International Couneil for the CM 1986/H:15 Exploration of the Sea Pelagie Fish Committee

HERRING SPAWNING GROUNDS AROUND THE SCOTTISH COAST

by

PW Rankine

Marine Laboratory Aberdeen Seotland UK

• SUMMARY This paper updates and enlarges on existing knowledge of the loeations of the main herring spawning grounds around the eoasts of Seotland. Information on the oecurrence of recently hatched herring larvae from the leES Herring Larval Data Base, from 1972 to 1985, on the loeations of known gravel deposits and information from earlier papers on larval hatehing size, growth rates, and drift and diffusion rates have been utilised in this study.

INTRODUCTION

Some of the spawning areas of herring around the Seottish eoast have been deseribed in previous papers based on the positions of eapture of haddoek whieh had recently been feeding on herring eggs (Bowman, 1922), the positions of drift net eatches of spawning herring (Wood, 1922) and the distribution of <10 mm herring larvae (Wood, 1971). Postuma et ale (1975) identified the general areas of spawning grounds in the . --

The previous works deseribed above ean only provide a general deseription of spawning • areas since both haddock with herring spawn in their stomaehs (referred to as "spawny" haddock) and <10 mm larvae may have been eaptured some distance away from the spawning sites. As noted by earlier authors the only eertain way of identifying a herring spawning site is by the actual loeation of herring eggs on the sea bed. Unfortunately, sueh reeords are extremely rare and the only doeumented oeeurrenees in Scottish waters are those from Ballantrae Bank, in the (Parrish et al., 1959). However, the consistency of the data given in previous papers, both between years and between methods, indicated that they gave useful first approximations of the loeations of some of the herring spawning grounds around Seotland.

This paper attempts to update and enlarge upon present knowledge of the loeations of the main spawning grounds around the Scottish coast using the occurrence of recently hatehed larvae from 1972 to 1985, the positions of known gravel deposits (particle size

1 ., .",

._- range 0.5-5.0 cm) and information hom previous papers on the occurrence of "spawny" haddock and stage VI herring. The positions of gravel deposits have been included in this study as earlier work (Holliday, 1958) has established that the Atlantic herring (Clupea harengus) actively selects gravel deposits for egg deposition. Gravel beds are important therefore as potential spawning sites.

MATERIALS AND METHODS

Assuming that small herring larvae are passive drifters the concentration of larvae above a spawning bed during the period immediately following hatching will be governed by three factors; a) the concentration of larvae at the time of peak hatching. b) the drift rate of the patch away from the spawning site. c) the diffusion rate of the patch.

If peak hatching densities of larvae can be estimated and information is available on likely drift and diffusion rates, hatching size and growth rates in each area, then the numbers of recently hatched larvae occurring at each station can be used to estimate • the position of a spawning site and thus to delimit the main spawning areas.

Larval length distributions contained in the ICES International Herring Larval Data Base from 1972 to 1985 were used in this study along with previous works on hatching size and growth rates. In addition, recent work on larval drift and diffusion rates were also utilised.

For comparative purposes the locations of gravel deposits were extracted from large scale Admiralty and Kingfisher charts and a summary chart based on previous works by Bowman (1922) and Postuma et al. (1975) was also made.

The distance between sampling stations in the leES survey is approximately 10 nautical mHes and the resolution of the sampling grid is therefore + 5 nautical mHes. If high densities of larvae, smaller than a particular length, a~ therefore recently hatched, are to be used to indicate a spawning site within 5 nautical mHes of astation then both larval density and length criteria must be consistent with diffusion and growth over the time taken for the patch to drift 5 nautical mHes.

Estimation of Larval Drift and Length Criteria Larval drift rates • To determine the likely time taken for a larval patch to drift a maximum of 5 nautical miles previous measurements on larval drift rates and residual current flows, in the Buchan, - and Division VIa(N) areas were utilised. These data are given in Table 1. Although. rather crude in area coverage these data give a general indication of the relative residual current speeds to be expected in each area over the spawning period. The data referring to Tiree and the Little Minch were, however, collected in early summer and may not be typical of larval transport rates in the autumn.

In the Orkney-Shetland area the figure given refers to the eore of the eurrent. This strong eurrent stream is relatively narrow, about 8.nautical mHes wide, and the residual current nows outwith the main current are very much weaker

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I I.. (Dooleyand McKay, 1975). Small larvae are mostly found between the current boundary and the coast (Heath et al., 1985) and it would therefore seem likely that most recentIy hatched larvae are subjected to residual current nows of less than 5 nautical miles per day. '

These data indicate that in order to limit larval drift, from a spawning site, to 5 nautical miles, drifting should be confined to aperiod of about two days in most areas, and perhaps to 1-11 days in the Orkney-Shetland area and to less than one day off Tiree. Limiting larval drift to one day will ensure drift of less than 5 nautical miles in most areas.

