The life cycle of the short fin squids of the North-west Atlantic, ranging in the area from the Florida Peninsula to Labrador as a single population (Romero and Amaratunga, 1981;

Froerman, 1983; Froerman, Dubinina, 1984), was studied. The data on the distribution and biological parameters of the population in different ontogenetic stages was collected du-ring the 1966 to 1983 period. Information on the distribution and biology of the short fin squids inhabiting the areas south of Cape Hatteras and north of Banquerreau Bank was mainly adopted from the literary sources

Froerman, Yu. M.

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NOT TO BE CITED WITHOUT PRIORREFERENCE TO THE AUTHOR(S)Northwest AtlanticSerial No. N913 Fisheries OrganizationNAFO SCR Doc. 84/IX/116 (Summary only)SCIENTIFIC COUNCIL MEETING - SEPTEMBER 1984 Life Cycle and Mechanism of  Abundance Fluctuations in Illex illecebrosus byYu. M. FroermanAtlantic Research Institute of Marine Fisheries and Oceanography (AtlantNIRO)5 Dmitry Donskoy Street, Kaliningrad, 236000, USSRSummary The life cycle of the short fin squids of the North-west Atlantic, ranging in the area from the Florida Peninsulato Labrador as a single population (Romero and Amaratunga, 1981;Froerman, 1983; Froerman, Dubinina, 1984), was studied.The data on the distribution and biological parameters of thepopulation in different ontogenetic stages was collected du-ring the 1966 to 1983 period. Information on the distributionand biology of the short fin squids inhabiting the	areas southof Cape Hatteras and north of Banquerreau Bank was mainly ado-pted from the literary sources.Results of seasonal and year-to-year analysis of theCYV)	distribution and biological parameters of the population in vario-cp us parts of the area are in agreement with the communication ofLange (1981) which says that the spawning of the short fin squidsoccurs throughout the year with a peak in a fall-winter-springperiod. Primarily, the spawning takes place in the continental slopepelagial between the Florida Peninsula and 40°N, which is sub-stantiated by the larva distribution pattern (Roper and Lu, 1978;Dawe and Beck, 1982; Hatanaka et al., 1982; Trites, 1982;Dubinina, Froerman,1983; Froerman, 1983; Froerman,Dubinina,1984).Some squids stay in the spawning area for feeding. Theother part of the spawning products represented as egg masses,larvae or juveniles is transported in the northern Gulf Streamedge to the north-eastern region of the species distributionarea. From the Gulf Stream juveniles migrate to Georges Bank,- 2 -Nova Scotia and Great Newfoundland Bank (GNB), where theyfeed and grow. At the end of the ontogenesis the modal mantlelength of the squid females, who feed in different parts ofthe distribution area, increases from 200-240 mm in the south(Mid-Atlantic Bight area) to 250-280 mm in the GNB area(Squires, 1957; Mercer, 1975; Collins and Ennis, 1978; Becket al., 1980-83; Amaratunga, 1980; Lange , 19811 our data).The maximum potential fecundity of the females increases inthe same direction (Durward et al., 1979; Burukovsky et al.,in press). Year-to-year fluctuations of abundance in theMid-Atlantic Bight area are insignificant (Lange and Sissen-wine 1980, 1983; ICNAP, NAPO SCS Doc. 1978-1984) increasingin feeding grounds of the area from Georges Bank to GNB(Squires, 1957; Hurley, 1980; Dave and Hurley, 1981; Amaratun-ga and Roberge, 1981; IMF, NAPO SCS Doc., 1978-84).The ontogenesis lasts 12 + 1-3 months (Squires, 1957;1967; Amaratunga, 1967; our data). The life cycle involvesfive ecologically isolated stages:Planktonic development stage (embrXogenesis, lar-va, juvenile, ML to 25-30mm). This phase passes in the pela-gial of the continental slope waters or in the northern Gulf.Stream edge.The duration of the planktonic stage is 20-30 days(Froerman, Dubinina, 1984). The distribution of egg masses,larvae and juveniles during this period depends on the Vecu-liarities of the water mass dynamics in the biotope.Nektonic pelagic development stage outside theshelf. The duration is 2,5 - 3,5 months. During this periodthe juveniles of 25-30 to 100-130 mm ML feed in the pelagial ofthe continental slope water and perform an active migrationtowards the shelf on completion of the pelagic stage. The meanmigration velocity is 2.5 miles per day.3. Nektonic stage of feeding and maturation above theshelf. The duration of this stage in the reproductive part ofthe area is 7-10 months, and in the feeding ground - 4-8 months.ML fluctuates from 100°330 to 220-300 mm. Rates of growth and- 3 -maturation change depending on season and feeding ground.Stage of