Autumn Spawning Herring Around Faroes During Summer 1991
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ICES 1991 PAPER C.M. 19911H:50 Pelagic Fish Cttee AUTUMN SPAWNING HERRING AROUND FAROES DURING SUMMER 1991 by Jan Arge Jacobsen Fiskiranns6knarstovan N6at6n, FR-IOD T6rshavn Faroe Islands • ABSTRACT Investigations in 1990 indicated that concentrations of herring feeding in the Faroese area from May through July were similar to North Sea herring. Again in 1991 North Sea type herring was distributed in Faroese waters in May-July and approximately 16.000 tonnes fished. Results from an exploratory fishery and a research vessel survey showed, as in 1990, that the herring might be an adult part of the auturnn spawning herring in the north-western North Sea migrating into the Faroese area to feed from May to late July. However, in 1991 more by-catch of herring was reported from the mixed industrial fishery than previous years. • 2 INTRonucnoN At prcsent two different groups of hcrring can be id~ntified as occurring at Faroes, one group of Iocal summer spawners found in the fjords', (Täning 1943), and one group of autimm spawners migrating into the Faroese area from May to late July, probably from the north-western North Sea around Shetland isles (Jacobsen 1990). A new fishery on this herring has been established south-east of the isles. The catch figure for 1990 was 5.500 tonnes and the preliminary catch in 1991 amounted to 'over 16.000 tonnes. ,' , ,I".' • In 1991 Fiskiranns6knarstovan received several re ports' on by-ciltches of herring in various fisheries for othcr species arourid the Faroes during the 'spring, and the reported herring was all of the Nortll-Sea type. There might be one exception to this, a sampIe of 22 herring caught north-east of the isles in March was exceptionally hirge herring with a mean length of 35 cm and mean weight of 400 g, all with maturation stage IU. This herring could be of Icelandic origin belonging to the Icelandic summer spawners. I increasing effort has been put ioto the research for herr~g the last few years. In 1988 during e an exploratory fishery with a Faroese purse-seiner, herring was found distributed in the south-eastern corner of the Faroese EEZ. Also in 1989 herring was found in the same area. Iri both years the distribution was liinited to a rather small area around 600 30'N and 6°00'W. , In June 1990 tbe Faroese research vessel Magnus Heinason carried out a siUVey in the Faroese area south-east of the isles, and herring was fourid to bc distribllted over a comparatively large area south and south-east of the isles. I • I . I In 1991 acommercial purse-seiner was hired to survey the area east and south-east of the isles from mid May to mid June, and RlV Magnus Heinason contiriued the research two wecks in June in thc same area. The aim of the investigations is firstly to deterrnine the origin ofthe herring and secondly to clarify when the h'erring enters the Faroese area, when it Icaves the area, and the abundarice in the Faroese ar~a during that pedod. j MATERIAL AND METHOnS .,' Magnus Heinason is equipped with a Simrad SU-2 15 kHz sonar and the new simi-hd • EK-500 38 kHz sounder. Both sonar and echo sounders were run continllously durmg the survey and echo intensities were integrated along the co'urse and mean integrator values (SA) were recorded for every 1 nm. I '. !' Sampies of herrlng were collected from the commercial purse-seine catches and analyzed for the followi.~g bioIogic~1 arid meristlc charactcrs: length, ,weight, otoliths, sex, maturi~y ~tage, and number of vertebrae. i'" . I I' ,I RESULTS :. t The survey area was limited to the eastern south-east~rn part of the Faroese EEZ. Cruise tracks with bottom trawl stations arid hydrographical stations are shown in Fig. 1. Magnus Heinason is not equipped with a purse seine, and sampIes were therefore collected from commercial purse-seiners fishing for herring in the same area (Fig. 1). I 3 , " The cruise tracks of the exploratory purse-seine fishery in May and June is shown iri Fig. 2, and positions with herring catches are indicated on the figure. Thetcmpcrature eC) at io m, 50 m, and 100 m dcpth is shown in Fig. 3a-c respeciively~ South-east of the Faroes the warmer North Atlantic watcr from south-west meets the colder water coming southward east of the isles (mixed North Atlantic arid Icelaridic water). This is the sccond year of investigations in this area in June, and the temperature at 10 m dcpth (Fig. 3a) is betweeri 1-2 °Ccolder in 1991 as compared to 1990 (Jacobsen 1990). The tcinperature at 50 