This paper not to be cited without permission oi the authors.

ICES C.M. 1994 ICES C.M. 1994/L:18 ,BiologicalOceanographyCommittee

ZOOPLANKTON PREY FIELD VARIABILITY DURING COLLAPSEAND RECOVERY OF PELAGIC FISH IN TIIE NORTHEAST

SHELF ECOSYSTEM

K. Sherman1, J. Green1

, A Solow2, S. MurawskP, J. Kane1, J. Jossi1, and W. Smith4

, INational Marine Fisheries ServiceNortheast Fisheries Science Center

Narragansett LaboratoryNarragansett, RI02882-1199

2Woods Hole Oceanographic InstitutionWoods Hole, MA 02543

3National Marine Fisheries ServiceNortheast Fisheries Science Center

Woods Hole LaboratoryWoods Hole, MA 02543

4National Marine Fisheries ServiceNortheast Fisheries Science Center

James J. Howard Marine Science LaboratoryHighlands, NJ 07732-0428 '

ABSTRACT'

Since the mid-1970s, three pelagie fish species--herring, Clupea harengus; mackerei,Scomber scombrus; and sand eel, Ammodytes spp.--have undergone significant flips inbiomass. The herring and inackerel stocks that declined from doininant to subordinatepositions in the 1970s recovered to a present eombined biomass exceeding 5 million metrietons. Sand eels, whose population levels exploded in the late 1970s and early 1980s, arenow in decline. The effects of several million metrie ton changes in biomass among thesethree species of zooplanktivores are examiried in relation to possible d~msity-dependent

variability in zooplankton and pelagie fish Within the Northeast Shelf Ecosystem.,

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CHANGES IN FISH ABUNDANCE WlTI-IIN TIIE ECOSYSTEM I .In ari effort to reduce scientific uncertairities in fisheries managemerit, increasing

atteritiori has been focused over the past few years on synthesizing available information onfactors .iIilluencing the naturril produetivity. of marine fish biomass from an ecosystemSperspective (AAAS, 1993)., One of the results of thiseffort haS been aseries of reports onthe large-scale changes in fishery biomaSs yields of the Northeast Shelf Ecosystem duringthe past 30 years (Sissemvine, 1986; Murawski, 1991; Anthony, 1993). I '

From 1963 to the present, high ~alued groundfish species (gadoids, flounde;s) havedeclined to unprecedented low levels. Other demersal species, with low market value (spinydogfish, Squalus, acanthias; and skates1) have increased in abundance (Fig. 1). Anothercategory of species, the principal pelagics, Atlantic herring (Clupea harengus) arid Atlanticmackerel (Scomber scombrus), have uridergone precipitous declines for the decade 1968through 1978, followed by population recoveries from the.1980s to the present time (Fig.1). Tbeir recovery is aüributed to reduced fishing effort (Murawski, 1991). Tbe observedbiorriass "flip" of the principal pelagics from high levels in the 1960s to historically low levelsin the 1970s and the present high biomass has been the dominant factor in the populationexplosion of sand lance, Ammodytes spp., during the latter half of the 1970s (Sherman et ril.,1981), and the predpitous 4-yr decline in abundance from 1982 to 1985. Based on predator­prey studies, it was concluded that predatiori of sand larice in their early, plariktoriicdevelopmental stages by the principal pelagics depressed the population of sand lance,

'following aperiod' of approXimately. 6 yr (1976-1982) when the predation pressure wassignificantly reduced (Fogarty et al., 1991) (Fig. 2).

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LONG-TERM TRENDS IN HERRING AND MACKEREL ABUNDANCE

Tbe principai pelagics arid s::irid lance are zooplanktivores (Bowrnan et al., 1984;Maurer, 1976; Michaels and Grossiein, in prep.). Tbe long-term inverse relationship in •abundance of the principal pelagics aS discussed by Skud (1982) appears tci have shiftedduring the past 30 yr from the trends in a 90 yr time series of landings data exaffiined from1870 through 1960. BaSed on an analysis of herring arid inackerel catches in relation totemperature trends, SkUd (1982) concluded that ,when one of the species was dominant, theother was subordinate in responSe to·u combination of temperature and derisity-dependeritcompetition between the two species. He argued that density (abundance) of the twospecies is "governed by the carr}ting capacitY of the ecosystem und [that] the species apparentresponse to clirriate change is dependent on itS position in the, dominance hierarchy .und itspopulation size relative to its equilibrium density." Since 1960, this apparent relationship has

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ISpecies of skates iriclude: little skate (Raja erinacea), wiriter skate (R. ocellata),barndoor skate (R. laevis), thorny skate (R. riidiata), briar skate (R. eglaiueria), leopard skate(R. garmani) and smooth-tailed skate (R. seiua).

