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Brazilian sardine, the most important resource along the southeastern Brazilian coast, presented great variations and declines in its stocks. The main factors contributing to this are: oceanographic structure changes; recruitment failures; excessive catches of juveniles and increase in fishery effort. In spite of this, no alterations in the density-dependent parameters were detected. Consequently, methods analysing the condition of the larvae coupled with methods determining growth using sagittae otolith increment width were applied to evaluate growth under experimental conditions. The results of the readings on the sagittae were compared with the age of the laboratory-reared sardine larvae and confirmed that increments are formed on a daily basis. Under poor feeding conditions, sardine larvae showed a low growth expressed by dry weight, RNA/DNA ratio and tryptic enzyme activity and by the narrow and low contrast increments in the otoliths. The results of the biochemical indices showed an unexpected decline in the feeding group coupled with a decrease in width of increment numbers 8 and 10. Other factors than food availability were affecting the condition of the larvae and might be indicative of physiological processes and ontogenetic changes occurring in sardine larvae. La sardinela brasileña, el recurso más importante en la costa sudeste del Brasil, presentó grandes variaciones y disminuciones en sus poblaciones. Los principales factores que contribuyeron a dicha situación fueron: los cambios en la estructura oceanográfica, los fallos en el reclutamiento, las capturas excesivas de juveniles y el aumento del esfuerzo pesquero. A pesar de ello, no se detectaron alteraciones en los parámetros denso-dependientes de la población. Consecuentemente, se aplicaron métodos de análisis de condición larvaria junto a métodos de determinación del crecimiento mediante la anchura de los incrementos de los otolitos sagitta para evaluar el crecimiento bajo condiciones experimentales. Los resultados de las lecturas de los sagitta se compararon con la edad de las larvas de sardinela cultivadas en el laboratorio y confirmaron que los incrementos se forman diariamente. Las larvas de sardinela sometidas a bajos niveles de alimentación mostraron un bajo crecimiento expresado en peso seco, relación RNA/DNA y actividad de la tripsina, y un bajo contraste de los incrementos en los otolitos. Los resultados de los índices bioquímicos mostraron un inesperado descenso en el grupo de larvas alimentadas, acompañado de una disminución en la anchura de los incrementos 8 y 10. Además de la disponibilidad de alimento, otros factores afectaron a la condición de las larvas y podrían indicar procesos fisiológicos o cambios ontogenéticos que tienen lugar en el desarrollo larvario de la sardinela.

Investigations of factors affecting feeding success in fish larvae require knowledge of the scales of variability of the feeding process itself and the indices used to assess this variability. In this study, we measured short-term (diel) variability in feeding rates of wild haddock larvae 4 times per day during a 10-day cruise in the northern North Sea. Feeding activity was evaluated using indices of gut fullness, prey digestive state and biochemical measurements (tryptic enzyme activity). The gut fullness and the enzyme activity indices indicated moderate-high rates of food consumption throughout the cruise. Time series analysis of the three indices showed significant diel variability in all indices and enabled identification of significant lags between food uptake and peak digestive enzyme activity. The typical pattern of food consumption and digestion was characterized by maximal ingestion of prey early in the evening (7 pm) and peak digestive enzyme activity at 1 am. The timescale over which enzyme activities reacted to prey ingestion was ca. 6 hours and is consistent with expectations from controlled laboratory experiments with other larval fish species. Significant diel variability in tryptic enzyme activity suggests that attempts to relate this measure of feeding success to other variables (e. g., food concentrations) should take care to accommodate natural cycles in feeding activity before making statistical comparisons.