Please note that ths s an author-produced PDF of an artcle accepted for publcaton followng peer revew. The defntve publsher-authentcated verson s avalable on the publsher Web ste Marne Polcy December 2014, Volume 50, Part A, Pages 207-214 http://dx.do.org/10.1016/j.marpol.2014.06.005 2014 Elsever Ltd. All rghts reserved. Archmer http://archmer.fremer.fr The effect of dscards and survval rate on the Maxmum Sustanable Yeld estmaton based on landngs or catches maxmsaton: Applcaton to the nephrops fshery n the Bay of Bscay Jord Gullen a, b, *, Clare Macher a,, Matheu Merzéréaud a, Spyros Ffas c, Olver Guyader a a IFREMER, UMR AMURE, Unté d Econome Martme, BP 70, F-29280 Plouzané Cedex, France b Insttut de Cènces del Mar CSIC, 08003 Barcelona, Span c IFREMER, Unté Scences et Technologes Haleutques, BP 70, F-29280 Plouzané Cedex, France *: Correspondng author : Jord Gullen, emal address : jordgullen@hotmal.com Abstract: MSY objectves are beng adopted as major fsheres management targets worldwde. However, n fsheres where dscards account for an mportant share of the catches, the yeld, effort, and bomass at MSY can be sgnfcantly dfferent when accountng or not for dscards and thus when, basng the yeld estmatons on landngs or catches. Also whether accountng for a certan survval rate, of dscards can lead to dfferent MSY targets estmates. Hence, both decsons have mportant, mplcatons on catch and effort reducton recommendatons, and consequently on the quota, calculatons for fsheres managed by quotas. Results from the Bay of Bscay nephrops fshery confrm, that the optmal explotaton level can vary sgnfcantly when optmsng for catches or landngs, and, by assumng a certan survval rate of the dscards. By dong ths, the present study also allows to, explore the bass to clarfy the defnton of MSY n fsheres where dscardng takes place. Hghlghts The presence of dscards affects the MSY estmaton. MSY levels change when basng the yeld estmatons on landngs or catches. Effort level s hgher when maxmsng catches than landngs. Hgher survval rates of dscards lead to hgher effort and quotas at MSY. Keywords: Maxmum Sustanable Yeld (MSY) ; Dscards ; Nephrops ; Survval rate 1
1. Introducton MSY s wdely nterpreted as the maxmum long term average catch that can that can be acheved under prevalng condtons, ncludng both the state of the ecosystem and sze selectvty of the fshery [1]. The target of fsheres operatng at MSY meets both bologcal sustanablty and producton maxmsaton objectves [2-3]. Achevng MSY has become a key ssue for fsheres polcy as agreed at the 2002 Johannesburg s World Summt that stated to acheve the MSY objectve for all stocks n 2015 [4], whch was re-terated at the Ro+20 Unted Natons Conference of Sustanable Development n 2012 [5]. As a result, MSY has been adopted as a fsheres management objectve by many countres and nternatonal management bodes. Ths has also resulted n the European Commsson proposal to set MSY as the man objectve n the reform of the European Unon s Common Fsheres Polcy (CFP). The extended practce of dscardng has been dentfed as one of the reasons for the falure of the past CFP [6-8]. Dscardng has prevented several EU fsh stock from recoverng, despte the low landng quotas [6-8]. Dscards occur for several reasons, manly because of low demand for non-target speces, and because of mnmum landng sze regulatons and hghgradng for target speces [7,9]. Latest worldwde estmates report about 30 mllon tonnes of dscards, accountng for 23% of the global catches [10], where catches are the sum of landngs and dscards. However, dscards are hghly varable dependng on fsheres, speces, and fshng gears [11]. Indeed, dscards can reach 80% of the catches n some fsheres. Dscardng s a common practce n demersal fsheres such as the nephrops fshery n the Bay of Bscay [12-14]. The mpact of dscards n a fshery depends however on the survval rate that s lnked to the speces and the fshng gear [15]. The survval rate of dscards s generally low for fshes and can reach 0% of the dscards for some