Investigating ecosystem dynamics with ECOPATH/ECOSIM

Transcription

1 BASS WORKSHOP ON THE DEVELOPMENT OF A CONCEPTUAL MODEL OF THE SUBARCTIC PACIFIC BASIN ECOSYSTEMS (Co-convenors: Gordon A. McFarlane, Rchard J. Beamsh, Akhko Yatsu and Andre S. Krovnn) At the PICES Sxth Annual Meetng, the BASS Task Team sponsored a symposum on the Ecosystem dynamcs of the eastern and western subarctc gyres. The purpose was to brng together avalable nformaton on the two gyres n a comparatve framework. Topcs ncluded: 1) ocean responses to clmate forcng, 2) nutrents and prmary producton, 3) structure of the lower trophc levels, the mesozooplankton communtes, and the eppelagc nekton, 4) the role of mdwater fshes, and 5) the mportance of these areas to marne brds and mammals. Papers presented at the meetng were publshed n 1999, n a Progress n Oceanography specal ssue enttled Ecosystem Dynamcs n the Eastern and Western gyres of the Subarctc Pacfc (Guest Edtors: R.J. Beamsh, S. Km, M. Terazak and W.S. Wooster). The followng key research problems were ponted out durng dscusson at the symposum: 1) the need for nformaton on short-term or seasonal changes n the mxed layer, 2) how clmatevaraton may be changng the stablty of the water column, 3) the role of ron: understandng transport mechansms, 4) communty dynamcs and the need for small scale det studes, and 5) bomass estmates of some key speces. Members of BASS Task Team felt that the next step should be to develop a conceptual model of the subarctc Pacfc basn ecosystems and begn to examne approprate models. A 2-day BASS Workshop on ths topc was convened pror to the Nnth Annual Meetng n Hakodate, Japan (October 20-21, 2000). The objectve of ths workshop was to dentfy approprate approaches, not only modellng approaches but also how to develop studes whch wll answer some of the questons. At the workshop, a number of presentatons were made on ecosystem models that partcpants had used. These models were revewed and dscussed wth respect to ther utlty for gyre systems. Trophodynamc lnkages that were lkely to be common, as well as those that were modelspecfc, were dentfed, and shortfalls were hghlghted. Dscussons ncluded dentfyng data groups and potental data sources, ncorporatng clmate and oceanographc change n models, and lnkng gyre models to coastal area models. The followng secton contans extended abstracts of papers gven at the workshop. Investgatng ecosystem dynamcs wth ECOPATH/ECOSIM Jeffrey J. Polovna Honolulu Laboratory, SWFRC, Honolulu, HI , U.S.A. E-mal: Jeffrey.Polovna@noaa.gov Two applcatons of the ECOPATH/ECOSIM modellng approach are presented. The frst constructs an ECOPATH model of the central North Pacfc pelagc ecosystem, usng ECOSIM to nvestgate the response of the ecosystem to fshng mpacts. The ECOPATH model s hghly resolved at the top trophc levels ncludng speces groups for each of the prncpal fshery target speces as well as fshery mpacts of 6 types of fshng gears. The ECOPATH model was used n the dynamc ECOSIM to smulate the response of the pelagc ecosystem to an elmnaton of all fshng: a return of the ecosystem to the early 1900s. The ECOSIM model smulaton found all the fshed speces ncreased n the absence of fshng but a number of prey speces ncludng squd, flyng fsh and lancet fsh, decreased as ther predators ncreased. Furthermore, for the top trophc level speces, large sharks and blue marlns, ther bomass ncreased more than all other fshed speces because they benefted from both an absence of fshng as well as an ncrease n 1

2 ther prey, small tunas and bllfshes, speces whch ncreased n bomass when fshng was halted. A second applcaton of these models was a bottom-up smulaton. An eastern tropcal Pacfc pelagc ecosystem model was constructed and used to smulate the ecosystem response to changes n ENSO perodcty and cadence, and long-term global warmng. The ENSO mpacts were smulated by changng the phytoplankton avalablty n the ECOSIM model. An El Nño was smulated by a 30% drop n phytoplankton durng one year and a La Nña was smulated wth a 30% ncrease n phytoplankton for one year. The global warmng scenaro was smulated by usng predcted changes n SST from a 100-year global warmng modellng exercse. The predcted eastern tropcal Pacfc SST was converted nto phytoplankton bomass wth an emprcal relatonshp between these parameters for the eastern tropcal Pacfc. The ECOSIM results found that bottom-up forcng propagates through the 6 trophc levels of the ecosystem wth tme lags and ampltude of the forcng whch vares by speces and can be greater or weaker than the ntal forcng. Changng the ENSO perod results n modest changes n trophc transfer and ecosystem structure. For example, less energy reaches the top of the ecosystem when El Nño events occur every 2 years compared to every 6 years. However, the global warmng scenaro whch predcted warmng of SST, an ncreased vertcal stratfcaton and hence reduced phytoplankton n the eastern tropcal Pacfc, was smulated by the ECOSIM model to result n a substantal decrease n the entre ecosystem bomass at all trophc levels. These and other experences wth ECOPATH and ECOSIM suggest that these models are useful tools to nvestgate the responses of complex ecosystems to both top-down and bottom-up forcng. However, ssues reman regardng how well the models capture the complexty of actual ecosystem dynamcs. More evaluatons of model results wth actual ecosystem dynamcs are needed. There s a consderable lterature on applcatons of these approaches and a web ste whch serves as a source for the programs and related lterature. ECOPATH as a potental tool for modelng the North Pacfc Gyre ecosystems Kerm Y. Aydn Alaska Fsheres Scence Center, NMFS, Seattle, WA U.S.A. E-mal: Kerm.Aydn@noaa.gov The subarctc North Pacfc oceanc gyres contan hghly productve pelagc ecosystems. These waters have been subject to past fsheres and are mportant rearng areas for Pacfc salmon (Oncorhynchus spp.), marne mammals and commercally harvested squd. Furthermore, the speces of the regon responds to oceanographc sgnals on the order of decades, and thus they present a case study for the nteractons between clmate and marne ecology. However, the ecosystems of these regons are poorly understood, n part due to the dffculty of obtanng consstent data across such large systems. Extensve bologcal data exst for these regons and have been gathered by PICES member countres. It would be extremely useful to assemble these data to provde a meanngful quantfcaton of ecosystem structure and functon. To ths end, the software package ECOPATH may be a useful tool, as t helps researchers n modelng oceanc food webs, and provdes a meanngful context for comparng estmates of bologcal producton across speces and regons. ECOPATH s smply a tool for comparng ndependent estmates of bomass, producton, consumpton and det, mgratons, and fsheres catch of the mportant players n a food web. Such comparsons help determne: - f avalable estmates are consstent between speces; - the relatve mportance of speces or gulds wthn an ecosystem; and 2

