Of Herring Larvae in the Northern North Sea, Changes in Recent Years

THE DISTRIBUTION AND ABUNDANCE OF HERRING LARVAE IN THE NORTHERN NORTH SEA, CHANGES IN RECENT YEARS By A. S a v il l e Marine Laboratory, Aberdeen, Scotland 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 Shetland 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 Orkney. 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.

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