THE DISTRIBUTION AND ABUNDANCE OF HERRING LARVAE IN THE NORTHERN , CHANGES IN RECENT YEARS

By

A. S a v il l e Marine Laboratory, Aberdeen,

ln 1951 the Marine Laboratory, Aberdeen, began a water column through which the sampler fished. For regular programme to investigate the distribution and the earlier material, taken with a 1 metre silk net, the abundance of autumn spawned herring larvae in the sample numbers were also converted to this basis using north-western North Sea. It was decided to sample the the conversion factors from 1 metre to Gulf III calcu­ area from 56°00' N to 59°45' N and from the Scottish lated from the large number of paired hauls made coast to the prime meridian as this would cover all of with the two gears in 1957. the known major spawning areas within Scottish The numbers of larvae in the sampled area on each waters (W o od 1930, C la rk 1933). This programme cruise were calculated by contouring at levels of 1, 5, has been carried out every year from 1951 to 1967 10, 50, 250 and 1 000 larvae per square metre, meas­ with the exception of 1956. Due to interruptions by uring the area in square metres within each contour, bad weather and by ships defects the intensity of raising these areas by the appropriate contour level and sampling and the proportion of the total area sampled summing the values so obtained. has varied somewhat from year to year. Part of the In the area under consideration hatching of herring material has already been presented in a paper to the larvae begins about the middle of August and continues Pelagic Fish (Northern) Committee of ICES. (Saville, to the middle of October. In those years when several 1966). Since then additional material has been col­ cruises were completed during this period the curve of lected and analysed which throws some new light on larval abundance against time strongly suggests that the matter and the subject is of sufficient importance the preponderance of hatching takes place in Septem­ to the objectives of this symposium for its re-presen­ ber. In all years the highest abundance of larvae in the tation to demand little in the way of apologies. 10-15 mm size range has also been found in Septem­ ber. As this is the month in which the area was best sampled, in this paper consideration of the abundance METHODS and distribution of larvae is confined to the cruises From 1951 until 1957 sampling was done by timed which took place in that month. oblique hauls from bottom to surface with a 1 metre The distributions of larvae in the smallest size cate­ net of bolting silk with 60 threads per inch. In 1957 gory suggest that the area sampled can be divided the modified Gulf III sampler (B r i d g e r , 1958), now into two clearly demarcated areas of larval production, widely accepted as the standard sampling instrument the Buchan area to the south of 57°40'N, and em­ for herring larvae by European workers, was intro­ bracing the well known herring spawning grounds of duced. In that year, so as to give a basis for com­ Turbot Bank, and Aberdeen Bank; and what will be parison with earlier material, at each station on all termed the area to the north of 57°40' N cruises a sample was taken with both the Gulf III and including the Clythness spawning ground and the and the 1 metre oblique net. spawning grounds around . In the 10-15 mm Larvae caught on these surveys were grouped into size group the two distributions are not so clearly three size intervals less than 10 mm, 10-15 mm, and separated, and in larger larvae than this they merge over 15 mm. Only the first two of these size groups completely, but for the 10-15 mm sizes they are still will be considered here as providing information on sufficiently distinct for it to be convenient to retain variations in the size of the spawning stock and its this subdivision. distribution at spawning. For Gulf III hauls the num­ Material is available from sampling in September bers in a size group were converted to the number for all years since 1951, except for 1956 in the Buchan below a square metre of surface from the volume of area, and for all years except 1956 and 1957 in the water filtered at that station, as measured by a Currie- Shetland area. In most years, at least two surveys of Foxton or T.S.K. flowmeter, and the depth of the the area were done in this month. 88 A. Saville

1000/m2 250/m2

Figure 35. Distribution of larvae less than 10 mm in September Figure 36. Distribution of larvae less than 10 mm in September 1951-54. 1955-1960.