3 - how targetng ncreased research effort on crtcal, keystone speces mght ad n mprovng our understandng of the system s structure and functon. In cases where two smlar ecosystems are to be compared, such as wth the eastern and western subarctc gyres, ECOPATH provdes a quanttatve bass for comparson. Creatng an ECOPATH model should be a strongly collaboratve process among partcpatng scentsts. The framework of ECOPATH lends tself to an teratve peer-revew process between the prmary modelers, the data collectors, and the wder scentfc communty. There are fve man steps n creatng a model: - determnng the model framework; - assemblng and documentng the ntal data; - assessng the data n prelmnary models; - peer-revew of data and prelmnary models; - use of models to test hypotheses. Determnng the model framework To set up a modelng effort, the boundares, tme perod, tme step, speces of nterest, and mportant hypotheses for the system must be dentfed. Whle these dentfcatons may change as the model develops, t s mportant to have some prelmnary framework to ad n collectng data. Ths stage may also dentfy key data sources and provders. Generally ths task may be completed n a seres of dscussons over a short tme perod. Assemblng and documentng the ntal data Data assembly and documentaton may take from weeks to months dependng on the ecosystem n queston. Ths process may nvolve contactng researchers n many nsttutons to provde data to a central source. At the same tme, the qualty of the data may be assessed and adjustments made to the model framework f necessary. Assessng the data n prelmnary models The ntal assembly of data for an ECOPATH model s best conducted n a workshop settng wth a lmted number of partcpants who are famlar wth aspects of the data. The purpose of such a workshop s to vew the peces of data as a whole for the frst tme, and make prelmnary judgements on the qualty of the data and the usefulness of the model. At ths stage, plans may be made to revst hypotheses or attempt to refne data estmates. Peer-revew of data and prelmnary models After an ntal workshop, partcpants should be able to show the ntal model to a wder communty over a perod of months. Ths process allows addtonal nput to be gathered to mprove the model. Successve teratons of data exchange durng ths perod wll substantally mprove the model. In addton, the model may be used to drect further data collecton. Use of models to test hypotheses When the peerrevew process has reached the larger research communty, the models may be dstrbuted and used to compare the relatve roles of anthropogenc effects, predator-prey nteractons, clmate changes, or dynamc functon of the ecosystem through a varety of modelng technques. ECOPATH, lke all models, s a smplfcaton of nature. However, the quanttatve, teratve peerrevew process has n many cases contrbuted to an ncreased understandng of the ecosystem s structure and functon. Overall, ECOPATH s a powerful tool for assemblng and syntheszng ecosystem data from dsparate sources. Modelng the western Berng Sea ecosystem wth help of ECOPATH software Vctor V. Lapko, Elena P. Dulepova and Vladmr I. Radchenko TINRO- Centre, Vladvostok, , Russa. E-mal: nterdept@tnro.wavenet.ru The purpose of our presentaton s to dentfy some methodologcal aspects substantally affectng, or even defnng, the quanttatve appearance of the model, and as a result the appled conclusons derved from the model. Ths modelng work was ntated by our colleagues from the Natonal 3

4 Marne Fsheres Servce (NOAA, U.S.A.) wth the goal of constructng an ecosystem model of one of the major fshng area n the northern Pacfc - Berng Sea. The co-operatve project stpulated that wth the help of the ECOPATH software, we should buld a model of the western Berng Sea (WBS) n addton to one of the eastern Berng Sea (EBS) that had already been created by US scentsts. Further they ntended to combne both models nto a general model descrbng the whole Berng Sea ecosystem. Followng accepted rules, we bult a WBS ecosystem model for the 1980s. Durng that decade TINRO-Centre carred out large-scale nvestgatons of marne bota and collected numerous data on dverse speces and groups of pelagc and demersal taxa. Those data allowed tracng seasonal and nterannual dynamcs of speces abundance, dstrbuton, mgratons, feedng etc. Ths nformaton became a base for our model. The entre Russan EEZ n the western Berng Sea was used as the model area. It covers a total of 702,200 km 2 and encompasses a wde range of marne habtats ncludng shelf, slope and deep basns, but t was treated as a sngle homogenous regon n the model. To descrbe the WBS ecosystem, we separated all taxa nto 48 functonal groups, composed of a sngle speces or an aggregaton of ecologcally smlar speces, coverng all trophc levels from phytoplankton to marne brds and mammals. The model uses annual averages,.e. all necessary data on abundance and feedng collected n varous habtats were combned proportonally to the areas of those habtats and averaged seasonally and annually to provde year-round annual average characterstcs, whch were entered nto the two man tables (basc nput and det composton). Commercal fshery catch was also ncluded nto the model. Furthermore our model was balanced and we have compared the results obtaned n the WBS and EBS (Trtes et al. 1999). Comparson was partcularly nterestng because both models are composed of a very smlar lst of functonal groups and relate to the same tme perod. Results are presented n Table 1. Total bomass n the WBS ecosystem was 1.75 tmes hgher than n the EBS, whle other mportant ecologcal ndces dffered n even greater proportons. For example, the sum of all bologcal producton was 4 tmes hgher, the sum of all consumpton tmes, the sum of all flows nto detrtus - almost by order of magntude greater and total system throughput tmes Table 1 Descrptve summary statstcs for the eastern (EBS), western (WBS) and partally changed (WBS 1 ) Berng Sea ecosystem models n the 1980s. Parameters\model EBS WBS WBS 1 Sum of all consumpton Sum of all exports 2.62 (?) Sum of all respratory flows Sum of all flows nto detrtus Total system throughput Sum of all producton Mean trophc level of the catch Gross effcency (catch/net p p.) Calculated total net prmary producton Total prmary producton/total respraton Net system producton Total prmary producton/total bomass Total bomass/total throughput Total bomass (excludng detrtus) Total catches Connectance Index System Omnvory Index

