No abstract

Low dissolved oxygen (DO) had a significant effect on specific growth rate (GS), length increment (IL) and haematocrit (Hct) of the mummichog Fundulus heteroclitus. Regardless of access to the water surface, F. heteroclitus maintained high growth rates (GS and IL) at DO concentrations as low as 3 mg O2 l−1. With access to the water surface, both GS and IL of F. heteroclitus decreased by c. 60% at 1·0 mg O2 l−1 compared to all higher DO treatments. When denied access to the water surface, a further decrease in GS (c. 90%) and IL (c. 75%) was observed at 1 mg O2 l−1. There was no effect of diel‐cycling DO (1–11 mg O2 l−1) with or without surface access on GS, IL or Hct of F. heteroclitus. Similar trends between GS and faecal production across DO treatments suggest that decreased feeding contributed significantly to the observed decrease in growth rate. Haematocrit was significantly elevated at 1 mg O2 l−1 for fish with and without access to the water surface. Increased Hct, however, was not sufficient to maintain high GS or IL at severely low DO. When permitted to respire in the surface layer, however, F. heteroclitus was capable of maintaining moderate growth rates at DO concentrations of 1 mg O2 l−1 (c. 15% saturation). Although aquatic surface respiration (ASR) was not quantified in this study, F. heteroclitus routinely swam in contact with the water surface and performed ASR at DO concentrations ≤3 mg O2 l−1. No hypoxia‐related mortality was observed in any DO or surface access treatment for as long as 9 days. This study demonstrates that surface access, and thus potential for ASR, plays an important role in providing F. heteroclitus substantial independence of growth rate over a wide range of low DO conditions commonly encountered in shallow estuarine environments.

The ratio of RNA to DNA (RNA:DNA) in white muscle tissue of juvenile summer flounder (Paralichthys dentatus) and weakfish (Cynoscion regalis) was used as a proxy for recent growth rate in an estuarine nursery. Variability in RNA:DNA was examined relative to temporal changes in temperature and dissolved oxygen (DO). Initial laboratory experiments indicated (i) a strong positive relationship between RNA:DNA and growth rate, (ii) a rapid response of RNA:DNA to changes in feeding, and (iii) no effect of hypoxia on the relationship between RNA:DNA and growth rate (tested in weakfish only). Diel cycling DO occurred in the nursery throughout the summers of 2002 and 2003. Canonical correlation analysis of field data indicated a strong positive relationship between RNA:DNA and mean DO conditions prior to capture in both species. Correlations were weak or insignificant between stomach content mass (an index of feeding) and mean DO and between RNA:DNA and stomach content mass and DO variability. These results suggest a strong functional relationship between DO concentration and the growth rate of juvenile fishes in an estuarine nursery. Furthermore, growth rates of wild-caught fishes (estimated from RNA:DNA) appear to be more negatively impacted by diel cycling hypoxia than would be expected from published laboratory data. Résumé: Le rapport ARN sur ADN (ARN:ADN) dans le tissu du muscle blanc du cardeau d'été (Paralichthys dentatus) et de l'acoupa royal (Cynoscion regalis) a servi de mesure de remplacement du taux de croissance récent dans une nourricerie estuarienne. Nous avons examiné la variabilité d'ARN:ADN en fonction des changements temporels de température et d'oxygène dissous (DO). Des expériences initiales en laboratoire indiquent (i) une forte relation positive entre ARN:ADN et le taux de croissance, (ii) une réaction rapide de ARN:ADN aux changements d'alimentation et (iii) une absence d'effets de l'hypoxie sur la relation entre ARN:ADN et le taux de croissance (testée seulement chez l'acoupa royal). Il y a eu un cycle journalier de DO dans la nourricerie tout au cours des étés 2002 et 2003. Une analyse des corrélations canoniques des données de terrain montre une forte relation positive entre ARN:ADN et les conditions moyennes de DO avant la capture chez les deux espèces. Les corrélations sont faibles ou insignifiantes entre la masse du contenu stomacal (un indice alimentaire) et la valeur moyenne de DO et entre ARN:ADN et la masse du contenu stomacal et la variabilité de DO. Ces résultats indiquent une forte relation fonctionnelle entre les concentrations de DO et le taux de croissance des jeunes poissons dans une nourricerie estuarienne. De plus, les taux de croissance des poissons capturés en nature (estimés d'après ARN:ADN) semblent être affectés plus négativement par l'hypoxie cyclique journalière que ne le laissent prévoir les données de laboratoire de la littérature. [Traduit par la Rédaction]