Flexible Seasonal Closures in he Norhern Prawn Fishery S. Beare, L. Chapman and R. Bell Ausralian Bureau of Agriculural and Resource Economics IIFET 2 Proceedings Given high levels of uncerainy associaed wih fish socks, predeermined access righs o he fishery may no deliver he mos efficien oucome. Real ime monioring of he fishery could allow effor o be expanded or decreased in accordance wih a se of performance indicaors. The poenial for using real ime performance indicaors in he Ausralian norhern prawn fishery is examined in his paper using a sochasic opimal conrol model of he fishery. The resuls indicae ha he benefis of using real ime performance monioring o conrol fishery access may be limied. However, his resul may depend criically on he relaionship beween surviving socks and fuure recruis o he fishery. Keywords: prawns, fisheries access, sochasic conrol, geneic algorihm. 1 INTRODUCTION 1.1 Descripion of he Fishery The norhern prawn fishery (NPF) is locaed in he Ausralian fishing zone beween Cape Londonderry, Wesern Ausralia and Cape York, Queensland (figure 1). Covering an area of around 1 million square kilomeres, he NPF is Ausralia s larges fishery and one of is mos valuable. For his fishery he gross value of prawn producion in 1997-98 was esimaed o be $116 million in nominal erms wih a oal harves of around 8 5 onnes (ABARE 1998). Over 9 percen of he cach is expored, wih he principal marke being Japan. More han 5 species of prawn inhabi Ausralia s ropical norhern coasline bu brown iger prawns (Penaeus esculenus), grooved iger prawns (P. semisulcaus) and whie banana prawns (P. merguiensis) accoun for over 8 per cen of commercial landings from he NPF. The wo iger prawn species are he focus of he discussion in his paper. The lifecycle and habia of iger prawn socks in he NPF is described in Crocos (1987a and 1987b). The iming and locaion of iger prawn fishing largely reflecs he physical availabiliy and size of he species hroughou he fishing season. A he sar of he banana prawn season in April, fishing is almos exclusively in he eas of he Gulf of Carpenaria, and he iger prawn cach a his ime is comprised mosly of brown iger prawns. Afer hree almos all banana prawn fishing in he Gulf of Carpenaria has ceased and effor is direced oward brown iger prawns around Morningon Island, in he souh of he Gulf. During he iger prawn season, which opens in early Augus, brown iger prawns are argeed for he firs few, afer which effor swiches o grooved iger prawns. This paern of fishing effor and harves has been relaively sable over he pas five years (Timcke e al. 1999). Currenly, around 13 vessels are acively paricipaing in he fishery. All vessels are purpose buil win gear oer rawls. The flee is echnologically advanced, employing modern packing and freezing capabiliies and sophisicaed elecronic fishing aids such as colour echo sounders, saellie global posiioning sysems (GPS) and ploers. Mos boas in he fishery operae beween 8 and 9 percen of he ime available for fishing wih many unloading heir cach and receiving supplies from moher ships. Recen sock assessmens have indicaed ha iger prawn socks are overfished (Taylor e al. 1999; Die e al. 1999). Tiger prawn landings began o decline in he second half of he 198s and over he pas decade have no recovered o levels of he early 198s. Recen iger prawn caches (269 onnes in 1997, 325 onnes in 1998 and 2986 Figure 1: Locaion of he Norhern Prawn Fishery
