The Biological Optimal Level of the Arafura Shrimp Fishery furwanto ) THE BIOLOGICAL OPTIMAL LEVEL OF THE ARAFURA SHRIMP FISHERY

This paper briefly describes the past development of the shrimp fishery in the Arafura Sea, including intensity of illegal fishing, and presents the impact of increasing fishing pressure on the quantity of catch and biomass. The maximum sustainable yield and the optimum fishing effort are estimated.

The fishing capacity of trawl fleet in the Arafura Sea was continuously developed.This has substantially reduced the abundance of demersal stocks in this area.Naamin (1984); Badrudin ef a/. (2002)estimated optimaleffort required to produce optimal yield from the utilisation of the shrimp stock in theArafura Sea.Meanwhile, Widodo et al. (2001)   evaluated the development of demersal fishery, including shrimp fishery, in the Arafura Sea and concluded that the stocks of demersal fishes and shrimps in this area were over exploited.The over exploitation of shrimp stock in the Arafura Sea has threatened sustainability of these resources (Widodo   et a|.,2001).
The condition of the Arafura shrimp fishery had been worsened by illegalfishing practices undertaken by Indonesians and foreigners resulting in economic losses and a fishery management failure.However, studies undertaken previously did not taken into account the magnitude of the illegal fishing practices in the assessment of shrimp stock and the optimal fishing effort.lf the level of illegal fishing practice was substantial, the result of assessment would be bias, and management measures formulated on the basis of the assessment would be misleading.
Therefore, this study estimated the intensity of illegal fishing practice in the Arafura shrimp fishery.The result of this estimation then used to assess the optimal utilisation of shrimp stock in the Arafura Sea and the impact of the illegal fishing on the fishery.

MATERIALSAND METHODS
Data used in this article consist of the number of shrimp trawlers and fish trawlers, and the catch of shrimps per unit vessel, during the year 1996-2005.Fishing power of the fish trawler to catch shrimps was about25o/o of the shrimp trawler.Therefore, fishing effort in theArafura shrimp fishery was standardized in the number of shrimp trawlers.Sources of data on the number of fishing vessels holding fishing license were (i)  Analysis undertaken on the basis of the Schaefer  (1957) production model.The model is also discussed in Clark (1976).The equations of the model used here are as follows: Natural population growth function: G(x)=1x.11-"/K).. .

Biomass function:
where: q, = estimated catchability coefficient in year t =-alb-(E,+E*,'112 = catch per unrt effort in year t = constant oarameter with a value of 2 for the Schaefer model = fishing effort in year t The average catchability coefficient over n years (q), parameters K, and rwere estimated as follows: q : expflf:rt tn1 rI.1 K=a/q. .. (9 r=q.a/b ......( 10As shrimp stock in theArafura sea was also fished by vessels operated illegally, the number of those vessels was estimated on the basis of the data on illegal fishing practices in the year 2003-2008 from the Directorate General of Fisheries Surveillance.A trend analysis was undertaken to estimate the intensity of illegal fishing in theArafura shrimp fishery.The relationship between the percentage of the number of fishing vessels operated without license in year t and the corresponding year was analysed using selected equation as follows: Y,=€lo+a1.{+2r.t2+ where: Y, = the percentage of the number of fishing vessels operated without license in year t t = the year, 6 for 1996, ..., and 18 for 2008 The number of illegal fishing vessels was estimated by using the following equation: 4,={[1 0O/(t OO-y,)]-1 ]-V* = the number of permitted fishing vessels operating fish trawl in year t = the number of fishing vessels operating fish trawl without license in year t 9r= x = the stock biomass r = the natural growth rate of population q = catchability coefficient K = the environmentally limited maximum biomass or carrying capacity E = fishing effort H = Yield or total catch Maximum sustainable yield and fishing effort to produce maximum sustainable yield (E"rr) can be estimated by using the following equations: MSY=r.K/4 ... ..(S E"rr=r/2q ............ ( 6Biological parameters r, K, and q were estimated using the value of coefficients a and b of the catch oer unit effort function as follows: U=a-b.E where: Coefficients a and b of this equation were estimated 4n by ordinary least square.Then, a method developed by Fox (1975) was used to estimate parameters q, r, Vo, and K as follows: (4a BO RESULTS