Length criterion

In order to allow the choice of a suitable length threshold to delimit newly hatched from older larvae the likely size variation of one day old larvae was examined using mean hatching lengths and growth rates of newly hatched larvae obtained by previous workers (Table 2). These data indicated that herring larvae in these areas hatched at a range of sizes between 6 and 7 mm. Larvae in the data base are measured to the nearest mm and larvae of '<7 mm include larvae up to 7.4 mm. On the basis of the growth rates given in Table 2 the majority of larvae of one day old will be less than 7.4 mm. Ir similar mean hatching lengths and daily growth rates occur in the Orkney-ShetIand area (no relevant data are available at present for this area) then it • was thought not unreasonable to utilise larvae of up to and including 7 mm for the purpose of this study.

Estimation of Initial Hatching Density and Diffusion Rates.

Hatching densities

In order to obtain relative estimates of the 'peak hatching densities of larvae over the herring spawning grounds around the Scottish coast, the mean concentration of the five densest patches of ~7 mm larvae caught in each area from 1972 to 1985 were ca1culated. The mean maxima were 21M/rn ~ in the Buchan area, 3068/m ~ in ICES area VIa(N) and 5894/m ~ in the Orkney-Shetland area.

Larval patch studies off the Hebrides (Heath et al., 1985) and in the Buchan area (Munk et al., 1986), where active searches were carried out in order to establish initial hatching densities, gave results of similar magnitude. Patches of 2400/m ~ were found off the Hebrides and 6000/m ~ in the Buchan area.

The maximum concentrations of "<:7 mm larvae encountered in the ICES surveys therefore appears to be typical of the range of initial larval hatching densities over Scottish herring spawning grounds.'

Larval patch diffusion rates

Larval patch diffusion coefficients have been estimated for the area west of the Hebrides and in the Buchan area in the patch studies mentioned above. (No relevant diffusion coefficients exist for the Orkney-ShetIand area at present.) Using the existing diffusion models the likely densities of ~7 mm larvae occurring after one day of drifting and diffusion can be estimated.

3 , " - In the Hebrides diffusion model the initial concentration of larvae at the centre of the patch, had decreased from 2.400/m Z to around SOO/m Z ~fter one day at the ?ighest estimated mortality rate of 0.2.. Although the relatlve effects of mortallty and diffusion are difficult to separate in such studies, the effect of different assumed mortality rates on larval density were, in fact, very small.

In the Buchan area diffusion coefficients were higher and patch dispersal correspondingly more rapid. At similar initial densities to those encountered off ~he Uists any stations yielding 500 larvae per m Z would therefore be closer to a spawmng site than in the Hebrides example.

The presence of rapid tidal mixing and strong current streams in the Orkney-Shetland area (Dooley, 1981) indicates that higher diffusion coefficients than those found off the Hebrides might be expected, resulting in more rapid dispersion processes.

The available data indicate, therefore, that larval patches off the Hebrides exhibit the slowest dispersal rate of the three areas. If higher initial hatching densities were present in the other areas then these denser patches would tend to be dispersed more quickly by the greater diffusion rates in those areas.

Therefore, since different initial densities may to some extent be compensated for by the different dispersal rates in each area it was thought to be valid to generalise for • the three areas and to use concentrations of >500 /(7 mm larvae to indicate the main spawning grounds around the Scottish coast.

RESULTS AND DISCUSSION

The maximum numbers of ~7 mm larvae occurring at each station from 1972. to 1985 were extracted from the ICES data base and are portrayed in Figure I.

Figures II and m give the knowri locations of gravel deposits derived from Admiralty and Kingfisher charts and a summary of past data, respectively.

Despite the assumptions made in this study the distribution of patches of small larvae of densities of >SOO/m Z is very similar to both the known locations of gravel deposits, and to some earlier data, lending support to the methodology used.

