Fisheries Centre, University of British Columbia, Canada

198 6727ISSN

Preliminary Mass-Balance Modelof Prince William Sound, Alaska,

for the Pre-Spill Period, 1980-1989

Preliminary Mass-Balance Modelof Prince William Sound, Alaska,

for the Pre-Spill Period, 1980-1989

by

Johanne Dalsgaard

and

Daniel Pauly

with editorial contributions by

Thomas A. Okey

Published by

The Fisheries Centre, University of British Columbia2204 Main Mall, Vancouver, B.C.

Canada V6T lZ4

ISSN 1198-6727

111

ABSTRACT

Ealancing of the model required fewsteps that went beyond the availabledata; nevertheless, the model is pre-liminary in that additional ecologicalinformation is available on PWS andthe functional groups of the organ-isms therein. Much of this informa-tion is not yet incorporated in themodel. However, the purpose of thismodel is to serve as basis for furtherwork, illustrated in two authoredappendices, one showing the closematch between the trophic levelsestimated by the models and esti-mates based on stable isotope ra-tios, and the other documentinghow inferences on the dynamics ofPWS may be derived from its staticrepresentation.

A mass-balance model of trophicinteractions among the key func-tional groups of Prince WilliamSound (PWS), Alaska, is presented,based mainly on published data re-ferring to the period from 1980 to1989, before the Exxon Valdez oilspill.The functional groups explicitly in-cluded in the model are: detritus,phytoplankton, macroalgae, smallzooplankton, large zooplankton,epifaunal benthos, infaunal benthos,intertidal invertebrates, demersalfish, herring, salmon fry fromhatcheries, wild salmon fry, salmon,other pelagic fishes, birds, sea ot-ters, other resident marine mam-mals, and transient marine mam-mals.

v

Director's ForewordThe first Mass-balance models ofmarine ecosystems in the North-eastern Pacific, covering the AlaskaGyre, the shelf of southern BritishColumbia, and the Strait of Georgia,were constructed in November of1996 at a workshop held at the UBCFisheries Centre (see Fisheries Cen-tre Research Report 1996, Vol. 4, No1). The present report extends thatwork by drawing up a preliminaryecosystem model of Prince WilliamSound, Alaska, in its most likelyform prior to the Exxon Valdez oilspill in 1989. ECOPATH models areforgiving in that they can be im-proved and enhanced using newinformation without having to becompletely reinvented. This reportputs forward a preliminary model,based on data from published litera-ture, whose sole purpose is to leadto an improved model based onmore complete and more accuratedata from the recent intensive workin Prince William Sound.

For many years single species' stockassessment of fisheries has reignedsupreme and separate from main-stream marine ecology, but, for ma-rine conservation, this approach andlack of integration has been con-spicuously unable to answer thecrucial questions of our time. Thesequestions include the interplay ofpredators, competitors and preywith human fisheries, the impactboth acute and chronic of marinepollution, and the effects of pro-gressive shoreline development onthe stability and value to humansociety of coastal ecosystems.

ECOPATH is a straightforward trophicmodeling approach to ecosystems,that balances the budget of biomassproduction and loss for each com-ponent in the system by solving aset of simultaneous linear equa-tions. The ECOPATH approach is theonly ecosystem model to obey thelaws of thermodynamics. It is basedon pioneering work by Dr GeoffreyPolovina from Hawaii in the early1980s, and developed by Dr DanielPauly when he was at ICLARM, Ma-nila, and Dr Villy Christensen fromDenmark. Dr Carl Walters at theFisheries Centre recently developiedECOSIM a dynamic version ofECOPATH.

A Preliminary Mass-Balance Modelof PIince William Soun~ Alaska, forthe Pre-Oilspill Peno~ 1980-1989 isthe latest in a series of research re-ports published by the UBC FisheriesCentre. The series aims to focus onbroad multidisciplinary problems infisheries management, to provide ansynoptic overview of the founda-tions and themes of current re-search, to report on work-in-progress, and to identify the nextsteps and ways that research may beimproved. Edited reports of theworkshops and research in progressare published in Fjshenes CentreResearch Reports and are distrib-uted to all project or workshop par-ticipants. Further copies are avail-able on request for a modest cost-recovery charge. Please contact theFisheries Centre mail, fax or email to'office@fisheries.com'.

Tony j. PitcherProfessor of FjshenesDuector, UBC Fjshenes Centre

VI

Table of Contents

ABSTRACT V

DIRECTOR'S FOREWORD VI

TABLE OF CONTENTS VII

UST OF EXHIBITS VIII

INTRODUCfION 1

PRINCE WILLIAM SOUND 1

THE ECOPATH APPROACH AND SOFfWARE 3

PREliMINARY MODEL INPUTS 5

PHYTOPLANKTON .'."""".' '.."""'...' , 5MACROALGAE , " ",., 6

DETRI11Js ,.,."",..., , ".,." 7ZOOPLANKTON "' 7

BENTmC INVERTEBRATES 8lNTER11DAL INVERTEBRATES "'..' ' "' '..".'.'."' 9DEMERSAL FISH 10HERRING SALMON FRY 11

ADULT SALMON 1Z

MISCELlANEOUS SMAll. PEIAGIC FISHFS 13BIRDs 13SEA OTTERS , ~ 16OTHER MARINE MAMMALS "' '."."."."""'..' 16

BALANCING THE MODEL 17

21RESULTS AND DISCUSSION ,

~

ACKNOWLEDGEMENTS .25

26REFERENCES '

