Length-based population parameters of shortfin scad ( Decapterus macrosoma ) in Bali Strait waters, Indonesia

. Shortfin scad ( Decapterus macrosoma ) is one of the most abundant catches of purse seiners in Bali strait. This study aimed to reveal the length-length relationship, the length-weight relationship, and to estimate the length at first maturity (Lm 50 ) and the length at first capture (Lc 50 ) for D. macrosoma in the waters of Bali Strait. The samples were collected from Pengambengan Fishing Port from June to December 2018. A total of 3,341 shortfin scads were fork length measured and weighed during the research. There were 420 fish from the total sample fish measured more for the total and standard length and dissected. The sex and maturation stage determination were identified by gonad visual characteristics. The relationship between lengths was determined using linear regression, whereas the length-weight relationship was determined using power regression. The length at first maturity and capture were determined using the logistic curve. The lengths (total, fork, and standard) were highly related to each other. The lengths were also highly related to the weight. The growth pattern of the shortfin scad was hyperallometric. The Lm 50 for D. macrosoma was 164 mmTL for males and 162 mmTL for females. They were shorter than the Lc 50 , which was 184 mmTL.


Introduction
Indonesia is one of the top producers of captured fisheries in the world.To optimize fisheries management, Indonesia divides the Fisheries Management Area (FMA) into 11 separate areas.One is FMA-573, which the Bali Strait is part of.This strait divides the Java and Bali Islands.High concentrations of chlorophyll-a [1] indicate that it is an area of high primary productivity.Small pelagic fish in these seas have high biomass and productivity due to the nutrient-rich water [2,3].One of the regions in FMA-573 with the highest density of small pelagic fish was thought to be the 200-300 m water column beneath the Bali Strait [4].
Many types of small pelagic fish can be found in Bali Strait, with Bali sardinella (Sardinella lemuru) being the most common species [5,6].However, since 2016, the production of S. lemuru has significantly decreased (Fig. 1).Therefore, the scads become one of the most prevalent species in the purse seiner haul.The most abundant scad in this area is the shortfin scad, Decapterus macrosoma [7].
Source: Pengambengan Fishing Port Statistical Report 2016-2018 Fig. 1.The landing composition of purse seiners based on Pengambengan Fishing Port Shortfin scad is widely distributed in Indonesia as it can be found from the eastern part [8] to the western part [9] of Indonesia.Purse seine is a fishing gear that commonly caught scads in Indonesian waters, such as the Malaka Strait [10], Jawa Sea [11,12], and the western waters of Sumatra [13].This fish has an important economic value.Apart from being consumed, D. macrosoma is also used as bait in tuna longlines [14,15]and pole and line fisheries [16].Based on the genetic analysis of the marker of DNA mitochondria, there are at least two subpopulations of D. macrosoma in Indonesian waters [17].
Standard length (SL), fork length (FL), and total length (TL) are commonly used to measure fish [18].The length-length relationship allows comparisons between studies that apply different length measurements and conversions between measurements (e.g., if the tail is eliminated) [19].Comparable studies on fish species found in estuaries [20,21], freshwater [22], and marine environments [19,23] have also been conducted.
Further data on the length-weight and length-length relationships of D. macrosoma in Indonesian waters are required.The information would be helpful before determining whether fishing activities could affect species' biological characteristics.It is also necessary for stock assessment models, which estimate biomass level [24] and the spawning potential ratio [25].According to [26], the size of the fish population reaches 50% maturity is the initial size of maturity.It is important to determine the size of fish that should not be exploited as juveniles to maintain sustainability.
This study aims to reveal the population parameter for D. macrosoma in Bali Strait waters using length-based data.This study is essential to predict the sustainability of D. macrosoma in this strait.The management of these fish in Bali Strait is also anticipated to benefit from this study.

Sampling site and period
The samples were collected from Pengambengan Fishing Port (Fig. 2).This port is one of the main fish landings for the catch around the Bali Strait waters.The samples were collected every month between June and December of 2018.

Data Collection
The fish were collected from a one-day fishing purse seine.An enumerator was placed at the fishing port to record the composition of the daily landed fish.The enumerator randomly picked some fish from random vessels to represent the landed vessel catch.The FL was measured using measuring tape to the closest of 1 mm.The body of the fish were weighted (BW) using a digital weight scale to the closest of 1 gram.In order to get additional biological features, some of the sampled fish (sub-sample) were analyzed further.Further data collected are standard length, total length (Fig. 3), and the gender of the fish (male or female).

Fig. 3. Length measurements collected for D. macrosoma in this study
The fish sub-sample underwent dissection to ascertain the sex and maturity stage of their gonads.Decapterus macrosoma is not sexually dimorphism and dichromatism.The color, shape, and size of the gonad served as visual cues for determining the sex and gonadal maturity.(Table 1 and Table 2).The testes are soggy, shrink in size, and the color is pinkish-white.Sometimes, the residual semen is still seen.The ovaries are thread-like, translucent, pinkish, and the eggs are invisible.The length of the gonad is around 1/3 of the body cavity.

