This paper not to bo citüd uithout prior reference to the author. Sprat Stock of tho Open Sea in the Southern Baltic.

Transcription

1 This papr not to bo citüd uithout prior rfrnc to th author ntrnational Council for th Exploration of th Sa C.U Sardin Committ No. 68 Sprat Stock of tho Opn Sa in th Southrn Baltic by Jan Elwrtowoki Sinc many yars, v.hn th sason of wintr catchs is ovr, tords th nd of April th fishn of th Polish cuttr fishris commnc th xploitation of vry larg conccntrations of sprat, prsnt both in th Bay of Gdansk and in th ara of th Gdansk Basin in thc opn sa, at a distanc of 50 to 80 sa mils from fishing ports. During thc top of spring sason ths conccntrations sprad in tho form of wid strip.from Gdansk Bay towards N.W. to tho rgions situatd in th oastcrn part of Slupsk Furrou and to th slops of Middl Bank. t appars fromsovit rsarch information that at tho sam tim larg and stabl conccntrntions of sprat arc also prsnt in th Gotland Basin and ovr th slopos of Hoburgs Bank, as wl as in th rgion off tho ontrancc to th Gulf of ltiga Tho most intnsivo fishing is carrid out by Polish fishrncn in th rgion situatd in th cast passag bctwn Eupsk Furrow and th northrn ara of Gdansk Basin. Tb fishing is prformd both with trawls and with two-boat sprat trawls (porjas), bottom and mid-wator ons, and in th valuation to comparabl indics vidnt and ssntial difforncs appoar for ths two typs of gar. n gnral, vory high yilds ar obtaincd, and thus for xamplc in May 1964 tho highst rcordd landing from a 3 hour trawling amountcd to 36 tons of sprat. Th total quantity landd is howvor, limitd by th nvailabl capacity of fishing ports to handl and proccss ths amounts of sprat. For ths roasono thc landings of sprat oscillat round th lvl of thousand tons, whras in particularly favourabl sasons thy can roach 15 thousand tons. Such a landing is cumulatd for th priod of 6-8 wks in April and lhly, and partly in Jun. Thc catchs ar composd partly by tho sprat in running stag, in majority by maturing sprat. Hnc spring concntrations may bc dfincd as spawning ons. Tho invstigations on lngth and ag-cornposition of sprat shoals, capturd in th opn sa, wr conrnncd in 1955; of a mor rgular and continuous charactr hav ths invstigations bn sinco n viow of thc fact that similar invstigations wür carrid out in th coastal fishing grounds of th Bay of Gdansk, th comparison of th shoals oxploitd by thc fishris in both rgions bcamc possiblc. n Figura 1 is shown thc mun lcngth and agc-composition of sprat shoals, capturd in Gdansk Bay und in thc opcn ara of Gdansk Basin (in th astrn part of Slupsk Furrow for thc priod of ). n spit of th fact that th priod, for hich thc man coposition was calculatcd, was not particulnrly typical (owing to th prsnc of a vry rich gnration of 1959 and thc scrics of thr poor gnrations of th ycars 1960, 1961 and 1962) th spccific charactristics of sprat concntrations in ach of th rgions invstigatcd bar prmannt and rgular charactcr, rccurring vory yrr. 1. Th fish, concntratcd during th spnwning priod of spring in thc coastal fishing grounds of thc Bay of Gdansk, ar mainly young ycar-classs of srnal1r siz. At thc sam tim, in th ara of Gdonsk Basin in thc opn sa thc basis of concntrations und of spaning stock mak largr fish, blonging to oldr yar-classs. 2. Th compositions, shon in Figurl,rlat to spaing shoals of sprat und thcrforc illustrot th proccss of rcruitmnt into th spaning stock, concntratd in th coastnl watrs of Gdansk Bay and in thc opn sa. n thc Bay of Gdansk this procss bgins in th first ycar und usual1y cnds in thc third ycar of thc lif of sprat, blonging to ach gnration, though it in as a rul most intcnsiv in thc sccond yar of lif, lss oftn in thc third ycar. n Fiurc 1 thc paks of intnsity of rcruitmcnt into th spa\ing stock in both rgions ar dclaycd by on yar, which is du, as alrady mcntionod, to a particular situation, which took plac in th6 sprat stock ll1thc ynrs (vcry nicb gnration fol1owd by thr succossiv poor gcnrations Moro dtnild