APPENDIX I 32

APPENDIX 2 33

VII

List of exhibits

Box 1. Basic equations and assumptions of the Ecopath approach 4Figure 1. Map of Prince William Sound, Alaska , 2Figure 2. Simplified flowchart of Ecopath model of PWS 22Figure 3. Mixed trophic impacts of groups in the PWS ecosystem model 23Figure 4. Results of an Ecosim run of the PWS file generated by Ecopath 24Table 1. Macroalgae biomass estimates for different PWS habitats 6Table 2. Mean Settled zooplankton volume in upper 20m 7Table 3a. Estimated biomass of benthic infauna, in PWS 8Table 3b. Estimated biomass of benthic epifauna, PWS 9Table 4. QjB estimates and diet matrix (% weight) of PWS zoobenthos 9Table 5. Estimated biomass of demersal fish species in PWS, 1989 10Table 6. Mean demersal fish catch from PWS, 1987-1988 10Table 7. Average harvest of herring in PWS from 1980 to 1988 11Table 8. Characteristics of wild salmon fry, and hatchery released fry 12Table 9. Harvest of salmon in PWS, 1980 to 1989 12Table 10. Estimated standing stock of salmon in PWS 13Table 11. Instantaneous oceanic mortality rates (Z) for salmon 13Table 12. Population statistics of ducks overwintering in PWS 14Table 13. Estimation of duck biomass in PWS; by season 14Table 14. Diet matrix for selected ducks in Prince William Sound 14Table 15. Seabird densities in PWS. 15Table 16. Diet matrix for the seabirds of PWS 15Table 17. Diet composition of sea otters in Montague Strait, PWS 16Table 18. Population statistics of marine mammals in PWS 17Table 19. Basic statistics for transient marine mammals in PWS 17Table 20. Basic statistics for resident marine mammals in PWS 17Table 21. Basic statistics for pinnipeds in PWS 18Table 22. Diet matrix for the marine mammals of PWS 18Table 23. Basic estimates and trophic levels of groups in PWS model 19Table 24. Diet matrix of trophic interactions in PWS 20

VIII

much of the relevant data from bothperiods as possible, for a largenumber of functional groups.

However, the model presentedherein, and the assumptions usedfor its construction are prelimi-nary. Weare aware of the exis-tence of better data for most of thefunctional groups in the model. Aprimary goal of the current effortis a collaborative partnershipamong PWS experts to incorporatethese better data thereby maximiz-ing the usefulness of the more de-tailed models, to be constructed in1998.

Pending these detailed models, thepreliminary model presented herewas assembled to :

PWS consists of a central basin, witha maximum depth of 800 m, sur-rounded by islands, fjords, bays, anda large tidal estuary system (Fig. 1);the mean water depth is 300 m(Cooney 1993, Loughlin 1994). Thesemi-enclosed nature of PWS justi-fies the application of a modelingapproach, such as Ecopath (Box 1),that assumes mass-balance amongthe various elements of the system,and limited (or at least well-quantified) exchanges with adjacentsystems (see also contributions inPauly and Christensen 1996).

INTRODucnONPrince William Sound

Prince William Sound (PWS) , locatednear the northern apex of the Gulfof Alaska and renown for its wildlifeand once pristine environment, be-came the center of the world's atten-tion on March 24, 1989, when thesupertanker Exxon Valdez ranaground on Bligh Reef, in the North-eastern part of the Sound (Fig. 1),spilling over 40 million liters ofcrude oil -the largest oil spill inUnited States history. During thefirst two weeks following the spill,the oil was transported the south-west through the western part ofPWS, and into the Gulf of Alaska,along the Kenai Peninsula, killingthousands of seabirds, marinemammals and vast numbers of fishand invertebrates (Loughlin 1994).

Early assessments of the impact ofthe Exxon Valdez oil spill (EVOS)were presented in the proceedings,edited by Rice et al. (1996), of asymposium held from February 2 to5, 1993. Major efforts have beenmade since, under the guidance ofthe Exxon Valdez Oil Spill TrusteeCouncil, to study the long-term ef-fects of the EVOS, and a second gen-eration of assessments is nowemerging which will address ques-tions left open at the 1993 sympo-sium.

The present report is designed tosupport these efforts by presentinga prelilninary version of what will bemore precise and realistic models oftrophic interactions among the ma-jor functional groups of PWS, for theperiods before and after the spill.These models will incorporate as

1

Figure I. Map of Prince William Sound, Alaska, showing locations mentionedin the text (modified from Sturdevant et al.1996).