5 hgher n the WBS. At the same tme, the WBS ecosystem was exploted much less - gross effcency was almost 20 tmes lower compared to the EBS. Judgng from these statstcs n the 1980s, the WBS ecosystem functoned much more ntensvely but less effcently compare to the EBS. However, t should be remembered that the EBS s generally warmer and shallower than the WBS, and therefore we mght have expected the opposte stuaton. What s the reason? We assume that the man reason les n hgher average ecologcal characterstcs, and frst of all n the values of producton/bomass (P/B) and consumpton/bomass (Q/B) ratos, we have appled to descrbe some functonal groups of speces n the WBS ecosystem. As follows from Table 2, the annual Q/B values, whch were defned for the WBS, are substantally hgher n hgher trophc level groups (pollock and herrng are partcularly ndcatve) and lower n lower trophc level groups, compare to the EBS. Another mportant dfference s n P/B ratos of phytoplankton (see Table 2). Apparently applcaton of all these values caused the aforementoned dfferences between the models. To test ths assumpton we have entered nto the WBS ecosystem model P/B and Q/B ratos from the EBS one, keepng bomass and det composton unchanged. Although the resultng model was found to be slghtly unbalanced, the descrptve statstcs, as we expected, had ntermedate values between both orgnal models (Table 1, last column). Thus, the values of the such mportant ecologcal parameters as P/B and Q/B ratos, entered nto the model, are crucal for ts Table 2 P/B and Q/B values appled n the WBS (above slash) and EBS (below slash) models. Group name P/B Q/B Phytoplankton 139 / 60 - Copepods 9.5 / / 22.0 Euphausds 3.13 / 5, / 22.0 Amphpods 2.5 / / 22.0 Herrng 0.7 / / 3.65 Cod 0.52 / / 2.04 Yellowfn sole 0.26 / / 2.96 Rock sole 0.24 / / 3.6 Halbut 0.25 / / 2.49 Juv. pollock 2.5 / / 8.3 Adult pollock 0.5 / / 2.64 Steller sea lon 0.06 / / 12.7 Toothed whales 0.02 / / resulted appearance, features, further smulaton of commercal explotaton rate and fnal concluson. It s dffcult to magne that trophc and productve characterstcs of the same taxa dffer by several tmes n the same bogeographcal area. Of course, the some dfferences should take place due to varous sze-age composton of populatons, food condtons, general temperature of envronment, etc., however, we presume they are not so drastc. It s qute possble that the ECOPATH software wll be accepted as a standard tool for modelng of ecosystems n dverse Pacfc areas. Pacfc waters are nhabted by varous fauna, but there are no doubt that almost everywhere predomnatng speces and groups of speces wll concde n hgh extent, especally for adjacent areas. Thus, t would be very useful for future modelng efforts to compare and dscuss methodcal approaches for determnng the most mportant ecologcal parameters of common speces and groups n the northern Pacfc. Changes n the Strat of Georga ECOPATH model needed to balance the abrupt ncreases n productvty that occurred n 2000 Rchard J. Beamsh, Gordon A. McFarlane, C.M. Nevlle and I. Pearsall Pacfc Bologcal Staton, Nanamo, B.C., Canada. V9R 5K6 E-mal: beamshr@pac.dfo-mpo.gc.ca ECOPATH s a trophc accountng model that s a practcal way of studyng the nteractons of all speces n an ecosystem. We used ECOPATH to study the dynamcs of the Strat of Georga ecosystem (area: 6,900 km 2 ), located between Vancouver Island and the Brtsh Columba manland. The Strat of Georga s probably the most mportant marne ecosystem on Canada s west 5

6 Table 3 Functonal groups, bomass, producton/bomass and consumpton/bomass values used n the Strat of Georga model and resultng ecotrophc effcences. Functonal Bomass (t/km 2 ) Producton/Bomass Consumpton/Bomass Ecotrophc Effcency Group (t/year) (t/year) Phytoplankton Kelp/Sea Grass Herbverous zooplankton Neocalanus plumchrus Pseudocalanus mnutus Shellfsh Crab Grazng nvertebrates Carnverous zooplankton Euphausd Predatory nvertebrate Shorebrds Herrng Small Pelagcs Lampetra ayres Seabrds Gulls Msc. demersal fsh Chum Coho Chnook Toothed Whales Hake Dogfsh Lngcod Pollock Leuroglossus Yelloweye Englsh Sole Sea Lons Seals Detrtus coast, as much of the populaton of Brtsh Columba lves wthn 10 km of ts shores and t s a key rearng area for Pacfc salmon, herrng, and other speces. Our ECOPATH model has 32 functonal groups. We estmated the bomass, producton/ bomass ratos, consumpton/bomass rates, and det compostons for each functonal group. We used a number of references and the unpublshed results of our own studes over the past 26 years to estmate these data. An mportant estmate for the lower trophc levels was the hydroacoustc estmate of euphausds made n 1999 and 2000 (Pearsall et al. 2001). The two domnant fsh speces n the Strat of Georga are Pacfc hake and Pacfc herrng. Relable bomass estmates exsted for both of these speces (McFarlane et al. 2000; Schwegert and Fort 2000). Pacfc salmon are both abundant as juvenles and mportant commercally, culturally, and poltcally. In recent years, juvenle salmon have reared n the Strat of Georga longer than n the past, but adult coho were vrtually absent. Chnook of larger szes and ocean ages greater than age 0 remaned n the Strat but ther abundance was much lower n the late 1990s than n the 1970s and 1980s. 6

