Variability in the annual egg production of hatchery-reared Atlantic cod (Gadus morhua) was determined under stable experimental conditions. Egg size increased with fish age as an approximate step function. Comparing first-and second-time spawners, the variance in egg dry weight was 32% within individuals (i.e., the seasonal effect, the cod being a multiple-batch spawner), 55% between years, and 12% between individuals. In several repeat spawners, the curvature of the seasonal egg size curves showed little difference between years. The seasonal decrease in egg size was typically smaller in recruit spawners than in repeat spawners. There was no empirical evidence to suggest that environmental temperature regulates seasonal variations in egg size. The extent of egg swelling (i.e., the egg dry weight/diameter ratio) indicated a strong genetic component. Investment in ovarian growth was influenced by previous allocations as exemplified by annual, sinusoidal fecundity oscillations. Larger fish showed significantly longer spawning periods. The combined influence of maternal factors and the annual temperature variations noticed in the field during early stages suggests that larger larvae at the onset of feeding are more likely to survive.Résumé : On a déterminé la variabilité de la production annuelle d'oeufs chez des morues de l'Atlantique (Gadus morhua) et élevage dans des conditions expérimentales stables. La taille des oeufs augmentait avec l'âge des poissons en suivant approximativement une fonction en escalier. On a comparé le poids sec des oeufs chez des géniteurs à la première et à la deuxième ponte, ce qui a donné une variance de 32% chez les individus (ce qui correspond à l'effet saisonnier, la morue ayant plusieurs pontes pendant la saison), de 55% entre les années et de 12% entre les individus. Chez plusieurs géniteurs ayant frayé plusieurs fois, la forme des courbes saisonnières de la taille des oeufs n'indiquait que peu de différences d'une année à l'autre. La diminution saisonnière de la taille de ceux-ci était typiquement plus faible chez les nouveaux géniteurs que chez les anciens. Rien ne permet de croire que la température du milieu régule les variations saisonnières de la taille des oeufs. L'importance du gonflement des oeufs (c.-à-d. le rapport poids sec/diamètre de l'oeuf) semblait indiquer l'existence d'un important facteur génétique. L'investissement dans la croissance des ovaires était influencé par des investissements antérieurs similaires, comme le montrent les oscillations annuelles sinusoïdales de la fécondité. Les poissons de plus grande taille présentaient des périodes de ponte significativement plus longues. L'influence combinée de facteurs maternels et des variations de la température annuelle notées sur le terrain au cours des premiers stades indique que les larves qui sont les plus grosses au début de la période d'alimentation ont les plus grandes chances de survie. [Traduit par la Rédaction]

Nordeng H, Bratland P. Homing experiments with parr, smolt and residents of anadromous Arctic char Salvelinus alpinus and brown trout Salmo trutta: transplantation between neighbouring river systems.Abstract -Olfaction is decisive for the homeward migration of anadromous salmonids. Two different olfactory hypotheses for explaining how this mechanism works have been proposed (the imprinting and the pheromone hypothesis), and they differ with regard to the origin of the odours that define home. The pheromone hypothesis assumes that fish respond to strain-specific pheromones and that no imprinting is necessary. The imprinting hypothesis assumes that juvenile fish become imprinted to odours of abiotic or biotic origin in natural rivers; the imprinting may be a single event or may occur sequentially during downstream migration as smolts. The two hypotheses were challenged by reciprocal transplantation of parr, residents and smolt of Arctic char (Salvelinus alpinus) and brown trout (Salmo trutta) between the Salangen and the Løksebotten rivers. Both rivers empty into the Salangen Fjord, Norway, about 10 km apart. Salangen Arctic char released in the Løksebotten River, and Løksebotten brown trout released in the Salangen River (parr, resident and smolt) initiated spontaneous homeward migration without preceding contact with home stream water. This indicates that site imprinting (single or sequential) could not have been involved. In the sea, the released fish apparently integrated into the migratory system of relatives from their river of origin. Recapture rates did not differ for fish allowed to receive single or sequential imprinting.

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