Types, number, size, biomass, and selection of food items by the larvae of Atlantic cod Gadus morhua and haddock Melanogrammus aeglefinus were determined by examination of gut contents from CO-occurring larvae captured in plankton samples from Georges Bank. Eggs, nauplii, and copepodite stages of copepods were the predominant food items for both larvae, but cod consumed larger prey at an earlier age than haddock. The smallest larvae were the most euryphagous, haddock more than cod. Cod and haddock feeding intensity reached a peak shortly before sunset, and both larvae selected against the copepodite stages of Oithona similis. Dietary niche breadth and overlap indices indicate that competition is severe between and among similar sized individuals of both species.
Kane, J. 2007. Zooplankton abundance trends on Georges Bank, 1977–2004. – ICES Journal of Marine Science, 64: 909–919. Interannual trends in Georges Bank zooplankton abundance are described and related to variations in environmental variables for the period 1977–2004. Total zooplankton counts increased to above average levels in 1989 and stayed over or close to average through 2004. This analysis identified a group of taxa including Centropages typicus, Metridia lucens, and Temora longicornis that had similar interannual patterns of abundance. All these taxa increased sharply in the early 1990s and remained high through 2001. Some taxa declined sharply in 2002, others have continued elevated through 2004. Total zooplankton counts in the past two years were also boosted by a substantial increase in the abundance of the copepod Calanus finmarchicus. Evidence is presented that these changes may be related to variations in Scotian Shelf inflow, which freshened water on the Northeast US continental shelf, perhaps increasing both primary production and the influx of zooplankton into the region. There was a positive correlation between the biomass of pelagic predators and the abundance of several zooplankton taxa, suggesting that bottom-up processes and advective supply are the key factors that regulate the Georges Bank foodweb.
In 2003, the Georges Bank stock of haddock (Melanogrammus aeglefinus) experienced the largest recruitment event recorded during its assessed history. Several hypotheses have been advanced to explain recruitment variability in this much-scrutinized stock, including variability in the retention of eggs and larvae on Georges Bank, the timing of haddock spawning, and variability in the spring bloom, which influences larval growth and survival. Although these processes may contribute to the formation of successful year classes, none of the factors associated with these previous hypotheses provides an adequate explanation of the 2003 recruitment event. We analyzed data on the dynamics of the fall phytoplankton bloom the year prior to spawning and show it to be highly correlated with subsequent recruitment. We suggest that the fall bloom affects recruitment through enhanced condition of adults and by increasing the quantity and quality of their reproductive output, which in turn leads to a higher probability of survival of their offspring. Although synoptic data on the fall bloom are limited and our analyses are correlative, our purpose is to stimulate a rigorous test of this promising ''parental condition hypothesis''.Résumé : En 2003, le stock d'aiglefins (Melanogrammus aeglefinus) du banc George a connu le plus important épisode de recrutement jamais enregistré durant toute son histoire connue. Plusieurs hypothèses ont été avancées pour expliquer la variabilité du recrutement dans ce stock bien étudié, et en particulier, la variabilité de la rétention des oeufs et des larves sur le banc George, le calendrier de la fraie de l'aiglefin et la variabilité du foisonnement printanier des algues qui influence la croissance et la survie des larves. Bien que ces processus puissent contribuer au succès de certaines classes annuelles, aucun des facteurs associés à ces hypothèses ne permet d'expliquer de façon adéquate l'épisode de recrutement de 2003. Nous avons analysé les données sur la dynamique du foisonnement du phytoplancton d'automne pendant l'année qui précède la fraie et nous trouvons une forte corrélation avec le recrutement subséquent. Nous croyons que le foisonnement d'automne affecte le recrutement en améliorant la condition des adultes et en augmentant la quantité et la qualité de leur rendement reproductif, ce qui par contrecoup accroît la probabilité de survie de leurs rejetons. Bien qu'il existe une quantité limitée de données synoptiques sur le foisonnement du phytoplancton d'automne, notre but est d'inciter la conduite d'un test rigoureux de cette hypothèse prometteuse de la « condition parentale ».[Traduit par la Rédaction]
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