2

(ii) Definition of the functionalgroups (i.e., 'boxes') to be in-cluded;

(ill) Entry of a diet matrix, express-ing the fraction that each 'box' inthe model represents in the dietof its consumers;

(iv) Entry of food consumption rate,of production/biomass ratio orof biomass, and of fisheriescatches, if any, for each box;

(v) Balance the model, or modifyentries (iii & iv) until input =output for each box;

(vi) Compare model outputs (net-work characteristics, estimatedtrophic levels and other featuresof each box) with estimates forthe same area during anotherperiod, or with outputs of thesame model type from other,similar areas, etc.

The Ecopath approach and soft-ware

The Ecopath approach and softwarewere initially developed by Polovina(1984, 1995), of the u.s. NationalMarine Fisheries Service (HonoluluLaboratory). V. Christensen and D.Pauly, then both at the InternationalCenter for Living Aquatic ResourcesManagement (ICLARM), improved onthis work (see Christensen and Pauly1992a), and made it widely availablein the form of a well-documentedsoftware for computer running MS-DOS (Christensen and Pauly 1992b),and later Windows (Christensen andPauly 1995, 1996). Both versionsallow rapid construction and verifi-cation of mass-balance models ofecosystems. The steps involved con-sist essentially of:

(i) Identification of the area' andperiod for which a model is to beconstructed;

3

Box 1. Basic equations, assumptions and parameters of the Ecopath approach

The mass-balance modeling approach used in this workshop combines an approach by Polovinaland Ow (1983) and Polovina (1984, 1985) for estimation of biomass and food consumption 01the various elements (species or groups of species) of an aquatic ecosystem (the original,'ECOPATH') with an approach proposed by illanowicz (1986) for analysis of flows between theelements of ecosystems The result of this synthesis was initially implemented as a DOS soft _

Iware called 'ECOPATH n', documented in Christensen and Pauly (1992a, 1992b), and more re-cently in form of a Windows software, Ecopath 3.0 (Christensen and Pauly 1995, 1996). Unlessnoted otherwise the word 'Ecopath' refers to the latter, Windows version. The ecosystem ismodeled using a set of simultaneous linear equations (one for each group i in the system), i.e.

Production by (i) -all predation on (i) -nonpredation losses of (i) -export of (i) = 0, for all (i). !

This can also be put as I f,~

D-M2 -P(I-EE)-EX =0 ...1)i I I I I

where P is the production of (i), M2. is the total predation mortality of (i), EEl is the ecotrophic Iefficiency of (i) or the proportion of the production that is either exported or predated upon, (l-EE) is the "other mortality", and EX is the export of (i).

i I

Equation (1) can be re-expressed as

B*P/B -LB*QjB*DC -P/B*B.(I-EE)-EX. =0 ...1)i I j j j Ij ill 1

;;~~jl!?;~I:?or '.7,".~!1if:'i;",

Bj*P IBi*EEj -1:JBJ*Q/BJ*DCij -EXI = 0 ...2)

where B is the biomass of (i), P lB. is the productionjbiomass ratio, Q/B. is the consumpI 1 I

tionjbiomass ratio and DCij is the fraction of prey (i) in the average diet of predator (j).

Based on (2), for a system with n groups, n linear equations can be given in explicit terms:

B P IB EE -B Q/B DC -B Q/B DC B Q/B DC -EX = 0I I I I I II 2 2 21 n n nl I

B2P/B2EE2- B1O/BpCI2- B2O/BpC ,-B Q/B DC -EX = 0n n n2 222

B P /B EE -B QJB DC -B QJB DC B QJB DC -EX = 0n n n I I In Z Z Zn n n DR n

This system of simultaneous linear equations can be solved through matrix inversion. InEcopath, this is done using the generalized inverse method described by MacKay (1981), whichhas features making it generally more versatile than standard inverse methods.

Thus, if the set of equations is overdetermined (more equations than unknowns) and the equa-tions are not consistent with each other, the generalized inverse method provides least squaresestimates which minimize the discrepancies. If, on the other hand, the system is undetermined(more unknowns than equations), an answer that is consistent with the data (although noti unique) will still be output.

Generally only one of the parameters Bi, P /Bi, QJB, or EEj may be unknown for any group i. Inspecial cases, however, QJB. may be unknown in addition to one of the other parameters (Chris-tensen and Pauly 1992b). Exports (e.g., fisheries catches) and diet compositions are always re-quired for all groups.

A box (or "state variable") in an Ecopath model may be a group of (ecologically) related species,i~e~, a functional ~o~~~icI:1~gle species, or a single size/age group of a given species.

4

These steps can be implementedeasily if basic parameters can beestimated (see also Box 1), especiallyas numerous well-documented ex-amples exist of Ecopath applicationsto aquatic ecosystems (see Pauly andChristensen 1993, and contributionsin Christensen and Pauly 1993). Werefer here frequently to three eco-systems that have much in commonwith PWS (the Strait of Georgia, thecoast of British Columbia, and theAlaska gyre), documented throughthe contributions in Pauly andChristensen (1996), and from whichmany parameter estimates were

taken. In the following pages, detailsare provided, by functional group,on how items (ii) to (vi) were imple-mented, following definition of theperiod (1980-.1989) and area to bemodeled (Fig 1).