onnes in 1999) are well below he esimaed maximum susainable yield (MSY) of around onnes per year. 1 1.2 Managemen of he Fishery The NPF is managed by he Ausralian Fisheries Managemen Auhoriy (AFMA). The presen managemen sysem is he resul of a series of inpu conrols and season and area closures. The experience of he NPF is ypical of many limied enry, inpu conrol fisheries in Ausralia and overseas. Inpu conrols alone, have failed o provide he necessary incenives required for a reducion in excess fishing capaciy. The echnological improvemen of fishing inpus on individual vessels and subsiuion of regulaed inpus wih unconsrained inpus (effor creep) has significanly increased he fishing power of he flee over recen years. Effor creep in he NPF has been esimaed o be as high as 5 per cen a year (Buckworh 1987, Taylor e al. 1999). As a consequence, here have been ongoing reviews and adjusmens o inpu and enry conrols o reduce effecive fishing effor. 1.2.1 Quoa Schemes Alernaive approaches o he implemenaion of inpu conrols and he measuremen of fishing effor have been proposed. One alernaive managemen insrumen which explicily avoids he problem of effor creep is an oupu based sysem of individual ransferable quoas (ITQs). ITQs are ransferable propery righs o harves a specified quaniy of fish. While a marke in ITQs can enable operaors o minimise coss by adjusing he size of heir fishing operaions hrough he purchase, sale or lease of quoa, managers mus specify he oal level of allowable cach (TAC). When managers face a high level of uncerainy regarding a susainable or economically efficien TAC, he benefis of an ITQ sysem may be grealy reduced. For example, if managers make ongoing adjusmen o he TAC o avoid he risks of over exploiaion, he risks of such adjusmen for operaors capial invesmens, including quoa purchases, may limi he efficiency of he flee. Alernaively, managers could purchase and sell quoa wihin he season. This may add significanly o managemen coss, as he coss of purchasing quoa when socks are low are likely o be higher han he value of sales when socks are high. However, allowing long erm access hrough fixed annual quoas may lead o susained lower or under exploiaion of he fishery. 1 Hisorical cach and effor daa for he iger prawn fishery is consisen wih over-exploiaion of socks. However, anoher possible explanaion is ha recruimen has been affeced adversely by as ye unidenified environmenal facors (Taylor e al. 1999). Variable Closures In conras o inpu conrols and a quoa sysem, season and area closures regulae he iming and locaion of fishing effor. The principal objecives of seasonal closures in he NPF have been o proec pre-spawning prawns, o allow prawns o grow o a more valuable marke size before harvesing, hereby ensuring he producion value of he fishery is as high as possible. In addiion o he principal season closures ha are in effec for he enire NPF, here are numerous region specific closures ha operae for eiher par of he year or permanenly (see AFMA 1997 for a deailed lising). Mos of hese closures are designed o proec juvenile prawn socks and he coasal marine environmen. The relaively shor life span of prawns means ha he iming of fishing effor in he NPF is criical from boh a biological and financial poin of view. The NPF season ypically opens in early April each year and closes a he end of November wih a mid-season closure from mid June o he end of July o proec pre-spawning iger prawns. The end-of-year closure is designed o preven caches of juvenile iger prawns and o allow banana prawns o grow o a larger, more commercially valuable size. The fixed season closures of recen years have been criicised by indusry as inflexible. Wih fixed season opening and closing daes, here is cerainy regarding he iming of fishing operaions. However, large flucuaions in socks beween seasons imply ha a flexible approach o deermining he sar and duraion of he fishing season may be advanageous. Wih ongoing improvemens in fishery daa collecion and knowledge of he biological developmen of socks, closures ha are responsive o he saus of arge socks and he marine environmen may be a feasible insrumen for managemen in he NPF. The choice of opening dae for he norhern prawn fishing season has been reviewed a number of imes and a variey of daes wihin he March o May period found o be opimal (see for example: Somers 1985; Brown e al. 1995; Somers e al. 1997). Somers (1985) found subsanial regional and annual variaion in he opimal opening daes for he banana prawn fishery. Somers e al. (1997) exended he Somers (1985) model o include he iger prawn fishery. They found ha he opimal iming of he enire fishing season is sensiive o changes in recruimen iming and naural moraliy. Improved esimaes of prawn recruimen and moraliy relaionships (Wang e al. 1996) mean ha i is now possible o deermine he ne benefis of increased recruimen hrough season closures. I is he srengh and naure of his relaionship ha deermine he coss and benefis of mainaining socks for he following season. 2