Development of Fishing Fleet
The main fishing gear to utilize shrimp resources in the Arafura Sea was the double rigged trawl.This fishing gear was introduced to exploit shrimp resources in the Arafura Sea in early 1970.The fishing operation of shrimp trawlers was concentrated in the waters along the western coast of Papua, and the waters around Aru and Dolak lslands, as shown The Biological Optimal Level of the Arafura Shrimp Fishery (Purwanto) by the recorded tracks of the vessels presented in Appendix 1.Another gear was the otter trawl, even though the main target of this gear was fish stocks.
The development of fleet permitted to catch shrimps in the Arafura Sea is shown in  The average'dize of shrimp trd'wlers g-ranted fishery association and some skippers, occurred in fishing license was relatively stable at about 130 GT the year l ggg-2000.The Association estimated that during the year 1996-2005.Meanwhile, the average the intensity of illegal fishing in 1996 was similar to size of fish trawlers operated legally in Arafura Sea the intensity in eOO+.A guelstimate of the number increased from 184 GT in 1996 to 218 GT in 2005. of fishing vessels operated in the Arafura Sea without license was about 5% of the total number of vessels There was illegal fishing practices in Arafura targeting shrimp stock in 1996.Meanwhile, the shrimp fishery that has been conducted before 1996.proportion of the number of fishing vessels operated The highest intensity of the illegal fishing practices without fishing license in Arafura Sea was about17"/o in this area, as informed by the lndonesian shrimp in 2OO3 (Figure 2).(13 R2=0.936; -t-statistics in the parentheses are significant at P<0.05By using equation ( 13), the proportion of the fishing vessels operated in the Arafura Sea without license and the number of these illegal fishing vessels were estimated and presented in Appendix 2 and graphically shown in Figure 3.The highest percentage of the number of fishing vessels operated without license was estimated to be 28.3"/o in 2000 (Figure 3a).Therefore, the number and the total vessel tonnage of this illegal fishing vessels in 2000 was about 312 vessels and 66,700 GT, respectively (Figure 3b).The intensity of the illegal fishing Yi'.r' I )J lr:."t1:,.'r't-.i rr"':r'i.... -.. I dt.:i , _.:,-.ilic|ilt.i"]r' (a) the proportion of the vessels operating fish trawl without license in 1996-2008 and (b) the estimated number and the total tonnage of vessels operating fish trawl without license in 1996-2005, in the Arafura Sea.