of them [15-17]. The survval rate s hgher for crustacean such as nephrops. The survval rate for the dscarded nephrops has been assessed around 30% [18-19], resultng n a mortalty rate of around 70%, whch has been dscussed n several papers afterwards [20-23]. Recent expermentatons show that ths survval rate could be hgher because of mprovements n the sortng process [24]. Many fsheres worldwde are managed by quota systems, ether catch or landng quotas. Quota levels are often set so that the stock sze tends towards the bomass level that produces MSY (B MSY ). For example, the EU s CFP has varous measures to manage fsh stocks, where landng quotas or total allowable catches (TACs) play a key role. However, n fsheres where dscards account for an mportant share of the catches, MSY estmatons (B MSY, E MSY and mortalty levels, and consequently the quota level) can be sgnfcantly dfferent when accountng or not for dscards and thus basng the yeld calculatons on catches or on landngs maxmsaton. Smlarly, f a fracton of the dscards survves, ths can also lead to sgnfcant dfferences n these optmal estmates. Consequently, both ssues, choosng between landngs or catches maxmzaton and accountng or not for survval of dscards, have mportant mplcatons when determnng the yelds that can be harvested annually from the stocks. They also have mportant mplcatons on quota calculatons (on fsheres managed by quotas), and effort reducton recommendatons towards MSY. In the new EU s CFP a landngs oblgaton (dscard ban) s mplemented gradually, ths way landngs and catches wll match and thus these ssues wll be mtgated n some fsheres. 2
Ths paper explores the mpact on MSY estmates of maxmsng catches or landngs, and the effect of the survval rate of dscards. Analyses are appled to the Bay of Bscay nephrops (Norway lobster, Nephrops norvegcus) fshery, a demersal mxed fshery that s characterzed by hgh levels of dscards [12-14]. Therefore, the analyses of ths nephrops fshery allow to obtan mportant nsghts of the effects that dscards and ther survval have on the MSY estmaton and quota determnaton n fsheres where dscardng occurs. In partcular, the focus on a speces (nephrops) that shows a certan dscard survval and the sensblty analyss of the results to the dscard survval (secton 3.2) let to extent the results of ths study to speces wth lower (e.g. fshes) and hgher (e.g. crustaceans such as crabs and lobsters) dscard survval. Ths allows to set the bass to clarfy the defnton of MSY n fsheres where dscardng takes place. 2. Materals and Methods 2.1. Case Study Nephrops n the Bay of Bscay s fshed by bottom trawlers (Fgure 1). In 2009, 197 bottom trawlers caught more than one ton of alve nephrops n the Bay of Bscay. Among them, 115 bottom trawlers target nephrops throughout the year and depend on nephrops for more than 40% of ther gross revenue whle 82 trawlers target nephrops only durng a certan perod of the year. The fshery s managed by TACs and quotas and by a lcence system. Bottom trawlers use poorly selectve gears whch lead to hgh amounts of underszed catches of target speces and to by-catches of other speces [25]. In 2009, 58% of nephrops catches n number (38% n weght) were dscarded n the Bay of Bscay [13]. Those dscards were manly composed of the smallest ndvduals that were below the mnmum landng sze (9 cm for nephrops) [13]. Dscardng has negatve mpacts on stock renewal as 70% of the nephrops dscards de [18-19]. 