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ICES Marine Science Symposia ICES mar. Sei. Symp., 192: 33-38. 1991 Factors affecting the vertical distribution of eggs Svein Sundby Sundby, S. 1991. Factors affecting the vertical distribution of eggs. - ICES mar. Sei. Symp., 192: 33-38. The spatial distribution of eggs and larvae is a function of the properties of the ambient water (the density, current, and turbulent diffusion) and of the physical properties of the eggs (the buoyancy and dimension). Study of the vertical distribution is the first step towards understanding the horizontal transport of eggs and larvae. Two models are applied to demonstrate how the physical and biological conditions influence the vertical distribution of the three main categories of eggs, here defined as pelagic, bathypelagic, and bottom eggs. In particular, the physical conditions affecting the distribution of bathypelagic eggs are studied. Wind-induced turbulence, the most important ambient factor for the vertical distribution of pelagic eggs and larvae, contributes to mixing the buoyant eggs and larvae throughout the wind-mixed layer. The vertical spreading of bathypelagic eggs depends mainly on the buoyancy distribution of the eggs. The model demonstrates that non-adhesive demersal eggs will be partly mixed into the water column, this mechanism contributing to the horizontal transport of demersal eggs. Svein Sundby: Institute of Marine Research, Department of Marine Environment, P. O. Box 1870 Nordnes, N-5024 Bergen, Norway. Introduction and understanding the vertical distribution of eggs and larvae. This paper demonstrates how the physical The spatial distribution of eggs and larvae is influenced properties of eggs and the ambient physical forces influ­ by a set of biological and physical processes which ence vertical distribution. Two models for the vertical together contribute to determine the fate of the year distribution of eggs (Sundby, 1983; Westgård, 1989) are class. The importance of the distribution and transport applied to demonstrate the three main types of vertical of the early stages for the recruitment of fish stocks has distribution: pelagic, bathypelagic, and bottom dis­ been pointed out by many authors: it has been suggested tributions. The physical conditions for each type of that unfavourable drift of eggs and larvae beyond the distribution are analysed. appropriate distributional area will cause permanent loss from the population (Hjort, 1914). On the other hand it has been shown that anomalous transport from Results and discussion one region to the other may transfer recruits from one Basic equation stock to another. This has been described by Hansen and Buch (1986), who found that export of cod larvae The vertical processes which influence the distribution from the Iceland to the Greenland area increased the of eggs are studied and it is assumed that all horizontal Greenland cod stock. The importance of larval retention gradients are zero. The basic equation is then reduced within specific geographical regions during critical to the vertical component of the diffusion equation: periods has been outlined by lies and Sinclair (1982). Considering the large vertical variations of the hori­ zontal flow field, it is evident that the vertical dis­ 8C(z, t) 3[w(z, t)C(z, t)] d rir, dC(z, t)‘ ------------------------------------- = — K(z, t) ----------- tribution of eggs and larvae is important for their at dz dz L dz . horizontal transport and spreading and their survival. + S(z, t) + M(z, t) (1) Cushing (1982) gave an example of how the vertical distribution itself directly influenced recruitment: the rising halocline in the Baltic caused cod eggs and larvae where C(z, t) is the concentration of eggs in numbers to rise closer to the productive layers where their food per unit volume; w(z, t) is the vertical velocity of the is produced. Consequently, an important step towards eggs; K(z, t) is the vertical eddy diffusivity coefficient; understanding the recruitment processes is describing S(z, t) is the spawning production of eggs; M(z, t) is the 33 egg mortality; z is the vertical component, positive Egg buoyancy towards increasing depth; t is time. To solve the equation, the vertical velocity of the General knowledge concerning the buoyancy of fish eggs, w(z, t), and the vertical eddy diffusivity coef­ eggs has increased substantially during recent years, ficient, K(z, t), must be known. The vertical velocity is in particular since the density gradient column was expressed as w = f(d, Ap, v), where d is the diameter introduced by Coombs (1981). This instrument enables of the egg, Ap is the difference in density (buoyancy) us to measure the specific gravity of individual eggs with between the egg, pe, and the ambient water, pw, and v high accuracy and resolution. The physiological causes is the molecular viscosity of the water. Hence, the two