THIS PAPER NOT TO BE CITED WITHOUT PRIOR REFERENCE TO THE AUTHOR. stock assessments of Haddock, Blue Whiting and Greater Silver Smelt on Rockall Bank

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1 THS PAPER NOT TO BE CTED WTHOUT PROR REFERENCE TO THE AUTHOR. nternatonal Councl for the Exploraton of the Seu C.M. 1983/ G : 29 Demersal Fsh Commttee Ref. Pelagc Fsh Cttee stock assessments of Haddock, Blue Whtng and Greater Slver Smelt on Rockall Bank by Segfred Ehrch nsttut für Seefschere, Bundesforschungsanstalt für Fschere, Abstract Palmalle 9, 2000 Hamburg 50, Federal Republc of Germany n January hauls were made by the German R.V. "Walther Herwg" on Rockall Bank n depths down to 700 m. Usng the "swept area" method, the stock szes of Haddock, Blue Whtng and Greater Slver Smelt were estmated to be about t, t and t resp. Resume En janver trats ont ete effectues par le N.R. Herwg" sur Rockall Bane "Walther jusqu'ä 700 m de profondeur. En employant la methode de "swept area", les stocks d'eglefn, de poutassou et d'urgentne ont ete estmes respectvement ä envron t, t et t. 1. ntroducton n the lust years an ncrease n fshng actvty could be recognzed n the deeper parts of the shelf west of Scotland and reland and on Rockall Bank, as a result of the restretons to the tradtonal fshng grounds. Besdes the spawnng stock of Blue Lng on Lousy Bank, the stocks of Haddock, and perhaps Blue Whtng, on Rockall Bank could be of nterest for the German trawlers.

2 '1at.ora.l and lplhodh The data used were collected on board of' the research vessel "Walther Herwg" n a survey carred out durng 4 January to 4.February hauls of' half' an hour duraton were made usng a 200 ft bottom trawl. The cod end was ftted wth a small-meshed lner. Tbe plateau of the Rockall Bank and ts slope down to 700 m depth were dvded n depth zones, each of' 100 m. The sze n nautcal square m1es of' each depth zone s shown n tab. 1. Tho hauls at each zone were randomly se1ected (see f'g. 1): Experments n 1981 and 1982 from the "Wa1ther Herwg" for measurng the 200 f't net resulted n a mean wng-end spread... of' about 21 m (KOCK, personal communcaton). Stock bomass estmates were made by the "swept area" method (see SAVLLE, 1977). The conf'dence level had to be set at 80%. 3" Results ;3.1. Haddock Haddock s the most mp6rtant stock n the shallower parts of' the Rockall Bank (estmated trawlable bomass: t). Tbe hghest relatve mean densty (t/nm 2 ) was found n depths between 100 and 200 m (see tab. 2 and fg. 2). n ths zone thehaddockoocupes nearly 85% of' the total f'sh bomass, and nearly two thrd of' ts own bomass. n the next deeper zone thedensty decreases by half' and n greater depths the catches became next to ~ero. No Hadd06k was found deeper than 500 m. n January 1983 about 70% of' the total catch n numbers consst of' fsh between 25 and 31 cm 1ength (see fg. ~).Accordng to JONES (1982) ths length range must nclude the strong yearclasses of'1980 and 1981, that meansthe 2 and 3 years old fsh, assumng spawnng to have occurred on the 1st of January. n realty spawnng takes place later n the year, for only 0.3% of the mature f'ema1e had gravd gonads n January 1983.