2.2. Smulaton model MSY estmatons for the nephrops fshery n the Bay of Bscay have been performed usng the bologcal component of the bo-economc model IAM (Impact Assessment Model for fsheres management) [12, 25-27]. The model s developed n R/C++ languages [28-29]. It s an ntegrated model couplng the bologcal dynamcs of fsh stocks wth the economc dynamcs to perform mpact assessments takng nto account the bologcal mpacts and the economc mpacts for fleets. Ths model assesses mpacts n the short, medum and long term, transton phases, and mpact dstrbutons between dfferent fleets. It s age structured, spatally aggregated and has an annual tme step. The model calculates at each tme step fshng mortalty, spawnng bomass, bomass, total catches, landngs and dscards accountng for survval of dscards, landngs by fleet, fleet and ndvdual economc performances, the total number of vessels by fleet, employment and crew salares. The model can assess the mpacts of varous management scenaros such as fshng gear selectvty mprovement, decrease n tme fshng per vessel, decrease n the number of vessels, and quota constrants. The model s used n Macher et al. [12] and Raveau et al. [25] to assess the mpact of selectvty mprovements n the nephrops fshery of the Bay of Bscay. It s also used wthn STECF to assess several scenaros of achevng MSY n the sole fshery of the Bay of Bscay [30], and n Gullen et al. [27] to estmate the MSY and MEY n multspeces and mult-fleet fsheres. Man equatons of the IAM model relevant to the current study are descrbed below for the nephrops case study. The IAM bo-economc 3
model allows to account for mult-speces and mult-fleets, but n ths study t s only modelled one speces (nephrops) and one fleet (bottom trawlers). The IAM model assumes proportonalty between the rate that ndvduals are captured by the fshng gear by age group (Fc ) and nomnal fshng effort (E): Fc me E (equaton 1) q where q s the catchablty by age-class () for a gven fleet. m s a multpler of the nomnal effort E used for smulatons of scenaros. The fshng mortalty correspondng to removals (landngs and dead dscards), denoted Fr, s gven by: Fr Fc K (equaton 2) where K s the corrected factor of the fshng mortalty that accounts for the survval of dscards, modelled followng Mesnl [31], such that: K ( sr d ) (equaton 3) 1 where sr s the survval rate and d s the percentage dscarded n number by ageclass, as seen before. Survval of nephrops s calculated followng Beverton and Holt as: Z, t N N e (equaton 4) 1, t 1, t where N s the total number of ndvduals and Z s the total mortalty (natural and removal fshng mortalty). Catches n number per age group ( Cn ), are related to fshng mortalty, by the Baranov equaton: Cn ( 1 Z e F N (equaton 5) Z ) Dscards n number (Dn), by age-class are estmated n the model by: Dn Cn d (equaton 6) where d s the percentage dscarded n number by age-class. In ths study t s assumed a constant dscardng behavour based on hstorcal data, the percentage of dscards by age s consdered constant over tme (each year). Landngs n number (Ln) are obtaned by the dfference between Catches n number and Dscards n number, n the model are gven by: Ln Cn Dn (equaton 7) 4
Dscards n weght (D) and Landngs n weght (L), are estmated n the model by: D L w Dn (equaton 8) D L w Ln (equaton 9) Where w D s the mean weght of dscards, and w L s the mean weght of landngs. Smulatons on catch and landng evolutons for the nephrops fshery accordng to a range of effort multplers were performed wth the IAM model. MSY estmates (maxmum catches and landngs) and the correspondng effort and bomass at equlbrum were obtaned by optmzaton wth the IAM model. 2.3. Data The IAM model was parameterzed wth the outputs from the stock assessments performed by ICES n 2012 (.e. fshng mortalty, stock numbers at age, mean weght at age, dscards at age) for the stock of nephrops n the Bay of Bscay [32]. Hockeystck stock-recrutment relatonshp was adjusted based on 1987-2011 data 1. Table 1 presents the man data on the nephrops fshery: total bomass, fshng mortalty, nephrops catches, landngs and dscards by the bottom trawl fleet n 2011; table 2 detals the man data by age-class. 2.4. Scenaros analysed The nephrops catches, landngs, dscards and fshng effort multplers at MSY are estmated under four scenaros (Table 3) accordng to the varable whch s maxmzed (landngs or catches) and the survval rate consdered (0 or 30% followng Guéguen and Charuau [18] and Charuau et al. [19]). 