In order for he NPF fishery o move o a variable season opening dae, sampling of socks prior o he season opening would be necessary. Based on he size of prawns sampled and cach raes, he opening dae for he fishery could be deermined. The benefis of such a program are likely o depend on he cos and accuracy of he sampling. Prior o 199, opening daes for he NPF were flexible and deermined by pre-season sampling. In 199 he opening dae for he fishery was fixed a 1 April, wih he proviso ha he season could be varied if excepional monsoonal rainfall occurred in he Gulf of Carpenaria. The decision o fix he opening dae recognised he high coss and logisical difficulies associaed wih pre-season sampling (Hill 199). To vary he mid-season and end of season closing daes, i may be possible o analyse logbook daa on volume of cach by size composiion for he curren season o ascerain boh he abundance of socks and he mauriy of prawns in he cach. The primary source of informaion on he saus of socks in he NPF is daily cach and effor logbook daa. Currenly, cach/effor logbooks are compleed manually by vessel skippers and daa on caches only becomes available o fishery managers afer a delay of several. 2 The posiion of he individual boas fishing in he NPF is presenly moniored in real ime using a global posiioning sysem. The developmen of elecronic logbooks may no impose prohibiive coss. 3 Fishers could use a message erminal on board he vessel o complee an elecronic version of he relevan logbook, and hen forward he logbook daa via he ransceiver used o ransmi posiional daa. One condiion for he effecive adjusmen of season closures based on real ime cach and effor daa is ha sock abundance can be accuraely prediced. Tha is, if cach and effor informaion provides some indicaion of he sae of iger prawn socks, hen he exploiaion of socks may be conrolled during he season by an adjusmen o season closing daes. Timcke e al. (1999) showed ha wihin five of he sar of he iger prawn season he annual iger prawn cach could be esimaed o wihin 5 per cen of he acual oal cach. This suggess ha if cach daa were available in real ime 2 For 1996, 1997 and 1998, AFMA repored ha hey received a record for every fishing day from every rawler in he NPF. The qualiy of daa provided in he NPF is regarded as excepionally high (Taylor e al. 1999). 3 Under AFMA s guidelines for he implemenaion of an Inegraed Elecronic Daa Managemen Sysem, indusry is required o mee he full cos of elecronic logbook hardware and 5% of oher coss. i would be feasible, a leas once he iger prawn season had sared, o se an opimal closing dae for he fishery. A second, more general condiion for an effecive closure scheme is ha he closure of he fishery a one sie locaion and ime does no lead o a significan reallocaion of effor o oher locaions and or periods. Given ha he inensiy of fishing effor is relaively consan hroughou he season (Timcke e al. 1999), season closures are unlikely o shif effor from one par of he year ino anoher. A change o he end of season closure o proec grooved iger prawn socks, for example, is unlikely o resul in an increase in effor earlier in he season wih a consequenial impac on brown iger prawn socks. 2 MODELING THE FISHERY The objecive of he modeling exercise was o represen he exising, as opposed o he ideal srucure of he fishery, and he response in fishing effor o policy insrumens inroduced under condiions of uncerainy regarding recruimen socks. The model draws upon an exising biological model of he fishery and overlays his wih saisically esimaed behavioral equaions o represen fishing effor. Economic reurns are hen calculaed using esimaed marke relaionships ha reflec he seasonaliy of prawn prices and fishing coss derived from surveys of he NPF fishing flee. A geneic algorihm (GA) was hen used o deermine opimal policy seing. 