarithm size at which fish are fully mature X : Log size increment Pi: Proportion of adult fish in the i th class (pi = ri/ni) Ri: Number of gonadally mature fish in the i th class Ni: Number of fish in the i th class Qi: 1-Pi

Length-based spawning potential ratio
The stock status of fish species can be estimated based on fish length using the Length-Based Spawning Potential Ratio (LBSPR).LBSPR analysis requires fish length-frequency data, natural mortality parameters (M), growth coefficient (K), asymptotic length (L∞), and size at first gonadal maturity (Lm).LBSPR analysis was also performed using a barefootecologist's toolbox.The LBSPR was obtained by calculating the ratio of the spawning stock biomass in the exploited stock and spawning stock biomass in the absence of exploitation as follows [21].

Tactical decision
Fisheries management analysis was made using a tactical decision approach.Tactical decision-making was determined based on a matrix referring to [22].The tactical decision matrix includes attributes, status, objective strategies with associated pressure, and tactics (Table 2).

Research results that are categorized based on reference point values
The strategy chosen will determine the best thing to do in the situation with regard to the status of the attributes.
Tactics or actions to implement the strategy that has been chosen Determining the status of the attributes studied based on the reference indicators of an ecosystem approach to fisheries management referring to [23].Attributes or indicators used are fish size trends and the proportion of captured juveniles.Each attribute has different criteria and assessment scores (Table 3).

Results and Discussion
Indian mackerel that were landed in the southern waters of West Java during the study had a variable length distribution.The total number of fish obtained was 1038, with 582 males and 456 females.The length-frequency distribution of Indian mackerel ranged from 125-280 mm (Figure 2). Figure 2 shows the highest length frequency distribution in the 190-202 mm class interval and the lowest in the 268-280 mm class interval.Indian mackerel is a common fish species caught by fishermen in the southern waters of West Java [24].Indian mackerel are caught using gill nets with mesh sizes ranging from 1 to 2.5 inches.The length distribution of fish during the study had a minimum length of 125 mm and a maximum of 280 mm.The dominant frequency of male and female fish was in the 190-202 mm.This size is similar to the size of fish previously studied.Research by [15] showed that the size of Indian mackerel in Palabuhanratu Bay was dominated by fish, with sizes ranging from 210-219 mm.Differences in fish size at different times and regions are caused by environmental conditions, including temperature, availability, and food quality [25].
The sex ratio of the Indian mackerel population was determined based on the total number of male and female fish obtained each month from the observation site.The ratio between male and female fish was 1:0.78 (Table 4).Based on the chi-square test, the value of χ 2 is more significant than 3.84 (α = 0.05).This value indicates that the sex ratio of Indian mackerel in the southern waters of West Java is not balanced.The sex ratio of Indian mackerel varied with each month of observation.The observed sample fish consisted of 582 males and 456 females.Based on the Chi-Square test with a 95% confidence interval, the ratio between the number of male and female fish in a population was 1: 0.78.This value indicates that the sex ratio of Indian mackerel in the southern waters of West Java is not balanced with the dominance of male fish.The dominance of male fish also occurs in Indian mackerel that are gonadally mature or ready to spawn.The sex ratio of maturity stage IV Indian mackerel is 1: 0.79, which consists of 166 male fish and 131 female fish.This is similar to research conducted by [26], which states that male fish dominate the sex ratio of Indian mackerel in the Madura Strait.Differences in sex ratio can be caused by fish reproductive behavior, environmental conditions, and fishing activities [27].
The ratio between male and female fish is expected to be in balance (1:1), or at least the number of female fish is greater than that of male fish [28].Sex ratio deviations are related to fish spawning behavior.According to [29], the difference in the sex ratio pattern from 1:1 is associated with the external spawning of fish with high fecundity, so more male fish are needed than female fish.Indian mackerel are classified as fish that spawn together in the open sea.Male and female fish will simultaneously release sperm and eggs in large quantities [30].
The number of Indian mackerel ready to spawn or in maturity stage IV is 296 fish, consisting of 166 males and 130 females (Table 5).The sex ratio between male and female fish is 1:0.79.Based on the chi-square test, the value of χ 2 is more significant than 3.84 (α = 0.05).This value indicates that the sex ratio of Indian mackerel maturity stage IV is not balanced with the dominance of male fish.The level of gonadal maturity describes the reproductive cycle and determines the timing of fish spawning.The level of gonadal maturity of Indian mackerel has five stages of development, which are TKG I immature (undeveloped), TKG II maturing (early development), TKG III (mature gonads), TKG IV (late development), and TKG V spent (spawning) (Figure 3).The gonadosomatic index (GSI) can be used as an indicator to determine the spawning season of fish.The GSI value is obtained from the ratio of gonad weight to total fish weight.The mean GSI of male fish ranged from 0.19-0.95,and female fish ranged from 0.45-3.41.The highest GSI values of male and female fish were in August (Figure 5).