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been modified by the high levels of fishing mortalitY from the late 1960s through the niid1970s for Atlantie herring arid mackerel (Murawski, 1991). Sirice 1982, the depressedmackerel stock has been undergoing significant rebuilding from an estimated stock biomasslevel of api>roximately 650,000 metric tonS (mt) in 1982 to 2.8 million metric toris (mrnt) in1992. The rebuilding of the herring stock during the same decade has progressed from lessthan 100,000 rnt to 2.7 inrnt. Neither species ean be considered subordinate. Both haveundergone recovery arid are at high abundance levels, simultaneously (Figs. 3 äiid 4), withas yet, no apparent adverse impact on the "carryirig capadty" of the ecosystem, as suggestedby Skud (1982).

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ZOOPIANKTON COMPOSmON AND ABUNDANCE

The zooplankton ofthe Northeast ShelfEcosystem has been exarriined for broad-areaspatial and temporal trends in aburidance and species corriposition since the mid-1960s(Sherman et al., 1983, 1988; Kane, 1993; Jossi änd Goulet, 1993), und dtiring four earlierperiods.. The first was the classie measurements of zooplankton voluines and species madeby Bigelow during the first two decades of the century (1912 through 1920). The second, thevolume measurements made by Bigelow arid Sears in the laie 19205 through the 1940s. Thethird eovered the late 1930s to the 19605, with thebiomass and sjJecies demographie stodiesof Fish (1936a,b), .Clarke and Zinn (1937), Clarke (1940), Redfield (1941), .. Clarke 6t al.(1943)~ Riley ririd Bumpus (1946), Deevey (1952, 1956, 1960a,b), Griee arid Hart in 1962;and the fourth, the more contemporary measurements of Judkins et al. (1980), Dagg rindTurner (1982), Davis (1984a,b), Townserid arid Carriffien (1988), and the recent studj ofDurbin and Durbin (in press) who examined the regulatory processes of the Northeast Shelfzooplankton (e.g., teIriperature, food, änd predation).

During the deeacte 1977 to 1986, sampies were collected using 0.333-nitil mesh bongonets towed obliquely through the water colurnn to a rriäxiIrium depth of 200 m. Tows of 15­min duraiion at 1.5 knots were made at 200 sampling stations in a grid network at 35 kmspacing Within the entire ecosystem, six or more times per year (Sibunka and Silvernian,1984, 1989) (Fig. 5). In addition to point-samplings ,with neis, the Northeast FishenesScience Center (NEFSC) staff have completed Inonthly Contiriuous Plankton Recorder(CPR) trariseets across the Gulf of Maine since 1961. . This transect series häS beenaugmented bya secorid transect from New York to Bermuda, across the Mid-Atlaritic Bightof the Northeast Shelf Eeosystem since 1976 (Jossi arid Goulet, 1993).

The total zooplankton measured. as displacement volumes during the MarineResmirce Moriitoring ASsessrnerit and Predieiion Program (MARMAP) surveys of the shelffrom 1977 to 1981 did not differ significaritly from the earlier estimates of zooplariktonsianding stock of the shelf ecosystein from 1912 through ihe 1960s (Sherman et aI., 1988).The zooplanktori eomposition inchided 394 taxa, with 50 dominant in at leastorie Iocationin one or more seasons, including copepods, chaetognaths, barnacle larVae, cladoceranS,

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appendicularia, doliolids, brachyurari larvae, echiriodermata larvae, and(Sherman et al., 1988).