2.1 Biological Model Socks of boh brown and grooved iger prawns are assessed annually using he cohor model described by Wang e al. (1996). Assessmens are based on weekly cach and nominal effor (days fished) log book daa for he wo iger prawn species. Acual cach daa are no recorded on a species specific basis. Cach samples were used o deermine he proporion of brown and grooved iger prawns a differen locaions. Log book daa was hen disaggregaed o he species level based on he recorded locaion of he cach. The observed cach daa were fied o he esimaed cach from a populaion model o provide esimaes of annual recruimen and spawning sock. These esimaes were hen used o esimae equilibrium yield as a funcion of effecive fishing effor (acual fishing moraliy). The populaion model is agesrucured and includes naural growh and moraliy, cachabiliy and seasonaliy in paerns of spawning and recruimen. 3
2.1.1 Linking Surviving Maure Biomass o Fuure Recruis Recruis o he fishery in he curren year were modeled as a funcion of he surviving maure socks (over 26 in age) from he previous year. Daa on recruis, R, and surviving socks, S, were derived from he Wang e al. (1996) cohor model and fied according o Ricker s equaion: J 2S 1 ¹ R J S e (1) 1 1 Residuals were normalised as a percenage of he prediced value. For he purpose of sochasic simulaion he error srucure was modeled using a gamma disribuion, wih sandard parameers alpha and bea. The residuals were ranslaed o a non-negaive domain using he minimum residual value. The resuls are summarised in able 1. Table 1: Parameers for socks o recruis relaionship Parameers Brown Tigers Grooved Tigers J 1 18.1* 13.7* J 2 -.16* -.16* R 2.33.8 Alpha 3.11 6.15 Bea.19.9. Minimum residual.5. * significan a he 5% level. The esimaed gamma disribuion for boh socks is righ skewed in level erms. The sochasic mean level of recruis is greaer han he deerminisic mean. 2.2 Prediced Effor/Cach Relaionships e 2 D e 2, D 53 e D K (3) 2, D 16 j 1 c 1, j D j 1 c 2, j where K i are equaion residuals. The effor equaions, esimaed as a sysem, are deailed in able 2. The fied equaions provide a reasonable represenaion of he hisorical effor daa (figures 2 and 3). Table 2: Esimaed equaions for fishing effor by species Variables e 1 equaion e 2 equaion (grooved igers) (brown igers) e i,-53.52699*.3915* e i,-16.2798*.3959* c 1.27277E-3* -.3156E-5 c 2 -.85917E-*.53E-3* R 2.83.93 consan.722 -.3767 * significan a he 5% level. Cach is given, wihou species and ime subscrips by: cach where m f sock 1 exp m o m o () m f e cachabiliy (5) and m f is fishing moraliy and m o is he sum of fishing and naural moraliy Naural moraliy and cachabiliy are hose given in Wang e al. (1996). Esimaes for weekly fishing effor for brown and grooved iger prawns were based on weekly, species specific cach and effor daa for he fishery for he period January 1987 o December 1998. Fishing effor, e, is expressed as oal boa days fished and is spli beween each species. Cach daa, c i, is he oal cach weigh of prawns of species i in week in kilograms. The weekly fishing effor for brown iger prawns (2) and grooved iger prawns (3) was esimaed o be relaed o effor in he corresponding week of he previous wo years and cach of each species over he pas four : e 1 D D (2) e1, 53 D e1, 16 D13 j 1 c1, j D c2, j K j 1
