SOME POPULATION PARAMETERS AND EXPLOITATION STATUS OF FOURFINGER THREADFIN ( Eleutheronema tetradactylum Shaw , 1804 ) IN TARAKAN WATERS , NORTH

Information on exploitation status of fourfinger threadfin (Eleutheronema tetradactylum Shaw, 1804) is important for sustainable gillnet fisheries management in Tarakan, North Kalimantan waters. In an attempt of providing scientific data and information on the exploitation status of this species, a research work was conducted from January to November 2016 in Selumit landing place. Fish sampling was done by trained enumerator on bottom gillnet catches landed through semi-regular observation. A total of 1964 specimens were collected and measured. The results showed that the size ranged between 16-70 cmFL with an average of 37.72 ± 0.36 cmFL. The length at first captured (Lc) of about 38.5 cmFL, was smaller than the length at first mature of female (Lm) of 39.6 cmFL. The growth pattern was negative allometry. By applying von Bertalanffy growth model, it was found that growth model for this species was L(t)=75,8(1-e). Exploitation rate (E) was 0,47. It showed that the exploitation status of fourfinger threadfin in Tarakan Waters was still sustainable.


INTRODUCTION
Tarakan waters, which is geographically located in eastern part of North Kalimantan province, is the fishing area of Fisheries Management Area (FMA) 716.One of the important economic species of demersal fishes belong to polynemids family that was exploited in Tarakan was fourfinger threadfin (Eleutheronema tetradactylum Shaw, 1804) or locally named as Senangin.About 84% of fourfinger threadfin caught in Tarakan waters was exported and about 16% was consumed by local resident in Tarakan so it provided high potential fish nutrition and contribute economical impact for local community.
Fourfinger threadfin (E.tetradactylum) in Tarakan was mainly caught by bottom gillnet.The estimate number of gillnet boats operated in Tarakan were 1.019 vessels and the production of fourfinger threadfin in 2014 was 483 tonnes or 0.8% from total production of fourfinger threadfin in Indonesia (DGCF, 2015).Exploitation status of demersal fisheries in FMA 716 was still at moderate level (Ministerial decree No. 50/ KEPMEN-KP/2017).Exploitation status by species, especially for fourfinger threadfin, could be as supporting information for local management plan of demersal fisheries in Tarakan.The exploitation status of fourfinger threadfin could be predicted by studying about its population parameters.
Information on population parameters of fourfinger threadfin (E.tetradactylum) in Indonesia was still limited.The previous information available particularly related to feeding habit and its exploitation status in East and South Sumatra (Rengi et al., 2015., Ridho et al., 2010).Population parameters of fourfinger threadfin in Tarakan waters hadn't been reported yet.Therefore, this study was needed to provide biological indicator as part of exploitation status for supporting sustainable fisheries management in this particular area.

MATERIALS AND METHODS
Samples of fourfinger threadfin were collected from small scale bottom gillnet catches landed in Selumit, one of the main fish landing places in Tarakan, North Kalimantan (Figure 1), during January-November 2016.All samples were measured fork length (FL, to nearest centimeter) and weight (to nearest gram), and were identified for sex and maturity level.Length at first maturityd (Lm) was analyzed based on logistic curve (King, 1995): Where a=the intercept of regression, b=the slope of regression, P Lm = logistic curve of proportion mature by length, Lm was determined by a/b.Growth parameters, including asymptotic length (L  ) and growth rate (K), were estimated by Electronic Length Frequency Analysis/ELEFAN I in FISAT II packages (Gayanilo et al., 2005).Theoretical age when the fish has zero length (t 0 ) was calculated based on the equation of Pauly (1983): Log (-t 0 ) = -0,3922 -0,2752 log (L  ) -1,038 log Where, Lt is the fork length at age t, is asymptotic fork length (cm), K is the growth rate of fish and t 0 is the point at time when the fish has zero length.