7 Sum Table 4 Det matrx n the models. Prey/Predator Phytoplankton Kelp/Sea grass Herb. zooplankton N. plumchrus P. mnutus Shellfsh Crab Grazng nvertebrates Carn. zooplankton Euphausd Predatory nvertebrate Shorebrds Herrng Small pelagcs Lampetra ayres Seabrds Gulls Msc. demersal fsh Chum Coho Chnook Toothed whales Hake Dogfsh Lngcod Pollock Leuroglossus Yelloweye Englsh sole Sea Lons Seals Detrtus

8 In ths report, we model two ecosystem states: one n 1998 and one n We show that the ncrease n producton that occurred n 2000 had a major mpact on the dynamcs of the trophc relatonshps n the Strat of Georga n The ncrease n productvty n 2000 probably resulted from a change n the clmate and a correspondng change n the oceanography. The 1998 model (Table 3) assumed a bomass of phytoplankton of 36 t/km 2 and a producton/ bomass rato of 130. The model was balanced wth 99.2% of the phytoplankton producton beng consumed by hgher trophc levels. On average, over 90% of the producton of the four herbvorous zooplankton groups (euphausds, P. mnutus, N. plumchrus and other herbvorous zooplankton) was consumed by hgher trophc levels. Carnvorous zooplankton (amphpods) contrbuted 58.1% of ts producton to hgher trophc levels. The bomass of Pacfc hake was 10 t/km 2. The major tems n the hake det were euphausds (70%) and carnvorous zooplankton (16%). Det composton for the models are summarzed n Table 4. Although adult hake fed on juvenle hake and herrng n the past, we have not found fsh remans n hake stomachs n the late 1990s. Pacfc herrng mgrate out of the Strat of Georga after about age 1, and return only to spawn n the wnter n ther thrd and subsequent years. We estmated that euphausds made up 58% of ther det. Euphausds were also an mportant prey for juvenle salmon, accountng for 22% to 30% of ther det. Another major fsh speces n the model was spny dogfsh. Dogfsh are omnvorous, but grow only a few mm/year, thus ther consumpton s small relatve to ther bomass. The model balanced for 1998 ndcated that 75.5% of the euphausds were consumed by hgher trophc levels. The hydroacoustc study ndcated that there was t/km 2 and t/km 2 of euphausds n the Strat of Georga n September/October 1999 and 2000, respectvely. The bomass estmates n Pearsall et al. (2001) were modfed for our ECOPATH model to approxmate our nterpretaton of ther lfe hstory. The maxmum bomass of euphausds n the Strat of Georga occurs late n the year and the mnmum bomass about June. The lfe span exceeds one year thus the P/B wll be lower than other groups of zooplankton. Therefore, we estmated the annual bomass to be 80 t/km 2 n 1998 and 160 t/km 2 n A varety of scenaros would be possble to balance the 2001 model wth the ncreased euphausd producton, but all scenaros would ndcate a substantal ncrease n the bomass of a number of functonal groups. The addton of the bomass of euphausds used n the 2001 model nto the 1998 model resulted n 37.8% of the producton beng consumed by predators and an mbalance of 115.8% of the phytoplankton producton. Because euphausds feed prmarly on phytoplankton, the ncrease n euphausd abundance most lkely was assocated wth an ncrease prmary productvty. Such an ncrease would also beneft larval copepod survval. We balanced the 2001 model by ncreasng the bomass of these and other functonal groups that would beneft drectly or ndrectly from ncreases Table 5 Changes n the abundances of functonal groups from the 1998 model to 2001 model, scaled to the mpact of the euphausd bomass ncrease. Functonal Group 1998 Bomass t/km Bomass t/km 2 Euphausd Phytoplankton N. plumchrus P. mnutus Shellfsh Crab Grazng nvertebrates Carnvorous zooplankton Small Pelagcs Mscellaneous Demersal Fsh Chum Coho Chnook Hake Herrng Herbverous zooplankton Leuroglossus

9 n euphausd bomass (Table 3). We adjusted the bomass of speces that mght be drectly affected by the ncreased productvty n the 2001 model (Table 3), but dd not change the dets used n the 1998 model. In our 2001 model, we assgned only 58.1% of the new euphausd producton to consumers, yet there were ncreases n bomass of 40% for herrng and hake, 50% for shellfsh, 100% for crab, chum, chnook and coho, 150% for mscellaneous demersal fsh, and 267% for small pelagc fshes (Table 5). It s possble that these ncreases mght take longer to develop, however, the model ncreases would be ndcatve of the possble changes n bomass. The changes n salmon abundance would be partcularly mportant. In another study we are testng the hypothess that salmon survval or producton s a functon of both predaton and summer growth. If our hypothess s correct, the mproved summer growth n 2000 wll ncrease marne survval, and the returns of coho and pnk salmon n 2001 wll be larger than prevous years. We note that f the prmary producton remans at the 2000 level, there wll be contnued major changes n the ecosystem as hgher trophc levels ncrease ther bomass n response to the ncreases n prey. References Pearsall, I.A., Macaulay, M., Beamsh, R.J., McFarlane, G.A., and Saxby, G An abrupt doublng of euphausds n the Strat of Georga, Brtsh Columba, Canada. Fsheres Oceanography (n preparaton). McFarlane, G.A., Beamsh, R.J. and Schwegert, J Common factors have opposte mpacts on Pacfc herrng n adjacent ecosystems. Proc. Lowell Wakefeld Symposum, Anchorage, AK (n press). Schwegert, J., and Fort, C Stock assessment for Brtsh Columba herrng n 1999 and forecasts of the potental catch n p. Seres Canadan Stock Assessment Secretarat Research Document; 99/178. Smulatng hstorcal changes n the Strat of Georga ecosystem usng ECOPATH and ECOSIM Steven J.D. Martell, Carl J. Walters, Alasdar Beatte, Tarun Nayar and Robyn Brese UBC Fsheres Centre, Vancouver, B.C., Canada. V6T 1Z4 E-mal: smartell@fsheres.com The Strat of Georga (SOG) ecosystem has been heavly exploted for the last 90 years and development n commercal fsheres has shfted the focus from top predators n the ecosystem to more abundant lower trophc level speces (Wallace 1998). Ths phenomenon s known as fshng down food webs (Pauly et al. 1998; Pauly et al. 2000). Salmon fsheres were by far the most mportant fshery n the early years of fshng development, and by 1897, Brtsh Columba was cannng more than 1 mllon cases of salmon a year (Lchatowch 1999). Both chnook and coho salmon have been heavly exploted n the SOG by the commercal net and troll fsheres, and by sports fsheres (DFO 1999a; DFO 1999b). Wth almost all SOG coho stocks n jeopardy, a coastwde closure for all coho fsheres was mplemented n 1998, wth the excepton of a sports fshery for hatchery fsh at the mouth of the Caplano Rver. As fshng technologes mproved, herrng fsheres and groundfsh fsheres grew rapdly n the 20 th century, wth precptous results. By the early 1960s, herrng stocks were beng harvested at unsustanable rates and the fshery collapsed n 1967 (Stocker 1993). Snce ths tme, however, herrng stocks have recovered to near hstorcally hgh levels (Schwegert et al. 1998). Pror to 1970, herrng were manly used for fshmeal, but after the collapse, a more valuable roe fshery was developed. Groundfsh such as lngcod and several rockfsh speces were also heavly exploted n the 1900s, and wth the ntroducton of trawl fsheres to the SOG n 1943, explotaton rates rose dramatcally (Cass et al. 1990; Martell 9