DISTRIBUTION OF LARVAE

Unfortunately it is not possible, in the space avail­ 60° able, to provide charts of the distribution of the larvae on all the cruises over the years 1951-1966. It has been decided to illustrate only the distributions of the smallest size group as providing the best index of the centres of, and changes in, the spawning sites. The 10-15 mm size category in any case, although showing 59* greater dispersion, normally have a very similar distri­ bution of the areas of greatest density to those of the larvae less than 10 mm. In Figures 35—37 are shown the generalised distributions of larvae less than 10 mm b 0 in three time periods, 1951-54, 1955-60, and 1961-66. 58° These were chosen as being periods within which the distributions did not change appreciably but between which there were quite marked changes. The contour levels of 1 000 and 250 larvae per square metre only 1000/m 2 are illustrated as giving the best index of major spawning in these years. 57* 250/m2 In Figure 35 are shown the areas within which more than 1 000 and more than 250 recently hatched larvae were taken below 1 square metre of surface in 1951- 54. It will be seen that at this time larvae were not very plentiful in the northern area. No concentrations 56" of larvae exceeding 1 000 per square metre were de­ tected around Orkney but there was a band of larvae at 250 per square metre to the north and east of the Figure 37. Distribution of larvae less than 10 mm in September islands. In the Buchan area a small patch of larvae at 1961-1966. Herring larvae in the northern North Sea 89

concentrations of 1 000 per square metre was located However, in the period 1957-63 the area was well close to the Aberdeenshire coast while a considerably sampled and the abundances of this size group in the larger patch of the same density was found over Turbot Buchan area were fairly constant at a level appreciably Bank and Aberdeen Bank. In the period 1955-60 (Fig. lower than in 1951-54. In 1964, as mentioned above, 36) larvae of this size group were more abundant there was a build up in abundance in the Buchan area in the Orkney area with a concentration of larvae to around the level found in the earlier period. In exceeding 1 000 per square metre of the east of the 1965, 1966 and 1967 there was a further sharp pro­ islands, enclosed by a large area containing more than gressive fall in this size category in the Buchan area to 250 larvae per square metre. In the Buchan area during an extremely low level. this period the patch of high density of larvae close to In the Shetland area (Fig. 39) the abundance of the the Aberdeenshire coast was not in evidence and only size group less than 10 mm has fluctuated more widely two small areas of larvae of more than 1,000 per square from year to year but in seems clear that in this area metre were found on Aberdeen Bank and to the east they were considerably less numerous on average in the of Montrose Bank. years up to and including 1955 than they have been In the period 1961-66, as shown in Figure 37, in the years sampled since then. this situation has developed still further with a marked When considering the size of the spawning stock in increase in the abundance of larvae of this size group the north-western North Sea the total production of around Orkney and off Clythness. In the Buchan area larvae within the area is of greater relevance. In during this period the distribution of larvae of this Figure 40 is shown a measure of this obtained by size category has been much more variable. From 1951 summing the mean September abundances for the to 1963 their distributional pattern has been very close Buchan and Shetland areas. Although there are major to that shown in Figure 37 with no concentrations variations from year to year there is little evidence exceeding 1 000 per square metre in the Turbot and here of any long-term trend of declining larval pro­ Aberdeen Bank area and the area enclosed by the duction over the area as a whole such as has been 250 per square metre contour diminishing still further. In these years, however, there has been evidence of 22 -, high concentrations of larvae in the area of Berwick

East Bank which was not apparent in the earlier 2 0 - periods. In 1964 larvae of this size group were con­ siderably more abundant in the Buchan area and their distribution reverted to that shown in Figure 36. In 1965 and 1966 on the other hand larvae became still 17 - scarcer in the Buchan area and no concentration of 16 _

larvae of even 250 per square metre was found there 15 - at all. The results of the international herring larval 14 - survey in 1967 (Sa v il l e , 1968) show that in that year there was no concentration of larvae less than 10 mm at all in the Buchan area.

LARVAL ABUNDANCE The abundance of larvae on each cruise made in September in the Buchan and Shetland areas has been computed separately as described on page 87. In Figure 7 _ 38 the mean abundances of larvae less than 10 mm for the September cruises in each year are shown for the Buchan area. It will be seen that larvae of this size group were plentiful up to 1955 when the apparent abundance fell very sharply. However, the 1955 value is probably seriously biased downwards as no sampling was done in that year south of 57° N. On the rather scanty evidence available of larval abundance in the

Buchan area in 1955 it seems probable that it was in 1951 '53 '5 9 ’63 fact lower than in the preceding four years but almost Figure 38. Abundance of larvae less than 10 mm in September certainly not as low as would appear from Figure 38. in the Buchan area. Ordinate: No. of larvae divided by 10u. 90 A. Saville