This is then followed by twoauthored appendices, by Kline andPaUly (1997; Appendix 1) and Paulyand Dalsgaard (1997; Appendix 2),illustrating some of the possiblefollow ups to the preliminary modeldocumented here.

PREUMIN AR Y MODEL INPuTS

PhytoplanktonThe phytoplankton community inPWS is usually dominated by dia-toms.However, Goering et al. (1973)found that in Valdez Arm (Fig. 1),the flagellate Phaeocysus pouchetilwas numerically dominant duringApril; also, during the less produc-tive July conditions, the phytoplank-ton community was dominated bythe dinoflagellate Cerauum lon-gjpes.Detailed seasonal data on phyto-plankton growth exist for selectedareas of PWS. Chi. a concentrationsand carbon production were meas-ured bimonthly from May 1971 toApril 1972 in Port Valdez and Val-dez Arm. A typical spring-bloomsequence was found in which bothstanding crop and primary produc-tion increased rapidly in April, de-pleting nitrate from the upper waterlayers. Production then de

creased during the reminder of theSummer, then increased again -atleast in several areas -in the Fall(Sambrotto and Lorenzen 1986).

The mean yearly primary productionof ,..,185 gCom-2 estimated by Sam-brotto and Lorenzen (1986), may beassumed to apply to the whole ofPWS (Cooney 1986), as the range is150-200 g Com-2.yearl (T. Cooney,pers. commo). Assuming 0.1 gC ~ 19WW, an annual primary productionof 185 gC. m-2 equals 1850 tww. km-2 0 yeari.

Olivieri et al. (1993) estimated theP/B ratios of large diatoms andsmall phytoplankton (cells < 5 ~;including cyanobacteria and flagel-lates) to range from 125 to 255yearI, with a mean of 190 yeari.From this, the phytoplankton bio-mass can be estimated as (1850 tww 0 km-2 0 yearI) I (190 yearI) ~ lOtww 0 km2.

5

Macroalgae surface area of approximately 8800In PWS, dense macrolagal assem- km2, th~ shallow subtidal. zone isblages are typical in the shallow 880 km .These assumptIons aresubtidal zone, which extends from ~eing revised during the next itera-the intertidal zone to depths of non to account for areas with noabout 20m (Dean et al. 1996). kelp cover and by decreasing the

areal proportion of shallow subtidalDe~ et al. (~996) compared the zone in PWS using GIS measure-dens:ty and bIomass of ments.subtIdal macro algae inoiled versus non-oiled Table 1. Macroalgae bio- Dean et al. (1996)(control) sites in PWS mass estimates for differ- found Agarum cn-one year after the Exxon em PWS habitatsa). brosum and lami-Valdez oil spill. While naria saccharina tothe relative species com- be the dominantposition of these assem- subtidal macroal-blages appeared to have gae in shelteredchanged, the authors' bays. Generally,study revealed no differ- these two speciesences in total density, --.constituted morebiomass, or percent I than 90% of totalcover. These results pro- macro algal bio-vide some justification for using the mass.

biomass values of this study's con- Agarum cnbrosum also dominatedtrollo'cations for a pre-spill baseline on exposed points (more than 60 %reference Use of these values as- in terms of number of individuals).sumes that they are representative Less abundant algae were laminariaof ~he shallow subtidal zone of the saccharina and l. groenlandica.entIre PWS for the entire decadefrom 1980 to 1989 prior to the oil Nereocystes habitats are located onspill. ' exposed sites, and the algal diversity

was higher than in the other two~hree type of habitats were identi- habitats. The kelp forest structure atfled by Dean et al. (1996): these locations consists of a canopy1. Agarum-laminaria beds ,!-~ of Nereocystes luetkeana with an

bays(2-11m, II-20m); understory ofl. groenlandica (61 %..of the biomass), l. yezoensis, Pleu-

2. AB:arum-lammana beds rophycus gardner~ and A. cnbro-pOInts (2-11m, II-20m); and sum.

3. Nereocystisbeds (2-8m). Based on the aforementioned as-

It is assumed here that each of these sumptions, the biomass of macroal-habitats cover 1/3 of the shallow gae was calculated to be 3,967 g. m-2subtidal zone (0 -20 m) of PWS. for 880 km2 of shallow subtidal area;Furthermore, we assume that the re-expressed for the PWS as a whole,shallow subtidal zone is 1/10 of the this corresponds to ,... 400 t .km-2.

surface area of PWS. Since PWS has a

Biomass(g ww.m-2)

Shallow DeeJ}~Habitats

Ba sPointsNereoc stisMeans

LZ2!?~~3565-,

529678

402a) Modified from Dean et al(1996).

ill

on

6

Estimates of the P /B ratio of PWSmac.roalgae were not found. Thevalue of 4.4 yearl used here, per-taining to Laminaria beds in theNorth Atlantic, is from Brady-Campbell et ill. (1984).