1 12 1 esimaed effor acual effor 8 6 2 boa days Figure 2: Fishing effor for brown iger prawns 1 12 1 esimaed effor acual effor 8 6 2 boa days Figure 3: Fishing effor for grooved iger prawns 2.3 Coss and Reurns The gross revenue from fishing was esimaed as he produc of he number of prawns caugh and he expor price obained. The majoriy of prawns from he norhern prawn fishery are expored o Japan and Japanese prawn prices are highly seasonal. Esimaes for he weekly price of prawns were based on hisorical daa for he primary wholesale price of iger prawns received ouside he Tokyo cenral marke from June 199 o Sepember 1999. In erms of price received by suppliers, here is no disincion beween brown and grooved iger prawns. Prawns are caegorised ino one of five grades and sold in 1.5 kilogram los, wih he number of prawns in a lo deermining he grade of he lo. The larges prawns arac he highes wholesale prices. Prawn grades considered are: (1) under 6 prawns; (2) 6-8; (3) 9-12; () 13-15; and (5) more han 16. For each grade, he weekly iger prawn price was deermined as: p g 5 2 E gk p g, E H for g = 1 o 5 (6) g 1 k 1 k where g is he grade of prawn, k is he number of lagged price erms included and H g is he equaion residual. Inclusion of higher order auoregressive erms was considered and rejeced. The esimaed equaions are deailed in able 3. g 5
Variable P1 equaion P2 equaion P3 equaion P equaion P5 equaion P1-1.6786* -.38 -.77 -.212.5 P1-2.2529*.169.8.1 -.82 P2-1.259* 1.877*.1158.37 -.29 P2-2 -.198* -.93 -.993.171.236 P3-1.95*.353.68*.11*.682* P3-2 -.228 -.21.3218* -.72* -.338 P -1 -.1595* -.887.1776*.8986*.598 P -2.89.718 -.29*.161 -.75 P5-1.1288.353.985.65.982* P5-2 -.81.12 -.656.175.39 consan -5.56 1.8285 85.7813* 9.6285 53.95* SE(e ) 96.32 76.12 98.135 76.693 6.825 R 2.969.982.935.9715.9778 LLF -282.7-2691.66-281.91-2695.29-2586.57 * Significan a he 1 % level. Table 3: Esimaed equaions for prawn prices by grade IIFET 2 Proceedings The cos daa used in he model were derived from ABARE surveys of he indusry. Capial coss were excluded from he model so esimaes of he coss associaed wih fishing are based on operaing coss. The operaing coss were separaed ino packaging coss, fuel coss and crew coss and are shown in able. Packaging coss are dependen on he weigh of he cach. Fuel coss are esimaed as a funcion of effor in erms of boa days and reflec boh he price of fuel in he region and he geographic spread of fishing effor. Crew coss are esimaed as a fixed percenage of cach revenue. 2.5 Opimisaion The objecive of his analysis is o examine he relaive effeciveness of alernaive seasonal closure rules. The firs is a fixed closing dae. The second is a real ime rule based on observable indicaors of he curren saus of he fishery (a performance rule). Brow n iger prawn cach (millions of prawns) 5 3 Table : Operaing coss associaed wih fishing Uni Value Packaging coss $/kg.5 Fuel coss $/day 85 Crew coss % of revenue.26 2. Hisorical Validaion 2 1 8 Brow n iger prawn effor (boa days) To assess he validiy and accuracy of he model, weekly cach and effor predicions generaed by he model were compared wih daa from Wang e al. (1996). Predicion errors were generaed for cach and effor from one year forward projecions beween he years 1995 o 1997. The relaive sandard errors of he predicions are repored in able 5 and acual and prediced effor and cach for he 1997 biological year are shown in figure. Table 5: Relaive sandard errors of cach and effor daa, November 199 Ocober 1997 Brown igers Grooved igers Cach.7.6 Effor.19.33 6 2 Figure. Coninued on page 7 6
Grooved iger prawn cach (millions of prawns) 5 relaively well, especially in he presence of uncerainy regarding biological parameers affecing he fishery. This uncerainy includes, iniial sock condiions and recruimen relaionships. 3 2 1 8 6 2 Grooved iger prawn effor (boa days) hisorical daa ------ model prediced daa When CPUE is increasingly subjec o random or oher unexplained sources of variaion, he reliabiliy of CPUE in a closure rule indicaor declines. The use of a performance based closure rule under hese condiions may sill be advanageous in ha i allows greaer harves during periods of abundance and reduced harves when socks are low. However, random variaion in he CPUE measure may lead o premaure closing of, or unwarraned exensions o, he fishing season which may adversely affec boh reurns and fish socks. The lack of reliabiliy of poin in ime measures may be parially overcome by he use of moving averages. However, as he lengh of he moving average increases, he measure can become an increasingly poor predicor of he curren sae of he fishery. This inroduces he need o consider an opimal window over which he performance indicaor is consruced. 