10 1999). Invertebrate fsheres have exsted n the SOG for the last 100 years, however, untl the 1950s the fsheres were manly focused on dungeness crabs and manlla clams (an exotc speces). Snce the 1950s, there have been developments n shrmp fsheres, geoduck clams, sea urchn, sea cucumbers and octopus fsheres (Ketchen et al. 1983). Stock assessment reports have attrbuted the observed declnes n abundance to factors other than overfshng. In fact, more attenton has been spent on tryng to explan envronmental processes that may have led to a reducton n marne survval rates n salmon (Beamsh and Boullon 1995), or changes n food avalablty assocated wth changes n physcal propertes (Robnson 1999). At ths tme, the occurrence of a regme shft, or long-term changes n prmary productvty n the Pacfc Ocean (Beamsh et al. 1999), s postulated as the major factor leadng to abundance declnes n the SOG. An obvous, but often unresolved, ssue s the role of trophc nteractons n suppressng recrutment or ndrectly changng natural mortalty rates (generally assumed to be constant). Among fsheres scentsts and academa, there s a growng consensus that we can no longer forge ahead and explot a resource wthout consderng trophc nteractons at an ecosystem scale (Walters et al. 1997). The majorty of data avalable, however, are usually restrcted to speces of commercal mportance. In the SOG alone for example, there are more than 250 dfferent speces of fsh, but fsheres statstcs are collected for less than 50 speces coast-wde (vertebrate and nvertebrate combned). Moreover, we have even less knowledge about the specfc nteractons among members n an ecosystem, a problem we are now forced to face. The objectve of ths paper s to carry out a retrospectve analyss of the Strat of Georga ecosystem and use data from sngle speces stockassessment programs to determne f the observed data suggest that changes n prmary productvty have occurred n the last 50 years. We address ths ssue by comparng reconstructed ecosystems from 1950 to 1998 usng two scenaros: 1) assume that there has been no changes n relatve prmary productvty, and 2) estmate relatve prmary productvty regmes that would better explan the observed data. Fnally, we compare the estmated prmary productvty regmes to envronmental correlates, specfcally wnd speed squared (a measure of sheer stress), and the Pacfc Decadal Oscllaton. Predctng bomass dynamcs usng ECOPATH wth ECOSIM The trophc mass-balance model used n ECOPATH uses a set of smultaneous lnear equatons that assumes the producton of group s equal to the consumpton of group by all predators j, plus export and non-predaton losses (ncludng fsheres landngs) of group, over a specfed tme perod. Ths functon s generally expressed as: Equaton 1 B ( P B) EE= Y + B ( Q B) n j= 1 j j DC Where B s the bomass of group, (P/B) s the producton bomass rato of group, (Q/B) j s the consumpton bomass rato of group j (predators of group ), and DCj s the average det fracton of prey for group j. EE s the ecotrophc effcency, or the fracton of producton that s consumed wthn the system, ncludng fsheres yelds (Y ). The followng dfferental equaton s used to predct dynamc changes n bomass and s dependent on whether the group s a prmary producer or a consumer n the system: Equaton 2 n ( B) M B F B c ( B B ) / dt = f o j = db, Here M o = (1 EE )*(PIB) represents the fracton of producton that s unaccounted for (other mortalty), F s the fshng mortalty rate, and c j s a functon used to predct consumpton of group by predator j, condtonal on the nteractons and abundance of the two groups (see Eqn. 4). For prmary producers, a smple saturatng functon s used to predct bomass producton: j j j 10

