Effect of ocean acidification on early life stages of Atlantic herring (Clupea harengus L.)

Due to atmospheric accumulation of anthropogenic CO2 the partial pressure of carbon dioxide (pCO2) in surface seawater increases and the pH decreases. This process known as ocean acidification might have severe effects on marine organisms and ecosystems. The present study addresses the effect of ocean acidification on early developmental stages, the most sensitive stages in life history, of the Atlantic herring (Clupea harengus L.). Eggs of the Atlantic herring were fertilized and incubated in artificially acidified seawater (pCO2 1260, 1859, 2626, 2903, 4635 μatm) and a control treatment (pCO2 480 μatm) until the main hatch of herring larvae occurred. The development of the embryos was monitored daily and newly hatched larvae were sampled to analyze their morphometrics, and their condition by measuring the RNA/DNA ratios. Elevated pCO2 neither affected the embryogenesis nor the hatch rate. Furthermore the results showed no linear relationship between pCO2 and total length, dry weight, yolk sac area and otolith area of the newly hatched larvae. For pCO2 and RNA/DNA ratio, however, a significant negative linear relationship was found. The RNA concentration at hatching was reduced at higher pCO2 levels, which could lead to a decreased protein biosynthesis. The results indicate that an increased pCO2 can affect the metabolism of herring embryos negatively. Accordingly, further somatic growth of the larvae could be reduced. This can have consequences for the larval fish, since smaller and slow growing individuals have a lower survival potential due to lower feeding success and increased predation mortality. The regulatory mechanisms necessary to compensate for effects of hypercapnia could therefore lead to lower larval survival. Since the recruitment of fish seems to be determined during the early life stages, future research on the factors influencing these stages are of great importance in fisheries science

A comparison of the nutritional condition of herring larvae as determined by two biochemical methods - tryptic enzyme activity and RNA/DNA ratio measurements

Two biochemical methods for measuring larval fish condition – tryptic enzyme activity and RNA/DNA ratio measurement - were applied to laboratory-reared and wild-caught herring larvae. The comparison of both methods when applied to laboratory-reared herring larvae showed that tryptic enzyme activity and RNA/DNA ratio are linear and positively correlated under constant nutritional conditions. Wild-caught larvae were transferred to the laboratory and used to compare both indicators in relation to shortterm changes in food availability and long-term starvation periods (13 days). In the starvation experiments with the wild-caught larvae the lowest trypsin values were obtained after 3–4 days and a significant decrease in RNA/DNA ratios was obtained after 5–6 days. Prolongation of the starvation time did not result in a further significant change in either parameter. The results of the study demonstrate the usefulness of both methods in monitoring nutritional condition offish larvae in field samples

Baltic sprat larvae: coupling food availability, larval condition and survival

Eggs and larvae of Baltic sprat Sprattus sprattus L. were collected during 14 cruises covering the spawning season in 2002 in Bornholm Basin. Main egg and larval production was in April, with a second small peak in June 2002. The in situ larval abundance was corrected for transport processes by hydrodynamic model runs. Corrected larval abundance estimates were compared to initial larval production to derive an index of larval mortality. This index suggested a much higher survival of summer- over spring-born sprat larvae, with pronounced differences in survival for larvae >11 mm. Independent evidence for this survival pattern was gained by measured RNA:DNA ratios in sprat larvae hatched from April to July 2002 and was linked to temporal variability in potential prey abundance. We found higher mean but less variable RNA:DNA ratios in spring- than in summer-born larvae, indicating a strong selection for fast growth in April and May but a less selective environment in June and July. Zooplankton data revealed high naupliar concentrations of Acartia spp. (a key dietary component of sprat) in April and May, but very low concentrations of larger prey items such as copepodites or adults. In contrast, abundance of larger prey increased considerably in June and July. The results suggest that larger sprat (>11 mm) in April and May 2002 may have been food limited and, therefore, had lower rates of survival, supporting the underlying hypothesis of size-specific, temporally limited ‘windows of survival’ linked to the availability of suitable prey

Improvements in the fluorimetric determination of the RNA and DNA content of individual marine fish larvae

The RNA/DNA ratio is a useful indicator of the nutritional condition of fish larvae. The presented analytical procedure is an improvement of Clemmesen's (Meeresforschung 32: 134-143, 1988) methodology which involves purification of fish larvae tissue homogenates and subsequent fluorescence-photometric measurements using specific nucleic acid dyes. The modifications concern the homogenization and nucleic acid extraction procedures. A 'shaking mill' was compared to a potter Elvehjem microhomogenizer and a reduction in the washing and purification steps was achieved. Treatment of samples with ribonuclease A and subsequent fluorescence measurement using ethidium bromide was given preference compared to the DNA-bisbenzimidazole determinations due to problems arising from high self-fluorescence of the samples and the influence of 'quenching' substances disturbing the DNA-bisbenzmidazole determinations. Different RNase concentrations and their influences on RNA and DNA were checked. Recovery rates of standard RNA and DNA 'spikes' were determined. Fish larvae samples were analysed with the previous and the improved modified procedure and a correction factor to compare results measured with the 2 procedures was calculated. With the presented method the physiological condition of individual larvae and the amount of variability can be determined

Comparison of biochemical and histological methods for the evaluation of the in situ nutritional condition of marine fish larvae

To estimate the importance of starvation induced mortality for recruitment of marine fish larvae three distinct methods were applied to determine the nutritional condition of fish larvae in situ. In addition to highly sensitive fluorescence techniques for analysing RNA/DNA ratios and tryptic enzyme activities histological standard methods were used to compare the nutritional status of fish larvae of the genus Vinciguerria (Photichthyidae) caught in two ecologically different areas of the Indian Ocean: In the central Arabian Sea and on the continental shelf of Pakistan. A comparison of the results elaborated by the distinct methods shows a trend towards better nutritional conditions for fish larvae from the offshore region