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I.thaliaceans,

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IECOSYSTEM LEVEL RESPONSE 1'0 PELAGIC PERTURBATIONS I,

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nie more recent observations of zooplankton voluines on the shelf, reported by Kalle(1993), for the Georges Bank area of the Shelf Ecosystem imd Jossi and Goulet (1993) forthe Gulf of Maine and the Mid-Atlantic Hight reveal interannual variability in zooplanktonabundance. Examination of the CPR trend-data for the trarisect across the Mid-AtlariticHight from 1976 through 1990 revealed a declining trend for the MARMAP decade 1977­1986 (p = 0.026). However, the interarinual variability observed from net tows for theentire time series was not statisticaUy sigriificant as a declining trend (p = 0.30) (Fig. 6).The numbers of zooplankton on the northern CPR transect aCfoss the Gulf of Mainegenerally increased in abundance from 1961 through 1990 (p = 0.054), interrupted by adoWnward trend during the MARMAP decude (p = 0.06) (Fig. 7). Based on MARMAPnet sampling for Georges B3.nk, Kane reported volumes higher than._ the 10-yr medianbiomass (cc/lOO m3

) for 1977, 1978, arid 1979, and lower volumes for 1982, 1983, and 1984.According to Kane (1993) the higher voluines may have coritributed to the increases iri sandlance, Ammodytes spp., during the late 1970s. In contrast, Payne et al~ (1990) repofted ariinverse relationship between sand lance and the copepod, Calanus finmarchicus, during thepopulation explosion of sand lance, suggesting density-dependent grazing control of thestanding stock of zooplankton. ,_ This obserVation; however, was limited to a relatively

_restdcted area of the Northeast Shelf Ecosystein kIloWn as the Stellwagon Bank area (Payneet al~, 1990)., On Georges Bank, where. sand lance, herring, arid mackerel are present inrelatively high nuinbers in spring, zooplankton biomass in spring, measured as displacemeritvolumes per 100 m3 was abundant during the population explosion of sand lance, 1971-1981,with median anriual volumes equal to or exceeding the 22-yr (1971 through 1992) long-teimmedian value of 43 cc/100 in3 (Fig. 8).

Examination of the temperature iriformaiion for the Northeast Shelf Ecosystemduririg the last two decades indicates that the temperatures dunng 1978 to 1982, aperiodof population depression for the principal pelagics, was intermediate betWeen a 'slightlywarmer period of themid-1980s (1983 to 1986) arid moderateIy cooler waters of the riiid­19605 (HolZwarth and Mountairi, 1992). The influence of temperattire on the survival of .early developmental stages of sand larice, mackereI, and herring iS not viell understood andrequires fui"ther study. The correlaiion betWeen temperatures and positive and negativecorrelations With herring arid mackerel dominarice reported by Skud (1982) is not supportedby the present study, wherein hoth species show population declines arid increases attributedto excessive flshirig effort in the 1970s arid inc1uded sharp. dec1ines in the economicallyimportant deinersa! species during the same period (Allth0ny, 1993; Mui-awski, 1991).Whereas the excessive flshing mortality has persisted _for the demersals, it has _beensignificantly reduced for the principal pelagic species, arid is considered the principal factorin their recovery (Murawski, 1991). The lowest period of larval herring abundance fr9m

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1971 through 1992 occurred from 1976 to 1984, spanning the fritermediate and warmtemperatuie periods.. Tbis is in contrast to the warriling :ind cooling correlaiionand analysisof changes in. herririg distrioution repofted by Skud (1982), and lends support to thepredominant influence of excessive fishing mortality to the decline in both herririg aridmackerel abundance (Fig. 9). Tbe influence of temperature on the iriterarinual vaclabilityof a dominant zooplallkter, the copepod, Calanus jinmarchicuS, on Georges Bank issuggestive of a relationship (Meise-Munns, et aL, 1990). Further studies are presentlyunderway to examine more cIosely the relatioriship betweeri seasonal and iriterarinualenvironmental variability and chariges iri zooplankton production (GLOBEC, 1991, 1992).