General Description on Data Collection
Numbers of specimens were measured on monthly basis, the biometric description of fourfinger threadfin showed that the length of fourfinger threadfin ranged between 13-68 cmFL.The weight ranged between 90 -4640 g (Table 1).
The data showed that the average of length for each month was between 32.1 -43.4 cm FL and the average of weight for each month was between 585.5 -1586.9g.The maximum average of length and weight for fourfinger threadfin was obtained in January.

Length Frequency
The total samples of gillnet fishery from January until November 2016 were 1,964 fish (Figure 2).The fork length (FL) of fourfinger threadfin (E.tetradactylum Shaw, 1804) ranged between 16-70 cm FL with an average of 37.72 ± 0.36 cm FL and the mode was 40 cm FL (Figure 2).

Length-Weight Relationship
Based on the monthly length-weight relationship, the result showed that the 95% confidence value of b was 2.42 -2.7 in March and 2.68-2.98 in June or the value of b was significantly different from 3 that indicated the negative allometry (p<0.05) in March and June.This condition showed that the growth in length in March and June was faster than the growth in weight.The length-weight relationship in the other months showed that the value of b was not significantly different from 3 (p>0.05)that means the growth in length was equal as the growth in weight ( The plot of the growth coefficient (b) monthly based length-weight relationship (Figure 3) showed that the value tends to be high in January then it was low in March, while ones in other months seem to fluctuate.The fluctuation of b in some months could be caused by spawning season and the growth (Uchiyama & Boggs, 2006).
Combining all monthly length and weight data, the relationship between length and weight of fourfinger threadfin in Tarakan waters was determined as W=0.0212*L 2.9054 (Figure 4).The95%confidence intervalof b was 2.86-2.95so the value of b was significantlydifferent from3 (p<0.05) or the negative allometry that means the growth in length was faster than the growth in weight.

Length at First Captured (Lc) and Length at First Maturity (Lm)
The monthly proportion of mature (stage III and stage IV) to immature (stage I and stage II) gonad showed that the fourfinger threadfin (E.tetradactylum) spawned along the year and the peak was estimated in January and June.The proportion of >50% mature fish occurred during January to June with range of proportion was 60 to 88% (Figure 5).The same period of spawning season in January and June occurred in Chilka lake and the other period of spawning season in March until September occurred in Malacca strait (Patnaik, 1970;Zamidi et al., 2012).
The estimation of length at first captured (Lc) of fourfinger threadfin (E.tetradactylum) is 38.5 cm and length at first matured (Lm) is 39.6 cm (Figure 6).The short gap between Lm and Lc indicated that almost 50% of fish caught by gillnet were already spawned.The growth parameters including the asymptotic length (L  ) and the growth rate (K) was obtained from the shifting modes for each month frequency distribution determined by ELEFAN I.The results showed that there are more than one mode for each month that means more than one cohort of fish were catched by gillnet in Tarakan.The estimated asymptotic length (L  ) was 75.8 cm with the growth rate of 0.3 year -1 (Figure 7).Theoretical age when the fish at zero length (t 0 ) was 0.035 year so the Von Some Population Parameters and…………… in Tarakan Waters, North Kalimantan (Tirtadanu & U. Chodrijah) Copyright © 2018, Indonesian Fisheries Research Journal (IFRJ) Ind.Fish.Res.J. Vol. 24 No. 2 December 2018: Bertalanffy growth model was determined as L (t) = 75.8(1-e -0.3(t+0.035)).
The maximum length was estimated to be reached at relative age about 10 years, while the length at first mature (Lm=39.6cm) correspond to the age of 28-30 months (2.5 years) old (Figure 8).The result of analysis suggested that the fish is long-lived species.

