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. Although detailed records of the substratum between the two regimes. In early October 1987, two for this area are limited, the British Geological Survey systems were deployed in similar positions to the 1985 has produced a chart of sea-bed sediments for the Tyne- deployments (Fig. 4c). Their subsequent movement, in Tees area (Anon., 1986) and some provisional data for contrast, had a much greater easterly component, in the Farne area north of latitude 55°N (Anon., in press). keeping with the position of the boundary between The British Admiralty, London, has compiled North mixed and stratified water at that time. Their rate of Sea Fisheries Charts that define areas of stones and drift was greater than the 1985 deployments with a mean rough ground. These data are summarized in Figure 2 velocity of 4.3km d~ 1 to 4 November, for the system to indicate the areas that are potential herring spawning launched at latitude 55°15'N, longitude OTll'W, and grounds. This confirms the coincidence of high numbers 9 km d_1 up to 16 October, for the system launched at of small herring larvae with most of these potential latitude 54°20'N, longitude 00°00'. This latter rig was spawning sites, and in particular with the areas of gravel badly affected by the severe storm between 16 and 19 around the Longstone Ground, the North-East Bank, October whilst it was over the relatively shallow water and off the Yorkshire coast. Areas where small herring of the Dogger Bank. The most northerly rig was larvae are rarely found, for example between the Tyne launched on 8 October in well-stratified water. It mean­ and Tees running offshore in a northeasterly direction, dered slowly west, south, and then east at 2 to 4 km d~1 are characterized by either a muddy or a sandy sub­ and was only 26 km south of its launch position after 36 stratum. Absence of small larvae in this area is a feature days. This indicates the potential for larvae to be of all the distributions (Fig. 1) and serves to separate retained for long periods in stratified water regimes. larval cohorts produced north of the river Tees from those produced off the Yorkshire coast. The area of the west-central North Sea in which the herring spawns is characterized by the presence of both Discussion thermally stratified and well-mixed water. Thermal stratification is well established by June/July and begins One of the more interesting findings of the larval surveys to break down during September/October (Harding in the west-central North Sea in recent years has been and Nichols, 1987). Larval distribution in relation to the reappearance of recently hatched larvae offshore on stratification is shown for the English surveys from 1984 the North-East Bank and along the western edge of to 1987 (Fig. 3). For similar sampling periods, there are the Dogger Bank. The disappearance of Dogger Bank interannual differences in the position of the boundary larvae in 1964/65 has been taken as evidence of a between stratified and mixed water, represented by the discrete subpopulation and Burd (1985) speculated that 10 J itT 3 isoline. This is particularly noticeable when its failure to recover might indicate the extinction of a comparing the data in early October 1984 and 1985. “genetically determined local stock”. It is possible that Since the spawning sites are fixed, they generate dif­ the reappearance of larvae in these areas is the result ferent distributions of newly-hatched larvae in con­ of a spread of spawning fish following full occupation trasting physical regimes. Compare, for example, the of other sites. It is also possible that these spawning peaks of larval production in 1985, which are mostly in groups have retained a separate identity throughout the stratified water, with those in 1984, 1986, and 1987, period of decline but that larval production there has where they occur in either well-mixed water or close to been too low to detect by the plankton surveys. If the the boundary between the two regimes. former is valid, then it is necessary to explain why

164 (b)

150 70 70 30)

2 8 /9

10/10 55“

4/11 2 9 /9 '

£—-26 /10

17/10 , 20/10 54°

24 /1 0 29/10

26/10-

19/10

Figure 4. The drift of "Argos” satellite-tracked buoys released in the centre of herring larvae distribution in: (a) 1984, (b) 1985, and (c) 1987. The stratification parameter 0 , for the period close to the deployment, is also shown.

production should have declined in these areas first and Evidence from the distributions of larvae <10 mm not evenly across all of the spawning sites. This decline since 1985 lends some support to the hypothesis of a has been attributed in part to a shift in spawning time density-related re-occupation of former spawning sites. from October to September, associated with a change This is indicated by the steady build-up of larval pro­ in the growth rate (van de Kamp and Corten, 1982). duction on the Dogger Bank east of longitude 1°E. This Van de Kamp and Corten suggest that this natural reached 500 larvae m 2 in 1988 when the large 1985 change could result in less favourable conditions for year class spawned for the first time (J. H. Nichols, larvae at hatching time on the Dogger Bank compared unpubl. data). Similarly, on the North-East Bank there with the near-coast spawnings. They recognize, has been a steady increase up to 1987. However, in 1988 however, that this could not entirely account for the production was greater offshore, with peak production decline and that, as Zijlstra (1970) suggests, the higher on the North-East Bank of 8600 larvae m-2 compared fishing mortality on the Dogger Bank spawners was a with 3800 larvae m '2 on the Longstone Ground (Ran- major cause. In this context it should be noted that kine, 1989). the larvae reappeared on the western Dogger in late Off the northeast coast of England herring larvae September 1985, generated from a spawning in mid- occur in all hydrographic regimes from stratified to September. mixed, but mainly in the upper part of the water column