Development of Fishing Effort, Catch Per Unit Effort and Total Catch
As shown in Appendix 3 the total fishing effort increased to the maximum (equalto 801 shrimp trawler units) in 2000, then continuously declined to 509 units in 2005 (Figure 4a).On the contrary, the observed catch per unit effort decline to the minimum (about 57.5 tonnes/vessel/year) in 2000, then increased to 89.5 tonnes/vessel/year in 2005.The relationship between the catch per unit effort (U) and the fishing effort (E) in theArafura shrimp fishery was significantly represented by the following equation: U=148,970-0.121E.......... (14  (20.377).*(10.617)".R2=0.934; ** t-statistics in the parentheses are significant at P<0.001.
The estimated catch per unit effort was also at the minimum (about 51.9 tonnes/vessel/year) in 2000.
Meanwhile, the estimated catch per unit effort in 2005 was about 87.1 tonnes/vessel/year (Figure 4b).
After reaching the maximum rate, decreasing the biomass resulted in lower growth rate (Figure 6a).The relationships between (a) shrimp catch per unit effort and fishing effort and (b) shrimp biomass and fishing effort in the Arafura shrimp fishery.
-J' '*f:r .:., ilr t,.l l-:1.! lalii i_r(r ';, tr,.',, l[]i'(, tr-,'r'.0.'.r Figure 6. The relationships between (a) biomass growth rate and biomass and (b) shrimp catch and fishing effort in the Arafura shrimp fishery.Similarly, the catch or yield increased to the maximum level with increasing fishing effort (Figure 6b).After attaining the maximum sustainable yield, increasing fishing effort resulted in lower catch.The relationship between the quantity of catch and fishing effort in the Arafura shrimp fishery was as follows: h=148,970 E-0.121 E2 (17 r_, 130 Jl0 i,,10 Ll0it li){i0 illjtlr f' r:liirtr rI ltlr t ( r.r1t iiil The estimated optimal levels of fishing effort, biomass, harvest, and catch per unit effort in the Arafura shrimp fishery are presented in Table 2.The maximum sustainable yield of the shrimp stock in the Arafura Sea was about 45,850 tonnes/year resulting from the operation of 616 units of shrimp trawlers, this was the biologically optimal level of fishing effort (E".").
Fishing effort, bromass, harvest, and catch per unit effort in the Arafura shrimp fishery in various years and their estimated biological optimal levels The actual fishing effort of fishing fleet operated in the Arafura shrimp fishery, taking into account fishing vessels operated illegally, was higher than the Er* during 1998-2003.In 2003, the effort was 647 units.Meanwhile, the lowest and the highest ones were 628 units in 1998 and 801 units in 2000, respectively (Figure 4a).Therefore, it could be concluded that the Arafura Sea shrimp biomass was over exploited during that period.As consequences, the catch per unit efforiwas lower thanthe catch per unit effortat maximum sustainable yield (Figure 5a), and the yield was lower than the maximum sustainable yield (Figure 6b), During 2004-2007,the shrimp stock in the Arafura Sea was biologically under exploited as operated fishing effort declined to the levels lower than the E*r" (Figure 4a).
The operation of fishing vessels without license created two kinds of direct losses, namely a decrease in the catch of shrimps landed by permitted vessels and a decrease in the domestic supply of fishes.In 2003, total fishing effort was 647 vessels producing 45.7 thousand tonnes of shrimps (Table 3).However, 2.9 thousand tonnes of shrimps was taken away by 41 vessels that was operated illegally, without fishing license.They could export the catches directly from The Biological Optimal Level of the Arafura Shrimp Fishery Qurwanto) fishing ground to the market overseas.This resulted in a decrease in the domestic supply of shrimps.
Meanwhile, 606 vessels holding fishing license landed 42.8 thousand tonnes of their catches in Indonesian ports before exportation.lf illegal fishing practice did not exist and shrimp stock was utilised by permitted vessels only, that was 606 units, this vessels would land 45.8 thousand tonnes of shrimps.Adecrease in the catch of shrimps landed by permitted vessels, amounting of about three thousand tonnes in 2003, was an impact of additional fishing pressure resulting from the operation of illegal fishing vessels that caused shrimp stock depletion.
The fishing effort in theArafura shrimp fishery could increase again to the level higher than the Er." as the Indonesian Directorate General of Capture Fisheries had issued a number of fishing allocation certificates (Surat ljin Usaha Perikanan) to provide fishing companies with additional fishing allocation.By 2007, the total fishing allocation that had been provided to the fishing companies consisted of 440 shrimp trawlers and 1112 fish trawlers (Directorate General of Capture Fisheries, 2007)that could result in 71 B units of fishing effort, the level higher than the Er.r.In 2007, only 341 shrimp trawlers and 500 fish trawlers had been operated (Directorate General of Capture Fisheries, 2007), resulting in 466 units of fishing effort.An amount of 252 units was latent effort that would increase fishing effort to 7'18 units when new fishing allocation consisting of 99 shrimp trawlers and 612 fish trawlers were operated.lf fishing effort was 718 units, biomass, catch per unit effort, and harvest would decrease to the levels lower than their optimal levels (Table 2).