In March-May, the upper layer ofzooplankton in PWS is dominated bythe copepods Neocalanus cnstatus,N. plumchrus, Eucalanus bungil,which are oceanic species, by Cala-nus marshaUae, Pseudocalanus sppwhich are not, and by the arrow-

.worm Sagitta elegans. Towards June,Detntus this community is gradually re-

Rough estimates of the standing placed by late stage copepodites ofstock of detritus in marine ecosys- Calanus marshaUae, the smaller co-terns may be obtained from: pepods Acaraa, Centropages, Torta-

nus and Pseudocalanus, Metndialog D = -2.41 + 0.954 .log PP + 0.8631og E okhotensis, M pacifica, and the cla-

where D is the standing stock of docerans Podon and Evadne. In thedetritus, in gC. m-2, PP the primary Fall and Winter, these are followedproduction in gC. m-2 .yearl, and E by Sagitta elegans and M pacifica.is the euphotic depth in meters Other groups also occurring in zoo-(Pauly et al. 1993). In the absence of plankton samples are amphipods,a readily available estimates of mean euphausiids (5 species), and coelen-euphotic depth for PWS, the detritus terates, among others (Anon 1980;standing stock estimated by this Cooney 1,986; Cooney 1993; Tedequation for the Strait of Cooney, pers. comm.).Georgia, of 7 t. km-2 (Venier Published estimates of1996),. is used. The precise Table 2. Mean zooplankton biomassvalue of this estimate has no settled zooplankton

ld t b f d f th.volume upper 20m cou no e oun or eeffect on the computatIon of of PWS (T. Cooney, period prior to the EVOS,detritus flows. pers. comm.)a) and we therefore relied on

Bacteria (incl. bacterioplank- the. data in Table 2, madeton) are not included in the available by Dr T. Cooneymodel: it is assumed that (pers. comm.):bacteria consume only detri- Settled volumes in mI. m-3tus, and that the fluxes asso- can be converted to g. m-3ciated with this consumption by assuming that 70% ofcan be treated as if they oc- settled volume in millili-cUffed in another, adjacent ters is equivalent to wetecosystem, i.e., that in which weight in grams (Weibe etdetritus accumulates when it al. 1975). Applying thisleaves PWS. This omission of conversion factor, 1.75bacterial fluxes has no im- mI. m-3 correspond topact whatsoever on the other 1.225 g ww. m-3.estimates of fluxes estimated G.

th t PWSIven a on averageby Ecopath. is 300m deep, and that

Zooplankton zooplankton occur at the

Year~

198119821983198419851986198719881989Mean

ml.mo3

1.452.432.652.394.481.531.42I~

.91 5.2

1.75a) Sampled using a0.25 mm mesh sizeand 0.5 m diameternet. The data areaverage values forMarch 15 to June15, and were taken2-3 times weekly inthe southern part ofthe Sound.

7

To account for substrate and otherhabitat differences, PWS was splitinto the following three areas withgenerally different habitat charac-teristics. For this initial exercise, onethird of the surface area was allo-cated to each area.

1. Central Basin and HinchinbrookEntrance;

2. Eastern fjords and bays;

3. Western fjords and bays.

The substratum in the central basin(1) is composed of sand, silt andclay, reflecting a depositional envi-ronment. The fjords in the west ofPWS (3) are impacted by glacial silt,and characterized by low infaunalabundance and biomass, at leastwhen compared to the communitiesin the east and north of PWS. Here,muddy bottoms dominate, sup-porting a primarily deposit-feedinginfauna (Feder and Jewett 1986).Table 3 and 4 shows the estimatedbiomass of benthic infauna and ben-thic epifauna respectively.

same density throughout the watercolumn, the biomass of zooplanktonper surface area would be: 1.225g.m-3. 300m = 368 g.m-2.

For the model, it is assumed that25% of zooplankton biomass con-sists of macro plankton, based onCooney (1993), who gave 25% as aconservative estimate of macro-plankton production to total zoo-plankton production. The remain-ing 75 % are assumed to consistsoverwhelmingly of mesoplankon(microzooplanton is not consideredhere). This leads to biomass esti-mates of 92 t.km-2 for macroplank-ton and 276 t.km-2 for mesoplank-ton.

Another input required by Ecopathis OlB, h~re assum~d to have thesame value, 10.5 yearl, as herbivo-rous zooplankton in the Strait ofGeorgia (Harrison et al. 1983). Fi-nally, we assume mesozooplanktonto .have a diet consisting only ofphytoplankton, while macrozoo-plankton is assumed to consume75% meso zooplankton and 25% phy-toplankton. For both zooplanktongroups, we set EE at 0.95, a defaultvalue for groups heavily preyedupon (Polovina 1984).

Benthic invertebrates

The values used herein for benthicinvertebrates larger than 0.5 mmare very tentative. Estimates forthese components need consider-able refinement.

a) Port Valdez and Valdez Arm;Derickson Bay (20 g.m-2); Blue Fjord (13 & 3 g.m-2); andMcClure Bay (6 g. m-2).

8

Feder and Jewett (1986, Table 12-9)indicate that the infaunal biomass atHinchinbrook Entrance, of 343 g.m-2,produces 4.6 gC.m-2.year1, corre-sponding to 222 g ww. m-2.year1.

