Rapp. P.-v. Réun. Cons. int. Explor. Mer, 191: 160-168. 1989 Herring larval studies in the west-central North Sea J. H. Nichols and K. M. Brander Nichols, J. H., and Brander, K. M. 1989. Herring larval studies in the west-central North Sea. - Rapp. P.-v. Réun. Cons. int. Explor. Mer, 191: 160-168. Herring larval abundance in the west-central North Sea, as shown by the annual indices from the international larval surveys, declined during the early 1970s to reach a low in 1975 and 1976. Following a ban on fishing for herring in the North Sea (1977- 1983), the spawning stock recovered. This was indicated both by a rapid rise in larval production and by a re-occurrence of larvae in areas where no larval production had been observed since the mid-1960s. Larval production reached a new peak in 1985 but has subsequently declined to about half that level. Spawning is confined to suitable substrata for egg adhesion. As a result, larval production occurs in thermally stratified water in the north, in well-mixed water further south, and in the boundary areas between these two regimes. Interannual variation in the position of these water masses during the hatching period produces variability in the environment that could affect larval survival. Some indication of the potential for variability is seen in the movement of satellite-tracked buoys. J. H. Nichols and K. M. Brander: Ministry of Agriculture, Fisheries and Food, Directorate of Fisheries Research, Fisheries Laboratory, Lowestoft, Suffolk NR33 OHT, England. Introduction Sea stocks. This resulted in a recommendation from the Assessment Working Group for a complete closure of The spawning areas of the herring in the west-central the North Sea herring fisheries. The closure came into North Sea are well defined from larval surveys both effect in 1977 and was lifted in 1983. By 1984, larval historically (Bückman, 1942) and in recent years from abundance in the west-central North Sea had increased, international larval surveys (Rankine, 1986). Spawning reaching a peak in 1985. Larval production has sub­ occurs along the northeast coast of England from the sequently fallen, both in 1986 and 1987, but is still well Longstone area south to the Dowsing (see Fig. 2 for above pre-1984 levels in this area. Detailed field studies, place names). Offshore, spawning occurs in the vicinity aimed at improving our understanding of the factors of the North-East Bank and along the western edge of governing recruitment, have run concurrently with the the Dogger Bank. The spawning season extends from international larval surveys since 1984. These studies August in the north progressing southwards to the were planned before production rose, and in some ways Dowsing in October. Zijlstra (1970) shows the position the high larval densities made such patch studies more and timing of spawning on the Yorkshire coast and the difficult to conduct. Dogger Bank from catches of spawning herring and This paper describes the changes in herring larval from larval surveys during the period 1957-1968. His distribution and abundance in the west-central North surveys demonstrate particularly the decline and dis­ Sea since 1976. The position and timing of spawning in appearance of larvae on the western edge of the Dogger relation to substratum and stratification is also exam­ Bank. ined. Since 1972, the ICES Herring Assessment Working Groups have established a relationship between larval Materials and methods abundance and spawning stock biomass for North Sea stocks. This has been used as a basis for stock assessment The ICES coordinated herring larval surveys have been over that period (Anon., 1987a). For the central North carried out in the west-central North Sea since 1970 by Sea, larval data have been collected on regular surveys England and The Netherlands with some participation during September and October. These surveys show a by Poland and Norway. Herring larvae are sampled steep decline in larval abundance in 1975 and 1976, using a Gulf III type high-speed townet (Gehringer, which coincided with a big reduction in all other North 1952) deployed at 5 knots, on a double oblique profile 160 from the surface to within 5 m of the sea bed. This are fixed in 4% formalin and the herring larvae sorted is fitted with a conductivity, temperature, and depth and measured at least two weeks after fixation. recorder monitoring continuously during the tow. Vol­ For the examination of distribution and abundance umes filtered are calculated from the readings of the of larvae, the international data set is grouped into four calibrated flowmeter in the mouth opening. Numbers two-weekly periods for September and October. of larvae per sample are