Discussion
Arafura Sea is the most important fishing ground for shrimp fishery in Indonesia, as it is very productive area for shrimp fishery that attracted fishers to operate in this area for fishing either in legal or illegal ways.
The optimal catch of penaeid shrimps resulting from theArafura Sea as estimated in this studywas about 45.850 tonnesiyear.lt is close to the optimal level of the penaeid catch stated in the Decree of Agriculture Minister number No.995 of 1999, that is 43,100 tonnes per year.The difference between those two values is 6.4%.The difference may be caused by the inclusion of the fishing effort of illegaltrawlers in the estimation of optimal catch undertaking in this study.
The total variability of annual catches harvesting from certain stocks is affected not only by fishing effort variations but also by environment variations (Freon,  1986).The result of a study conducted by Purwanto (1997), using modified von Bertalanffy growth model, showed that growth of Penaeus monodonwas affected by various environmental variables, namely salinity, water temperature, and water transperancy.
Meanwhile, Okey & Poloczanska (2008), from their review, identify that there are likely to be significant climate change impacts on the biological, economic, and social aspects of prawn fisheries of northern Australia.As the study presented here did not take into account environmentalvariables that may affect the catch of shrimps from Arafura Sea, it is suggested to incorporate the environmental variables into the model used in the future assessment of shrimp stock in theArafura Sea.
The result of this study also shows the positive impact of the improvement on the fisheries management undertaking by the Indonesian Government during 2000Government during -2005. .The improvement covered activities to improve fisheries licensing services, to re-register all fishing licenses and to increase fisheries surveillance capacity and activity, and law enforcement.The objectives of this activities were minimising the number of illegal fishing vessels operated by Indonesian fishing companies and minimising the intensity of illegal fishing practices in the Indonesian waters in order to optimise the utilisation of fishery resources for Indonesians'welfare.
Therefore, the number and the total tonnage of the shrimp trawl and fish trawl vessels that could be granted fishing licences decreased during year 2001-2005.The intensity of the illegalfishing practices also decreased during that period.The decrease in illegal fishing practices decreased catch losses.
Consequently, productivity of fishing vessels, as indicated by catch per unit effort, increased considerably.Unfortunately, the intensity of illegal fishing practices by foreign vessels increased since 2007.This was a coincidence that the Government of Indonesia did not renew fishing licenses for foreign vessels since 2007.However, this problem could be solved by appropriate tactic and strategy on fisheries surveillance and consistent law enforcement.
Further improvement on fisheries management especially on the management of licensing services should be undertaken to avoid the over allocation on fishing capacity, as happened in 2007, in order to avoid possible over exploitation on shrimp stock.This improvement would cover at list the data and information management and the decision mechanisms.When deciding the amount of vessels that would be granted licences to harvest shrimps, however, the impact on other species should be taken into account, as species composition of catches $Irimp trawlers and Ar .l'resulti ns from trggn;r,,flXx E=S9:rl:*.-lEESSEEQFet :;;,rt;.@c")sr(E o fish trawlers consists of shrimps and demersalfishes (Widodo et al., 2001).The optimal number of vessels that would be granted fishing licences to operate in the Arafura Sea could be estimated by using a multispecies multigear fishery model, in which the result of this study could be incorporated.

CONCLUSIONS
1. On the basis of the result of the analysis shows, it can be concluded that the maximum sustainable yield of the shrimp stock in the Arafura Sea was about 45,850 tonnes/year resulting from the operation of 61 6 units of shrimp trawlers.Although, the Arafura shrimp stock were over exploited in 2000, as an impact of mis-management and illegal fishing practices, the improvement on the fisheries management undertaking by the Indonesian Government during 2000-2005 had resulted in the positive impact.ln 2007, the shrimp stock had recovered from over exploitation.However, fishing effort in the Arafura shrimp fishery could increase again to the level higher than the optimal level as total fishing allocation in SIUP that had been provided to the fishing companies was higherthan the optimal allocation (over allocation) by 2007.

Further improvement on fisheries management
especially on the management of licensing services should be undertaken to avoid the over allocation in order to avoid possible over exploitation on shrimp stock.
the statistic of Indonesian-flag and Foreignflag Fishery Vessels (Directorate General of Fisheries, 1998), (ii) Indonesian Fisheries Statistic (Directorate General of Fisheries 1999, 2000; Directorate General of Capture Fisheries, 2001), and (iii) various Corresponding author: Jl.Pasir Putih I, Ancol Timur-Jakarta 14430, E-mail: rccf_office@indo.net.idInd.Fish Res.J. Vol.16 No. 2 Desember 2010: 79 -90 unpublished records of data on permitted fishing vessels in 1999-2005, from the Directorate of Business Services of the Directorate General of Capture Fisheries.Meanwhile, data on the catch of shrimps per unit shrimp trawler were from the Indonesian Shrimp Fishery Association.
(a)  The number of trawl vessels granted license to operate in Arafura Sea in1986-2005   and   (b) their total tonnaqe durinq1996-2005.
(a)  The fluctuation of fishing effort and (b) catch per unit effort in the Arafura shrimp fishery, 1 996-2005.The value of biological parameters of population growth and production of shrimp stock in the Arafura Figure 5.