migration for spawning and complete matura-tion. The duration is 1 to 4 months. ML of the females is 190to 130 mm, and of the males 160 to 250 mm. The mean migrationvelocity is 11.1 miles per day.Spawning. Occurs throughout the year, the majorperiod lasting from September to May. The spawning ground islocated between the Florida Peninsula and 40°N in the pelagialof the continental slope.In the continental slope area south of 40°N the reproduc-tive and feeding parts of the distribution area coincide. This isthe "core" of the species range. The shelf waters from GeorgesBank to GNB are a feeding or adaptive ground. The squids migrateto this ground from the oceanic and shelf waters, where the eggmasses, larvae and juveniles transported by the Gulf Streamaccumulate (planktonic stage of the ontogenesis). The area,where the stages of planktonic development and nektonic pelagicfeeding proceed can be identified as preadaptive region.Theamount of spawning products transported to the adaptive regiondepends on the Gulf Stream strength and itsorientation relative to the mean long term position. For theperiod from 1960 to 1982 the mean temperature of the slopewaters (16°C) in the area between 66° and 50°W was taken asthe index of the mean long term position of the Gulf Stream.The more the surface water temperature in that area exceeds16°C the greater the Gulf Stream strength and the closer itsnorthern edge to the spawning area; as a result, the distancecovered by the larvae, and juveniles in the north-east direc-tion is greater and the amount of the latter larger. Havingattained the 25-30 mm length, juvenile squids migrate fromthe northern Gulf Stream edge towards the shelf. During theJanuary to June period length classes of juveniles in the areabetween the continental slope and the Gulf Stream east of 66°Wwere represented by 4 or 5 cohorts. Each next cohort with themodal length of 25 -35 mm appears at the northern edge of the- 4Gulf Stre at 20. to 30 day intervals. The rate of lineargrowth of, the squids in cohorts during the period preceedingtheir movement onto the shelf is 1 mm per day on the average.Daily growth rate (DGR) of squids measuring 15 to 120 mm decreases from 12.8 to 2,3% during this period. The squids moveto the shelf waters of Georges Bank, Nova Scotia and GEB at100-130 mm Mi. The time, when numerous enough cohorts appearabove the shelf, can be determined by recording the decreaseof mean sizes of squids in the areas where they aggregate, andalso using the method of selection of normal distributions(Harding, 1949; Cassie, 1954).The squids in cohorts, which originated from the No-vember to January spawning migrate to the shelf in :rch- y,when the euphausiid biomass is the highest there (Hollingsheadand Corey, 1974; Kulka and Corey, 1978) Euphausiids are the mainfood item in the short fin squid ration at this stage of the onto-genesis (Squires, 1957; Ennis and Collins, 1978; Amaratunga, 1980;Froerman, 1983). Daily feeding rate (DAR) of the squids 140-1.60 mmin length feeding on euphausiids in May comprises 8% of the rawbody weight on the arerage, and DGR is 2%. The squids in cohortsresulting from the spawning that takes place after January migrateto the shelf when the food reserve becomes poorer. In July DFR forthe squids with 140-160 mm ML is 2.8%, and DGR may decrease to0.63%. In certain cases, when the density of squids of various co-horts fluctuates from 200 to 700 thousand specimens per km2 , thecannibalism of the squids of older age cohorts (170-220mm) consti-tutes 21 to 99% of all the food consumed. The length of the preysquids is less than 75-80% of mantle length of the predator squids(Froerman, 1983). Therefore, during the formation of summer aggre-gations the squids of younger cohorts originating from the spaw-ning that takes place after December-January suffer most Growthrates of the squids of younger cohorts are lower, but their matura-tion rates exceed those of the squids of kidder cohorts (Rowe andMangold, 1975). Therefore, at the end of the ontogenesis the po-tential fecundity of survivors of younger cohorts is lower than thatof the squids originating from the November to January spawning.11.