54 1 50 - 52 - 48 - 50 46 - 48 44 - 46

42 - 44 -

4 0 - 42 - J 38 40 -

36 - 38

34 - 36

32 34

30 32 - 30 28 - 28 26 - 26 24 - 24 22 22 20 - 20 18 18 16 - 16 - 14 - 14 12 12 - IO - 10 -

8 - 8

6 - 6

4 4

2 2 -

fes 1951 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 Figure 39. Abundance of larvae less than 10 mm in September Figure 40. Abundance of larvae less than 10 mm in September in in the Shetland area. the north-western North Sea. Ordinate: No. of larvae divided bv 10u . shown by G u s h in g (1964) for the Southern Bight stock. However, both the 1966 and 1967 figures are lower abundances of this size group in the September cruises than those for any of the preceding years. This may are shown in Figure 41. This largely supports the not be of any significance but would suggest that the conclusions drawn from the smallest size group that in situation should be carefully watched over the next recent years there has been a sharp reduction in the few years in view of the greatly increased exploitation abundance of larvae in the Buchan area. The main to which this spawning group has been subjected since differences between Figure 38 and Figure 41 are 1965. that in the 10-15 mm category the reduction in larval Because of the short time that larvae less than 10 mm abundance in this area would appear to have originated remain in the plankton, and the limited spatial extent in 1954. The 1955 figure is again probably seriously of high density concentrations of larvae of this size, biased downwards by inadequate sampling. However, their estimation may be subject to major errors due it is clear that with this possible exception and with to small variations in the timing and positioning of the exception of 1963 this size category was very much sampling. The 10-15 mm category although probably less abundant in the Buchan area in the period 1954- giving a poor indication of the major centres of spawn­ 67 inclusive than in the period 1951-53. Figure 41 ing should be less prone to chance variations of this also confirms the further drop in larval abundance in sort in apparent abundance and it would seem worth­ this area in 1966 and 1967. In the 10-15 mm size while looking at the abundances of this size group to group in the Shetland area, also shown in Figure 41, see how far the conclusions reached from the smaller it would again appear that herring larvae have been size category are supported by these data. The mean much more abundant in this area from 1961-65 than Herring larvae in the northern North Sea 91

69 88-7 Aberdeenshire coast from Kinnaird Head to Collieston, 66 and on the offshore banks of Turbot Bank, Aberdeen Shetland 63 Bank, Montrose Bank and Berwick East Bank. It Buchan would appear, however, that there has been a major 6 0 change in the importance of these different spawning 57 localities during the late 1950’s. In the early years of 54 these investigations the heaviest spawning took place off the Aberdeenshire coast and on Turbot and Aber­

48 deen Banks whilst spawning in the more northerly regions was at a fairly low level. Since about 1955 45 there has been a gradual reduction in spawning in the more southerly areas. The patch of recently-hatched larvae regularly found in September close to the Aber­ 36 _4 deenshire coast and referred to by both W ood (1930)

33 and C la r k (1933) has not been observed in recent years, and the abundance of larvae on Aberdeen and 3 0 _i Turbot Banks which was a consistent feature of Scottish 27 sampling up to the mid 1950’s has decreased sharply thereafter until in 1966 and 1967 it was completely absent. This reduction in larval production in the Buchan area has, however, been compensated to some extent by an increase in the abundance of recently hatched larvae around Orkney and off Clythness. There is some evidence also of greater spawning in the 1960’s around Berwick East Bank than in the early 1950’s although this area is mentioned as a spawning site also by W ood (1930), and C l a r k (1933) so that spawning there is probably not a new phenomenon. The results of the international herring larval survey Figure 41. Abundance of larvae of 10-15 mm in September in 1967 (Sa v il l e , 1968) suggest that although spawn­ in the Buchan and Shetland areas. Ordinate: No. of larvae ing around Shetland was not on a large scale in that divided by 1011. year appreciable spawning did take place in that area. C la r k (1933) took no recently hatched autumn spawned larvae north of and some exploratory in earlier years. Figure 41 again shows a sharp fall sampling to the east of Shetland in the early 1950’s to low abundances of this size group in the Shetland also failed to produce recently hatched larvae. In the area in 1966 and 1967. The summed totals for the two past few years occasional catches of maizy herring areas as an index of larval production on the north­ have been reported from grounds off the east coast of western North Sea are also shown in Figure 41. Shetland from to Sumburgh Head but prior to Here again there is no evidence of a declining trend this ripe fish were of rare occurrence in catches taken in larval production which would suggest a reduction north of Fair Isle. All the evidence would suggest that in abundance of the mature stock of Buchan spawners herring spawning, on any appreciable scale, in the area at least up to 1965. Very low levels are again shown north of Fair Isle has been a feature only of the last for the area as a whole in 1966 and 1967. four or five years. It would therefore appear that the “Buchan” stock have been gradually deserting the DISCUSSION more southerly of their spawning grounds which were In general the spawning areas for autumn-spawning previously the ones most extensively used and have herring found in the present investigation correspond been spawning further north off Clythness and off the closely to those described by W ood (1930) and C la r k coasts of Orkney and perhaps in the last few years (1933) for the years between the wars. The main centres have used grounds formerly unused or used only to a of spawning in the present data in the northern area very minor extent off the east and west coasts of Shet­ are to the west of the Pentland Firth, to the north and land. A question which must be left open at this east of Orkney extending from Fair Isle close to the juncture, but which will be examined carefully in east coast of Orkney, and off the mainland coast to future, is whether the fish spawning on these widely just south of Clythness, in the Buchan area close to the ranging spawning grounds from Shetland to Berwick 92 A. Saville