II (Maturing)
The ovaries are yellowish.The oocytes are hard to see with the naked eye.The length is around 1/2 of the body cavity.III (Mature) The ovaries are reddish-yellow.The blood vessels are visible.The oocytes are seen clearly with the naked eye.The length of the gonad is around 2/3 of the body cavity.IV (Spawning) The ovaries are huge, plump, and the blood vessels are visible.The ovaries are yellow to orange, covering over 3/4 of the body cavity.The ovaries are full of oocytes, which can be seen with the naked eye.The egg size is bigger than the previous stage.When the stomach is squeezed, eggs can come out of the ovaries.V (Spent) The ovaries are deflated, shrink in size, soggy, and have a feeble wall; the color is yellowish clear or dark yellow.At this stage, the residual eggs could still be seen.

Data Analysis
The relationships between several types of length measurements (SL-FL, SL-TL, and FL-TL) were established by applying linear regression, represented by the equation y = a + bx, where x and y represent the respective length measurements.The length-weight relationship was determined using the regression model W = aLb.The formula utilizes the following variables: W, representing body weight measured in grams; L, representing body length measured in millimeters; b, representing the slope of the connection; and a, representing the intercept.
The student t-test was applied to assess the statistical significance of the value of b in the regression equation pertaining to the length-weight relationship, specifically to determine if it was significantly equivalent to or distinct from 3. The study employed the use of two hypotheses, namely H0 and H1.H0 posited that the value of b is equal to 3 (b = 3), indicating an isometric growth pattern.On the other hand, H1 suggested that b is not equal to 3, signifying an allometric growth pattern.It is considered hyperallometric if the value of b is more than three and hypoallometric if it is less than 3.The regression slope b of the lengthweight relationships between the maturity (juvenile and adult) were compared by the analysis of covariance (ANCOVA).The chi-squared test was used to identify the sex-ratio divergence from the expected value of 1:1 (male: female).
The length at which 50% of the population (represented by the sample size) is in the mature stage is known as the length at first maturity.The length interval was fitted with the proportion of mature fish using the nonlinear least square regression method described in reference [27], and a logistic model was created to determine this.The length at first captured (Lc50) of the fish from the purse seine in this location was likewise ascertained using the logistic model.Using Microsoft Excel, all statistical analyses were assessed for this study at a significance level of P < 0.05.

Results and Discussion
During the observation period, 1,880.12tons of Decapterus spp.were landed in Pengambengan Fishing Port from 1,544 days of purse seine trips.There were three species of scads identified, namely D. macrosoma, D. kurroides, and D.russelli.D. macrosoma is the most significant catch among the other scads (Fig. 4).D. macrosoma is recorded as one of the most abundant fishes in Bali Strait [7].This fish was also reported as the main catch from purse seine in some waters in Indonesia, such as Sibolga waters [13], South China Sea [27], Kendari waters [28], Java Sea [29,30], Prigi waters [31], and Watampone waters [32].In the South China Sea, D. macrosoma was already classified as highly exploited [33], while it was overexploited in the Java Sea [34].