3. Results Man results are presented on table 4. Secton 3.1 landngs or catches maxmsaton compares scenaros A and B, and scenaros C and D. Secton 3.2 Accountng for dscard survval compares scenaros A and C, and scenaros B and D and provdes a senstvty analyses of landngs to effort and survval rate. Results show that the optmal explotaton level can vary sgnfcantly when optmzng for catches or landngs, and also whether assumng a certan survval of the dscards or 1 Hockey stck stock-recrutment relatonshps have been chosen because they offer a shape closer to the producton functons usually used n more theoretcally economc models and more realstc when consderng mpact of hgh effort levels on recrutment. It should be noted that the nflexon pont parameters have been arbtrarly chosen from the nephrops stock assessment data because hstorc recrutments have proven to be robust and there s no percepton of nflexon ponts on the data. The optmal reference ponts for ths nephrops fshery have shown to be ndependent of the nflexon ponts n the stock-recrutment relatonshps. 5
not. In fact, optmal nephrops stock levels (B MSY ) vary from 16,239 to 28,478 tonnes and E MSY from 0.57 to 1.39, potentally resultng n a 43% decrease on the current effort to a 39% ncrease to maxmze the yeld (landngs or catch respectvely) consderng survval of dscards or not. Table 4 shows that maxmum yeld estmatons for the nephrops fshery n the Bay of Bscay would result n 5,641 tonnes of catches (or 4,135 tonnes of landngs) when amng at maxmsng catches assumng no survval of dscards; or 5,480 tonnes of catches (or 4,334 tonnes of landngs) when amng at maxmsng landngs assumng no survval of dscards. When a 30% survval rate of dscards s consdered n the estmaton, yelds can ncrease up to 6,464 tonnes of catches (or 4,266 tonnes of landngs) when amng at maxmsng catches; or 6,131 tonnes of catches (or 4,792 tonnes of landngs) when amng at maxmsng landngs. These yeld estmatons would be equal to the quotas at equlbrum f the fshery was optmally managed, and there were no nter-annual varatons n the recrutment 2. 3.1. Landngs or catches maxmsaton Fgure 2a compares the curves of sustanable nephrops landngs and catches as a functon of effort assumng no survval of dscards n equlbrum. Catches are ether retaned or de when dscarded n ths case; n both cases yelds are compared. Fgure 2b compares the maxmum sustanable nephrops landngs and catches as a functon of effort assumng a 30% survval rate on dscards 3. Both fgures show that the effort level that maxmzes landngs s lower than the effort that maxmzes catches n equlbrum. Lower effort levels correspond to a larger nephrops stock sze. The dfference between catches and landngs n both fgures s equal to the dscards. Dscards ncrease dsproportonally hgh when effort ncreases because the average weght of nephrops n the stock, and consequently n the catches, decreases. When assumng no dscards survval, the E MSY s reduced from 0.86 of the current effort when optmsng catches, to 0.57 when optmsng landngs. Thus, current effort needs to be reduced by 14% to 43% to maxmze catches or landngs yelds, respectvely. Correspondng value of MSY based on landngs s 34% lower than MSY value based on total catches. When assumng survval of the dscards, the E MSY decreases from 1.56 to 0.66 of the current effort accordng to the varable to be maxmzed, catches or landngs respectvely. The MSY calculated on landngs s 58% lower than the MSY calculated on catches assumng dscards survval. Maxmzng catches nstead of landngs can lead to an ncrease of dscards. In ths case, dscards ncrease by 31% compared to maxmsng landngs wth no dscard survval and by 64% when dscard survval s assumed. The B MSY s 37% larger when optmzng landngs nstead of catches and assumng no survval of dscards. It s 71% larger when optmzng for landngs compared to optmzng for catches when assumng a 30% survval rate for dscards. The average weght of nephrops bomass s 44% larger when maxmsng landngs than when 2 Under normal condtons, of nter-annual varatons n recrutment, the annual quotas are unlkely to be equal to the MSY yeld estmatons, because the annual TAC wll be roughly equal to F MSY * B and bomass (B) wll not be constant from one year to the next due to the varaton n recrutment. Therefore, quotas wll vary annually around the MSY estmatons. 3 The odd knks n the yeld versus effort curves (fgures 2a, 2b, 3a and 3b) are due to the knk n the assumed hockey-stck shape of the stock-recrutment relatonshp curve. 6