2.5.2 Simulaion Design Figure : Comparison of model prediced weekly cach and effor wih hisorical daa, November 1996 o Ocober 1997 2.5.1 Indicaors Ideally, indicaors used in closure rules should be objecively and accuraely measurable a any poin in ime. Mos indicaors will be subjec o measuremen error which may be sysemaic in some cases (for example, under-reporing). However, even an ideal se of indicaors is unlikely o provide full informaion on he sae of he fishery because uncerainy will always be a key aspec of fisheries managemen. The indicaor considered here is oal cach per uni effor (CPUE). The CPUE rule was implemened using he combined oal of brown and grooved iger cach as boas do no presenly record separae cach by hese species. Wihin he fishery model he relaionship beween CPUE and cach may be specified in a number of ways. When cach is in fixed proporion o he level of effor and socks, CPUE is equivalen o a measure of curren socks. Hence, i is reasonable o expec ha a CPUE closure rule under hese condiions will perform As discussed previously, he NPF is currenly subjec o opening, mid-season and final season closure rules, as well as regional closures. The resuls presened here are resriced o he deerminaion of an opimal final season closure rule, eiher a a fixed poin in ime in each season or when a k period moving average measure of CPUE falls below an opimal olerance level (ol). cpues d ol (7) k s k The wo closure alernaives are compared under hree scenarios. In he firs scenario, he relaionships beween (1) surviving prawn socks and recruimen; and (2) prediced effor and prediced cach, are purely deerminisic. In he second scenario he relaionship beween socks and recruimen is sochasic while he relaionship beween prediced effor and prediced cach is deerminisic. Tha is, where here is a reasonably good performance measure under uncerain condiions. Finally, in he hird scenario boh relaionships are uncerain, wih he proporion of socks caugh per uni of effor allowed o vary uniformly by 2 percen. The geneic algorihm discussed below is easily adaped in he sochasic simulaions o selec he size of he window o compue he average CPUE value as well as he criical value. 7
The iniial condiions for he simulaion were based on cach, effor and surviving biomass for he 1997 and 1998 seasons. rials, and assigned a finess value equal o he average finess over he rials. Following Goldberg, a cross-over rae of.6 and muaion rae of.1 was used. 2.5.3 Use of Geneic Algorihms A geneic algorihm (GA) is a search echnique ha has been successfully applied o problems wih complex dynamic srucures ha canno be easily handled wih radiional analyical mehods. The GA approach was firs developed by Holland (1975) and has subsequenly been widely employed in economics and finance research as a flexible and adapive search algorihm (see for example: Alemdar e al. 1998; Beare e al. 1999; Beare e al. 1998; Birchenhall 1995; Ching-Tzong e al. 1997). The approach provides a globally robus search mechanism wih which o opimise over a decision process involving uncerainy in he form of a lack of a priori knowledge, unclear feedback of informaion o decision makers and a ime varying payoff funcion. A GA performs a muli-direcional search by mainaining a populaion of individual sraegies, each wih a poenial soluion vecor for he problem. An objecive funcion is employed o discriminae beween fi and unfi soluions. The populaion undergoes a simulaed evoluion such ha a each generaion, he relaively fi soluions reproduce while he relaively unfi soluions die ou of he populaion. During a single reproducive cycle, fi sraegies are seleced o form a pool of candidae sraegies, some of which undergo cross over and muaion in order o generae a new populaion. Cross-over combines he feaures of wo paren sraegies o form wo similar offspring by swapping corresponding segmens of he parens. This is equivalen o an exchange of informaion