3 - J - J.2. Blue Whtng The stock O secondary mportance on the Rockall Bat~ s that O Blue Whtng. The,llghest densty O ths speces was ound n the depth zone between JOO and 400 m (tab J and g. J). ts.total trawlable bomass was estmated to about t, excludng probably apart O age group 0 (see g. 6). Usng the growth curve O BALEY (1982) the rst maxmum must consst O about 9 to 10 month-old Blue Dltng. Spawnng s unlkely to take place beore March" as only O.~O mature emale had gravd gonads J.J. Greater Slver Smelt The stock O Argentna slus on Rockall Bank has only a trawlable bomass O aboutl5 000 t and s t~lereore O loss mportance. n depths' between 400 and 500 m ths speces obtans the hghest relatve mean densty O about 4.8 t/rull 2 (tab. 4 and g. 4). Fg. 7 shows that the proporton O smaller sh on.the stock s very low. 4. Conclusons Accordng to the "swept area" method used,the gven data on ~ stock szes are only data or the trawlable bomass and, there ore among other actors, related to the shp and thegear. No correctons could be made and the calculatons are based on a catchablty actor O 1. Thereore the results on the stock szes must be c'onsdered wth cauton. Durng the 58. cruse O FFS "Walther Herwg" n May 198J~ some hauls wth a bg Engel rndwater trawl were made on Rockall Bank n addton to the use O a 200 foot bottom trawl. At the same bottom trawlng statons as the day boore, the mdwater trawl was shot wth ts ground rope 4 to 6 m above the bottom; ths benrr the same vertcal heght above the bottom as the headlne 01 the hottom trawl.

4 , n a depth of' 208 m, :320 kg of' Haddock and 140 kc- of' 131uo Whtncwere caught by the,bottom trawl, whereas no,lladdock and only 2 kcof' Blue Whtng was f'ound n the mldwater one the f'ollowng day. No Haddock were taken by ether nets'ln 400 m depth; the,amount of' D1ue Whtng decreasng f'rom 24 kg n the mdwater one. t appears that n a 72 kg n the bottom trawl to vertca1 drectlon, Haddock are'taken comp1ete1y by the bo t tom trawl, whereas apart of' tw Blue Whtng stand llgher than 6 m above the bottom. The above f'ndng must resu1t n an underestmaton of' the Blue Whtng stock. JONES (1982) has estmated the Haddock stock sze of' Rockall Bank n 1981 to be of' about t, wthn the depth ranc-e of' 130 to 220'm. The mean catch rate of' the R.V. "G.A. Reay" n 1981 was 0.8 tonnesper 1 rate of' the R.V. hour hau1, whereas the mean ~atch "Wa1ther Herwg" n 1983 n the same depth range was 1.26 t/h. Because no data cou1d be f'ound of' the gear Used durng the "G.A. Reay"- survey n 1981, a drect comparson between the df'f'erent catch rates s mpossble. Comparng the 1ength dstrbutons of' laddock n 1981 (Augu~t) and 1983 (january, f'g. 5) and usng the age composton data of JONES (1982), t can be stated that the year-classes 80 and 81 are very abundant, and that tlle catches cou1d ncrease to about t n 1983, as expected by JONES (1982). But ths re1atve- v h~l catch can be obt~ned only f'or one 01' perhaps two years more, because of' the very 10w abundance of' year-c1ass 82. t seems 1ke n prevous years, that the abundant year-c1asses of' 80 and 81 are f'ollowed by two 01' three year-c1asses of' 10w abundance. Sn'ce 1974 the R. V. "Walther lerwc-" caugh t Uue Whtng on Rockall Bank all over the years and seasons (EHRCH, up to around ) n amounts t/h. Df'f'erences n the maturato~ curve of' the D1ue Whtng f'rom Rockall Dank compared wth that of' Dlue Whtng south of' the Porcupne Bank 01' wth that of' B1ue Whtng mgratng n sprng rrorn the north nto these lattudes ror spwawnng (ERCH and RODLES, 1982; EHRCH and SCHÖNE, 1983) rel'er to a separate populaton n ths area ~f PAWSON the Rockall Trough (see also et al., 1978), 01' whch the Blue Whtng on Rockall Bank could be on1y apart.