, Earlier observatioris indicate ihat the zooplankton component of the Noitherist SheifEcosystem has been relatively coherent in resilience, abimdance, and biodiversit}r from thefirst decade of the. centuly to the 1980s (Sherman et aL; 1988). Based on the presentanalysis of riearly 10,000 zooplankton volume ineasurements (9,942) for the MARMAPdecade, 1977 through 1986, we find thai the zooplailkton was in a decliriiiig trend during the1980s. Tbe declimng trerid is highly significant for the ",hole ecosystein (Fig. 10a), and forthree.of the four subareas, the trend is significant at the p < 0.05 level (Fig. lOb, c, arid d).Tbe declinirig trend for Georges Bank was more variable than for the other three subareas(p = 0.10), (Fig. 10e). Tbe persistent dowriward trend coincided with 'the increrise inabundance level of the priricipal pelagics, and. the iricrease in biomass of another pelagiczooplanktivore, the butterfish, Peprilus iriacanthuS, (Fig. 11): During the years 1977, 1978,and 1979, the highest riumbers of zooplankters, based on CPR sampling (Fig. 6), coinddedwith the first three years of the severi-year depressed state of the herririg and mackerelbiomaiis (Figs. 3 arid 4).. From atop-down examination of the ecosystem, tbe zoopla11ktoiicomponent has been suffici6ritly ~obust,. even tbro~gh the period of decliriirig trend,. to,

.sustain the recovery cif both herring arid mackerel to a level of 55 mmt. In addition to thisbiomriss, 'thezooplankton '. prey field is needed to' support' the recovery ci!. the depressed .gadoid and flounder stocks during their larval stages, posing another significant biomass tobe considered in e'stimating the carrying capacity ,of the ecosystem. Whether the declinereflects aresponse to increasirig predation by the growirig ,biomäss of· the pelagic fishspecies, or a biofeedback response to an environmental sigrial remains an importantuminSwered questiori. From the spring and riutuinri volume data, baSed on bongo tows overGeorges Bank from an extended data set encompassing 1971 through 1992, it appears thatzooplankton standing stock may rigain be incieasing (Fig. 12a and b).

For insights to the possibility oi an enviionnientaiIy induced biofeedback, the,zooplankton time series was examined for shifts in biodiversityusing the annual time seriesof log abundance of 5 zooplankton species--Calanus jinmarchicUs, PseudocalclliuS spp.,Ceriiropages typicus, Metridia lucens" änd Celuropages hainaruS--for four regions: GeorgesBank, Gulf cif Maine, Mid Atlantic Bight, arid Southem New Englarid, foi the pencid 1977­1987. AiIDual time sefies were constructed by averagirig all sampies withiri a year arid 3lsoby taking the maxirIlUm sampie Withhi a Year. In terms of temporal behaVioi, there wereno real differences between these two approaches, so the. average values wen~ anri.lyzed.The inethods used are described in Solow (1994) to extract an overall trend from. the five

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species for each of the four regions. Tbere was rio significani trend for the Mid-AtlariticBight (p = 0.785), or Southern New England (p = 0.128). There were significant trends forGeorges Bank (p ,= 0.031) arid the GuIfof Maine (p = 0.014). The trends ure shoWn inFig. 13 and ure highly correlated. Tbe correlationS oetWeen these trends and the fivespecies are given in Table 1. From an examinaticiri cif biodiversity, it appears that thedominant copepod community' is imdergoing a shirt from Calanus jinmdrchicus,Pseudocala;lus Spp., Aletndia lucens, arid Centropages iypicus, to\vard an iricreasing abundanceof Celitropages lzaJ1Iatus on Georges Brink und in theGulf cif Maine (Fig. 14). Tbis apparentshift in species abundance may be environmeritally induced, aS it is unlikely that theprincipal pelagics would selectively avoid pre)ring on C. luimatus. Tbe ecosysteni morutoringprogram presently underivay by the Northeast Fisheries Science Center, in eooperation withEPA, is designed to provide more definitive irifoi-mation on denSity-dependent predationbehveen the principal pelagics and their prey field, and densit)'-independent enviror1mentalsignals that may be the eause of ripparent chririges in the dominance structure' of thezooplankton comriiunity. Pedator-prey studies being eonducted as part of NOAA's COaStalOeean Prograni are examining the fine-sCale temporal and spatial interrelation between theprincipal pelagics and their prey-field (Michaels ririd Grossiein, in prep.).