Fig. 5. Gonadosomatic index of the Indian mackerel
The Indian mackerel observed during the study consisted of fish at stages I to V, dominated by fish that had not yet reached gonadal maturity.[31] states that some small pelagic fish are only temporary in some regions.That region is only used as a feeding, nursery, or spawning area.Based on the results, it can be seen that the GSI value of female fish is greater than that of male fish.According to [33], the average GSI of female fish is greater than that of male fish due to the increase in ovarian weight, which is greater than the weight of the testes.The increase in ovarian weight in female fish is related to the process of vitellogenesis.In contrast, the increase in testicular weight in male fish is associated with spermatogenesis and semen volume in the seminiferous tubules.
An increased GSI value is an indicator of the spawning season.[34] stated that the GSI value will increase and reach a maximum limit when spawning occurs and then decrease again after spawning.The GSI value of Indian mackerel during the study peaked in August.This indicates that the peak of Indian mackerel spawning in the southern waters of West Java is in August.This is reinforced by the high frequency of stage IV maturity that month.The same results were found in the research of [12], which stated that the peak of Indian mackerel spawning in the Sunda Strait was in July and August.
Fecundity calculations were conducted on the gonads of 131 female fish in maturity stage IV or ready to spawn.Based on the analysis, the fecundity value of Indian mackerel ranged from 27,027 to 87,837 eggs.The relationship between fecundity and total length was expressed by the equation F = 0.0115L 2.8278 with a coefficient of determination of 85.11%.The relationship between fecundity and fish weight was described by the equation F = 942.79W+ 0.8163 with a coefficient of determination of 85.94% (Figure 6).Fecundity analysis aims to determine the number of fish eggs produced by female fish.The fecundity of Indian mackerel in the southern waters of West Java ranged from 27,027 to 87,837 eggs, with an average of 40,216 eggs.The results of previous research conducted by [15] stated that Indian mackerel in Palabuhanratu Bay had a fecundity ranging from 5,349 to 67,423 eggs.This number is relatively lower than the Indian mackerel in Biak Waters, which has a fecundity of 64,965-692,139 eggs [35].Fish fecundity can differ based on species, time, and observation location.However, fecundity analysis of fish species in the exact location can have different results due to differences in the observation time [11].The difference in fecundity was probably due to the influence of other environmental conditions, especially related to the availability of food in the previous season [36].
Fecundity can also be influenced by fish length and weight, egg size, spawning frequency, environmental conditions, and population density [37].The results show a significant relationship between fecundity and fish's length and body weight.The fecundity increases with the fish's increasing length and body weight.This is reinforced by research conducted by [35], which states that the relationship between fecundity and the length and weight of Indian mackerel in Biak Waters has a close correlation.In general, fish fecundity will increase during growth.Fecundity will increase up to a certain age and then decrease.Maximum fecundity occurs in young fish and then will reduce as the fish get older [18].
Eggs diameter analysis shows the spawning type of the fish.The study of 193 female fish obtained egg diameter distribution, which ranged from 0.025-0.840mm (Figure 7).Based on the figure, it can be seen that the distribution of egg diameter has one peak at each level of gonad maturity, so it can be seen that Indian mackerel has a total spawner spawning pattern.Fish spawning type can be determined by analyzing the distribution of egg diameter.The egg diameter of Indian mackerel at stage III, stage IV, and stage V have one mode.Based on egg diameter, it can be seen that the spawning type of Indian mackerel in the southern waters of West Java is a total spawner.These results follow previous research conducted by [35], which stated that Indian mackerel have a total spawner spawning type, where the fish will release eggs simultaneously in one spawning season.However, in contrast to previous research conducted by [15,12], which states that Indian mackerel have a partial spawner spawning type, spawning is carried out gradually in one spawning period.Changes in spawning patterns in fish are thought to be a response to environmental changes and the influence of fishing [11].
The size of the first mature gonads of male fish is 198.5 mm, while the size of the first mature gonads of female fish is 197 mm (Figure 8).Based on the length distribution of Indian mackerel, male and female fish start to mature in the 190-202 mm class interval.Estimating the size of the first maturity determines the fish that can be harvested.The size of the first mature gonad of male fish is 198.5 mm, and that of the female is 197 mm.Based on the results of this study, it can be seen that female Indian mackerel experience gonad maturity shorter than males.The size of the first maturing gonads in fish is influenced by environmental conditions, regional differences, water quality, and fishing pressure [37].The difference in the size of the first maturing gonads is one of the fish reproduction strategies to restore the balance of the population in nature due to changes in environmental conditions and high fishing activity [38].
The LBSPR value of the Indian mackerel in this study was 12%, indicating the overexploited status.Meanwhile, this study's proportion of fishing mortality to natural mortality (F/M) was 3.52, higher than the threshold of 1 (Figure 9).This means that the fishing activity of Indian mackerel is under pressure.Tactical decisions are made based on the objectives to be achieved, in this case, the sustainability of Indian mackerel.Tactical decision-making is based on the reference point of each attribute or indicator.Fish size trends that were caught based on interviews with fishermen and supported by research conducted by [15] showed a steady increase in fish size and relatively fixed.The proportion of juveniles (<Lm) caught using gill nets was in an excessive category with a percentage of >60%.Tactical steps that can be implemented as a management strategy for Indian mackerel in the southern waters of West Java include limiting fishing activities during the spawning season (July, August, and September) and setting gillnet mesh sizes above 1.5 inches (Table 6).The proportion of fishing mortality to natural mortality (F/M) is more than 1.This indicates that Indian mackerel are overexploited.This phenomenon is also supported by the high proportion of juveniles caught, which suggests that the captured fish have not yet had the opportunity to spawn.According to this result, fishing mortality (F) needs to be balanced with natural mortality (M) so that the proportion is close to 1. [39] classified the stock status based on LBSPR values.For management purposes, an LBSPR of 30-40% is recognized as the intended target, while an LBSPR of 20% is accepted as the reference point limit.
Management strategies that can be used to ensure the sustainability of Indian mackerel resources in the waters use a tactical decision approach.Reduced fish populations in the future can be caused by fish caught that have never spawned or are just about to spawn [37].The indicator of the proportion of juveniles or fish that have not matured gonads is in excessive of conditions (> 60%) [14].Tactical steps can be taken by organizing the use of selective fishing gear.Fish that can be caught have exceeded the size of first maturity, with a length of > 198.5 mm.This restriction can be applied by using a mesh size above 1.5 inches.This size is used based on the reference size of the body height of fish that have reached gonadal maturity during the study.
Another tactical step that can be taken is to limit fishing activities during the Indian mackerel spawning season.Based on the research, it can be assumed that the Indian mackerel spawning season in the southern waters of West Java lasts from July to September, with peak spawning in August.Limiting fishing activities during the fish spawning season was also suggested in a study [12] in the waters of the Sunda Strait.It reduces the intensity of fishing during the spawning season of Indian mackerel in locations suspected of being spawning areas to prevent the capture of fish that are maturing gonads and protect fish breeding sites in the waters.

