Effect of food deprivation on distribution of larval and early juvenile cod in experimental vertical temperature and light gradients

Abstract

How and when food limits the growth and survival of larval fish are essential questions which link oceanography and fisheries science, and theory suggests that food availability may affect survival through behavioural responses. We tested the effect of food deprivation on the vertical distribution of larval cod Gadus morhua in light and temperature gradients during larval and early juvenile stages. Groups of fed (ad lib) and unfed (16 h) larval cod at 16, 30 and 37 d post hatch (dph) and early juvenile cod at 44 dph were repeatedly observed in experimental columns with temperature gradients (110 × 15 cm, 6 to 8°C) for a period of 3 h while the directionality of the light was manipulated. At earlier stages (16 and 30 dph), larvae were distributed mainly in the upper part of the column regardless of the water stratification. The presence of a thermocline reduced the likelihood of cod being observed in the lower part of the column at 37 and 44 dph, but this response was not affected by feeding history. Unfed larvae distributed deeper in the column than fed larvae, but this effect was small (on average, 4.3 cm difference in a 115 cm column). Video analysis of individual swimming behaviour at 44 dph indicated that, while unfed cod would not stay longer or swim deeper into the columns than fed cod, there was an increased number of fish swimming up and down in the column, suggesting an increased motivation to sample new environments. We conclude that short-term food deprivation (16 h) has a small but significant effect on larval vertical distribution, most likely as a consequence of increased prey-search behaviour, but that temperature and ontogeny were the key factors in determining distribution patterns.