Table 3b. Estimated biomass of ben-thic epifauna, PWS (from Feder and

1986).

a) Near Port Etches;b) No estimate available; biomass assumed tobe the same as for eastern fjords and bays.

This leads to a P /B estimate of 0.6yearI, which we apply throughoutPWS. The same table in Feder andJewett (1986) gives a P/B ratio of 2.0yearl for the epifaunal macrofauna,here also applied to the entire PWS.

Trowbridge (1996) gives the follow-

ing catchesof benthic invertebratesfrom PWS:

1) Epifauna (including pink andother shrimps), king crab (red,blue, brown), and tanner crab:0.143 t.km-2.yearl;

2) Infauna (razor clam and others):0.003 t .km-2 .yearl.

It is here assumed that similarcatches were made in the 1980-1989period.

Intertidal invertebrates

Intertidal invertebrates, dominatedby barnacles, gastropods, and bi-valves (especially Mytilus edulis) ,are, in PWS, an important source offood for birds and sea otters, amongothers predators. The only biomassestimate we have identified, 624g. m-2, is from Stekoll et al. (1996,Table 2), and was obtained during astudy carried out from the Spring of1990 to the Summer 1991, to assessthe impact on the intertidal zone ofthe EVOS, and of the cleanup effortswhich followed.

Assuming the intertidal zone makesup 1% of the surface area of PWS,the total size of the zone is 88 km2.Averaged over the total area of PWS,the invertebrate biomass is 6.24t .km-2. P /B and QjB are assumed

equal to the values forTable 4. QjB estimatesa and diet matrix b (% weight) of epifaunal benthos (seePWS zoobenthos. above).

O'Clair and Zimmer-man (1986, Table 11-4)list the followingfeeding groups of in-tertidal benthos ofrocky areas of the Gulfof Alaska: herbivores(22%), suspension

a) From Guenette (1996, based on several sources);Based on Table 3a for infauna (mainly deposit feeders) and Table 3bfor epifauna (mainly predators: tanner crab is a scavenger/predator;pink shrimp feed on small polychaetes and crustaceans; and sun-flower feed mainly on mollusks).

9

feeders (23%), carnivores (26%), de-posit feeders (16%), omnivores (6%),and scavengers (7%).

used here are simple averages for1987. and 1988 (Table 6). These sug-

Table 5. Estimated biomass of demersalfish species in PWS- 1989a. -

a) Adapted from NMFS (1993);b) Big skate, Bering skate, Alaska skate, Aleutianskate, Pacific halibut, rex sole, Pacific cod,rougheye rockfish and others;c) Based on the 95% confidence interval (walleyepollock and sablefish)' and the fact that the walleyepollock biomass was only slightly greater than thatof flathead sole (NMFS 1993);Based on arrowtoothflounder contributin~ 60% of total biomass

gest, for PWS as a whole, a catch ofdemersal fishes of 0.037 t.km-2.yeari.

Adult halibut and cod are apexpredators, and feed on a variety ofmedium to large fish such as pol-lock, flatfish, and sculpins. Theyalso feed on invertebrates, such as

crabs, shrimps and krill,~ 6. Mean demer- and small pelagic fishes1 catch from PWS, such as smelt. Walleye

_1987-1988 pollock and many rock-fish species feed pre-dominantly on small tomedium-size nektonicprey such as large am-phipods, copepods, krill,smelt, and other smallfish. Pollock is alsoknown to be cannibalis-tic. Sablefish is an om-nivore and scavengerfeeding on fish and in-

Demersal fish

Demersal fish, as defined here, in-cludes true bottom fish such as flat-fish and skates and semi-demersalfish such as pacific cod, walleyepollock and others.

In PWS, the two dominant demersalfish species are arrowtooth flounderand walleye pollock. Table 5 showsthe biomass estimates resultingfrom a trawl survey conducted inPWS in 1989. As reported by NMFS(1993), "arrowtooth flounder madeup the greatest proportion of totalbiomass at every site except CentralBasin and Port Wells. It accountedfor 67% of total biomass in the areaof Knight Island/Montague Straitand 65% of total biomass in the areaoutside PWS" (NMFS 1993). Based onthis, arrowtooth flqunder was set tocontribute 60% of total demersalbiomass in Table 5.

Based on Table 5, the density ofdemersal fish in PWS wascalculated as: 83,000 ton-nes / 8800 km2 = 9.4 t. km- Ta~le2 sal fist

Species Catchtj!:y~

45.63193.576

174.7809.663

.0.4394.948

329.037

RockfishSablefishPa ..

FlaUnat erTotalAdapted from Bechtol(1995).

Pending detailed analysesof data from PWS, the P /Bvalue of 1 yearI, and theQ/B value of 4.24 year-I,pertaining to the demersalfishes of the Strait of Geor-gia, were taken from Venier(1996, Table 36).