2 analysis of th proccss of rcruitmnt and simlutanous aftr wintring is givn in Tabl l. sgrgation of tho stock Thc comparativ dtl hav bn prpard for thro yars (1955, 1959 nnd 1961) whn thr rich gnrations, born in th yars 1953, 1957 and 1959, appard in th stock as two-yars-old fish. Th changs of frquncy of ths gnrations and th gnrations fol1o,ring thm in thir conscutivc lif phass: wintcring phas and spavning phas - ar rproscntd in Tabl 1. n rlation to initial stag (wintring phas) ths changs must b attributd to th procss of rcruitmnt into th spawning stock according to th spaning grounds, i.. according to thc dpth and th distancc of tho spawning ground from th coast. Thc changs consist of th diminishing proportion of on-and two-ycars-old fish and of th rcspctivo incras in thc numbr of oldor fish in th ratio. Both ths procsss tak placc in a loss vidnt form in th Bay of Gdansk, hroas thy ar distinctly pronounccd in thc fishing grounds of th opn sa, which is, in our opinion,th rsult of a cumulatcd cffcct of two natural scgrgating factors, that is a physiological factor - sxual maturity, and a physica1 factor - tho dpth of thc rgions of spaning and thir distanc from thc coast. Thc ffct of th lattr factor upon particular gnration in various yars of th fish lif is givcn in Figur 2. t may bc sn from this Figur that among youngr yarclasss (to-and thrc-yürs-old fish) most of thc fast-growing fish of th actually largst siz appar alrady in th spawning stock in th opn sa and hnc tho right portions of tho curvos, rprcscnting th composition of this stock ar shiftcd tonrds gratcr lcngths. n rspct of th oldcr yoar-classs (four-and fiv-yoars-old), which complctoly or almost compltly startd a dp-v.ntr mannr of lif in th opn wators of Gdansk Basin, thc curvos of thc lngth distribution of tho invstigatod gnrations ar idontical both for tho Gdansk Bay and for tho opn soa. Conclusions 1. n tho months of spring (spawning phas) in tho fishing grounds of thc opn sa oldr nnd 1argcr sprat appcar compard,vith concntrations of this spocios, cxploitod in th coastal fishing grounds of Gdansk Bay; 2. Th rcruitmnt into thc spawning stock of th opn sca bgins in tho sccond-third ycar of fish lifo, nds in its fifth yar of lif, whrby thc most intnsiv rcruitmont taks placc in th fourth yar of lif of oach gnration. 3. Thc most carly and most intnsiv migration from coastal fishing grounds into th spawning stock of tho opn sa is obsrvd in thc two-and thrcc-yars- 4t old fish (rcruits) of th fastst growth.

3 -... Tab1 1. Lngth- and Ag-Composi tion of tha Sprat Caths in tha Bay cf Gdansk and in th opn Sa (astcrn part of Slupsk Furrow) in th yars 1955, 1959 and Phas of Lth-.(cm) - Aga (rs)! Y R lif 6 i l2t tcta :':;'4-=..Lr--=5-r---;;6:-- aars' gons cyc1 frqu3ncy in %0 Tot 0.1 CJ'l n brackts: th populations obsrvd in th catchs during particu1ar yars.

4 - 4 - Figur 1. Avrag 1ngth and ag-composition of th sprat catchs in th Bay of Gdmlsk and in th opn sa takn during spring sasons of th priod A. Bay of Gdansk cm yars l il t--- - B. Opn sa (astrn part of Slupsk Furrow) ' C. Diffrncs bctwn Bay of Gdunsk und opn sa i-, -- -""""'-t.."".-t a 9 cm yars

5 " Figur 2. Lngth distribution of particu1ar gnrations'caught in th d Bay of Gdansk and in th opn sa (astrn part of th Supsk Furrow) l 2 yars old in th spring sasons of th yars 1955, 1959, j O,..l( 400.l( 1957 gnrations '/ i- i j t b) 3 yars old -li t 1952 f.'t-\ 'f. gnrations 0 cm '/ c) 4 yars old i-..l( 'f- \' 'f \ 200 'f.. )( A- 'f gnrations.r -r 200 / d) 5 ars 1 ;/ ' t 0..J... t f.....;j f J -f t - )< Bay of Gdmlsk Opn soa (astrn part of Supsk Furrow) 'i. "- i3 i 15 1 j