beween differen poenial soluions. Muaion inroduces addiional variabiliy ino he populaion by arbirarily alering a sraegy by a random change. In deermining he opimal harves sraegy each GA sraegy or sring conains a possible seing of he decision rule or rules for closing he fishery. The lengh of each sring corresponds o he number of parameers o be esimaed. Those srings ha give a relaively high ne revenue are given greaer weigh in he formaion of he nex generaion of srings. Afer a number of generaions, he soluion may converge, wih he bes individual srings represening he opimum soluion. The geneic search algorihm was implemened in MaLab using he approach described in Goldberg (1989). Given he compuaional ime required, a small sample sraegy was adoped. The search was conduced over 5 generaions using 2 populaion srings. For he sochasic simulaions, each sring was evaluaed over 15 3 RESULTS The resuls from he wo closure rules under various levels of uncerainy are presened in able 6. Under deerminisic seady sae condiions, an opimal closure dae and an opimal CPUE based closure rule are equivalen managemen insrumens. The small differences observed beween he wo opions in he model simulaion are due o he fac ha he model begins from a disequilibrium posiion of below average spawning socks. Wih he inroducion of sochasic recruimen relaionships, effor and cach levels are higher reflecing he righ skewed disribuion of he Ricker equaion errors. Under he closure dae regime, he opimal season lengh is expanded o exploi he addiional socks which resul in high recruimen years, when compared o he deerminisic opimisaion. The season lengh expands and conracs wih he level of socks under he CPUE closure rule. However, he opimal season lengh is unchanged, on average, using he CPUE based closure rule, when compared o he deerminisic opimisaion. This reflecs he esimaed Ricker recruimen relaionships. While here is a greaer probabiliy of a large increase in recruimen han a large decline, he marginal value of surviving socks increases as populaions decline. Hence, he benefis of reducing effor when socks are low balance he benefis of expanding effor when socks are high. Neverheless, he ne benefis of he CPUE based rule are relaively small. Over he 15 year horizon, ne revenue increases by abou $18m. Wih he inroducion of a 2 per cen uniform variaion in cach for a given level of socks, here is no significan change in he opimal closure dae regime. For he CPUE closure dae rule, he opimal lengh for he moving average was four. On average, he opimal seasonal lengh is reduced when compared o he previous opimisaions. The difference in ne revenue beween he opimal closure dae and he CPUE closure rules declines o abou $m. This reflecs he reduced reliabiliy of he CPUE performance measure as an indicaor of he saus of fish socks. The recruimen relaionships in he model allow socks o recover quickly regardless of he fishing pressure placed on he prawn populaions. This is poenially a key assumpion in any assessmen of boh he biological and economic susainabiliy of he fishery. If, for example, 8
reducions in sock levels below criical levels resuls in longer erm impacs on he fishery, he benefis of limiing access based on real ime monioring of fish sock may be much more subsanial. CONCLUSIONS The uncerainy associaed wih surviving socks and fuure recruimens reduces he effeciveness of mos managemen opions for he NPF. Variable closure rules, based on real ime monioring rules, may allow managers o preserve socks when hey appear o be low and exploi socks during abundan seasons. However, he subsequen impac of reduced curren cach on fuure socks is highly variable. As a consequence, he benefis of esablishing access righs based on real ime performance measures may be limied. However, wihin he biological framework of he model, he impacs of exploiaion are readily reversible. If over exploiaion resuls in susained damage o he reproducive capaciy of prawn socks, he value of real ime monioring could be subsanially higher. Table 6: Numerical opimisaion resuls Opimal Closure Dae Opimal CPUE Closure Rule Brown Grooved Brown Grooved uni Mean Sd Dev Mean Sd Dev Mean Sd Dev Mean Sd Dev Deerminisic Season wks 1 na 1 na 1 na 1 na Effor a days 67 na 27 na 6528 na 25 na Cach m 62. na 36. na 62. na 36.3 na Revenue $m 13.5 na 9.9 na 13.5 na 1.1 na Sochasic 1 Season wks 16 na 16 na 13.9.7 13.9.7 Effor a days 6917 19 5231 881 713 161 599 1236 Cach m 69 29 5 13 7.9 31.5 6.9 15.6 Revenue $m 15.2 7. 