11 Equaton 3 ( B ) = r B ( 1 B h ) f / + (a) Whereas f group s a consumer then n ( B ) g c j ( B B j ) j= 1 f =, (b) here g s the growth effcency and must satsfy B P B = g Q. the relatonshp ( ) j j Predctng consumpton n ECOSIM stems from the "Foragng Arena" concepts proposed by Walters and Juanes (I993). At equlbrum the consumpton of by j s: Equaton 4 c j ( B, B ) j a = 2 j ( v + a B ) j v j B B j In Equaton 4, a j s the mass acton consumpton rate, and vj descrbes the exchange rate process from "avalable" to "unavalable" behavoural states. Usng ECOPATH estmates (Q j, B, and B j ) the mass acton consumpton rate can be estmated by re-arrangng Equaton 4. Therefore, the only user specfed parameter s the behavoural exchange rate parameter (v j ). Equaton 4 s structured such that consumpton s dependent on both predator and prey bomass. If predator bomass s low then consumpton reduces to a mass-acton flow, and f predator bomass s hgh then consumpton approaches a "donor control" rate represented by the behavoural exchange rate process v j (Walters et al. 1997; Walters et al. 2000). As v j approaches 1, the rate of predaton s dependent on the bomass of the predator (topdown control), and as v approaches 0, prey spend a larger fracton of ther tme budget hdng from predators and predaton s lmted by bomass of prey n the system (bottom-up or donor control). ECOSIM uses a Marquardt non-lnear search algorthm wth a trust regon modfcaton for each of the Marquardt steps to estmate relatve forcng nputs and v j. To evaluate the dfferences between predcted and observed data, ECOSIM uses a logleast-squares crteron, whch we refer to as SS ft to the data. We allow the search routne to estmate v j parameters and, when we assume there have been changes n relatve prmary producton j j over tme, a relatve forcng tme seres that s appled to the prmary producton. Estmatng v j s equvalent to estmatng observaton errors n a sngle speces stock assessment approach, and the relatve changes n prmary producton s equvalent to estmatng process errors. Changes n prmary productvty The observed tme seres data, shown as crcles n Fgures 1 and 2, are better explaned when we assume that there have been substantal changes n prmary productvty over the 50-year tme seres (also see Table 6). Under the constant prmary producton hypothess the log sum of squares ft to the data was , and under a fluctuatng prmary producton hypothess the ft to the data was (roughly a 35% reducton). The probablty of ths reducton n the SS by chance alone s 0.006,.e. the observed data do suggest a change n prmary productvty must have occurred. Under the constant prmary producton hypothess, model bomass predctons generally agree wth the observed data. However, t fals to capture recent observatons n harbour seal abundance and southern resdent kller whales. Marne survval rates for coho and chnook salmon have also declned through the 1990s, and a more parsmonous explanaton s that there has been a severe declne n prmary producton startng around 1990 (Fg. 2). The estmated changes n relatve prmary productvty are shown n Fgure 3, and we compared ths estmated ndex wth other envronmental correlates that are lnked to prmary producton. We were unable to fnd any sgnfcant correlatons between prmary producton and Fraser Rver dscharge, wnd speed, sea surface temperature, upwellng, or the Pacfc Decadal Oscllaton. However the overall downward trend n prmary producton s very smlar to the downward trend observed n average wnd speed squared (Fg. 4). The square of the wnd speed s a measure of sheer stress between ar and the water surface, whch results n vertcal mxng of the water column allowng entranment of nutrents used n photosynthess (Blackett, 1993). A smlar downward trend s also observed n the Pacfc Decadal Oscllaton ndex (PDO). 11

12 Fg. 1 Predcted and observed abundance and total mortalty ndces assumng the relatve prmary producton has remaned constant from 1950 to SS=

13 Fg. 2 Predcted and observed abundance and total mortalty ndces usng relatve prmary productvty sequence shown n Fgure 3. SS=

14 Table 6 Sum of square devatons (SS) between model predctons and observed data for bomass and mortalty. SS no model s equvalent to fttng a straght lne through the data, and SS no envronment assumes constant prmary producton. Data Type SS No Model SS No Trophc Interactons SS No Envronment SS all Effects Adult Chnook Bomass Adult Coho Bomass Hake Bomass Lngcod Bomass Harbour Seal N Adult Herrng Z Juvenle Coho Z Juvenle Coho Z (PSARC) Juvenle Coho Z (Beamsh) Juvenle Chnook Z Seabrd N Res. Orca N Res. Orca Z Adult Herrng Bomass Juvenle Herrng Bomass Total Dscusson The observed tme seres for 11 of the 15 data types (Table 6) suggest that large fluctuatons n prmary producton must have occurred n the Strat of Georga over the last 50 years. Declnes n average wnd speed, and the Pacfc Decadal Oscllaton ndex also support the declne n prmary productvty hypothess. Ideally, ths study should nclude drect estmates of prmary productvty over the entre Strat of Georga; unfortunately, we were unable to fnd these data, f they exst. Nevertheless, t s clear, from our understandng of ecosystem dynamcs that observed declnes are better explaned by assumng prmary productvty has declned. The fsheres stock assessment data used here were not made for the purposes of studyng the role of clmate effects on ecosystem dynamcs. It s mportant to note that these data are lmted n use, as predctors of relatve changes n prmary productvty. Untanglng the complcated trophc nteractons, clmate effects, and mortalty patterns n ecosystem analyss s dffcult; and drect observatons on each of these processes wll be requred to mprove our understandng of ecosystem dynamcs. Many physcal oceanographc studes have been completed, and currently n progress, n the Strat of Georga. Incorporatng these data nto the analyss wll greatly mprove our understandng of ecosystem responses to changng physcal envronments. Probably one of the more dffcult tasks, however, wll be to study small-scale processes of changng trophc nteractons that are related to spatal and temporal abundance of anmals. Predator-prey nteractons play a key role n determnng optmal explotaton rates n that we need to understand how reducng the abundance of one speces effects mortalty rates for other speces n an ecosystem wth varable productvty. 14

15 Fg. 3 Relatve changes n prmary productvty from 1950 to Fg. 4 Relatve prmary productvty, average annual wnd speed squared n the Strat of Georga (a measure of sheer stress) and the Pacfc Decadal Oscllaton (PDO) from 1950 to Sold lnes represent lnear trends lne for each ndex. References Beamsh, R.J., and Boullon, D.R Marne fsh producton trends off the Pacfc coast of Canada and the Unted States. In Clmate Change And Northern Fsh Populatons. Edted by R.J. Beamsh. Can. Spec Pub. Fsh. Aquat. Sc. 121 pp Beamsh, R.J., Noakes, D.J., McFarlane, G.A., Klyashtorn, L., lvanov, V.V. and Kurashov, V The regme concept and natural trends n the producton of Pacfc Salmon. Can. J. Fsh. Aquat. Sc. 56: Blacket, A. W Wnd nduced entranment n the Strat of Georga and the possble consequences for fsh survval. M.Sc. Thess, Unversty of Brtsh Columba. 82p. Cass, A.J., Beamsh, R.J. et al Lngcod (Ophodon elongatus). Can. Spec. Pub. Fsh. Aquat. Sc. 40 p. DFO, 1999 a. Fraser Rver Chnook Salmon. DFO. Scence Stock Status Report D6-11 (1999). DFO, 1999 b. Coho Salmon n the Coastal Waters of the Georga Basn. DFO Scence Stock Status Report D6-07 (1999). Hay D. E., McCarter, P.B., and Danel, K Pacfc herrng taggng from : A reevaluaton of homng based on addtonal data. Canadan Stock Assessment Secretarat Research Document 99/