165 — Feb 1985 Feb 1986 (1984 year class) (1985 year class)

Figure 5. O-group Ur herring sampled by &- Isaacs-Kidd midwater Feb 1987 Feb 1988 trawl during the (1986 year class) (1987 year class) International Young Fish Surveys in 1985- 1988 (Anon., 1987b, — 0-1 ■ 2-25 • 26-50 5 1 - 1 0 0 ■ 1988b).

(J. H. Nichols, unpubl. data). They do not conform to 1985 and 1987. Their subsequent drift is noticeably a simple relationship with hydrography or onshore- different, with a stronger easterly component in 1987 offshore circulation, and the only obvious common fac­ than in 1985. tor is substratum. Even over the fairly short period It is clear from these studies that there is the potential covered by these surveys it is clear that the larvae do not for variability in the retention of larvae which may be remain in the area where they hatch and the difficulty of related to stratification in this area. Examination of the following and identifying discrete patches implies that distribution of 0-group herring from Isaacs-Kidd trawl eddy diffusion is mixing larvae of different origins in hauls in the North Sea in February (Anon., 1987b, time and space. Changes in the seasonal position of the 1988a) show the annual variation from 1984 to 1987 in front, or the timing of its breakdown, do appear to the numbers of larvae still present in the west-central affect the direction of drift and the subsequent fate of North Sea at that time. However, it is not clear either the larvae. This is indicated by the movement of the from the general distribution, or the size distribution “Argos” buoys launched at the same spot and time in (Anon., 1988b) over the North Sea, whether those

