Appraisal of reproductive strategy and fecundity is necessary to evaluate the reproductive potential of individual fish species. To estimate reproductive potential, one needs to consider a variety of attributes including onset of maturity, fecundity, atresia, duration of reproductive season, daily spawning behaviour and spawning fraction. In this contribution, we review several methods currently used to estimate fecundity of marine fishes collected in the field in relation to their reproductive strategy. The advantages and disadvantages of each method are provided. Requirements are given to appropriately sample gonadal tissue that will enable researchers to establish incidence of sexual maturity and estimate fecundity.

Large-scale climatic conditions prevailing over the central Baltic Sea resulted in declining salinity and oxygen concentrations in spawning areas of the eastern Baltic cod stock. These changes in hydrography reduced the reproductive success and, combined with high fishing pressure, caused a decline of the stock to the lowest level on record in the early 1990s. The present study aims at disentangling the interactions between reproductive effort and hydrographic forcing leading to variable recruitment. Based on identified key processes, stock dynamics is explained using updated environmental and life stage-specific abundance and production time-series. Declining salinities and oxygen concentrations caused high egg mortalities and indirectly increased egg predation by clupeid fish. Low recruitment, despite enhanced hydrographic conditions for egg survival in the mid-1990s, was due to food limitation for larvae, caused by the decline in the abundance of the copepod Pseudocalanus sp. The case of the eastern Baltic cod stock exemplifies the multitude effects climatic variability may have on a fish stock and underscores the importance of knowledge of these processes for understanding stock dynamics.

There is accumulating evidence to suggest that spawning stock biomass (SSB) may not be directly proportional to reproductive potential. The wide-ranging implications of this conclusion necessitate that it be tested for as many stocks as possible. Undertaking such tests is complicated by the fact that fish stocks vary in the amount and type of information that is available to estimate reproductive potential. In this review, nine stocks illustrate the range of J. Northw. Atl. Fish. Sci., Vol. 33: 161190 1 Present address: University of Aberdeen, Zoology Dept., Tillydrone Ave., Aberdeen, AB24 2TZ, Scotland, UK. Fish. Sci., Vol. 33, 2003 162 approaches that are being taken to developing alternative indices of reproductive potential from existing data resources. Three stocks had sufficient data to reconstruct a time series of total egg production (TEP), whereas, the remaining stocks were limited to estimating proxies for stock reproductive potential. For some of the case studies the alternative indices explained a higher amount of recruitment variation than did SSB. Other case studies provided evidence that characteristics of the spawning stock, e.g. age diversity and female-only SSB, influence recruitment in ways that are not properly accounted for by using SSB as the sole index of reproductive potential. This is further evidence that the assumption of proportionality between SSB and TEP is invalid. The data-rich stocks showed the relationship between SSB and TEP to be variable and characterized by distinct time trends. This variability will impact the ability of biomass-based reference points to conserve reproductive potential. Consequently, management protocols should be adapted to incorporate the detailed information on reproductive potential that is increasingly becoming available rather than being restricted to approaches that have been designed for data-poor situations.

SUMMARY:The recruitment processes of Baltic cod and sprat were analysed and critical periods were identified by addressing the major impact factors on individual early life history stages separately and relating observed abundance data between successive stages. For cod, recruitment appeared to be dependent on egg survival, with low oxygen concentration in dwelling depths and predation by clupeids as the major causes for egg mortality. Surviving egg production and larval abundance were weakly correlated, whereas larval abundance was significantly related to year class strength. This indicated that the period between the late egg and the early larval stage is critical for cod recruitment. A potential variable identified to affect this life stage was prey availability for larvae. For sprat, early and late egg stage production as well as late egg stage production and larval abundance were significantly related. However, year class strength was largely independent of larval abundance. Thus, the period between the late larval and early juvenile stage appeared to be critical for sprat recruitment. Potential variables identified to affect this life stage were ambient temperature and wind stress. Environmental factors showing statistically significant covariance with the survival of one of these critical life stages were incorporated into stock-recruitment models for individual spawning areas separately and for the Central Baltic combined.