16 Ketchen, K.S., Bourne, N., and Butler, T.H Hstory and present status of fsheres for marne fshes and nvertebrates n the Strat of Georga, Brtsh Columba. Can. J. Fsh. Aquat. Sc. 40: Lchatowch, J Salmon wthout rvers. Island Press, Washngton D.C., 317p. Martell, S.J.D Reconstructng Lngcod Bomass n Georga Strat and the Effect of Marne Reserves on Lngcod Populatons n Howe Sound. M.Sc. Thess, Unversty of Brtsh Columba. 89p. Pauly, D., Chrstensen, V., Dalsgaard, J., Froese, R., and Torres, F. Jr Fshng down marne food webs. Scence 279: Pauly, D., Chrstensen, V., Froese, R., and Palomares, M.L Fshng Down Aquatc Food Webs. Amercan Scentst, 88: Robnson, C.L.K., and Ware, D.M Smulated and observed response of the southwest Vancouver Island pelagc ecosystem to oceanc condtons n the 1990s. Can. J. Fsh. Aquat. Sc. 56: Sansbury, K Lvng marne resource assessment for the 21st Century: What wll be needed and how wll t be provded? In Fsheres Stock Assessment Models. Edted by F. Funk, T. J. Qunn 11, J. Hefetz, J. N. lanell, J. E. Powers, J. F. Schwegert, P. J. Sullvan, and C.I. Zhang, Alaska Sea Grant College Program Report No. AK-SK-98-01, Unversty of Alaska Farbanks. Schwegert, J.F., Fort, C., and Tanaschuk, R Stock assessments for Brtsh Columba Herrng n 1997 and forecasts for potental catch n Can. Tech. Rep. Fsh. Aquat. Sc. 2217: 64 p. Stocker, M Recent management of the B.C. herrng fshery. In Perspectves on Canadan Marne Resource Management. Edted by L. S. Parsons and W. H Lear. Can. Bull. Fsh. Aguat. Sc. 226, pp Wallace, S.S Changes n Human Explotaton of Marne Resources n Brtsh Columba (Pre-Contact to Present Day). In Back to the Future: Reconstructng the Strat of Georga Ecosystem Edted by D. Pauly, T. Ptcher, D. Prekshot, J. Heame. Vol 6, pp Walters, C.J., Chrstensen, C., and Pauly, D Structurng dynamc models of exploted ecosystems from trophc massbalance assessments. Revews n Fsh Bology and Fsheres. 7: Walters, C.J., Pauly, D., Chrstensen, V., and Ktchell, J.F Representng densty dependent consequences of lfe hstory strateges n aquatc ecosystems: Ecosm II. Ecosystems 3: Prelmnary mass-balance ECOPATH Model n the Boha Sea Lng Tong 1, Q-Sheng Tang 1 and Danel Pauly 2 1 Yellow Sea Fsheres Research Insttute, Qngdao , P. R. Chna. E-mal: tonglng@ysfr.ac.cn 2 Fsheres Centre, Unversty of Brtsh Columba, 2204 Man Mall, Vancouver, B.C., Canada. V6T IZ4 Introducton The Boha Sea (Fg. 5) s a sem-closed contnental water of Chna, whch s nearly encrcled by land only wth a mouth about 90 km at the eastern apex that connects t to the Yellow Sea. The Boha Sea s located n the temperate water regon between ~ N wth 77,000 km 2 n area and the average depth of 18.7 m and the maxmum water depth of 70 m. Water temperature changes a lot resultng from the mpact of the land clmate. The hghest SST s 26~30 C n September and the lowest one s 1.2~4 C n February. Much of the fresh waters run nto the Boha Sea from about 20 rvers, for example the Yellow Rver, Lao Rver, Raoyang Rver, Lng Rver, Luan Rver, and other rvers. The runoff of fresh water was 31.4 bllon m 3 per year n the 1970s and half of t came from the Yellow Rver. The sea s an ocean space wth dstnct productvty, strong fshng actvty and complcated relatonshp of food web, and s also polluted by ndustry and lvng sewage recently. 16