of buoyancy in marine fish eggs have been studied by physical properties of the eggs, the buoyancy and the Craik and Harvey (1987). diameter, are key variables for modelling the vertical Since pelagic eggs have a specific density which is distribution. lower than that of the upper mixed layer of the sea, In the following sections it will be shown that changes they tend to rise towards the surface. Only a small in the buoyancy, Ap = pw - pe, result in the most pro­ fraction of such eggs, however, are found at the very nounced changes to the solutions of the basic equation surface, because the turbulent forces of the mixed layer (1). For this reason it is useful to classify eggs into three counteract the buoyant forces of the eggs. The eggs will main groups (see Fig. 1): A. Ap > 0, B. Ap = 0, C. concentrate to varying degrees towards the surface, Ap < 0. In this paper, group A will be defined as depending on the magnitude of the counteracting forces. “pelagic” eggs, group B as “bathypelagic”, and group Examples of eggs with such distributions are North Sea C as “bottom” eggs. Since there are few descriptions of plaice eggs (Pommeranz, 1973), North Sea mackerel the vertical distribution of eggs in relation to their eggs (Iversen, 1973; Coombs, 1981), and Northeast buoyancy, much of the literature is inconsistent in this Arctic cod eggs (Solemdal and Sundby, 1981). The question. It seems that eggs are often described as vertical distributions of these species have been mod­ “pelagic” if they are found in the water column and elled by Sundby (1983). “demersal” if they are found on the bottom. However, Bathypelagic eggs have a higher specific density than the solution to Equation (1) will show that all three the upper mixed layer of the sea, but lower than the groups defined above are found in the water column. density of the bottom layer. They are therefore dis­ Even demersal eggs, given they are non-adhesive, will tributed at mid-depths and quite frequently in the be mixed into the water column. In addition to the three pycnocline. Examples of species with this type of egg main types of eggs in Figure 1, there are also mixed distribution are Pacific halibut (Hippoglossus steno- types. Species with a buoyancy distribution between lepsis) (Thompson and van Cleve, 1936), the Baltic cod pelagic and bathypelagic have been reported by Coombs (Kändler, 1949), and the Atlantic halibut (Hippoglossus etal. (1985). hippoglossus L.) found in Norwegian fjords and coastal waters (Haug et al., 1984, 1986). Kendall and Kim (1989) have demonstrated how bathypelagic eggs of the f walleye pollock ( Theragra chalcogramma) substantially 0,5 changed their vertical distribution mainly due to changes in buoyancy during their development. 0.3 Bottom eggs have a specific density which is higher 0,1 than that of the bottom layer. In principle, demersal 30 31 32 33 36 37 38 3935 eggs are one kind of bottom eggs. However, it is essen­ I I i tial to distinguish between adhesive and non-adhesive eggs. With respect to the nature of the vertical forces acting on the eggs, there is no difference between non­ adhesive demersal eggs and pelagic eggs. Non-adhesive demersal eggs, unless buried in the seabed, will be mixed to varying degrees into the water column dep­ ending on the magnitude of the bottom turbulent mixing. One species with non-adhesive demersal eggs is the saffron cod (Eleginus gracilis) in the northeast Pacific Ocean (Dunn and Matarese, 1986). Even adhesive eggs may separate from each other and from 100 the seabed and be mixed into the bottom layers and transported away. This has been reported for Barents Figure 1. Buoyancy distribution of three categories of eggs Sea capelin eggs (Bakke and Bjørke, 1973). (upper part of the figure) in relation to the salinity profile (lower part of the figure). A: pelagic eggs, B: bathypelagic The density distribution of an egg population can eggs, C: bottom eggs. often be described by a Guassian distribution function. 34 Eggs from the Northeast Arctic cod are neutrally buoy­ distribution of air bubbles in the sea. Although their ant at an average salinity of 31.0 and standard deviation results differ to some extent, it may be concluded that of 0.55 (Solemdal and Sundby, 1981). Eggs from the the eddy diffusivity coefficient ranges from about Atlantic halibut are neutrally buoyant at an average 10 cm2 s“1 at wind speeds near zero to about 103 cm2 s-1 salinity of 34.2 with a standard deviation of 0.52 (Haug at wind speeds of approximately 20 ms-1. et al., 1986). The buoyancy of sprat eggs off the southern In the pycnocline the eddy diffusivity coefficient is coast of Great Britain ranged over about eight salinity inversely related to stratification and directly dependent units during the end of the egg stage (Coombs et al., on energy input. However, the functional relationship 1985).
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