In the Buchan area there are two main spawning grounds, one inshore, between • Aberdeen and Peterhead and the other further offshore at Turbot Bank. The inshore grounds, extensively used in the past, have not shown appreciable larval production in recent years although there are indications from the 1985 surveys that these grounds are now being re-utilised. Although some spawning appears to have taken place on Turbot Bank in earlier years it is only in the past three years that this area has been heavily re-colonised and it is now the most important spawning ground in the Buchan area. The station immediately off Banff, in square 44E7, was an important autumn spawning ground in earlier years but has not shown any major spawning activity from 1972 to the present day. In Figure I the station in square 41E8 contained recently hatched larvae for the first time in 1985. Again, this station is situated in an area of gravel deposits and these.data may indicate a spread of the spawning area southwards by the increasing Buchan population.

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In the Orkney-Shetland area there are numerous spawning grounds throughout the area, most of which are indicated by previous data. Recently hatched larvae were caught in the area to the east of Fair Isle for the first time since the larval surveys began; in the autumn of 1985. This may represent a recolonisation of old spawning sites since the northern part of the larval distribution corresponds with past data on autumn caught stage VI herring and "spawny" haddock. Again this may be due to an increase in stock size in this area. Previous data (Bowman, 1922) suggested spawning to the north west of Shetland in the spring and autumn spawning on the east side of Shetland. No firm conclusions could be drawn as to changes in autumn spawning distributions, however, as the larval sampling in these areas is incomplete. The occurrence of larvae between Wiek and Helmsdale eorresponds closely to the distribution of both stage VI herring and "spawny" haddock indicating that the spawning sites have remained statie in this area.

In ICES subdivision VIa(N) there would again appear to" be numerous spawning sites occurring over a wide area off the north eoast of Seotland, off Lewis and the Uists, and at St Kilda and Tiree. With the exeeption of the bank off Tiree larval numbers were low in the Minch suggesthlg that the main spa\~Jning banks He outside the Minch itself. The spawning grounds at Tiree and also at St Kilda showed marked annual variations in the appearance of >SOO/m Z patches which might perhaps be expected due to the greater variability in the timing of sampling in these areas and also to the rapid dispersal rates off Tiree. Although larval numbers were relatively low the data • suggested that present day spawning is restricted to the east side of the Minch, while earlier data suggested substantial spawning on the west side. However, the paucity of data in this instance makes it impossible to say for certain that areal shift in spawning sites has taken piace. The same reason makes any conclusion regarding a changeover from spring to autumn spawning uncertain also, although most early data refer to spring spawning and present data to autumn spawnirig. Off the north coast, present day autumn spawners appear to be using the same gravel deposits utilised by spring spawning herring in earlier ye"ars. The distribution of >SOO/m Z patches and the locations of known gravel deposits show a particularly good correspondence in this ICES area.

In the extraction of the data used in Figure I it was evident that certain stations Z consistently produced >SOO k7 mm larvae per m , each year. This phenomenon was most marked in the Buchan area where 21 instances of densities >SOD/m Z were recorded from 8 stations over the period 1972 to 1985, out of a total 73 stations in the survey grid• • REFERENCES Bowman, A. 1922." The occurrence" of "spawny" haddock and the locus and extent of herring spawning grounds. Fishery Board for Scientific lnvestigations. HMSO Edinburgh.

Dooley, H.D. and McKay, D. 1975. Herring larvae and currents west of the Orkneys ICES CM H:43. Pel. Fish Cttee.

S t.

Dooley, H.D. 1981. The role ofaxially varying vertieal mixing along the path of a eurrent in generating phytoplankton produetion. Phil. Trans. Roy. Soe. London. A302; 649-660.

EIlet, D.J. and Edwards, A. 1983. Oeeanography and Inshore Hydrography of the Inner Hebrides. Proe. Roy. Soe. Edinburgh, 83B, 143-160.

Gamble, J.C., MaeLaehlan, P., NicoIl, N.T. and Baxter, I.G. 1979. Growth and­ Feeding of Atlantic Herring Larvae Reared in Large Plastie Enc1osures. International Symp. on the Early Life History of Fish, Woods Hole, USA, lCES ELH Symp/I:1, 2-5 April.

Heath, M.R., Henderson, E., Hopkins, P., Martin, J .A. and Rankine, P.W. 1985. Hydrographie Influenees on the Distribution of Herring Larvae in the Orkney-Shetland Area. ICES CM H:29. Pel. Fish Cttee.

Heath, M.R. and MaeLaehlan, P. 1985. Growth and survival rates of yolk sae herring larvae from a spawning ground to the west of the Outer Hebrides. ICES CM H:28, Pel. Fish Cttee.

