The Size and The Shape of Sagittal Otolith of Redtail Scad, Decapterus kurroides Blekeer 1855 from Kema Bay, North Minahasa Regency, North Sulawesi, Indonesia

This study aimed to examine the difference in the otolith size and otolith shape of redtail scad Decapterus kurrodes between sex in North Sulawesi. The otoliths of D. kurroides were studied for sagitta otolith pair samples of 34 juveniles, 58 females and 75 males from Kema Bay. These pairs sagitta otolith images were interpreted using ImageJ tool to describe the otolith size (length, width, perimeter, and area), then we calculated the shape index (form factor, roundness, circularity, rectangularity, ellipticity and aspect ratio or length-width ratio). No significant difference was found in otolith length between left and right otoliths of juveniles of D. kurroides or called symmetrical but significant differences or asymmetrical were found in all sample, females and males. The regressions of total length against otolith sizes (otolith lengt, otolith width otolith perimeter and otolith area of D. kurroides follow a power function. The growth patterns showed dominant allometric growth in total length–otolith sizes relationships of D. kurroides from Kema Bay.


Introduction
Fish is one of the water resources contributing protein to human life, this commodity can be eaten by various levels of human life and so all religions do not forbid him to eat. Redtail scad, Decapterus kurroides as a small pelagic fish known locally as 'malalugis anggur', this fish is not only consumed by humans but are useful as bait fish in catching tuna and skipjack. Scad fish production in Indonesia is high and with a wide market share, including in this area. The economic value is generally in the medium price category (Sumaila et al. 2007). Redtail scad is a fish which has prominent red colour on the tail and is in one genus with other red-tailed scads, such as D. akaadsi, D. tabl and D. smithvanizi n. sp. (Kimura et al, 2013) It is called Decapterus that comes from the word deca = ten and pteron = finlet (Rumero 2002). The fish is categorized in class Actinopterygii, order Perciformes, suborder Percoidei and family Carangidae (Masuda et al., 1975(Masuda et al., , 1984; World Registered Marine SpeciesWORMS: http://www.marinespecies.org/ aphia.php?p= sourcedetails&id=234250) and fishbase (https://www.fishbase.se/search.php).
Biologically in fish found ear stones (otolith), functions as a hearing aid and regulates body balance. In its development, otolith is also used as a very useful tool for determining fish age and growth. Even Begg and Brown (2000) can also be used to identify stocks. Otolith is part of the inner ear of fish including scad fish.
The otolith study on various marine fishes in tropical regions, especially Indonesia, has not been significantly conducted. Wright et al (1990) have firstly reported it on Stolephorus heterolobus in Java Sea, then on several species of eels (Anguilla spp.) around Sulawesi waters and others (Arai et al. 2000(Arai et al. , 2003Sugeha et al 2001;Kuroki et al. 2005;Lee et al 2008;Chino & Arai 2010), bluefin tuna (Thunnus maccoyii) around Bali waters (Shiao et al. 2009;William et al. 2013;Farley et al. 2014 (Wujdi et al. 2016), skipjack tuna (Katsuwonus pelamis) in the southern waters of Java and the Sulawesi Sea (Wujdi et al. 2017(Wujdi et al. , 2018Mogea et al. 2019), and Selar crumenophthalmus in Manado Bay (Bahri et al. 2018). Otolith study does not merely aim to get information on fish aging and fish growth but also to identify or discriminate the fish stock. According to Campana & Casselman (1993), the size and the shape of otolith are also beneficial to use as stock identity indicator for varied growth rate among fish stocks. This study aimed to characterized otolith and examine the difference in the otolith size and otolith shape of redtail scad Decapterus kurrodes between juvenile, male dan female from Kema Bay, North Sulawesi. This study was carried out from September 2017 to September 2019. Redtail scads D. kurroides were obtained from fishermen's catches in Kema Bay (Geographical 1°15 '-1°22' North / 125°05'-125°15' East) ( Figure 1). The fish were caught with vertical multiple hook-handline and mini purse seine. They were proportionally taken to represent the exploited fish body size. Data collected included sex and total length (TL). The redtail scad that were used as the research sample consisted of 167 individuals, measuring a total length with an average of 159 mm with total length an interval of 97 mm-203 mm. Redtail scad are then taken to the Laboratory of Fish Health and Fish Health, Environment, and Toxicology of FPIK UNSRAT. Redtail scad ascertaining their sexes by observing their gonads. The head is then separated and the otolith will then be taken out from just below the skull near the spine. Otolith is cleaned with H 2 O 2 to dissolve the blood, and then washed with deoionized water (WaterOne) then air dried using 70 % alcohol. The otoliths to be observed under a stereo microscope Olympus SZX7-DP21 microscope camera, the pair of left and right otoliths documented into photographic image. The image of an otolith is used by the imageJ facility to obtain the size of the otolith, in 4 parameters, length (OL), width (OW), perimeter (OP) and area/breadth (OA), here in after shall be referred to as otolith size as presented in Figure 2. The shape index calculation includes the roundness index and the aspect ratio.