12.5. 15.7 7.8 13.2 5. Sochasic 2 Season wks 16 na 16 na 13..2 13..2 Effor a days 6919 113 5229 881 692 1616 5333 17 Cach m 68 3 5 13 69. 31. 5.6 1.2 Revenue $m 15.2 7.5 12.5. 15.3 7.9 12.7. a effor measured in boa days. 5 REFERENCES ABARE 1998, Ausralian Fishery Saisics, Canberra. AFMA 1997, Norhern Prawn Fishery 1997, Informaion Bookle for masers and owners of NPF rawlers, carrier boas and processor boas, Canberra. Alemdar, N. and Ozyildirim, S. 1998, A geneic game of rade, growh and exernaliies, Journal of Economic Dynamics and Conrol, vol. 22, pp. 811 832. Beare, S., Bell, R. and Fisher, B. 1998, Deermining he value of waer: he role of risk, infrasrucure consrains, and ownership, American Journal of Agriculural Economics, vol. 8, no. 5, pp. 916. Beare, S., Bell, R. and Harrison, S. 1999, Sochasic opimal conrol in bio-economic simulaions: a comparison of soluion echniques, ABARE conference paper 99. presened a he combined 3 rd annual Ausralian and 6 h annual New Zealand Agriculural and Resource Economics Sociey Conference, Chrischurch, New Zealand, 2 22 January 1999. Birchenhall, C. 1995, Modular echnical change in geneic algorihms, Compuaional Economics, vol. 8, pp. 233 53. Brown, D., Pascoe, S. and Whiham, J. 1995, Seasonal Closures in he Norhern Prawn Fishery, ABARE repor o he Norhern Prawn Managemen Advisory Commiee, Canberra, Sepember. Buckworh, R. 1987, Changes in fishing effor and caching power in DMZ iger prawn fishery, in Norhern Prawn Fishery Informaion Noes, Number 1, CSIRO, Cleveland, Queensland, pp. 2 3. 9
Ching-Tzong, S. and Wen-Tsuan, T. 1997, A geneic algorihm approach employing floaing poin represenaion for economic dispach of elecric power sysems, ModSim Conference, Hobar. Crocos, P. 1987a, Reproducive dynamics of he grooved iger prawn, Panaeus semisulcaus, in he norhwesern Gulf of Carpenaria, Ausralia, Ausralian Journal of Marine and Freshwaer Research, vol. 38, no. 1, pp. 79 9. Crocos, P. 1987b, Reproducive dynamics of he iger prawn, Panaeus esculenus, and a comparison wih P. semisulcaus in he norh-wesern Gulf of Carpenaria, Ausralia, Ausralian Journal of Marine and Freshwaer Research, vol. 38, no. 1, pp. 91 12. Die, D. and Bishop, J. 1999, Saus of iger prawn socks a he end of 1998, NPFAG Working Paper 99/1, CSIRO, Cleveland, Queensland. Goldberg, D. 1989, Geneic Algorihms in Search, Opimisaion and Machine Learning, Addison- Wesley Publishing Company, New York. Hill, B. (ed.) 199, Norhern Prawn Fisheries Assessmen Repor 1993, AFMA, Canberra. Holland, J. 1975, Adapaion in naural and arificial sysems, Universiy of Michigan Press. Somers, I. 1985, Maximising value per recrui in he fishery for banana prawns, Penaeus merguiensis, in he Gulf of Carpenaria, in Rohlisberg, P.C., Hill, B.J. and Saples, D.J. (eds), Second Ausralian Naional Prawn Seminar, Cleveland, Ausralia, pp. 185 91. Somers, I. and Wang, Y-G., 1997, A simulaion model for evaluaing seasonal closures in Ausralia s mulispecies norhern prawn fishery, Norh American Journal of Fisheries Managemen, vol. 17, pp. 11 3. Taylor, B. and Die, D. (eds) 1999, Norhern Prawn Fishery: 1997 and 1998 Fisheries Assessmen Repor, AFMA, Canberra. Timcke, D., Harrison, S., Bell, R. and Chapman, L. 1999, Applying ESD principles in he norhern prawn fishery: an economic assessmen, ABARE repor o he Fisheries Resources Research Fund, Canberra. Wang, Y-G. and Die, D. 1996, Sock-recruimen relaionships of he iger prawns (Penaeus esculenus and Penaeus semisulcaus) in he Ausralian norhern prawn fishery, Ausralian Journal of Marine and Freshwaer Research, vol. 7, pp. 87 95. 1