Tbe present state of the ecosyst~m remains stressed. Tbe shift since the 1960s froma demersaI assemblage of fish dominated by gadoids io the present state dorriinated by spinydogfish rind skates. coinciding \vith the iricrease in the abundance level cif the pelagic fishspecies (Fig.l), raises an ecological question. How will fishirig erforl reduction measuresinitiated in 1994 (New England Fishery Mariagement Council, 1994)äffeet thc recoveryprocess for the depressed state of highly-vaIued demei-saI species (e.g., cod, haddock,flciiinders)? Tbe eonsequence of reduced biomaSs of gadoids and flounders has been'~mincrease in iargeiing fisheries on abundarit elasinobranch species. Nevertheless, thesespecies are at historicallyhigh levels. Implications of the removal of an iricreasing fractionof the elasmobrrinch biomass ori other fish coinponeniS of the ecosystem \vill be clciselymonitored. Tbe ecosystem level,corisequences associated with this adaptive managementstrategy have been reviewed by Sissenwine and ,Cohen (1991), who list severaI alternativespecies that eould become major fish predators. With regard to the zooplankton eomponeniof die. system, the ehanged state of the fish eommuiIity hriS introduced uncertainty to anyforecaSt of recovery fcir the demersal populatioris, dependent on the zooplallkton prey-fieldduring the early developmental stages of eod, hacldock, arid flounders. Unlike the gadoidsand flounders, the present high biorriass of elasmobrunchs are iridependent of thezooplankton prey field during early development. Tbe estimated biomass level of hemngand mackerei, prior to the 1960s is noi weIl docuinented and is likely, acccirdirig to datapreserited by Skud (i982), to have been h~ss than the present estimated level of 5.5 ri11l1.t.Tbe questionS posed by SkUd with regard to the "earrying capacit}r" of the ecosystem Vlillrequire rurther study as the NortheaSt Shelfundergoes asignificaritly aItered ecologic3J. state .'within which greater numbers of predators Will be dependerit on a zoopirinkion prey fieldthat may be undergoing a shift in biodiversity. Tbe recent simulations of the pelagiccomponents of th6 eeosystem by Overholtz et aI.. (1991) Will be exterided to incIude thezooplarikton eomponent in an errort to examine the cäpacity of the ecosystem to suPP?rt

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recovering populations of pelagic and demersal fish and baleeri whale species.

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Sissenwine, M. P. 1986. Perturbation of apredatör-eontroUed eontinental shelf eeosystem.pp. 55-85. In K Sherman and L. M. Alexander (Eds.) yariability and managementof large marine eeosysterns. AAAS Seleeted Symposium 99, Westview Press, Ine.,Boulder, CO. 319 pp.

: .Sissenwine, M. P., and E. B. Cohen. 1991. Resouree produetivity and fisheries management

of the northeast shelf eeosystem. In K Sherman, L M. Alexander, and B. D. Gold(Eds.) Food ehains, yields, models,. arid management of large marine ecosysterns.Westview Press, Ine., Boulder, CO. 320 pp. •

Skud, B. E. 1982. Dominanee in fisheries: A relation between environment andabundance. Scienee 216:144-149.

Solow, A. R. 1994. Deteeting change in tlle composition of a multispecies eommunity.Biometrics 50:556-565. :

Townsend, D. \V., and L. M. Cammen. 1988. Potential importanee of the timing of springplankton bloorns to benthic-pelagic coupling and recruitment of juvenile demersalfishes. Biol. Oceanogr. 5:215-229. ;

10

80,------- ---,

60

40

20

PRINCIPAL GROUNDFISHAND FLOUNDERS

20 ornER flNFlSH

•120

80

<40

SKATES AND SPINY DOGFlSH

o-t-""'--r-"""-.--.....-........,r--r-..,-,....,...........,r--r-..,-,....,...........,r--r-..,-,....,...........,r--r-.,.-I62 &4 66 68 70 72 74 76 78 80 82 34 86 88 90 92

YEAR

Fig. 1. Trends in indices of aggregate abundance (catch in weight per survey trawl haul) forfour species groups, reflecting the major changes in fishery resources, 1962-1992. (From theNEFSC, 1993.)

11

2600-.-----------------------1 2

•

soJ-

1.5 a:w0...

Iu~()

z«w~

oWLL

0.5 I-::«a:J­CI)

.:( SAND LANCE

---"\ / "! " \/ ...·••/

/MACKEREL

~ ... /\'\HERRING / \/

............, : //'\..".............

~-----------o4---=~ ............::..--==-r-------r--~-_r- r_-_,_- r_-+ 069 71 73 75 77 79 81 83 85 87

. YEAR

600

2000

i=~

o~ 1500Cf)

~

o:c!:::Cf)Cf) 1000«~oro

Fig. 2. Trends in biomass of mackerel (age 1+) and herring (age 3+) derived from virtualpopulation analysis and trends in relative abundance (stratified mean catch per tow (kg»of sand lance (age 2+) based on research vessel surveys. (From Fogarty et al., 1991.)