converted to numbers below Specifications for the “Argos” satellite-tracked, drift­ one square metre of sea surface. All plankton samples ing buoys deployed in the centres of herring larvae 8-16 Sept 1981 3-12 Sept 1985 31 Aug - 4 Sept 1987 1976 32-30 Sept 1981 21-28 Sept 1985 S.W^Patch 29 Sept - 12 Oct 1981 6-12 Oct 1987 j Herring larvae nm " 2 1-27 27 -2 4 3 243-2187 > 21B7 Figure 1. Distribution of herring larvae <10 mm as numbers m 2 for the years 1976, 1981, 1985, and 1987. The area of the survey is also outlined. patches in 1985 and 1987 are described by Booth and Table 1. Abundance indices of <10 mm larvae (xlO 9) for the Ritchie (1983). Briefly, the window drogue used meas­ west-central North Sea, 1976-1987. ures 7.5 m deep by 5 m wide with its surface float sep­ Year Larval index Year Larval index arated from the transmitting buoy by a 5 m tether line. The top of the drogue is set between 12.5 and 16 m from 1976 76 1982 338 the surface. 1977 174 1983 661 1978 462 1984 1055 1979 188 1985 3802 1980 214 1986 2027 Results 1981 364 1987 1970 Larvae distribution and abundance The abundance of herring larvae <10 mm in length is In 1976, the highest abundance of larvae <10 mm in shown for the first three sampling periods in 1976,1981, length during the season rose to only 91 m-2 in a small 1985, and 1987 in Figure 1. These distributions show area close to the coast between the Fame Islands and the low level of larval production in 1976, and the the river Tyne in early September (see Fig. 2 for place gradual recovery following the closure of the adult names). By mid-September, some hatching had fishery between 1977 and 1983. occurred off Bridlington Bay. By 1981, the picture in 56° \Farne Is. [Farne Deeps! 55” ;S.W. Patch River Tees W hitby* Flamborough 54° More than 80% gravel 30%-80% gravel Dowsing] 0 o\ 5% -30% gravel M ud 1 [Less than 5% gravel S and Stones and rough ground 53“ Figure 2. A summary of available data on the substratum off the northeast coast of England. 162 early September had changed considerably with dense and recently-hatched larvae were present in all four patches of small larvae in the Longstone area and off sampling periods. The most notable feature was their Whitby. Abundance of larvae reached a peak of reappearance well offshore at the northern edge of the 1120 m-2 off Whitby in the first half of September survey area (North-East Bank) and on the western edge 1981. After the end of September, no further hatching of the Dogger Bank in the vicinity of the South-West occurred. Patch. Larval abundance, although low in early Sep­ By 1985, larval production had increased considerably tember, reached a peak of over 12 000 m^2 off the 21-28 Sept 1985 Herring larvae nm Stratification parameter 10,30,50,70.90.Jm -s 2 7 -243 2 *3-2 187 > 2187 Figure 3. The distribution of herring larvae <10 mm on the English surveys from 1984-1987 and the contours showing stratification parameters 0 (J m~3) (see Simpson, 1981). 163 Yorkshire coast by the end of September. Larval pro­ Drift of larvae duction declined in 1986, although hatching continued on the North-East Bank and along the western edge of Differences in the position of the boundary areas and the Dogger Bank. In 1987, there was a further reduction in the timing of the breakdown of stratification may also in overall larval production, although abundance in the affect the drift of larvae and the retention times within Whitby area reached 18 000 tr r 2on 1 October. Hatching the sampled area. Such potential interannual differences continued on the North-East Bank and there was still can be seen from the tracks of “Argos” satellite-tracked evidence of spawning activity on the western edge of buoys deployed in the area in 1984, 1985, and 1987. In the Dogger Bank. The larval index for the area for the 1984, a single system was deployed in a larval patch in years 1976-1987 ( from Rankine, 1988) is shown in Table well-mixed water off Flamborough Head on 13 October 1. This index is based on the mean number of <10 mm (Fig. 4a). This buoy moved generally southeastwards at larvae and provides a useful summary of the trend in 7 k m d ^ ‘ until it was recovered on 29 October. Two larval production over this period. systems were launched in late September 1985 in the De Groot (1980) reviewed the relation between centre of larval patches and on the edge of mixed and spawning herring and substratum in the southern North stratified water (Fig. 4b). Their movement over the next Sea and established their requirement for a coarse, six weeks indicates a very slow south to southeasterly stable substratum generally consisting of some gravel movement averaging 2.5 km d~1 following the boundary or stones.