| Summary: | The horse-mussel, Modiola modiolus (L.) is a bivalved mollusk dosely related to the sea mussel, Mytilus edulis L. and common in northern waters. In Norway there are considerable beds of this mussel along the western and northern coasts, from just below the littoral region to about 80 meters of depth. The horse-mussel plays an important part as bait in the set-line fisheries, especially in the Lofoten "skrei"- fisheries. As some of the mussel beds have been exhausted, and others are in danger of being depleted, investigations have been initiated by the Fisheries Directorate of Norway in order to study the biology of the horse-mussel, especially the growth and propagation of the mussel and the renewal of the beds. In some places the horse-mussel together with other organisms form a special community, the Modiola modiolus community, which is described more closely. The food of the horse-mussel consists of plankton, mainly small phytoplankton forms and protozoa, and detritus, the latter playing an important part. The chief enemies of the horse-mussel are the starfishes Asterias rubens and A. glacialis. Other enemies are some sea-urchins, the whelk, dog whelk, the crab Cancer pagurus, the cat fish and the eider duck. Seaweed (Laminaria digitata) sometimes smother the mussels and heavy sea may sweep them up on the shore. In individuals from shallow water the siphones and mantle edges are sometimes coloured green by a small, globular flagellate of a species not yet determined. The sexes are separate, hermaphrodites occurring in 2-8 ‰ of the individuals. The proportion of males to females is usually 1 : 1, but on some occasions more males than females are found among the smaller individuals and more females among the larger ones. The horse-mussel reaches maturity from the 3. to the 8. year of life, the majority at an age of 5 to 6 years. The genital tissue has the same arrangement as in the sea mussel except the mantle lobes, which are thin and membranlike, similar to those of young sea mussels. The genital tissue of male and female is described. A female mussel of 110 mm length will yield about 20 millions of eggs. Spawning usually takes place in March-April and may be accomplished within one day. All mussels in a bed will usually spawn at the same time. Most often there will be an interval of two or more years between each spawning. Larval development is supposed to be accomplished within a month. The spat fall is small compared with that of the sea mussel. The growth has been investigated both experimentally and on the base of the growth rings. These have proved to be annual rings except in a few cases, when secondary rings are formed. These are, however, easily distinguished from the annual rings. Growth is comparatively rapid from July to October-November, when it slows down, and almost entirely ceases between December and April, in which period the winter rings are formed. In May-June the growth is resumed. The most rapid growth is usually shown by the smaller individuals up to 55 millimeters of length (about 8 years of age) Which have an average annual growth of 6-10 mm. The maximum growth found experimentally was 15 mm. After the 10. year growth slackens, and in individuals 14 years old or more the growth scarcely exceeds 2-4 mm a year. In the localities examined individuals 10 years old on an average measured 65-79 mm, 18 years old 98-118 mm. The best growth was recorded from mussels living at a depth of 6-15 meters in fjords and sounds with strong water movement and frequent renewal of the water layers. The horse-mussels commonly used as bait measure from 80 to 140 mm of length and are from 10 to 20 years old. The normal length of life is estimated to be about 23 years, but individuals have been found up to an age of 36 years. The age composition of the stocks of mussels is very varying in different localities, the percentage of younger specimens sometimes being very small. The proportions of the horse-mussel are discussed. Between length and height there is the following relation: H = L/(a+bL) where a and b are constants. Between breadth and length the relation is B = cL, c being a constant. The outer volume can be calculated from the formula V = L.B.H.d or V = L³cd/(a+bL) or approximately V = L³k. For values of the constants a, b, c, d, and k see page 43 and 48. Mussels of good quality yield 1/6-1/10 of the total volume as salted meat. Analysis of weight and chemical composition have been undertaken. Different tools for collecting horse-mussels are described. In preparation for bait the meat is removed from the shells and salted in casks of 30 liters. Commonly 600-800 nmssels will yield one cask. The horse-mussels are used as bait in the set-line fisheries mainly in the Lofoten area ("skrei"-fisheries) but also in other coastal set-line fisheries. The method of preserving the meat with salt was first employed commercially in 1883. Commercial fishing on a bigger scale for horse-mussels at first took place in the fjords and streams near Bergen on the west coast of Norway (1883) but soon spread to other localities. The counties which yield most part of the horse-mussels are Hordaland on the west coast and Trøndelag and Nordland in the northern part of Norway. The total catch in the years 1914-1943 varied from 678 to 5 876 hundreds of liters (salted mussels), the economic value of the catches from 73 000 to 440 000 kroner. The protection and possible cultivation of the horse-mussel beds are discussed. No laws of protection have as yet been enacted. Cultivation of the horse-mussel will probably be difficult on account of the tardy growth and small power of regeneration.
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