12

Atlantic HerringCoastal stock Complex

3000-,----------------------,

o " .~ ~ ro n ~ n n M ~ M U M 00 ~

YEAR

1500

1000

,,,,,,,,II

II

STOCK BIOMASS I(ACE 2+) ",

I;

;,,;

-'

soo

2500

'lil'ä 2000o&(/)zf2o

~::E

•Fig. 3. Atlantic herring commercial landings and stock biomass, 1967 through 1992(thousand metric tons). (From the NEFSC, 1993.)

13

Atlantic Mackerellabrador-North Carolina

3000

II

2500 II

I'iii' Ib 2000 I0

I2-(/) ~

Z 1500 -...... I

~ I \ STOCK 810MASS I,\~(AGE 1+) ;0

~, I

1000 , ,I,

:::E I , I .... , I, , '" , ,;

500I

I TOTAL;- lANDINGS_.... 0 -k~~:;:"'-~~I"1~::::;::::;:::::;:::.:r::;:::;::::;::;::;:;::;:;=~

62 6.( 66 68 70 n 7,( 76 78 80 82 M 86 88 90 92

YEAR

Fig. 4. Atlantic mackerel commercial landings and stock biomass, 1963 through 1992(thousand metric tons). (From the NEFSC, 1993.)

14

"

•

•

Fig.5. Zooplankton sampling locations on MARMAP·surveys of the Northeast ContinentalShelf Ecosystem.

15

MAß CPR9.6

9.4

9.2

9

8.8

8.6

8.4

8.2

8

7.81976 1978 1980 1982 1984 1986 1988 1990 •

Fig. 6. Continuous Plankton Recorder trend data for the transect across the Mid-AtlanticBight, 1976-1990. Rank Corre1ation Test for Trend; p = 0.026, 1977-1986; P = 0.30, 1976­1986. The ordinate values represent lo~ transformed numbers of total zooplankton per 100m3/yr of water strained.

16

GOM CPR10.5

10

9.5

9

8.5

8

7.5

7

6.51960 1965 1970 1975 1980 1985 1990

Fig. 7. Continuous Plankton Recorder trend data for the transect across the Gulf of Maine,1961-1990. Rank Correlation Test for Trend; p = 0.054, 1961-1990; P = 0.062, 1977-1986.The ordinate values represent 10& transformed numbers of total zooplankton per 100 m3/yrof water strained. .

17

VOLUMESGeorges Bank Early Spring

100

90+

...- 80 /Ci)+<

/ 1\E 700 / +,0 60...- / +-...

I \0 50 +0 +........ Medianc 40CiS

/'+"0 30<D2 20

+-+ +10

071 72 73 74 75 76 7778 79 80 81 82 83 84 85 86 87 88 89 90 91 92

Year

Fig.8. Annual zooplankton median volumes (cc/lOO m3) for Georges Bank during early

spring, 1971-1992, with the 22-yr median for the total time series; N = 733. Data for 1976are not available.

18

...

Fig.9. Changes in abundance of Atlantic herring Clupea harengus lalVae in the GeorgesBank area between 1971 and 1990. Intervals represent multi-year periods that reflect themost apparent changes in spawning patterns.

19

AREA WEIGHlEO TOTAL

a

co

~.n 'u, "79 1Ieo ,,., .tU 1063 ,Me 181$ ....

GOM

b

so

30

2S

liTT 1171 ure neo 'M' 1M2 'lU ,.... '''5 ,IM

MAßu.,----- ----,

dos

co

':'•.l=T7-,="',.---,.,"'~.....~...=,-:,=...-=.....:-=,..:-:-.-,,,=,-:-:I....

Gß •e

.~•.l=n-:.=...,......=...:--:-:-:'...~...:-:-,-:c:'_~....=-=,=...-='....,--l,_

Fig. 10. Mean annual volumesin cc/100 m3 for the ~ortheast Continental Shelf Ecosystem.,1977-1986. Data were combined for the four subareas--Gulf of Maine, Southem NewEngland, Mid-Atlantic Bight, and Georges Bank--based on a Rank Correlation Test forTrend for the area weighted total (a) p = 0.001. The results of the test are given separatelyfor each of the four subareas, b) Gulf of Maine, p =0.006; c) Southem New England, P. =0.006; d) Mid-Atlantic Bight, p = 0.040; and e) Georges Bank, p = 0.130. .