17 Fg. 5 Boha Sea regon. The Boha Sea ecosystem depends on the amount of nput of solar energy and the organsms mported from several rvers. NO 3 -N and PO 4 -P are basc nutrents supportng the prmary productvty n the Boha Sea. The producton of organc carbon of the sea s 112 gc/m 2 /year. The productvty, lke other marne ecosystem, s characterzed by seasonal and spatal varablty wth hgh levels n sprng and fall and n the southern part of the sea, but not much change between years. In the Boha Sea, the domnant small zooplankton are nertc brackshwater speces, such as Sagtta crassa Tokoka, Labdocera euchaeta Gesbrecht and Centropages mcmurrch Wlley. The Boha Sea small zooplankton bomass has two seasonal peaks, n June and September, and the bomass of other ndvdual speces has only one seasonal peak (Ba et al. 1991). The fshng effort n the sea has been ncreasng more and more snce 1962, and has led to a sgnfcant varaton n the abundance and dstrbuton of the most speces n the area. The ntensve fshng resulted n the decrease of bomass of demersal speces wth hgh economc value (large predatory speces), such as Pseudoscaena polyacts and Trchurus haumela, whle harvestng more of smaller pelagc speces, such as Setpnna taty and Engrauls japoncus. Methodology and the ECOPATH model The frst ECOPATH model was developed to descrbe a coral reef ecosystem (Polovna 1984) and was further developed by Chrstensen and Pauly (1992) at ICLARM to make t wdely avalable as a well-documented software runnng on a mcrocomputer. Lately the ECOPATH model has been ntegrated wth ECOSIM for dynamc smulaton modelng based on a mass-balanced model by Walter, Chrstensen and Pauly (1997). In an ECOPATH model t s assumed that the ecosystem modeled s n steady state for each of the lvng groups, whch mples that nput equals output,.e. Q = P + R + U, where Q s consumpton, P - producton, R - respraton, and U - unassmlated food. The above equaton can be structured around a system of lnear equatons for expressng mass-balance wth the smplest form. It can be expressed for an arbtrary tme perod and for each element of an ecosystem by Equaton 1 (see Martell et al. ths report). It s the smultaneous lnear equatons used n ECOPATH to state that the producton and consumpton are balance wthn an ecosystem. The ECOPATH model allows rapd constructon and verfcaton of mass-balance model of ecosystem. The mass-balance model not only verfes the prevously publshed bomass estmates, but also dentfes the bomass requred for assessment of marne carryng capacty. Constructng an ECOPATH model ncludes the followng steps: 1. Identfcaton of the area and perod for whch the ecosystem model wll be constructed; 2. Defnton of all functonal groups (boxes), from prmary producers to top predators, n the ecosystem to be ncluded for the thermodynamc balance; 3. Settng parameters of producton/bomass rato (P/B), consumpton/bomass rato (Q/B), bomass (B) and ecotrophc effcency (EE) for each functon group, but only three of them are necessary as the basc nput parameters n the model, and also entry of the catches to every fshng speces; 4. Entry of a det consumpton matrx (DC) expressng the det fracton of predator/prey relatonshp n the model; 5. Modfy the entres of P/B, Q/B, EE or the bomass, to balance the ECOPATH model (repeatng steps (3) and (4) above) untl the mass nput equals output for each box. 17

18 Structure of the Boha Sea ECOPATH model The resources composton n the Boha Sea changed a lot along wth the fshng effort ncrease to mult-speces fsh communtes after The CPUE (catch per horse power) was 7.61 tons n 1962, but t went down to 0.88 ton n The tradtonal speces fshed n the area, such as small yellow croaker, slender shad, cutlasfsh, were hgh valuable n the market, but the bomass of them declned then. The small pelagc fsh and small crustacean speces appeared more n the landngs and fluctuated much more annually. The hghest annual landng of Acetes (a sergestd shrmp) can be 100 hundred metrc tons (1.3 t/km 2 ) n the sea. The hghest catch of jellyfsh reached 280 hundred tons durng the 1970s. Ths reflects a gradual transton n catch from long-lved, hgh trophc level pscvorous bottom speces toward short-lved, low trophc level nvertebrates and planktvorous pelagc speces. The Boha Sea s an example of an overfshed marne ecosystem leadng to smaller, hgh-turn-over speces. It s a pecularty of the sea that small pelagc fsh and jellyfsh replace large table fsh n an over-exploted ecosystem (Ptcher 1998). The mass-balance model of the Boha Sea s amed at constructng a quanttatve descrpton of trophc structure and the relatonshp among the dfferent groups n the whole Boha Sea. The model s based on the data of the Boha Sea ecosystem survey project completed durng Aprl 1982 to May The project collected the data monthly by the bottom trawlng and manly made assessment of the commercal mportant speces and ther bologcal characterstcs study. As ths s the frst ECOPATH model of the Boha Sea, t only presents a prelmnary revelaton of the trophc structure and flow n the sea between dfferent functonal groups. The functonal groups n the model covered the man trophc flows among the lvng marne speces and detrtus, but the group defnton s very rough because of the lmted type of survey data avalable n the regon. The functonal group determnaton s based on the speces dstrbuton n the water and ther feedng behavour after nspectng the stomach contents of 54 speces from 1863 samples. Consderng the lmted data and no exstng mass-balance model n the Boha Sea, the model only has 13 functon groups. One prmary producer of phytoplankton was dentfed. Zooplankton was splt nto two groups, mcrozooplankton and macrozooplankton. The former ncludes small herbvorous and carnvorous zooplankton and the latter manly conssts of jellyfsh and Acetes. Benthc nvertebrates were dvded nto small mollusca, large mollusca, small crustacean and large crustacean, most speces of whch were commercal harvest n the sea but the landng data were not readly avalable. There were no bomass data for some speces n the small nvertebrate groups so ther bomass were estmated by the model usng the fxed ecotrophc effcency (EE=0.95). Bomass for the two large groups were obtaned by summng up the bomass data from the survey. Fve fsh functon groups were dentfed n the model on the bass of 31 fsh speces whch hold about 90% of total bomass for the fsh communty n the Boha Sea. The herbvorous feeders group ncludes manly Mugl cephalus and Lza haematochela. The other four groups were small pelagc fsh, demersal fsh, benthc feeders and top pelagc feeders, whch were mportant commercal fshng targets. The detals of 13 functon groups (box) n the Boha Sea ECOPATH model are summarzed n Table 7. Many speces are ncluded n one box of the model so t s hard to fnd P/B and Q/B from one speces for the whole group. The P/B and Q/B parameters were based on the parameters from smlar functon groups n the models of the Strat of Georga (Dalsgaard 1998), the Brune Darussalam, South Chna Sea (Slvestre 1993) and the Georges Bank (Sssenwne 1984). The basc parameters of bomass (wet weght t/km 2 ), P/B, Q/B, EE and harvest for the ECOPATH model of the Boha Sea ecosystem are presented n Table 8. Detrtus s estmated from prmary producton of carbon by equaton A5 of the emprcal relatonshp method (Pauly, D., M.L.Sorano-Bartz et al. 1993). Phytoplankton was estmated from Boha Sea prmary productvty of 112 gc/m 2 /year converted to g wet weght phytoplankton m -2 year -1 by a wet weght:carbon rato of 10:1. 18