Number of Larvae Number of Larvae

1 Catch I Catch

Figure 42. Abundance of larvae up to 15 mm and catch of Figure 43. Abundance of larvae up to 15 mm and catch of stages VI and VII per 100 net shot in the Buchan area. Left- stages VI and VII per 100 net shot in the Shetland area. Left- hand ordinate: No. of larvae divided by 1011. Right-hand ordi­ hand ordinate: No. of larvae divided by 10n . Right-hand ordi­ nate: Catch of stages V I and VII/100 nets. nate: Catch of stages VI and VII/100 nets.

Bank are in fact all members of a single spawning close to the coasts where the tidal currents are strong stock, as has been generally assumed in the past. If and shooting drift-nets is fraught with difficulties and this is not true it could well be that what we have been (2) there is a tendency for ripe herring to stay deeper observing is not a northwards shift in the spawning in the water even in darkness thus reducing their grounds of a single stock but rather a change in the chance of capture by surface drift-nets. Because of relative abundances of different spawning stocks with these factors ripe herring never form a large proportion the more northerly ones becoming dominant in recent of the Scottish drift-net catch even during the spawn­ years. ing season and because of lack of outlets for fish in The evidence of a shift in spawning grounds from this condition the fishery normally stops as soon as larval distributions is borne out by a similar trend in spents form an appreciable proportion of the catches. the distribution of fishing and of high catches by the In spite of these factors, which should be constant Scottish drifter fleet. Particularly in August when the from year to year, a shift in spawning grounds of the fishery is predominantly on full and ripening fish the extent suggested by the larval data should be detectable previously prolific grounds off the Aberdeenshire coast in the catch data. This is shown in Figures 42 and have given poor catches and the most heavily fished 43 which show for the Buchan and Shetland areas and most productive grounds have been further north respectively the abundances of larvae up to 15 mm -between Clythness and Shetland. As pointed out by long as the best index of larval production in the W o od (1930), in the Scottish drift-net fishery the areas, and the catch of Stage VI and VII fish per 100 average catches during August and September are a net drift-net shot in each area as an index of spawning poor index of the density of the spawning stock. This stock size. It is evident that there is no clear relation­ can probably be accounted for by two factors: (1) the ship in yearly variations between the two factors. spawning grounds lie within the 50 fathom contour However, it is equally clear that in the Shetland area Herring larvae in the northern North Sea 93

there has been a marked increase both in larval pro­ in Scottish waters”. Rapp. P.-v. Réun. Cons. perm. int. Explor. duction and in spawning stock size after 1957 whilst Mer, 85: App. 3, 11—68. C u s h in g , D. H., 1964. “The decline of recruitment to the Downs in the Buchan area both indices have decreased very herring stock”. ICES C M 1964 Herring Cttee, Pap. No. 100 appreciably from about the same time. (mimeo). Sa v il l e , A., 1966. “The distribution and abundance of herring larvae in the northern North Sea”. ICES CM 1966, Herring Cttee, Pap. No. 31 (mimeo). S a v il l e , A., 1968. “A report on the international herring larval REFERENCES surveys in the North Sea in 1967” . ICES CM 1968, H : 20 B r id g e r , J. P., 1958. “On efficiency tests made with a modified (mimeo). 10 pp. Gulf III high-speed tow-net”. J. Cons. perm. int. Explor. Mer, W o o d , H., 1930. “Scottish herring shoals: pre-spawning and 23 (3) : 357-65. spawning movements”. Scient. Invest. Fishery Bd Scotl., C l a r k , R. S., 1933. “Herring larvae. The mixing of the broods 1930 (I) 71 pp.