166 larvae hatched and were retained in that area, or to lower prey densities in mixed water. Kiørboe et al. whether they were immigrants from further north. (1988), working in a herring larval patch in the Buchan There is no doubt that many of the larvae from the west- area of the North Sea, found the most favourable con­ central North Sea spawning patches are not retained in ditions of prey density and larval survival in boundary the local inshore area. However, the question of zones between mixed and stratified regimes. However, whether they remain as separate groups, either from even in these areas larval growth was limited by food each other or from the products of other North Sea availability. spawnings, must remain open. Quite clearly the studies in the west-central North There is difficulty in identifying individual stocks of Sea serve only to identify areas where more research herring in the North Sea either as larvae, pre-recruit effort could usefully be directed. In this context, priorit­ fish, or as adults. This does present a problem in assess­ ies are to measure the nutritional condition of larvae in ing the effects of mechanisms studied on a local scale, the wild and to be able to identify the origins of late which might generate variability in larval survival and larvae and under-yearlings in the central and eastern subsequent year-class strength. It is accepted that this North Sea in February. Field measurement of RNA/ can only be measured for the whole of the North Sea DNA ratios in single larvae (Clemmesen, 1988) is poten­ and attempts to do separate analyses are no longer tially the most rewarding index of larval condition made. In spite of this, there are differences in advection because it can identify the starvation at an early stage. on a “local scale” which could have generated variability in larval survival between 1984 and 1987. For example, the apparent rapid easterly movement of water in 1987 is coupled with a low larval index (Table 1), and low numbers of 0-group herring in February 1988 (Fig. 5, References 1987 year class). In contrast, larval production occurs Anon. 1986. British Geological Survey 1:250 000 series. Sea mainly in stratified water in 1985 with very little easterly Bed Sediments and Quaternary Geology Charts. Tyne-Tees, movement indicated by the “Argos” buoys. This gen­ sheet 54°N-02°W. British Geological Survey, Keyworth, erates a high larval index and high numbers of 0-groups UK. on the east coast in February 1986 (Fig. 5). The 1985 Anon. 1987a. Report of the Working Group on Herring Larval Surveys, South of 62°N. ICES CM 1987/H: 7, 21 pp. year class has, in fact, turned out to be high for the (mimeo). whole of the North Sea (Anon., 1988a) but it is not Anon. 1987b. Report of the Herring Assessment Working possible to apportion the contribution of the west-cen­ Group for the Area South of 62°N, Parts 1 & 2. ICES CM tral North Sea to this increase. 1987/Assess: 19, 206 pp. (mimeo). Anon. 1988a. Report of the Herring Assessment Working For 1984 and 1986 the stratification for early October Group for the Area South of 62°N, Parts 1 & 2. ICES CM only is available. This indicates conditions similar in 1988/Assess: 17, 205 pp. (mimeo). both years to 1987 rather than to 1985, although the Anon. 1988b. Report on the International Young Fish Survey “Argos” buoy in 1984 shows no easterly movement. in the North Sea, Skagerrak and Kattegat in 1988. ICES CM The larval index for the area (Table 1) is much lower 1988/H: 8, 49 pp. (mimeo). Anon, in press. British Geological Survey 1:250 000 series. Sea for both years than that of 1985, although year-class Bed Sediments and Quaternary Geology Charts. Fame, strengths for the North Sea do not entirely reflect this sheet 55°N-02°W. British Geological Survey, Keyworth, trend (Anon., 1988a). Similarly, 0-group abundance for UK. the whole of the North Sea in February in those years Booth, D. A., and Ritchie, D. 1983. SMB A satellite-tracked buoy and drogue. Mar. Phys. Grp. Rep., Scot. Mar. Biol. does indicate a survival much higher than in 1987. Ass., (22), 15 pp. (mimeo). Clearly, there are discrepancies between the larval index Buckley, L. J., and Lough, R. G. 1987. Recent growth chemi­ for the area and subsequent total North Sea year-class cal composition and prey field of haddock (Melanogrammus strength which can only be resolved by the ability to aeglefinus) and cod (Gadus morhua) larvae and post larvae separate the products from each area. on George’s Bank May 1983. Can. J. Fish. Aquat. Sei., 44: 14-25. The evidence from 1985 and 1987 suggests that there Biickmann, A. 1942. Die Untersuchungen der Biologischen is a causal link between larval distribution in relation to Anstalt über die Ökologie der Heringsbrut in der südlichen stratification and advection and larval survival. This link Nordsee, 1. Teil. Helgoländer Wiss. Meeresunters., 3: 1- could well be through short-term retention mechanisms 57. Burd. A. C. 1985. Recent changes in the central and southern keeping larvae in more favourable feeding conditions. North Sea herring stocks. Can. J. Fish. Aquat. Sei., Buckley and Lough (1987), studying three spawning 42(Suppl. 1): 192-206. sites of cod and haddock on George’s Bank, showed Clemmesen, Catriona M. 1988. A RNA and DNA fluorescence that better conditions existed for growth and survival of technique to evaluate the nutritional condition of individual marine fish larvae. Meeresforsch., 32: 134-143. haddock larvae above the thermocline in stratified water Gehringer, J. W. 1952. High-speed plankton samplers. 2. An compared with mixed water. They concluded that the all-metal plankton sampler (Model Gulf III). Spec. Scient. stratified spring conditions were more favourable for Rep. US Fish. Wildl. Serv., 88: 7-12. haddock larvae, but that cod larvae were able to adapt Groot, S. J. de 1980. The consequences of marine gravel

167 extraction on the spawning of herring, Clupea hurengus Rankine, P. W. 1986. Herring spawning grounds around the Linné. J. Fish Biol., 16: 605-611. Scottish coast. ICES CM 1986/H: 15, 6 pp. (mimeo). Harding, D., and Nichols, 1. H. 1987. Plankton surveys off the Rankine, P. W. 1988. A report on the ICES herring larval north-east coast of England in 1976: an introductory report surveys in the North Sea and adjacent waters in 1987/88. and summary of the results. Fish. Res. Tech. Rep., MAFF ICES CM 1988/H: 62, 14 pp. (mimeo). Direct. Fish. Res., Lowestoft, (86): 56 pp. Rankine. P. W. 1989. A report on the ICES herring larval Kamp, G. van de, and Corten, A. 1982. A hypothesis for the surveys in the North Sea and adjacent waters in 1988/89. decline of the Dogger Bank herring stock. ICES CM 1982/ ICES CM 1989/H: 5, 6 pp. (mimeo). H: 30, 16 pp. (mimeo). Simpson. J. H. 1981. The shelf-sea fronts: implications of their Kierboe, T., Munk. P.. Richardson, K.. Christensen, V., and existence and behaviour. Phil. Trans. R. Soc. 302(A): 531— Paulsen, H. 1988. Plankton dynamics and larval herring 546. growth, drift and survival in a frontal area. Mar. Ecol. Prog. Zijlstra, J. J. 1970. Herring larvae in the central North Sea. Ser.. 44: 205-219. Ber. Dt. Wiss. Komm. Meeresforsch., 21 H: l^t, S: 92-115.

168