Introduction 51Terminology 51Types of spawning omission 56Identifying spawning omission 57 External appearance of gonads 58 Gonad indices 59 Histology 59 Importance of sampling time 61 Causes of spawning omission 62 Non-annual spawning as an adaptive trait: trade-offs between reproduction and survival 64 Significance to fisheries science 66Abstract It is often assumed that iteroparous fishes spawn annually once reaching sexual maturity, but this is not always the case. This paper reviews available information on skipped spawning in female teleost fishes. All instances of non-annual spawning are described as one of three types (retaining, reabsorbing, resting), depending on where in the normal spawning cycle development has been interrupted. Retaining ripe eggs is caused by conditions experienced during the spawning season (fish density, mate availability, pollution), whereas failure to start vitellogenesis (resting) or the breakdown of all oocytes that enter into vitellogenesis (reabsorbing) is caused by factors experienced prior to the spawning season (primarily temperature and poor nutrition). It is speculated that the relative shortage of data on non-annual spawning may be because of difficulties in identifying non-reproductive individuals.In an attempt to rectify this situation, the criteria needed to identify females undergoing the three forms of spawning omission are presented in terms of external appearance of gonads, gonad indices, and histological analysis. The energy saved by not spawning in a poor year may lead to increased survival and the probability of spawning in subsequent years. As the cumulative number of progeny gained by surviving to spawn in multiple subsequent years outweighs the number of progeny lost by not spawning in a given single year, occasional omission of spawning may constitute an adaptive trait in long-lived iteroparous fishes.

Atlantic cod Gadus morhua were collected from Smith Sound, Newfoundland in January 1999. Visual examination of females (n=150) and males (n=126) revealed that some large fish (42-79 cm) had underdeveloped gonads. Histological examination of underdeveloped ovaries indicated that the majority of these females were undergoing mass resorption of oocytes and would not have spawned in 1999. Fish in this condition included females that were aborting their first attempt at maturation and females that had spawned the previous year but were failing to re-ripen. Somatic and liver condition were significantly lower (P<0·05) for fish undergoing mass oocyte resorption than ripening females, suggesting that the interruption in the maturation cycle may have been related to insufficient nutrient storage. In males, testes of some adult fish were considered to be non-reproductive as they showed no signs of ripening and probably would not have spawned in 1999. Liver condition was significantly higher (P<0·05) for non-reproductive males than those that were ripening. Disruptions in male and female reproductive cycles may also have been related to water temperatures that were too cold (0-0·5 C) for successful gamete development. Immature gametes (perinucleolar oocytes in females; spermatogonia in males) showed no signs of breakdown in non-reproductive individuals, suggesting that they retained the potential to develop and spawn gametes in

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The reproductive status of winter flounder was analyzed in individuals collected at regular intervals throughout the year. Although spawning occurred in May–June, sperm could be activated in January, and male gonadosomatic indices in November were approximately twice those recorded immediately before spawning. Oocyte recruitment occurs close to spawning, and oocyte changes are compatible with a 3-year cycle. Vitellogenesis of the oldest year class of oocytes is initiated during the summer feeding season and maintained over the nonfeeding winter season. Some females after spawning do not enter the vitellogenic stage but overwinter with two year classes of immature oocytes. This nonreproductive phase in females in related to condition rather than age. Nonreproductive males also occur in the population sampled and in the fall there was a trend towards an increased number with advancing age. The protracted gametogenesis in winter flounder demonstrates the need for caution when assessing spawning condition and reproductive capacity of some fish stocks.

With 3 plates and 1 figure in the text) When female winter flounder (Pleuronectes arnericanus) were subjected to periods of satiation feeding alternating with starvation it was found that the first part of their normal six-month feeding period could be associated with subsequent gametogenesis, whereas feeding in the later part of the normal feeding period was not necessary. Conversely if females were not fed during the first part of the normal feeding season they were likely to become non-reproductive, although high condition fish or a non-spawner could become gametogenic in spite of starvation at this time. Feeding restricted to the later part of the normal feeding season was therefore not generally associated with successful gametogenesis for the following year's spawn. When the first part of the feeding period was further subdivided with satiation feeding limited to one or more months within it, most fish became non-reproductive. The exceptions which became reproductive were females which had high post-spawn condition maintained in the month following the immediate spawning period for the individual fish. A nutritionally sensitive period for early gametogenesis in female winter flounder therefore appears to occur in the early part of the normal feeding season, close to the normal spawning period. The experimentally produced non-reproductive females were generally halted in a previtellogenic stage of development for the most advanced oocytes which is consistent with a nutritionally related inhibition of gametogenesis close to the previous spawning period.