maxmsng catches and accountng for dscard survval. Whle the average weght of nephrops landngs s 28% larger when maxmsng landngs than catches and accountng for dscard survval, t s a 15% larger when t s assumed no dscard survval. 3.2. Accountng for dscard survval Fgure 3a compares the nephrops landngs as a functon of effort both wth and wthout survval of dscards; fgure 3b compares the nephrops catches, also wth and wthout survval of dscards. Fgures 3a and 3b llustrate the effect of dscards n the assessment models, when accountng for a certan survval rate (30% n the case of nephrops) of dscards. When accountng for a 30% survval rate of dscards, E MSY s hgher than E MSY when assumng no survval. Our results show that for Bay of Bscay nephrops, effort that maxmzes the catches s 62% hgher when assumng survval of dscards than when assumng no survval of dscards. Effort that maxmzes landngs s 19% hgher when assumng survval of dscards than when assumng no survval of dscards. Ths leads to optmal target yelds (quotas) that are hgher when assumng the survval of dscards. Optmal catches are a 15% hgher and optmal landngs are 11% hgher when assumng survval of dscards. As a consequence, the B MSY accountng for dscards survval s lower than the B MSY when assumng no survval (22% and 2% lower for catches and landngs respectvely). Ths also results n the average weght of nephrops bomass beng 16% and 3% lower and the average weght of nephrops landngs beng 14% and 3% lower when accountng for dscard survval when maxmsng catches and landngs respectvely. As shown on Fgure 4, for speces wth low survval rate of dscards, the E MSY s obtaned at lower fshng mortalty (F MSY ) and thus lower fshng effort. Ths leads to a hgher bomass at sea. Furthermore, landngs at MSY ncrease slghtly wth ncreasng survval rate and the percentage of dscards from the total catches ncreases (because of a lower average sze of the catches) when the fshng effort and survval rate ncreases. 4. Dscusson The mportant dfference n the effort needed to maxmze catches or landngs occurs because often dscards are not proportonal to landngs weght, but to the dstrbuton of the catches by age-class. Nephrops dscards on the Bay of Bscay fshery nvolve the smaller ndvduals that are under the Mnmum Landng Sze, as t happens n many fsheres. Therefore, the dscards evoluton (and thereby the dfference between catches and landngs) s explaned because when fshng pressure ncreases the average sze of the catch declnes and dscards ncrease when assumng a constant rate of dscards by age group. The dscards' ncrease observed when the average sze of catches decreases s lnked to mnmum landng sze regulatons. Constrants of the processng ndustry for smaller ndvduals and a lower demand for small ndvduals can also be other reasons for dscardng [7,9]. MSY s wdely nterpreted as the maxmum long term average catch that can that can be acheved under prevalng condtons, ncludng both the state of the ecosystem and 7
sze selectvty of the fshery [1]. Therefore, sze at age and weght at age are assumed to reman stable, and consequently not to be nfluenced by the fshng actvty, as n ths study. However, long term hgh levels of fshng pressure and sze-based dscards could lead to evolutonary responses [33-35]. Induced phenotypc adaptaton to fsheres could lead to a reducton n the long-term yelds. However, there s hgh uncertanty on the level of these potental changes and when they wll occur. Moreover, these potental changes due to phenotypc adaptaton should not have a relevant mpact on the sgnfcant dfferences between yeld estmatons based on landngs or catches. Accountng for a certan survval rate of the stock also leads to changes n the target bomass, effort