20

ButterfishGulf of Maine-Middle Atlantic

30......--------------------...,....20

25

'<ii'o 20oS(I)

Z 15f2Ü

g:w 10:::E

5

LANOINGS\.

AUTUMN iSURVEY "" f~ ::

INO(X I\ ...:..:t\_/ ,\":::::: A: : :f\,.lA-r t,j,~A l.t1 ': ~\./ t........ : \ f ..•..J

" ~ :.--..;j\\7t.j \J

o 062 6.( 66 68 70 n 74 76 78 80 82 84 86 88 90 92

YEAR

Fig. 11. Commercial catches of butterfish for the Northeast Continental Shelf Ecosystem(1000 mt) and stratified mean catch per tow (kg) based on the autumn bottom trawl surveyindex, Northeast Fisheries Science Center, 1963-1992. (From NEFSC, 1993.)

21

A

71 727374757677 7B 79 BO B1 B2 838485868788899091 92

Year

VOLUMESGeorges Bank Early Autumn

50 ,.---------------------.

B

...

C? 40(

Eoo 30..-­oo'--'

c 20l1:l

"'0Q)

:::'i: 10

71 727374757677 78 79 80 81 82 838485868188 8g gO g1 g2

Year

Fig. 12. The allllUal median zooplankton standing stock in displacement volumes/100 m3

for: a) early spring, 1971 to 1992, and b) autumn, 1971 to 1992, based on bongo net towsmade during spring and fall bottom trawl surveys of the Northeast Continental ShelfEcosystem. Data missing for 19~6, 1989, and 1990.

22

.. .

GOt,ur---------__---,i.

Gß2

GOMt

GOM2 MAEJ2 SNE2

G03 GOM3 MA03 SNE3

GB4 GOM4

GOMS

MAB4

MABS

SNE4

SNES

Fig. 13. Abundance trends for five species of dominant zooplankton species:-Calanus finmarchicus (1),Pseudocalanus spp. (2), Centropages typicus (3), Metridia lucens (4), and Centropages hamatus (5)-­wilhin four regions of the Northeast ShelIEcosystem-Georges Bank (GB), GulI of Maine (GOM), Mid-AtlanticBight (MAß), Southern New England (SNE)--for the period 19TI through 1987. Annual time series are basedon avcraging all samplcs within one year. Plots show relationship between log., transformed sampie means (towertrend line) andlo& transformed maximum sampIe abundances (upper trend line) for each year within the timeseries. Number designations for each specics are given in parenthcscs.

23

'>

21""----------- -.,

•878685

.-.-....--i,

.........-- \\........... \\

8482 83

1 ,..................

..•......••..

,..-,IlOT---------------:H--- ~

/;l

~~

..............., .....,.,

-1

-2~':J::-----::c--=r=--.--_._-_._-_._-_._-_._-_,._-_._-.J77 78 79 80 81

1.5

0.5

-0.5

-1.5

YEAR

Fig. 14. Minimum-maximum Autocorrelation Factor (MAF) plot showing the monotonetrend toward Centropages hamatus from 1977 to 1987, for Gulf of Maine (dotted line) andGeorges Bank (solid line), (standardized to mean 0 and variance 1). Randomization testprobabilityvalues calculated following Solow (1994) were: p = 0.031 for Georges Bank, andp = 0.014 for the Gulf of Maine. The trends were not significant for Southem NewEngland, p = 0.128, and P = 0.785 for the Mid-Atlantic Bight subareas and are not plotted.

24

.. ..

Table 1. Correlations between the five copepod species and the time trends found by MAFanalysis for Georges Bank and the Gulf of Maine subareas of the Northeast ShelfEcosystem(Solow, 1994).

Species Georges Bank

Calanus finmarchicus -0.32

Pseudocalanus spp. -0.82

e Centropages typicus -0.44

Metridia lucens -0.69

Centropages hamatus 0.70

Gulf of Maine

-0.64

-0.54

-0.35

-0.77

0.76

25