Captive Atlantic cod (Gadus morhua) were fed at four ration levels (starvation, maintenance, moderate, and excess) for periods of 6–9 mo prior to and during spawning. At the end of the experiment, water, protein, glycogen, and fat contents of the liver, white muscle, and ovary were estimated. The depletion of the white muscle and liver was related to oocyte maturation and in particular to the proportion of eggs spawned (PES). The influx of protein into the ovary, the mean hydrated egg diameter, and the egg dry weight all reached maximum values at 10% PES and subsequently declined. Mean vitellogenic oocyte diameter was small prior to spawning but rose to a maximum at PES = 10%. Cod with high condition factors produced more previtellogenic oocytes and used a larger fraction during vitellogenesis. Actual fecundity of specimens deprived of food during the spawning period was between 20 and 80% of the potential fecundity depending on the nutritional status of the fish. Intensity of atresia in moderate ration fish increased from 0% at PES = 0% to 33% at PES = 80%. Estimates of durations of α-atretic vitellogenic oocytes varied between 10 and 13 d at 8 °C assuming a spawning period of 50 d.

This study indicates that the development of the vertebrae in the Atlantic salmon requires the orchestration of two sources of metameric patterning, derived from the notochord and the somite rows, respectively. Before segmentation of the salmon notochord, chordoblasts exhibit a well-defined cell axis that is uniformly aligned with the cranio-caudal axis. The morphology of these cells is characterised by a foot-like basal projection that rests on the notochordal sheath. Notochordal segments are initially formed within the chordoblast layer by metameric change in the axial orientation of groups of chordoblasts. This process results in the formation of circular bands of chordoblasts, with feet perpendicular to the cranio-caudal axis, the original chordoblast orientation. Each vertebra is defined by two such chordoblast bands, at the cranial and caudal borders, respectively. Formation of the chordoblast segments closely precedes formation of the chordacentra, which form as calcified rings within the adjacent notochordal sheath. Sclerotomal osteoblasts then differentiate on the surface of the chordacentra, using them as foundations for further vertebral growth. Thus, the morphogenesis of the rudiments of the vertebral bodies is initiated by a generation of segments within the chordoblast layer. This dual segmentation model for salmon, in which the segmental patterns of the neural and haemal arches are somite-derived, while the vertebral segments seem to be notochord-derived, contrasts with current models for avians and mammals.

The teleost backbone consists of amphicoelous vertebrae and intervertebral ligaments, both of which include notochord-derived structures. On the basis of a sequential developmental study of the vertebral column of Atlantic salmon ( Salmo salar L.) from the egg stage up to early fry stage (300-2500 day-degrees) we show that the vertebral body consists of four layers or compartments, two of which are formed through mineralization of preformed collagenous tissue (the notochordal sheath and the intervertebral ligament) and two of which are formed through ossification. The three inner layers have ordered lamellar collagen matrixes, which alternate perpendicularly from layer to layer, whereas the outer layer consists of cancellous bone with a woven matrix. The bone layers also differ in osteocyte content. In this study we describe the structural details of the layers, and their modes of formation.The results are compared with previous descriptions, and possible phylogenetic implications are discussed.

Cod larvae, Gadus morhua L., were reared in the laboratory and released to a large marine enclosure4 to Sdaysafter hatching(6-8" C). Thedevelopment ofthedigestive system was studied until day 24 after hatching. Morphological investigations of the jaw apparatus and the digestive tract showed that the larvae are able to absorb ingested food well before exhaustion of the yolk sac. The foregut, and especially the midgut, were particularly active in lipid absorption, and the hindgut was characterized by pinocytotic activity. During the first days of feeding, no distinct prey organisms were observed in the gut. and signs of food absorption in the epithelial cells of the gut were sparse. A distinct red fluorescence, restricted to the hindgut, was observed from day I 1 to day 19. On the basis of changes in absorptive pattern in the gut we suggest that changes in digestive and absorptive abilities, as well as in nutritional needs, take place around days 15-17 after hatching.In starved larvae, signs of degeneration of the gut tissue were first visible in the foregut. By day 9 after hatching, microvilli was degenerated to such an extent that the ability to absorb food must have been severely restricted. Iflarvae are starved longer than this, they will probably not survive.

Egg specific gravity is a function of egg volume, chorion volume, perivitelline space (PVS) and the specific gravity of chorion and ovoplasm. Experimental studies on cod (Gadus morhua L.), a marine multiple batch spawner, showed that approx. forty per cent of the chorion volume is incorporated between batches and that the chorionic material has a mean specific gravity of 1.20 (range: 1.14-1.35). The chorion volume and specific gravity are low at the end of the spawning period in extremely fecund fish. Specific gravity of ovoplasm was observed to be 1.01 7 in marine eggs and calculated to be about 1.008 in brackish water eggs. Inherent variation in PVS is found to have little influence on egg specific gravity. With the other variables constant, egg specific gravity is inversely related to egg volume supported by studies on single batches of eggs. Measurements on every batch of eggs reveal that the production of large eggs (> 1.3 mm) gives low density eggs initially and high density eggs towards the end of spawning, while the production of small eggs ( < I .3 mm) gives moderate specific gravity. These observations are discussed in relation to vertical distribution of the eggs and reproductive success of recruit and repeat spawners. There are indications that repeat spawners produce larger eggs.

In the Baltic Sea successful spawning of cod is restricted to the deep basins, and egg buoyancy is regarded as a major factor that allows eggs to avoid the stressful oxygen conditions that often prevail in the deep layers. In this study eggs of 3 to 5 yr old spawners (the basis of the spawning stock of Baltic cod Gadus morhua L. nowadays) maintained neutral buoyancy at a salinity of 14.5 i 1.2 ppt, with great variation among eggs from different females. Egg buoyancy was significantly correlated with yolk osmolahty and chorion thickness and weakly correlated with egg size. Measurements of egg buoyancy in successive batches from the same female indicated that egg buoyancy increased with batch number. The significance of neutral buoyancy for cod egg survival in prevailing conditions in the Baltic was evaluated by comparing the ability of eggs to achieve neutral buoyancy in the Baltic cod spawning areas during June of the 6 years 1977-1980, 1982 and 1987. The results suggest that the ability of eggs to maintain neutral buoyancy should be included in attempts to find explanations for fluctuations in recruitment of Baltic cod.

No abstract

This study describes the long-term effects of surgical ablation of the pineal gland on the spine of 3-yr-old Atlantic salmon (Salmo salar L.) with a mean weight of 3.2 kg. Radiographic examinations showed that 82% of the pinealectomized fish developed marked lateral (scoliosis) and dorso-ventral spinal curvatures. The proportions of the individual vertebral bodies and their mechanical properties were also altered. The stiffness, yield limit and resilience of the vertebral bodies, as measured by compression in the cranio-caudal direction, were significantly lower in the pinealectomized than in the sham-pinealectomized group. Calcium, phosphorous and total mineral content of the vertebral bodies were also significantly lower in the pinealectomized fish, while these parameters were similar in scales in the two groups. Alterations of the spinal curve accompanied by changes in the proportions, mechanical strength and mineral content of the vertebral bodies of the pinealectomized salmon indicate that melatonin has several functions related to vertebral bone growth. As the lesions found in salmon are similar to the spinal malformations observed in avian species and mammals after pinealectomy, this study strengthens the hypothesis of a phylogenetically conserved function of the pineal gland related to skeletal development.