and yeld. In fact, when t s consdered that 30% of the dscards survve n the case of nephrops, target yelds (landngs) ncrease by 11% compared to the stuaton where no survval s assumed. At the same tme, when assumng dscard survval, B MSY s 22% smaller. Therefore, mprovements n the sortng of dscards that could lead to ncreases n ther survval would allow hgher yelds from the fshery. Nephrops often has a hgher survval rate for dscards compared to other speces, especally fsh [15-17], but lower compared to other crustaceans such as crabs or lobsters [36-38]. Ths mples that the detrmental effects of dscards are even larger for most fsh speces, whle lower for some other crustaceans (.e. crabs and lobsters) than the ones shown n ths paper for nephrops. So, for speces wth hgh survval rate of dscards, t s benefcal to dscard small sze ndvduals to capture them at a later stage wth a hgher sze; whle, for speces wth low or null survval rate of dscards, t s not gong to be possble to capture them later. However, most of the dscards are due to the catch of underszed ndvduals that need to be dscarded because of the mnmum landng sze regulaton. Even f a regulaton that encourages dscards of underszed ndvduals may not seem benefcal for speces wth a low or null survval rate of dscards, such a regulaton could be benefcal because t may have a deterrence effect to avod the capture of large amounts of underszed ndvduals. So, t could be possble that f the mnmum landng sze regulaton was not n place, more underszed ndvduals would be captured, leadng to a lower level of landngs. Normally, n fsheres, dfferent speces are caught jontly wth the target speces, and some of these by-catch s dscarded. Hence, n order to avod dscards, together wth reducng fshng effort [27], t s mportant to mprove selectvty n the fsheres, for both target and by-catch speces. Indeed, mprovng selectvty, n partcular reducng retenton by the gear of the smallest ndvduals, results n medum to long term hgher yelds per unt of fshng effort and n greater catch values [14, 25, 39-43]. However, t seems that the adopton of selectve devces by fshermen s hampered because of the uncertanty of potental future gans, but also because ndvdual fshermen suffer the loss of catch due to the escapement of ndvduals wth commercal szes, whle benefts are gathered by the collectvty [25]. So ndvdual fshermen have ncentves not to adopt selectve devces. It should be consdered that because the nephrops stock s currently overfshed [27], a transton perod wll be needed to let the spawnng bomass grow to reach the B MSY level. Because the current stock bomass s less than B MSY, durng ths transton perod the TACs wll need to be reduced to allow the stock to grow. Therefore, when the stocks are at low levels, to move towards MSY wll requre a perod of reduced catches, amng to recover ths nvestment wth hgher quotas once the stock s at B MSY. If the stock was underexploted, there would be the need to set the quotas above MSY levels so that the stock bomass can shrnk to B MSY. 8
The term overfshed s commonly used to refer to a low level of stock abundance [3]. A stock s often consdered overfshed when ts stock abundance s lower than the Bomass at Maxmum Sustanable Yeld (B MSY ) [44-45]. Therefore, a fsh stock beng overexploted mples that the stock s producng less n bologc terms than what could be obtaned f t was optmally managed (.e. wth bomass at sea at B MSY ). However, the assessment of a stock could change from overexploted to underexploted (or vce versa) dependng on the assumpton of the survval rate of dscards and whether maxmsng landngs or catches. As can be seen n ths analyss, B MSY at sea when maxmsng catches, ranges from 20,852 tonnes when no survval s assumed to 16,239 tonnes when estmatng survval, compared to the 16,665 tonnes of the current stock. In ths context, stock status assessments are mportant because they can be decsve regardng whether a management plan needs to be developed, and t wll defne the effort and tme needed to reach the management