5 Lterature cted laloy, H.S. ~H~::?: 'rho populat.on b.ology or Blut' Whtng n tho North Atlantlc. Adv.Mar. lhol.,.1.2.: Ehrch, S. 1983: On the occurrence of some fsh speces at the slopes of the Rockall Trough. Arch. FschWss., 11(3): Ehrch, S. and Robles, R. 1982: nve3tgatons on maturty of Blue Whtng populatons between 42 0 N (Vgo, Span) and 61 0 N durng February and March lces, C.M /H:44.Ehrch, S. and Schöne, R. 1983: Gonap waturaton and spawnng of Blue Whtng (Mcromesstus poutassou) n the Rockall Trough area n 1983~ lces, C.M.1983/H:8 Jones, B.W. 1982: A stock assessment of Haddock on Rockall Bank. lces, C.M.1982/G:2 Savlle, A. 1977: Survey methods of apprasng fshery resources. FAO Fsh. Tech. Pap., 111:1-76

6 Tab. 1: Sze 01' Llll areu aud ul1mbnr 01' haul~ (n) p)r d()plh "'lllg on Rockall Bank depth range area n (m) ') (nm'-) Tab. 2: Relatve mean densty(ß), trawlable bomass (TB) and confdence lmts (CL) perdepth range of Haddock. Sonfdence level = 80% depth range ß TB CL (m) (t/nm 2 ) ( t) (:: %)

7 - 7 - Tab. 3: Blle Whtllf': mhan dpl::dly (0), LrawLable hollla::;:-; (TB) and con'dnllce lml::; (CL) pol' dl'plh range. Con'dence level = 80% depth range T '} TB CL (m) (t/nm'-) (t) ( %) Tab. 4: Greater Slver Smelt. Relatve mean densty (D), trawlab1e bomass (TB) and confdence lmts (CL) per depth range. Confdence level: 80% depth range D 2 TB CL (m) (t/nm ) (t) (:: %)

8 er 57", L :... -,', j wt sso 54" L..._..,,....._...._._._..._..._._._.._.._.._._._ 15._ J Fg. 1: Rockall Bank. Postons of trawl hauls. Depth n metres'.

9 59'N ls8 0 57" L..._..._..._..,.._..._..._..._"_,"_,"._.w._",",_", 17,,~_,,._.._...-_._~_"-_."_-_,,,,_.-_.,,-_,,._-_.,,-_,,_-_.'_-_"-_"'_-_'"'-_'''_'-_''. '_-_'-_'_''_,~_-_"-_",_,-_"-_,,_,,-_,,,-_,,,-_.J..J " Rocl<al Ben< Fg. 2: Haddock. Catches n kg/h. o = 0 kg [) = ~g ~ = 1-50 kg = kg => 1000 kg

10 .. j 1 l 57" w.. w 54" l _._.._..._._.._.,,.._.. _..._... 17" 15 14". Fg. J: Blue Whtng. Catches n kg/h. Declaraton of sgn8 see fg. 2.

11 WH _.._._-._.-.._-._._...- _. M" J 51" 56" " L.._.._,.._" _.. _ ~ _...._.... fly.nw.18" " Fg. q: Greater Slver Smelt. Catches n kg/h. DeclaratoD of sgds see fg. 2.

12 total length F~. 5: Haddock, Length dstrbutons of the total catch of "G.A. Reay" n 1981 (JONES, 1982) and "Walther Herwg" n 1983 on Rockall Bank. "Walther Herwg" (1983; 1 cm-class) males (n = ) females (n = ) "G.A. Reay" (1981; 3 cm-class)... sex combned (n = ) -_ _ _._-_ _ _ _._-._ _---_..-..._----_..---_.--_..-_.._---_._.._

13 (n) , 00 ~~ Ị. ',._-_ : D= ~~: D= total length (cm) Fg. 6: Blue Whtng. Sex separated length dstrbuton of the total catch.

14 ~ ~~~~~~~~~~~~~~ (n) 1500 " n = total length (cm) Fg. 7: Greater Slver Smelt. Sex combned length dstrbuton 01 the total catch.