XIX. Fluctuations in the Stock of Young Sprat off the West Coast of Norway and its Relation to the Sprat Population as a whole. By Paul Bjerkan. The sprat fishing districts of Norway are divided into two areas, a smaller eastern one, the Oslofiord and environs, and a larger western one, from the Naze to Statland and even further north to Trondhjemsfiord. The sprat is found in the intervening districts too, but the fishery is insignificant owing to the configuration of the coast and the lack of fiord systems. The most lucrative fishing districts are the fiords of Ryfylke and Sunnhordland, which latter might be said to be the centre of the sprat fishing districts of the west coast. North of Statland mostly mixed schools, sprats and young herrings, are found, and this fact I must ask you to bear in mind. Some years, however, very good catches have been made as far north as Molde. In fig. 1 the distribution of the yield of the sprat fishery along the coast is shown as an average of 20 years (1908-—27). The yield of the sprat fishery displays large fluctuations as shown in fig. 2. Some of the fluctuations shown are, however, caused by technical improvements, such as the use of the purse seine since about 1908 and new activities as the increase in the fishery of the eastern area since 192S. The decrease in the yield, even in periods of extended fishing activities, however, shows that natural conditions, and then most surely fluctuations in the stock are at play too. In this connection I may especially point out the bad fishing seasons 1919 and 1928 and the decrease in the yield from 1912. The sprat fishery off the west coast of Norway is most decidedly a fishery of a young fish stock. This has been shown by S u n d and K o e f o e d (1908—09), and the lact is accentuated by the recent investigations, in 1920 and onward, which have shown that the one-year-old sprat, the sprat in its second summer, provides the Norwegian sprat fishery with more than 95 °/o of the yield in ordinary years. This fact shows that a renewal of almost the whole stock takes place every year, and as there is mostly very little spawning observed along the west coast of Norway, — 174 — Trondhjem » .Âlestmd OO OOO OO O oooo oooo q OOOO OOOOO OOOOOO OOOOOOOOOOOOO o o OOOOOO QO .Stavanger , OOOO V 'o oooo oooo o o o c ■Kristiansai Fig. 1. Sprat Fisheries of Norway. Q = 1000 hi. catch, average of 20 years 1908—27. Note: The average yield, 6000 hi., off Aalesund (More) is too high which is caused mostly by extra­ ordinary high figures for 1915. According to later interrogations those figures are erroneous and the quantity ought to be reduced with 1/s-—*/s to 3—4000 hi., which is more in accord­ ance with the yield of recent years. — 175 this renewal must largely be dependent on immigration from the principal spawning districts, the Skager Rack and the North Sea coasts. This gives opportunity to very marked fluctuations. In order to understand these fluctuations, it is necessary to know how the renewal of the stock is effected. Some variations in the average size of the sprat from the different districts and in sprats from pure schools and from schools mixed with herrings give some clues. In schools mixed with young herrings the sprats are larger than the sprats of 191 r - 15 1916-20 i 1923-25 >l<)2f>-28 250 250 200 200 150 150 + + + 4- -I + +■ I+ + + 4 + 100 100 50 1 1901 1910; wo: Fig. 2. Sprat Fisheries of Norway. Catch in 1000 111. + + + Average of 28 years (1901—28). Average in 5 years’ periods. the same year-class from pure schools. North of Statland where such mixed schools are the rule as well as in pure schools from Northern fiords, the sprats are larger too. By calculations from the scale readings this is shown to be largely due to variations in the growth during the first summer. The fiord stock of young sprats is renewed by immigrations during the autumn. Mostly in the first part of September the young fish push into the outer fiord systems and later on further into the deeper fiords. By direct measurement it has been shown that the young fish in the schools are then very variable as to average size and to a great extent that the schools with more developed young fish are found farther north along the coast. This 1 have interpreted as showing that the first hatched and more developed young fish have had time to be carried farther away from the spawning places. That sprats of different age, in weeks and months, are found we know. The sprat has been found spawning between February and September. In this connection I may refer to the investigations of H e r r ic k (1923), who found that the spawning of one sprat might cover 21/2 month, the sprat spawning in batches with intervals of 8—9 days. — 176 — Off the Norwegian coast fry of 15 mm.s length have been found in the middle of March off the south coast, while fry of 25 mm.s length have been found off the west coast in the first part of September. This gives a wide scope as to variation in age of the same year- class and accordingly in size. Fig. 3 shows a young sprat of 3.5 cm. length and another of 8.5 cm. length from the month of September taken from the same school, the latter with a reproduction of the scale to show that it is a young fish. The one is a baby fish, the other a good-sized sprat. I may remark that in other years I have found young sprat as large as 9.5 cm. caught during the same month, September. The larger young sprats are thus fully mar­ ketable fish in the first autumn at least during some years. Fig. 3. Generally speaking I have found that a very variable stock of young sprats are dispersed along the coast according to size and development at the time they push into the fiords. The oldest and largest young sprats are mostly found in the mixed schools. They arrive in the western coast waters at a time when the young herrings are of about the same size order. They mix and form homogeneous schools, which are to a large extent carried northward with the prevailing current. Later arrivals mostly form the pure sprat schools, with larger, more even young fish in the northern and varied and mostly smaller sprats in the southern parts of the West Norwegian sprat districts. During the last sprat season a very good material of young sprat was procured. From the month of September, 1928, to January, 1929, in all 29 samples with about 10,000 specimens. Fig. 4 gives the result as to relative length of the sprat from pure schools and from mixed schools from each month for this collection. As may be seen the sprats from the mixed schools are on an average larger and more developed than those from the pure schools, this is most marked for the month of December with sprats of both categories. The growth is continued up to the month of November. From the month of November and onward no growth is found in any of the categories. The length-weight relation of sprats from mixed schools and pure schools from — 177 — the months of November to January taken for each length group, also confirms the idea that the young sprats from the mixed schools are more developed and, as I take it, older than those from the pure schools as shown in fig. 5. As stated above the autumn immigration constitutes the fiord stock of young sprat. Most years there is a more or less pronounced spring immigration too. When such an immigration occurs there are mostly runs and rises in the fiord stock as well and good fishing results. The spring immigrants mostly mix with the fiords stock later on and *#56739 cm. % S prat 9o Youngfish (year c/ass 1928) Sept 192B- J a n . 192 9 50 -------- from pure schools 20 \ -------- — ~ m ixed ------- °k w JO 10 l\ \ Vi September, 22 / V South coast 1 VJi W 20 7* sp 4.79 cm. 2 0 0 li i\\\\ O c to b e r 1 1 -1 5 / I' 1 lU li M est c o a s t °A / io so -/ it 11 'A. 5SS 5P 6 ** cm 11/ \\ (i November, 1 0 - 2 8 If \\ 1 i W est c o a s t o 20 Jf l\ \ \ isim p . 7.2Ï cm. i December, /o -29 \ 1 w e st c o a s t 2 050 sp 6S7 cm 10 A V N . /592 » 7.2* - I January 3-/0 V Wes!1 co a st \ !0t3 sp. 6.9* cm. * 5 6 7 S 9 cm Fig. 4. this often brings confusion in the size groups and more variation in the whole stock. Generally the spring immigrants are smaller, but the contrary might sometimes be found. That a fishery depending on such immigrations as sketched above might be sub­ ject to fluctuations is easily understood. The fluctuations affect the amount of stock and as might be understood by the facts stated above also the size and development of the sprat.