-- 5 -Spawning migration from the feeding ground begins in Oc-tober or November, when the females and males attain maturitystages III and V, respectively (Burukovsky et al., in press;Nigmatullin et al©, in press). The fact that several spawningpeaks occur during the fall-winter-spring period can be relatedto the existence on the shelf, between 40°N and GNB, of some are-as of high biological productivity (Georges, Browns, Emerald,Sable Island, Banquerreau, etc.), where the squids concentrateduring the feeding period, until the spawning migration begins.The comparison between time intervals at which the juvenile co-horts appear in the northern Gulf Stream edge (20-30 days) andthe distance between the conventional center of the spawning gro-und (37°N, 75°W) and conventional centers of Georges Bank, agroup of banks in Div. 4X and 4W, Banquerreau Bank and GNB givesthe mean migration velocity of 1101 miles per day. This is consis-tent with the communication by Dawe et al. (1981) concerning therate of spawning migration of females from GNB area (12 miles perday)®The duration of the development of a reproductive systemin females from maturity stage III to spawning can be calculatedfrom the formula:TSM	DSMVSMwhere TSM is time of spawning migration (time needed for matura-tion of females from stage III of gonad development to spawning);DSM is distance of spawning migration;VSM is velocity of spawning migration.The same formula was used to calculate the duration ofmaturity stage V in males from these areas.The analysis of interaction of abiotic factors and intra-population and ecosystem processes in space and time makes itpossible to draw the principal regularity of the mchanism ofmacroscale fluctuations of the short fin squid abundance. •C,During the intensification of the Gulf Stream its nor-thern edge shifts north-westward and approaches the spawning area.Accordingly, the transportation of the spawning products to thenorth-eastern part of the distribution area increases. This pro-vides for the increase of total potential fecundity of the popu-lation, and the optimal temperature conditions (13-22°C) in theareas, where the larvae and juveniles spawn and feed, favour theincrease of the recruitment abundance.Redistribution of the maximum abundance observed duringsome years in the period of feeding migrations from the southernparts of the area to the north, and the back migration for spaw-ning at the same velocity is responsible for a gradual shift ofthe spawning peak to a later title. However, as it was shown above,the recruitment from cohorts of later spawning is subject tohigh mortality. That is, if the abundance in the north of thefeeding ground rises above the average level, which correspondsto the level of balance of biological productivity in the area,directingbiological mechanisms the vector of the process of abundancevariation	towards the mean value begin to act. During thisperiod the action of the biological mechanism for optimization ofthe abundance coincides with the return of the Gulf Stream tothe mean long term position, while further deviation of thestream to the south-east from the mean reduces the transporta-tion of the spawning products to the areas of Georges Bank,Nova Scotia and GNB. As a result, the total fecundity of thepopulation decrAes , but the time of peak spavningis reverted to the optimum period. The abundance in the feeding(peripheral) area of the distribution range (Nova Scotia, GNB)decreases to the mean value and lower, and in the reproductive(central) area it rnIgng the mean optimum (supporting) level.Over the period from 1960 to 1982 the deviation of the GulfStream from the mean long term value both in north-west and south-east directions persisted for 5-6 years® The periods of high andlow (unstable) squid abundance in the areas of. Nova Scotia andGNB were of the same duration. The inertia of the mechanism ofinteraction of biotic and abiotic parameters changing the directionof the vector of abundance variation is 3years. The abundancefluctuations are most clearly pronounced in the periphery of thespecies distribution range (GNB area). In the center of the area(Mid-Atlantic Bight), where the feeding and reproductive zones arein immediate proximity, the year- to-year amplitude of abundancefluctuations is not wide, because of high stability in securing therecruitment during the unfavourable years of high abundance. Dura-bility of both the periods of varying abundance and the period ofinertia, in the course of which the direction of the vector ofabundance variation changes, depends on the state of the GulfStream - Labrador system within the range of the long term varia-bility (11-12 years). Protracted position of the Gulf Stream inthe extreme north-western part of the area would change the recur-rence of abundance fluctuations. In such a case short periods(2-3 years) of low abundance would correspond to 5-6 year longperiod of high abundance.

Life Cycle and Mechanism of Abundance Fluctuations in Illex illecebrosus

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