Abstract
The percentage of cells dividing in a specific tissue of individual larvae can be estimated by analyzing DNA per cell by flow cytometry. An experimental test was carried out with cod (Gadus morhua) larvae, with brain as the target tissue, to validate this technique as an appropriate growth index for larval fish. Standard length (SL), myotome height, and %S-phase (% of cells in the S-phase of the cell-division cycle) variability were analyzed, with temperature (6 and 10°C), food level (high- and no-food) and larval developmental stage (first feeding, pre-metamorphosis and post-metamorphosis) as independent factors. Cod larvae grew faster (in SL) and presented a higher %S-phase under high-food conditions. Larval SL increased with temperature in rearing and experimental tanks. However, there was a significant interaction between temperature and food in the %S-phase. There were no significant differences in the %S-phase between 6 and 10°C at high-food levels. We suggest that this result is a consequence of temperature-dependency of the duration of the cell cycle. In the absence of food, larvae at 10oC had a lower %S-phase than larvae at 6°C, which may be related to increased metabolic costs with increasing temperature. Considering the effect of temperature, the mean % S-phase explained 74% of the variability in the estimated standard growth rate.
Highlights
The processes affecting survival of larval and early juvenile stages are the main factors that determine recruitment of pelagic fish
We considered that 5 days of starvation vs. high feeding conditions would cause a clear effect on larval growth without causing a large larval mortality in starvation treatments that may put into question any significant differences between treatments
T and Food showed a significant effect on Ln standard length (SL) of larvae from the experimental treatments (ANOVA, p < 0.05, Table 4)
Summary
The processes affecting survival of larval and early juvenile stages are the main factors that determine recruitment of pelagic fish. A variety of growth and condition indices have been developed in order to understand how environmental conditions affect individual growth rates (see review from Ferron and Leggett, 1994). These include morphometric and histological indices, otolith microstructure analysis and others based on a variety of organic compounds that correlate to growth, such as nucleic acids and certain enzymes. The RNA/DNA index may reflect potential protein synthesis more than growth itself (Ferron and Leggett, 1994)
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