Manginsela
There were 6 parameters of the shape indices calculated from the size of the otolith = OL/OW. Difference in otolith size and shape index between the left and the right ones was analyzed using pair t-test, while the difference in the otolith size and shape between sex was tested with t-test. The relationship of otolith size against the fish TL was estimated using equation Y = a X b , where Y is otolith size (OL and OW) and X is fish total length (TL), a is intercept, and b is slope. Parameters a and b were calculated using Least Squares method after the data had been transformed to natural logarithm as ln Y = ln a + b ln X. The linearity and the goodness of fit were analyzed using ANOVA, whereas the regression line comparison used analysis of covariance (ANCOVA) (Scherrer 1984;Draper & Smith 1998). To examine whether b equals to the theoretical value of b = 1 (for the relationship of OL and OW), t-test was applied. If t calc. is bigger than t tab ., the growth pattern is allometric or the otolith growth increment does not go along with the body length increment whereas if it is the opposite, the growth pattern is isometric or the otolith size increment is in line with the body

Results and Discussions
3.1 Total length of fish.
The redtail scad that were used as the research sample consisted of 167individuals, measuring a total length with an average of 159 mm with total length an interval of 97 mm-203 mm. The fish samples were then grouped into three groups, juveniles, females and males. There are 34 juvenile fish with average: 110 mm (range 97-136 mm), 58 females average 175 mm (range 151-199 mm) and 75 males average 169 mm (range 126-203 mm). Comparison of two samples with t-test for total length of fish (TL) just males and females. There are significant differences (assuming variance is not the same) between female scad compared to male scad (female vs male) (t cal = 37.064 and P = 0.000).
The sample size of this fish is smaller than the average length of redtail scad caught as reported by fishbase which is at common total length 30 cm and can reach a maximum size of 45 cm. In the Masuda et al. (1975Masuda et al. ( , 1984 report, it was mentioned that there were male fish capable of reaching a maximum TL of 300 mm even 400 mm. Information on the size of this mature sex fish is not yet available in fishbase but Maulita et al. (2013) reported that the male scad maturity is at 32 cm and 33 cm on female. This research was able to distinguish females and males by checking at their gonads. Females at the smallest size were 175 mm while males at the smallest size were 151 mm. Thus, were smaller than the size of the first mature gonads that have been reported.