target (often MSY). It should be noted that these effects would be mtgated n fsheres wth a lower proporton of dscards, consequently, more smlar optmal yelds and effort levels are to be expected n the estmaton of maxmum catches and landngs. Results obtaned set the bass to dscuss the defnton of MSY n fsheres where dscards takes place. ICES [1] states that MSY s wdely nterpreted as the maxmum long term average catch that can be acheved under prevalng condtons, ncludng both the state of the ecosystem and sze selectvty of the fshery. In addton, ICES [1] asserts that the determnaton of Yeld between catches and landngs n MSY depends on the management objectve: the choce of Yeld (Y) as catches or landngs s a matter for polcy: f Yeld s consdered to be that whch s removed from the stock MSY (FMSY) should be based on maxmsng catch; f Yeld s consdered to be the utlsed component from the stock, the amount contrbutng to economc and socal beneft, then Yeld should be taken as landngs and FMSY calculated accordngly to maxmse landngs. We beleve that nterpretng MSY as the Maxmum Sustanable Total Catches (ncludng dscards of commercally valueless and llegal ndvduals) s awkward; nstead Yeld should be clearly related to landngs. From the socetal pont of vew, the value to be maxmzed s the amount of food that the fshery provdes or the rent from the fshery whch means a maxmzaton of the dfference between gross revenue and costs (equvalent to the Maxmum Economc Yeld, MEY). Therefore, t s clear that not catches, but landngs should be maxmzed, whch are the part of the catches used by the socety; there s no sense to maxmze harvest f a part of the catch s dscarded afterwards. Ths also explans that n the potental presence of dscards, t would be better to base TACs and quotas on the fsh removed from the stock (catches) and not on landngs, because dscards may vary overtme, and consequently landngs may not be proportonal to catches. However, catches and the potental survval of dscards can be proven dffcult to montor. By ncludng economc consderatons (.e. revenues from the landngs and fshng costs), the optmal explotaton level that maxmses the rent (MEY) would be characterzed by a lower fshng effort and a larger optmal bomass than at MSY. Fshng at MEY would therefore lead to lower fshng costs and lower dscards compared to MSY [27, 46-48]. These effects could be accentuated when consderng that prces depend on sze [49], snce the average length of specmen would ncrease by havng a hgher bomass and a lower fshng effort [10,23]. 5. Conclusons In ths paper, based on the nephrops Bay of Bscay fshery, t s analysed the senstvty of MSY reference ponts (B MSY, E MSY and yelds) to the optmzed varable -landngs or 9
catches- and to the survval rate of dscards. Results are relevant for any fshery where dscards take place. The detrmental effects of dscards are larger for speces wth lower survval rate (.e. fshes), whle effects are lower for speces wth hgher survval rate (.e. crabs and lobsters). It s concluded from ths analyss that () dfferences can be mportant and can mpact the recommendatons of quotas and effort reductons to reach MSY and () that landngs (and not catches) should be maxmzed, as part of the catches that contrbute to the welfare of the socety, ether drectly by ther consumpton or through the value chan. Acknowledgments Ths work has been carred out wth the support of the French Research Agency, ANR, (Agence Natonal pour la Recherche) through the research program enttled Adhoc and wth the support of the European FP 7 project Benthc Ecosystem Fsheres Impact Study (BENTHIS). References [1] ICES. Report of the workshop on mplementng the ICES Fmsy framework, 22-26 March 2010, Copenhagen, Denmark. ICES CM 2010/ACOM:54. 83 pp. 2010. [2] Beverton RJH, Holt SJ. On the dynamcs of exploted fsh populatons. Unted Kngdom Mnstry of Agrculture and Fsheres, Fshery Investgatons, Seres II, Volume XIX. London; 1957. [3] Hlborn R, Stokes K. Defnng overfshed stocks: Have we lost the plot? Fsheres 2010; 35: 113-20. [4] Unted Natons. Report of the World Summt on Sustanable Development. Johannesburg, South Afrca; 2002. [5] Unted Natons. The future we want. Unted Natons Conference on Sustanable Development. Brazl. 