Fig. 4. The composition of scads caught by purse seiner in Bali Strait
During this study, a total of 3,341 D. macrosoma were measured and weighed.The length ranged from 110 -220 mmFL with 177 ± 23.16 mmFL average (Fig. 5).The average fish sample used in this investigation (177 cmFL) is bigger than D. macrosoma that was caught in the Natuna Sea using purse seine [35].The mesh size of the fishing gear, which is correlated with fishing selectivity, is the cause of the variation in size distribution.The smallest fish weighed 12.7 grams, while the largest was 133 grams.Further measurements and dissections were conducted for 420 individuals, comprising 161 males, 172 females, and 87 individuals with indeterminate sex.Among the sex detected of D. macrosoma, the total sex ratio was 1:1.06 for males and females.After being tested by the chi-square, it is known that D. macrosoma deviated significantly from a 1:1 ratio based on length (Fig 6 ).In fisheries management, the length-length relationship is important, particularly when comparing data from studies that employed various methods of length measurement.For D. macrosoma, fork length was utilized by some researchers [12,35], while total length was used by others [36,37].The standard length, fork length, and total length of D. macrosoma correlated favorably with body weight.For every measurement, the correlation coefficients derived from the length-weight relationship were nearly equal to one.Similar results were also reported in Matang mangrove estuaries [37].Total length was shown to be more accurate in sizing up weight than standard length by [38], who examined the relationship between standard length and total length for weight estimation.The formula is applicable for D. macrosoma within the sampled region if more precise data were unavailable.Extrapolation from fish larger or smaller than those sampled is not advised for this type of investigation [39].All types of length (total, fork, and standard) had a strong relationship with the weight, with R 2 -values > 0.97 (Fig. 9).The power regression coefficient ranged from 3.2590 (SL) to 3.3871 (TL).The slope (b) for all measurement types differed significantly from 3 (P > 0.05).After being tested with ANCOVA, it is known that there is no significant difference in the regression slope (b) between juveniles and adults in this study.The growth patterns of D. macrosoma remained constant across various maturity, sex, and length measurements, indicating that they followed a hyperallometric pattern (Table 3).In this study, the slope (b) formed from the length-weight relationship of D. macrosoma was always more than 3.0, leading to a hyperallometric growth pattern based on the maturity level, sex, and measurement method.The hyperallometric growth pattern was also reported for D. macrosoma in Matang mangrove estuaries, Malaysia [37].[40] reported that D. macrosoma from the Masalima Sea was isometric.Some factors that can affect this relationship are season [41][42][43], habitat and water quality [44,45], and availability of food [20,46].
The length at first maturity is the length at which 50% of a population becomes sexually mature for the first time.The logistic curve indicated that the size at maturity of male shortfin scad in Bali Strait was 164 mmFL and that of females was 162 mmFL (Fig. 10).Meanwhile, the length at first captured (Lc50) was bigger, which is 184 mmFL (Fig 11).In Bali Strait, it has been observed that D. macrosoma attain sexual maturity at a lesser size compared to individuals found in the Java Sea [47], South China Sea [48], and southern waters of Kyushu [49].In contrast, it is bigger than from the coastal waters of San Fernando, Philippines [50] and the southwest coast of India [51].The length at first maturity for male D. macrosoma (164 mmTL) was bigger than that for females (162 mmTL).A similar finding that the male fish population reaches maturity at a bigger size is also reported by the coastal waters of San Fernando, Philippines [50] and the southwest coast of India [51].The different result, where the female would reach the length at first maturity larger than the male, was reported by [49] for D. macrosoma from the southern waters of Kyushu.Furthermore, [51] reported that the age of D. macrosoma, where it first reached maturity, was 1.05 years, while [49] reported it as two years.This difference could happen because of the different methods of estimating the age and first size of reproduction.Aside from environmental factors that can cause variations in gonad development, the fishing location also affects the length at 50% maturity.
[52] stated that one practical and affordable way to assess how fisheries affect fish stocks' maturation is by looking at the length at first maturity.Unfortunately, due to the absence of prior research in the Bali Straits and adjacent waters, it is not possible to establish a comparative analysis of the first size of maturity by time.According to [53], a reaction that may be observed in seas with high fishing activity is a decreased length for fish reaching its first maturity.
By the length at first captured of D. macrosoma in this study (184 mmFL), we know that most of the caught fish were large fish.A similar condition was also reported in which most of the captured D. macrosoma were large [40].The Lc50 in this study is bigger than reported from the purse seine catch in the Java Sea (153 mmFL) [54] and Sibolga waters (131 mmFL) [13].The difference in the length at first captured could be due to the different sizes of net mesh of the purse seine and the fishing ground.The decrease of D. macrosoma length at first captured was reported by [55] from 131 mmFL in 2004 to 110 mmFL in 2008 in Sibolga waters Sumatra, indicating stock depletion.This parameter could also be used to record whether there is a shift in the fishing ground or a change in the specification of the fishing gear.
Fish size structure is an indicator of good commercial fish environmental status fish [56].According to [57], the prevalence of capturing juvenile fish poses a significant risk of growing overfishing, hence jeopardizing the long-term viability of fish populations due to the escalating fishing pressure.The decline in fish reproduction can be attributed to the capture of potential parent fish during their growth phase.In the Java Sea, it is reported that almost all of the caught D. macrosoma by the purse seiner were juvenile [12,54].Fortunately, this study revealed the contradiction.The adult D. macrosoma dominated the captured fish in Bali Strait waters.Most captured D. macrosoma was above the length at first maturity.It is also shown by comparing Lc50 with Lm50.The D. macrosoma were harvested by the purse seine after they grew and reached maturity.This implies that they have been able to reproduce at least once during their lifespan.

Conclusion
The three types of length (total, fork, and standard length) of shortfin scad, D. macrosoma, significantly correlate with body weight.The growth pattern of D. macrosoma in this study was hyperallometric.The length at first maturity for D. macrosoma was 164 mmTL for males and 162 mmTL for females in Bali Strait.It was less than the length at first capture, 184 mmTL.

Fig. 2 .
Fig.2.Location of the study area for D. macrosoma in the Bali Strait.

Fig. 6 .Fig. 7 .
Fig. 6.The sex ratio of D. macrosoma from Bali Strait based on length size class

Table 1 .
The maturity stage of male D. macrosoma reproductive organ by this study visual observation

Table 2 .
The maturity stage of female D. macrosoma reproductive organ by this study visual observation

Table 3 .
The length-weight relationships by sex, maturity, and length measurement method for D. macrosoma from Bali Strait waters