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Observed fluctuations in relative fecundity of Eastern Baltic cod (Gadus morhua L.) were related to food availability during the main feeding period and were used to develop a predictive model that explained 72% of the interannual variations in fecundity. Time series of sex ratios, maturity ogives, and relative fecundity were combined with mean weights-at-age and stock sizes from an analytical multispecies model to estimate the potential egg production (PEP). Relationships between PEP and independent estimates of realized daily and seasonal egg production from egg surveys were highly significant. The difference between estimates of potential and realized seasonal egg production was of a magnitude corresponding to the expected loss of eggs as a result of atresia, fertilization failure, and early egg mortality. The removal of interannual variability in sex ratio, maturity, and fecundity on estimates of PEP deteriorated the relationships in all three cases. PEP proved to be superior to spawning stock biomass as measure of the reproductive potential in a stock-recruitment relationship of Eastern Baltic cod. PEP in combination with the reproductive volume explained 61% of the variation in year-class strength at age 2.Résumé : Les fluctuations observées dans la fécondité relative de la morue franche (Gadus morhua L.) dans l'est de la Baltique ont pu être reliées à la disponibilité de la nourriture durant la période principale d'alimentation et elles ont servi à établir un modèle prédictif qui explique 72 % de la variation de la fécondité d'une année à une autre. Des sé-ries temporelles des rapports mâles:femelles, des ogives de maturité et de la fécondité relative combinées à des données de masses moyennes pour un âge donné et de taille des stocks obtenues d'un modèle analytique multispécifique ont permis d'estimer la production potentielle d'oeufs (PEP). Les relations entre PEP et les estimations indépendantes de production journalière et saisonnière d'oeufs obtenues à partir des inventaires d'oeufs sont hautement significatives. La différence entre les estimations de la production saisonnière potentielle d'oeufs et celles de la production réalisée sont d'un ordre de grandeur qui correspond aux pertes qu'on peut attendre de l'atrésie, de l'absence de fécondation et de la mortalité précoce des oeufs. Le retrait de la variation annuelle du rapport mâles:femelles, de la maturité ou de la fé-condité du modèle d'estimation de PEP affaiblit la relation dans les trois cas. PEP s'avère être une meilleure mesure du potentiel reproducteur que ne l'est la biomasse du stock des reproducteurs dans la relation stock-recrutement de la morue de l'est de la Baltique. PEP, en combinaison avec le volume reproducteur, explique 61 % de la variation de l'importance de la classe d'âge de 2 ans.[Traduit par la Rédaction] Kraus et al. 1920

Several biological and environmental factors have direct or indirect influence on maturation, fecundity and egg viability of marine fishes. The relationships observed between these reproductive characteristics and specific biological and/or environmental indices may provide alternative methods of estimating annual variations in the reproductive potential of fish stocks in the absence of specific data, or represent a more rapid and less costly method of monitoring egg production. The utility of these relationships to serve as proxies for egg production will depend on the predictive power of the relationships, the effort required to measure selected proxies, the scale of application, and the availability of pre-existing target data. We believe that alternative methods to measure reproductive output could be developed for some reproductive characteristics needed to estimate egg/larval production using data already available. This review, largely based on gadoid fish species, indicates that relationships between indices of nutritional condition (e.g. condition factor, liver index) and potential fecundity meet the criteria of predictability and low effort and thus are candidate proxies for measuring egg production. No relationship examined was a suitable proxy for egg/larval viability and age-and size-at-maturity. The development of alternative measures of reproductive characteristics of fish could result in more precise estimates of reproductive potential of fish stocks that could be used for hindcasting and predicting egg and/or larval production and viability.

In this study, we explore the impact of four alternative indices of reproductive potential (RP) on perceptions of population productivity for eight fish populations across the North Atlantic. The four indices of RP included increasing biological complexity, adding variation in maturation, sex ratio, and fecundity. Perceptions of stock productivity were greatly affected by the choice of index of RP. Population status relative to reference points, RP per recruit, and projections of population size all varied when alternative indices of RP were used. There was no consistency in which index of RP gave the highest or lowest estimate of population productivity, but rather, this varied depending on how much variation there was in the reproductive biology of the population and the age composition. Estimates of sustainable harvest levels and recovery time for depleted populations can vary greatly depending on the index of RP.