No suitable data set beingavailable for the periodprior to 1987, the catches

10

vertebrates. Flatfish and sculpinshave overwhelmingly benthic diets.Soles consume small invertebrates(worms, snails, clams, brittlestars,etc.), while flathead sole feed onshrimp, krill, herring, and smelt(Alton 1981).

Based on such feeding habits infor-mation, as well as diet informationpresented in various contributionsin Pauly and Christesen (1996), thefollowing diet composition was de-rived for the demersal fish of PWS(in % weight): benthic invertebrates(25); pelagic fish (25); macrozoo-plankton (15); mesozooplankton(15); herring (10); and demersal fish(10; cannibalism).

from the above figures. Natural mor-tality (M) was estimated as 0.53 yearI, as the means of age-specific esti-mates (age 3 to 8) for herring in theGulf of Alaska (Wespestad and Fried1983). Since PIB, under equilibrium,equals total mortality (Z; Allen1971), and Z = F + M, the P/B ratiofor herring can be estimated from0.53 + 0.14 = 0.67 yeari.

The value of Q/B used here, of 18year-I, is the same as that used forsmall pelagics (mainly herring) inthe Strait of Georgia (Venier 1996);the diet consists of zooplankton:euphausiids, copepods, mysids, andamphipods, among other (White-head 1985).

Table 7. Average harvest of herring inPWS from 1980 to 1988."

From Morstad et al. (1996).

Salmon fry

Both wild fry and hatchery-releasedsalmon fry occur in PWS. The hatch-ery released fry is mostly pinksalmon, which are believed to residein the Sound from early April tojuly/early August (the hatcherystock is released in late April/earlyMay). Table 8 summarizes key fea-ture of salmon fry in PWS.

HerringHerring spawns in PWS in Spring,from mid-April to early May (Mor-stad et al. 1996). This is also theseason for seine and gill net fisher-ies, for sac roe, and for two spawn-on-kelp fisheries. Another fishery,for food and bait, occurs in the Fall.Table 7 presents mean catches forthe period from 1980 to 1988.

Based on Table 7, the catch of her-ring in PWS equals 1.136 t.km-2;

The corresponding biomass, also forthe 1980~1988 period, was esti-mated as 71,341 t or 8.107 t. km-2,based on an age-structured analysis(Morstad et at. 1996, AppendixH.11).Given that, under equilibrium, F =catch/biomass, a fishing mortality(F) of 0.14 year-1 can be estimated

11

Initially, salmon fry consume meso-plankton; calanoid copepods are apreferred prey, but diversity of thediet increases with size of the fish(Cooney et al. 1978). For the presentmodel, it will be assumed that the

Table 8. Characteristics of wild salmon fry Iand hatchery released frya

Density (t.km2)b, Survivors after 120 davs (%)I Consumption (() !cI:l120 days Ir O/B (year-I) I

a) From Cooney (1993); both wild and hatchery re-leased fry are about 25 g, and remain in PWS for 120days when entering PWS;b) Assuming that fry are evenly distributed withinPWS; Mean release of (mainly) pink salmon, 1980-1989(Morstad et al. 1996).

Table 10 presents minimum esti-mates of the mean biomass ofhatchery and wild pink and chumsalmon in PWS from 1980 to 1989,based on wild stock escapement(minimum estimates), hatchery re-

turns and catches.

Biomass estimates for thethree other salmon species inPWS could not be found.However, pink salmon is thedominant species, contrib-uting about 83% of the catch(in weight), while chum con-tributes about 13%. Assumingthe same percentages applyto the biomas, the mean totalstanding stock of salmon inPWS (wild and hatchery) from1980 to 1989 was 42,000tonnes, or about 5 t.km-2.

Total mortality estimates forthe oceanic phase of the variousspecies of salmon caught in PWS aregiven in Table 11; their weightedmean is 2.37 year-I, which serves asour estimate of P /B for salmon as a

Table 9. Catches of salmon in PWS, basedon data from 1980 to 1989 (commercial~

subsistence fisherY).

diet of young salmon consists of75% mesoplankton and 25% macro-plankton; also EE was set as 0.78 forwild fry and 0.63 for hatchery re-leased fry (see section on 'Balancingthe model').

Adult Salmon

Adult salmon appear in PWS fromJune through September, while ontheir spawning runs. All parametersin the text below were thereforesubsequently corrected to a yearlyaverage as required for the model(multiplied by 4/12).

Table 9 shows the average harvest ofsalmon in the Sound from 1980 to1989.

From Table 9, the average catch ofsalmon per area of PWS for the pe-riod 1980-1989 is calculated as 4.2t.km-2.

a) Average catches in PWS from 1980 to 1989,based on Morstad et ill.(1996, Appendices £.2 & G.2);Weighted means, based on Morstad et ill. (1996,Appenrux A.S).

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Table 10. Estimated standing stock ofhatchery and wild pink and chum salmonin PWS. 1980-1989.Stock Mean wtb

(kg)1.6

~

.

ChumTotal

Population"(N)

212691841632316

Ef;!Ql,5003.6(1.8)

Biomass

34 6945 338

39. ,32Based on Morstad et al. (1996, Appendices E.S &E.9);Weighted mean, based Morstad et al. (1996,Appendix A.S).