m depth (Fig. 3b) where the main bulk of herring was observed (ref. Fig. 4) was 1 °C colder thm the sUrface temperature, with no clear thermocIine. The temperature at 109 min 1991 (Fig. 3c) was apinoximately the sarne as in the 1990 sUlveY. Iri Fig. '4 is shown tbe distribution and dens'ities of herring from the survey in 1991. COInmercial by-catch of herring at bottom are shown as dotted areas, and areas with herrmg schools are hatched, while areas with large numbers of schools or significmt pw-se-seine . " catches äre einphasized (crosshatched) on the figure. The herring schools were disii-ibuted in the upperinost 5-50 in below sea surface in riumerous rather small schools (10-40 toriries), with a few large schools up to 300 tonnes as measured by commercial purse-seiners. Herring . • was caught in the bottom trawloperated by Magnus Heinason in areas where no schools were observed and far outside the areas where the purse-seiners were operating, refcr to the dotted areas in Fig. 4. The biological resuits are divided into two parts, the first part is from the arialysis of the sarnples provided by the commercial purse-seiners from mid May to mid July 1991; arid the sccond part isfrom the analysis of the sampIes caught with bottom trawl, mainly as by-ciltch in the indusmal fishery in April 1991. Thc reason to divide the sarnples is thai tbe räiher . small sampIes, a total of 347 herring in April, is not representative for the mam purse-seiric fishery later in theseason. also the total by-catch of herring during tbe spring is thought to be insignificani. Therefore the results arid the discussion later in the paper will mainly be on the bäsis of the daia from the purse-seine catches while the other results will be meniioned only briefly. Tbc length distribution of the recorded herring from the purse-seine catches is shown in Fig. 5. The length ranged from 26-38 cm with mean length of 30.0 cm and a mean weight of 244 g (Table I, first part). The length distribution of thc hcrring from the bottom trawl catchcs • is shown in Fig. 6. Tbe length ranged from 25-37 cm with mean length of 30.8 cm and a mean weight of 220 g (Table 1; second part). The size distribution was generally slightly larger in the noI-them part.of the surveyed ärea. The age distribution (nurnber of winter rings) from the purse-seine fishery arid from the by-catch is shown in Fig. 7 arid 8 respectiv ely. Approximatelyone third of the herring was 4 ringers and orie forth was 5 ringers while praciically no small herring less than 3 winter rings was observed. Maturity stages by montb is given in Fig. 9. The shift from stages VIII arid 111 domiriäiirig in April and May towards maturity stages iv and V in June and July is evident from Fig. 9. Th6 average vertebral coUnt of 823 herring was 56.63 vertebrae (Fig. 10). The weighi distribution from purse-seine catches in june is depicted in Fig. 11 and from boitom trawl catches in April in Fig. 12.' . Tbc mean iength and mean weight by age is give~ iri Tabie 1. It ls notlceable that the meän wcight by äge for ages 3 and 4 in June 1991 is father high compared to similar figw-es given for previous years (Anon. 1991a) and also compared to the Faroese investlgations last ycar (Jacobsen 1990). I' 4 ~ No attempt was made to estiinate the biomass of herririg from the acoustlc recordings, the reuson for this will be dealt with in the discussion. I' I I ! DISCUSSION I I . ''', ,,'" i' ",', ', ,""' The areal distribution of herring schools iri 1991 (Fig.,4) was limited to the south-eastem part of the Faroese area, approxiniately the same distribution as in 1990 (Jacohsen 1990). ' However, i.It 1990 good herring catches were obtairied 'on Sandoyarbanka east of the isles and the northem limit of distribution iri 1990 was up to 62°N while the limit was aplnoxi mately 61~0'N in 1990 with no schools over SandoYarbankti. One reason to the more southerly distribution of th6 herring might be fOUnd iri the temperature distribution this year (Fig. Ja-b). As the temperatUre at 10 indepth was generally I-2°C colder in 1991, in addition io the steady wind conimg from north~easi duririg late spring and early sUmmer, the herdng mighi have stopped its migration fuIther south' this year. An analysis of plllI1kton sarnples taken with aHensen net during the sUrVey shows that plenty of copepods in several stages and other plankton was available further north ririd east of the isles. Therefore it'is not assWned that insufficient amount of prey is' the reason to this more southerty distribution (the herring was mairily fee ding on copepods).