Conclusion
Indian mackerel spawn annually with total spawner spawning type.Female fish mature faster than male ones.Management strategy of Indian mackerel in the southern waters of West Java can be done through limiting fishing activities by reducing fishing effort in July, August, and 13

Fig. 1 .
Fig. 1.Map of the study area for Indian mackerel reproductive biology in the southern waters of West Java, Indonesia ratio M: Number of male fish (ind) F: Number of female fish (ind)

Fig. 2 .
Fig. 2. Length-frequency distribution of the Indian mackerel in the southern waters of West Java, Indonesia

Fig. 3 .Fig. 4 .
Fig. 3. Gonadal morphology of the Indian mackerelFigure4shows the percentage of Indian mackerel gonad maturity level in the southern waters of West Java.Based on the observation of the level of maturity of Indian mackerel gonads in July, August, and September, stage IV dominated with 52.83%, 60.66%, and 51.55%.Stage I dominated in October, November, and December with percentages of

Fig. 6 .
Fig. 6.The relationships between fecundity and total length (a) and fecundity and weight (b)

Fig. 7 .
Fig. 7. Eggs diameter distribution of the Indian mackerel in different gonad maturity levels

Fig. 8 .
Fig. 8.The size of the first mature gonads of male (a) and female (b)of the Indian mackerel

Fig. 9 .
Fig. 9. Length-based spawning potential ratio of the Indian mackerel

Table 3 .
Attributes, data sources, and status for tactical decisions.

Table 4 .
Sex ratio of the Indian mackerel in the southern waters of West Java, Indonesia

Table 5 .
Sex ratio of the Indian mackerel in maturity stage IV

Table 6 .
Tactical decision for the Indian mackerel management strategy