Mortality and Exploitation Rate
The instantaneous total mortality (Z) and natural mortality (M) were 1.2 year -1 and 0.64 year -1 , respectively.The fishing mortality (F) was estimated Figure 8. Age and growth of fourfinger threadfin (E.tetradactylum) in Tarakan waters, 2016.at 0.56 year -1 and the exploitation rate (E) was 0.47 (Figure 9).The optimum exploitation rate based on Gulland (1983) is 0.5, so the exploitation rate of fourfinger threadfin (E.tetradactylum) in Tarakan waters was still at sustainable level.

Discussion
One of demersal fishes that economically important in Tarakan is fourfinger threadfin (Eleutheronema tetradactylum Shaw, 1804) It contributes 11.2% to the total demersal fishery in North Kalimantan Province (DGCF, 2015).The fork length of fourfinger threadfin, which was caught by gillnet in Tarakan waters, was ranged between 16-70 cm FL that was larger than size caught in Malacca strait (38-55.5 cm) by the similar type of fishing gear (Zamidi et al., 2012).The different of fish size was caused by environmental condition and level of exploitation (Campbell et al., 2014).The size was relatively large probably due to different level of exploitation and not overfished yet.
The growth coefficients of fourfinger threadfin were fluctuated between January until November and the highest growth coefficient occurred in January.This was probably related to spawning season that the highest proportion of mature female occurred in that period.Uchiyama and Boggs (2006) noted that this species reaches its highest growth coefficient at the beginning of the spawning season.Combining all monthly length and weight data, the growth pattern was negative allometry.The growth pattern of this species in South Sumatera waters and Chilika, India, was positive allometry, while the growth pattern in Persian Gulf was isometry (Ridho et al., 2010;Karna et al., 2012;Kazemi et al., 2013).The variation of the growth pattern could be caused by season, different ecosystem, and food availability (Ndiaye et al., 2015;Kazemi et al., 2013).
The estimated asymptotic length (L  ) of fourfinger threadfin in Tarakan waters (E.tetradactylum) was 75.8 cm with the growth rate (K) of 0.3 year -1 .The asymptotic length and the growth rate of the species in the present study were very close to the estimated values found by Pember et al., (2005) that were 76.2 cm FL and 0.351 year -1 .E. tetradactylum was hermaphrodite species that changes sex from male to female at size of 28 cm FL (Motomura, 2004).The length at first mature of female E. tetradactylum in the present study was 39.6 cm and reached that size at the age of 2 years.The length at first mature (Lm) of female E. tetradactylum based on Kagwade (1970) ranged between 36-39 cm and reached that size at the age of 2 years.The maximum age of fourfinger threadfin in Tarakan waters was estimated by more than 10 years so it was presumably as the long-lived species.
The annual fishing mortality (F=0.56 year -1 ) of fourfinger threadfin in Tarakan waters was smaller than the annual natural mortality (M=0.64 year -1 ) so the annual exploitation rate (E) of fourfinger threadfin was determined as 0.47.The exploitation rate of fourfinger threadfin in Tarakan was below the optimum exploitation rate based on Gulland (1983) that is 0.5.It means that the exploitation status of fourfinger threadfin in Tarakan waters was still sustainable.This study suggested that the fishing of fourfinger threadfin inTarakanwaters withselectivegear could beanalternative option for the changing of shrimp trawl to gillnet.

CONCLUSION
The size of fourfinger threadfin (Eleutheronema tetradactylum Shaw, 1804) ranged between 16-70 cm FL.The growth pattern was negative allometry.The gillnet that was used to catch this species was relatively selective which has the length at first captured (Lc=38.5 cm) close to the length at first maturity (Lm=39.6 cm).Asymptotic length of the species (L  ) was 75.8 cm and the growth rate (K) was 0.3 year -1 .The estimation of maximum age was more than 10 years old.The exploitation rate (E) was 0.47 so it showed that the exploitation of E. tetradactylum in Tarakan waters was still under sustainable condition.

Table 1 .
The average of length and weight per month for fourfinger threadfin (E.tetradactylum) in Tarakan waters, 2016.