Heath, M.R. and MaeLaehlan, P.M. 1986. Studies on herring larvae off the north eoast of Seotland. lCES CM 1986 H:18. • Henderson, P.A., Whitehouse, J.W. and Cartwright, G.H. 1984. The growth and morta1ity of larval herring, Clupea harengus in the River Blaek Water Estuary 1978-1980. J. Fish Bioi., 24, P 613-622.

Holliday, F.G.T. 1958. The Spawning of the Herring. Seot. Fish. BuH., 10, 11-13.

Munk, P., Christensen, V. and Paulsen, H. 1986. Studies of a herring larval pateh in the Buehan area. Dana 6, p 11-24. Part II Growth, morta1ity and drift of larval herring.

Munk, P. and Rosenthai, H. 1983. Variability in size of herring larvae at hatehing -the influenee of egg deposition patterns and parental fish. ICES CM L:33, Pel. Fish Cttee.

Postuma, K., Saville, A. and Wood, R.J. 1975. Herring Spawning Grounds in the North Sea. lCES CM H:46. Pel. Fish Cttee. •

Wood, R.J. 1971. Autumn Spawning Grounds of Herring to the West of Seotland. Rapports et Proees-Verbaux, Vol. 160, 197.

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Table I Larval Drift Rates and Residual Current Flows

Area Larval drift rate Reference residual current flow

nautical mHes/day

Tiree Passage 5.9 EIlet and Edwards (1983) Little Minch 0.4-2.6 EIlet and Edwards (1983) North Coast of Scotland 1-4.0* Heath and MacLachlan (1986) Orkney-Shetland 5.0 Dooley and McKay (1975) Buchan 1.8-2.4* Munk et ale (1986) Uists 1.5* Heath and MacLachlan (1985)

• * Measured from patch studies

'. Table 2 Hatching Length and Growth Rates of Herring Larvae

Area Mean Growth Reference hatching rate mm/day length (mm)

Uist 6.2 0.17 Heath and MacLachlan (1985) Buchan 6.7 0.14 Munk et ale (1986) Thames EstuaryS 6.8 0.18 Henderson et ale (1983) Baltic Sea 7.3 Munk and RosenthaI (1983) Clyde EstuaryS 6.5 Rankine (unpublished data) MinchCS 6.6 0.35, 0.27 Gamble et ale (1979)

c cultivated eggs • s spring spawned herring

•

______--- 1 ,J '~ E2 E6 E1 E3 E4 ES E7 ES E9 51 • • • • • • • • • • 50 • • • • • • • • • • • 1'?>3 49 • •• • 24l. • • • 44b.. ~b_.. • • • 48 • • • lIb •• Sb1 -1000/m 2 ~ , ~ • I • 47 0 m'2 <'100 • • • 1'l3 • • Ja? • 354 Ilob • • • 111 IIb • 3'+5 ' 46 115 • • • • • • J?>~ 33~ • • • • 45 • • • • • • • • • ....· • • • • • •• • • • Itfl 154 • • 44 101 • • • - • • • - 43 • • • JJ::5 • • • • 1!oI-+ c 0 T LA ND • • • lSb • • • • • 42 • • • • • • • • • • 15~ • • • -,. • • • • • 41 • • • 40 ...... ~ \, o.;.iii •

Figure I

, r·'aximum numbers of 7 mm, and smaller Inrvne. oer m- occurring at eam.... station in iC~urvul survey? 1972-85 f , E1 E2 E3 E4 ES E6 E8 E9 51

I ,I I 50

.-----'~-----l-~--._+----_+----_t_----_+_~:-'r._--+_-a..~ ~'4..IJ--'---_+----+_ ~:;,:

/,'/'1, '.1, f"" 49 :11,~/'I1'1 '/ ,:::t;J 1------+------t------+------+------i--7~~,f<--+------t-="',"',/1'" ,:~" ':::1", ." ,,~ ~., 1f!; 48

47

46 ", ", '" '"'., " 45

44

43

42

.~ 41

40

Figure Ir

Location or known_gravel deeosits r , _'_'II:-r ~-_..,..--___:_--__:_------~ "~j E1 E2 : E3 E4 ES E6 E7 E8 E9 51

I •••• •• t •••••• •• • ••••• •• 50

1-- ---4- ~-.-.---+- 'spawnil :. spring 49 hadd ~clj . i'l;; QU tu1mn i I ! I i I stage: VI ;o(autumn) 48 'I I herr" I I

t':o- ••••• ••• ••

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Figure III

Summary of information on herring spawning groundsfrom Bowman (1922) and Postuma et al. (1975) -----.-.. -.-..~.----.--.