The otolith size and shape index of redtail scad
The total number of intact otolith samples collected was 167 pairs. Range of otolith length left-right: 2.523 mm -5.259 mm and 2.517 mm -5.173 mm, mean of otolith length left-right: 4.146 mm and 4.137 mm are small otolith (Furlani et al., 2007). Range of otolith width left and right: 1.150 mm -2.078 mm and 1.153 mm -2.089 mm, otolith perimeter (OP): left and right: 6.910 mm-13.926 and 6.108 mm -14.312mm and left otolith (OA) area 2.170 mm -6.954 mm and right 2.215 mm -6.966 mm. The analysis of the shape indeces of the otolith form factor has been calculated that were the form factor (FF) left and right : 0.401 -0.567 and 0.385-0.50 or FF < 1 which shows scad otolith is circular, roundness (RS) left the right : 0.304 -0.420 and 0.318-0.422 or RS < 1 indicates that the otolith is not round, circularity (CS), Rectangularity (RE) left and right : 0.634 -0.703 and 0.619 -0.796 : and Ellipsticity (EL) left and right : 0.334 -0.45 and 0.347 -0.46 and the aspect ratio of left AR : 2.004-2.808 and right AR 2.065 -2.816 or AR > 1 indicates the otolith is not square (AR = 1) but is elongated. All otolith size and shape indices show no real difference between left and right side (t cal < t tab (0.05, 165) ) ( Table 1). Comparison of size and shape index of otolith pairs redtail scad otoliths to characterize or differentiate otolith. Table 1. calculation of t-test for the mean comparison of all left-right paired otolith samples redtail scad turned out to be not significantly different (t cal < t (00.5, n-2) ) on otolith perimeter, form factor, roundness and circularity or there was difference between the left and right otolith (t cal >t (00.5, n-2) ) on otolith length, otolith width and otolith area, thus it could be said that left otolith and right otoliths were asymmetrical.
The comparison of otolith pairs (left and right side) is also done by doing the same grouping of juveniles (34 ind), females (58 ind) and males (75 ind) as presented in Table 2. Calculation of t-test for the average comparison of all left-right paired otolith samples redtail scad both on the size and on the shape index turned out to be not significant (t <t (00.5, n-2) ) on juvenils otolith pairs was symmetrical. But there was  (Mamuaya et al. 2017a andTaliawo et al. 2018), Cakalang (Katsuwonus pelamis) in Bitung (Mamuaya et al. 2017b) and in Manado (Mogea et al. 2019). Otherwise, otolith pairs of female and male difference size or asymetrical like otolith of plaice Pleuronectes platessa and turbot, Psetta maxima (Helling et al. 2005).  There was significant difference in ratio between the left otolith and right otolith, therefore the analysis was carried out using only the left side consistently in the t-test of male and female samples on otolith size and shape index in male and female fish. The results that differed from the results of the t-test comparison mean of male and female presented in Table 3 turned out to be the size of the otolith OL, OW, OP and OA all significantly different are presented in Figure  3 (t > t (0.05, 167-2) ). In contrast, the shape index CS, RS and AR were not significantly different (t < t (0.05, 165 ). It can be said that between males and females, there were no difference in the shape index between males and females.  regulated.Average male AR = 2.134) and female AR mean = 1.129 are both AR > 1 shown elongated otolith scad. The form of otolith was specific to each species was given a name. Specifically, redtail scad otolith that are not round, not circular, and are elongated. Redtail scad has a unique form are called fusiform shape (Furlani, et al. 2008 andTuset et al. 2003). 3.3. Relationship of total length and otolith size Regression between TL and OL, TL and OW, TL and OP and TL and OA of D. kurroides from Kema Bay follows a power (multiplicative) function (Figure 4) or linear function after transformation to natural logarithm (Table 4).  Figure 5.
To compare the linear regression lines of ln (TL)-ln (Otolith size), ANOVA was employed (Table 5). For the regression of ln(TL)ln(OL), because the p-value for the intercept and the slopes is greater or equal than 0.05 (a : F-ratio = 1.030; P = 0.311 and b : F-ratio = 2.230; p = 0.137), there are not statistically significant differences between for the intercept and the slopes for the various values of female and male at the 95% confidence level. Similarily with the regression of ln(TL)ln(OP), there are not statistically significant differences between for the intercept and the slopes for the various values of female and male at the 95% confidence level The comparison of linear regression of ln(TL)-ln(OW) and ln(TL)-ln(OP) there are statistically not significant differences between the intercepts for the various values of females and males. Because the p-value for the slope of ln(TL)-ln(OW) and ln(TL)-ln(OP) are less than 0.05 (F-ratio = 7.79; P = 0.006 and F-ratio = 6.250; P = 0.014), there are statistically significant differences between the intercepts for the various values of females and males at the 95% confidence level.

Conclusion
The otolith size and the shape index of D. kurroides from Kema Bay were successfully characterized. Significant difference was found in otolith size and shape index except otolith perimeter, rectangularity, ellipticity, and aspect ratio otolith between of D. kurroides. Therefore, the otolith size and shape of left-right sides of redtail scad from Kema Bay were asymmetrical. No significant differences were found in otolith size and shape index between left and right otoliths of juveniles or symmetrical, but significant differences between left and right otoliths of male and female D. kurroides from Kema Bay. The regressions between total length and otolith sizes (otolith length, width, perimeter and area) of D. kurroides follows a power function. The growth patterns showed dominant allometric growth in total length-otolith length, total length-otolith width total lengthotolith perimeter, total length-otolith area relationships of D. kurroides from Kema Bay.