Size, condition, and age of female-Icelandic cod Gadus morhua were correlated to the size of their eggs and newly hatched larvae. A positive relationship was detected between egg size and some larval viability parameters, including the age at first feeding, successful development of a swimbladder, and specific growth rates during the first 15 days after hatching. These results reveal that the viability of cod larvae is related to attributes of the spawning females and that this information is important to our understanding of stock-recruitment relationships.1998 The Fisheries Society of the British Isles

Evidence accumulated over the last decade indicates that reproductive potential of marine fish stocks is not correctly represented by spawning stock biomass. Fundamental variables that affect reproductive potential of fish stocks (i.e. proportion mature at age, fecundity, and offspring size and viability) have found to vary with age, size, and condition of spawning fish, and/or spawning experience. Modeling initiatives that attempt to describe reproductive potential by incorporating all available information on reproductive variables require extensive, high quality data of a temporal and spatial origin. In this study, we explored the influence of female age, size, and condition on several reproductive variables for the cod Gadus morhua, including maturity, fecundity, egg density and size, and larval size and growth. In addition, we described the potential for annual variation in these relationships to increase our knowledge of temporal stability for fundamental variables that influence reproductive potential.

The size of the Icelandic cod stock has been gradually declining since the middle of this century. Recruitment has been poor over an extended period of time and much below the long-term average since 1985. Except for the concurrent decrease in stock size and recruitment during this period, the stock size - recruitment relationship is weak. This relationship is improved by including the age composition of the spawning stock. Spawning stock age diversity in each year from 1955 to 1992 was estimated with the Shannon index using the number of mature fish in each age group. By including information on age composition, 31% of the total variation in recruitment was accounted for by the model with stock size, age diversity, and the interaction between the two, compared with less than 15% by single factor models of either age diversity or stock size. These results indicate that age diversity is an important component in stock-recruitment models and that one of the management goals for fish species should be to maintain high age diversity in the spawning stocks.

Allelic variation at nine microsatellite loci and the Pan I locus provides evidence that Atlantic cod (Gadus morhua) around Iceland is genetically structured (F ST = 0.003 and F ST = 0.261, respectively). A total of 2534 cod were sampled at 22 spawning locations. For both types of markers, most of the significant pairwise F ST resulted from northeastern-southwestern comparisons. A multidimensional scaling analysis based on F ST , a spatial hierarchical analysis of molecular variance (SAMOVA) and a hierarchical analysis of molecular variance (AMOVA), conducted on both types of markers confirmed a genetic differentiation between cod from the northeastern and southwestern regions. Genetic discontinuities were revealed across two main current fronts in the southeast and northwest, where the cold and warm water masses meet. The AMOVA also detected genetic differences with depth. Expected and observed heterozygosity of microsatellite loci significantly decreased with depth, whereas the B allele frequency at the Pan I locus increased. A tagging experiment of spawning fish conducted during the same years as the genetic work revealed that tagged individuals released in the southwestern region seldom migrated to the northeastern region and vice versa, suggesting that the southwestern and northeastern populations of Atlantic cod around Iceland represent two distinct spawning components.Résumé : La variation allélique de neuf microsatellites et du locus Pan I met en évidence une structure génétique chez la morue Atlantique (Gadus morhua) dans les eaux islandaises (F ST respectifs, F ST = 0,003 et F ST = 0,261). Un total de 2534 morues a été échantillonné sur 22 sites de ponte. Pour les deux marqueurs génétiques utilisés, la majorité des valeurs significatives de F ST par paires de populations est due à des comparaisons nord-est et sud-ouest. Une analyse multidimensionnelle basée sur les F ST par paires de populations, une analyse spatiale hiérarchique de variance molécu-laire (SAMOVA) et une analyse hiérarchique de variance moléculaire (AMOVA), réalisées sur les deux types de marqueurs génétiques, révèlent une différenciation significative entre les échantillons du nord-est et du sud-ouest. La recherche de discontinuités génétiques indique la présence de deux barrières au flux génique correspondant aux principaux fronts de courants océaniques localisés dans le nord-ouest et le sud-est du pays aux points où les masses d'eaux chaudes et froides entrent en contact. L'analyse AMOVA détecte aussi des différences génétiques en fonction de la profondeur. Une analyse approfondie montre que les hétérozygoties observées et attendues des microsatellites diminuent significativement avec la profondeur, alors que la fréquence de l'allèle B du locus Pan I augmente. Une expérience de « capture-recapture » réalisée sur les sites de pontes en même temps que les études génétiques révèle que des individus marqués et relâchés dans la région du sud-ouest migrent rarement vers la région du nord-est et vice versa, ce qui confirme que les population...

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.