As no standing stock estimates isavailable for the small pelagics ofPWS, their biomass is left as an un-known to be estimated by themodel; given the important role theyplay in the diet of larger vertebrates,this estimation of biomass will bebased on an assumed value of EE =0.95; the other parameters for thisgroup are set as' for the smallpelagics of the Strait of Georgia, ie.,P /B = 2 year-l and QjB = 18 year-l

(Venier 1996).

The diet of the small pelagic group(in % weight) is also adapted fromthat in Venier (1996), with modifica-tions as required to balance themodel: mesozoolankton (80); largezooplankton (10); and small macro-benthos (10).

functional group.

Adult salmon only feed a short timewithin PWS; in the model, their low

trophic impactTable 11. Instantane- can be capturedous oceanic mortality by giving themrates (Z) for five spe-' a low value ofdes of salmon occur- Q/B, here 1ring in Pwsa. yearI, corre-

sponding to1/12 of theannual valuefor pinksalmon in theAlaska gyre (L.Huato, pers.comm.; Chris-tensen 1996,Table 10). Adiet composi-tion (by weight)of pink salmonof 85% small

pelagics and 15% macrozooplanktoncame from Huato (1996).

Miscellaneous small pelagic fishes

This group includes capelin, eu-lachon, smelt, and other smallfishes. While not fished, smallpelagics are important in PWS, asthey are the major preys of pin-nipeds, cetaceans, birds and ofmany larger fishes.

~ Chinook !Birds

Of the 219 species of birds recordedin the Northern coast of the Gulf ofAlaska and in PWS, 111 are primarilyassociated with water bodies. Alarge number of birds concentrate inthe PWS area during the Spring mi-gration and smaller numbers duringthe Fall migration. Shorebirds andwaterfowls are especially numerous.During Summer, many nesting spe-cies utilizes the PWS area. The mostcommon of these are alcids, black-legged kittiwakes, cormorants, glau-cous-winged gulls, and arctic terns.In Winter, waterfowls such as "mal-lards, greater scaup, common andbarrow's goldeneye, buffleheads,oldsquaws, harlequin duck, white-winged -surf -and common scoters,and common and red-breasted mer-gansers and alcids" use the inshore

a) From Huato (1996),based on Ricker (1975) andBradford (1995);b) Weighted by the catchesin Table 9.

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14

Table 17. Diet composi-tion of sea otters in Monta-gue Strait, PWS (adaptedfrom Calkins 1976 andGarshelis et al. 1986).

density estimates for PWS,by season.

Based on Table 8 in Kelsonet at. (1996), the averageweight of seabirds is set at0.5 kg, resulting in an overalldensity of 0.017 t.km-2 overPWS.

I Molluscs

I

Crustaceans

I

Echinodernls

suming a femalemale ratio of 1:1,leads to a densityof 0.017 t.km-2 forPWS as a whole.

Further, given dailyrations of 5.3 kgfor the females and7.0 kg for themales (Calkins1986), Q/B is esti-mated as 92 yearl.The major preyorganisms of seaotters in PWS aremollusks, crusta-

ceans, and echinoderms (Garsheliset al. 1986, Calkins 1978); Table 17summarizes the results of an analy-sis of the diet composition of seaotters in Montague Strait.

I

Others inverts

Other marine mammals

The marine, mammals of PWS, be-sides sea otters, can be split intothree groups: resident, transient,and pinnipeds. The first group com-prises killer whales, Dall's porpoise,and harbor porpoise. The secondgroup comprises fin whales, hump-back whales, minke whales, belugawhales, and killer whales, occurringonly seasonally in PWS. The thirdgroup is comprised of harbor sealsand Steller sea lions. Table 18 sum-marizes key population statistics ofthe different species. Note that inthis table, the QJB values are basedon sex ratios of 1:1, except forSteller sea lion, where there are 1.2females per male (Calkins 1986).

Assuming, with DeGangeand Sanger (1986), that sea-birds between 200 and 600gconsume about 30% of theirbody weight per day, leadsto an estimated total foodconsumption of 2.157 ton-nes annually, correspondingto a QjB estimate of 110 yeari. Thediet matrix, indicating the fooditems thus consumed, is given asTable 16.

The summary statistics used for thecombination of waterfowls and sea-birds are: biomass = 0.021 t.km-2(see Tables 13 and 15); PjB = 0.1yearl (from Muck and Pauly 1987);and QjB = 103 (weighted mean,given 88 % seabirds and 12 % ducks).The mean (weighted) diet composi-tion derived from Tables 14 and 16for this combined group, is: pelagicfish, including cephalopods (45.9 %);invertebrates (21.5%); demersal fish(13.6%); euphausiids (18.7%) algae(0.2%); and insects (0.1%).

Sea otters

Two estimates of the populationsize of sea otters in PWS exist forthe period considered here: 4000-6000 in 1985, and 5000-10,000 in1989 (Burn 1994). The mean of themidranges, multiplied with themean weight of adults (females 21,males 28 kg; Calkins 1986) and as-

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