2012. https://ro20.un.org/stes/ro20.un.org/fles/a-conf.216l- 1_englsh.pdf.pdf. [6] Commsson of the European Communtes (EC). Green Paper on the Reform of the Common Fsheres Polcy. Brussels, 22.4.2009, COM(2009) 163 fnal. 2009. [7] Catchpole TL, Frd CLJ, Gray TS. Dscards n North Sea fsheres: causes, consequences and solutons. Marne Polcy 2005; 29: 421-30. [8] Daw T, Gray T. Fsheres scence and sustanablty n nternatonal polcy: a study of falure n the European Unon s common fsheres polcy. Marne Polcy 2005; 29: 189-97. [9] Pascoe S. Bycatch management and the economcs of dscardng. FAO Fsheres Techncal Paper. No 370. Rome, FAO. 137 pp. 1997. [10] Nellemann C, Mac Devette M, Manders T, Eckhout B, Svhus B, Prns AG, Kaltenborn BP. The envronmental food crss - the envronment s role n avertng future food crses. A UNEP rapd response assessment. Unted Natons Envronment Programme; GRID-Arendal. Avalable at www.grda.no. 2009. [11] Rochet M-J, Trenkel VM. Factors for the varablty of dscards: assumptons and feld evdence. Canadan Journal of Fsheres and Aquatc Scences 2005; 62: 224-35. [12] Macher C, Guyader O, Taldec C, Bertgnac M. A cost-beneft analyss of mprovng trawl selectvty n the case of dscards: The Nephrops norvegcus fshery n the Bay of Bscay. Fsheres Research 2008; 92: 76-89. 10
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Fgures Fgure 1: Nephrops fshery n the Bay of Bscay 13
Fgure 2a: Nephrops landngs and catches as a functon of effort wthout dscard survval and MSY levels (scenaros A and B) 14
Fgure 2b: Nephrops landngs and catches as a functon of effort wth dscard survval and MSY levels (scenaros C and D) 15
Fgure 3a: Nephrops landngs as a functon of effort wth and wthout dscard survval and MSY levels (scenaros A and C) 16
Fgure 3b: Nephrops catches as a functon of effort wth and wthout dscard survval and MSY levels (scenaros B and D) 17
Fgure 4: Maxmum sustanable landngs as a functon of the fshng effort and survval rate. The colour scale shows the percentage of dscards n the catches 18
Tables Table 1: Man data of the nephrops fshery n 2011 (ICES, 2012) Bomass nephrops (Tonnes) 16,666 F nephrops 0.48 Catches nephrops (Tonnes) 4,822 Dscards nephrops (Tonnes) 1,263 Landngs nephrops (Tonnes) 3,539 Table 2: Nephrops stock data n 2011 (ICES, 2012) Age-class Stock sze ('000 N) Mean Weght of bomass at age (gr) (w B ) Fshng mortalty (F) % of dscards (d) Mean Weght of landngs at age (gr) (w L ) Mean Weght of dscards at age (gr) (w D ) Natural mortalty age 1 655,480 3.5 0.02 100.0 0.0 3.7 0.30 0 age 2 295,066 9.0 0.31 95.4 11.0 9.0 0.30 0 age 3 237,356 16.9 0.54 37.0 18.0 14.3 0.25 75 age 4 123,078 26.6 0.57 11.2 26.9 23.0 0.25 100 age 5 48,505 36.9 0.50 5.8 37.3 28.0 0.25 100 age 6 21,820 48.6 0.43 3.4 48.9 36.0 0.25 100 age 7 10,639 61.0 0.38 2.8 61.4 39.7 0.25 100 age 8 4,926 71.5 0.42 1.9 71.9 49.7 0.25 100 age 9+ 6,253 91.2 0.42 1.4 91.8 47.0 0.25 100 Maturty rate (%) Table 3: Summary of the scenaros tested Maxmsng Landngs Catches dscard survval 0% Scenaro A Scenaro B 30% Scenaro C Scenaro D 19
Table 4: Man results from the maxmsaton of catches and landngs for the nephrops fshery Varables Current Explotaton Level Maxmum Landngs No survval Scenaro A Maxmum Catches No survval Scenaro B Maxmum Landngs 30% survval Scenaro C Effort multpler 1 0.57 0.86 0.68 1.39 F nephrops 0.48 0.27 0.41 0.33 0.67 Catch nephrops (Tonnes) 5,148 5,480 5,641 6,131 6,464 Maxmum Catches 30% survval Scenaro D Landngs nephrops (Tonnes) 3,827 4,334 4,135 4,793 4,266 Dscards nephrops (Tonnes) 1,321 1,147 1,506 1,339 2,198 Bomass nephrops (Tonnes) 16,665 28,478 20,852 27,769 16,239 Average Weght of nephrops Catches (gr.) 19.5 23.8 20.5 23.2 17.6 Average Weght of nephrops Landngs (gr.) 27.2 29.9 26.1 28.9 22.5 Average Weght of nephrops Landngs Bomass at Sea (gr.) 11.9 15.8 12.8 15.4 10.7 20