Effect of prey concentration and light on the foraging behaviour, growth and survival of Atlantic cod larvae (Gadus morhua) under laboratory conditions

Puvanendran, Velmurugu (1999) Effect of prey concentration and light on the foraging behaviour, growth and survival of Atlantic cod larvae (Gadus morhua) under laboratory conditions. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis describes experiments on the responses of Atlantic larval cod to two important ecological variables, prey concentration and light in terms of behaviour, growth and survival. The first ecological variable investigated was light intensity and its effect on the foraging behaviour, growth and survival of Atlantic cod larvae from two geographical regions in the Northwest Atlantic. Larval cod originating from different geographical locations responded differently to light intensity. Larvae originating from the Scotian Shelf (SS origin) foraged, grew and survived better in low light intensity while larvae from the Northeastern Grand Banks (NF origin) performed better in high light. This difference in response to light intensity may be explained by the different spawning seasons rather a than latitudinal difference. -- The next ecological variable investigated was prey concentration. Earlier studies on larval fish indicated that growth and survival of the larvae vary with prey concentration. However, the shortcoming of most of these studies involving cod larvae was that they were short term experiments. Thus, I investigated the ontogeny of foraging behaviour of Atlantic cod larvae exposed to different prey concentrations from hatching to metamorphosis. Larvae exposed to higher prey concentration outperformed the larvae reared in lower prey concentrations in all the foraging Modal Action Patterns (MAP's) investigated in this study. But the magnitude of the foraging MAP's increased as the larvae grew regardless of prey concentration. Results also indicated development of foraging behaviour was not affected by prey concentration. -- Next, I investigated the growth and survival of Atlantic cod larvae reared in a wide range of prey concentrations. My previous experiment showed that the highest prey concentration used (4000 prey L⁻¹) may not be the optimal prey concentration to rear the cod larvae in the laboratory. In this second experiment, prey concentrations of 8000 and 16000 prey L⁻¹ were included. Results indicated no difference in growth when prey concentration above 4000 prey L⁻¹ were used. Initially no difference was found in the survival of larval cod among the three highest prey concentrations (4000, 8000 and 16000 prey L⁻¹) but continuous use of prey concentrations above 4000 prey L⁻¹ beyond 3 weeks post-hatch reduced the survival considerably. Initially, mortality rates of cod larvae were higher in prey concentrations lower than 4000 prey L⁻¹. Beyond 3 week post-hatch no significant difference was found in mortality rates among any of the treatments. Observations on foraging behaviour of larval cod indicated that larvae reared in higher prey concentrations foraged more efficiently than larvae reared in the lower prey concentrations. Observations from this study emphasize the importance of behavioural observations to explain any difference in growth variables between the treatments. Results indicated that for intensive rearing larval cod require a prey concentration of 4000 prey L⁻¹ to sustain reasonable growth and survival. -- I also investigated foraging, growth and survival of Atlantic cod larvae (NF origin) reared at varying light intensities and photoperiods. Behavioural observations were also carried out in an attempt to explain any differences in the performance of cod larvae under varying light intensities. Cod larvae grew and survived better in higher light intensity (2400 lux) and 24L:0D photoperiod. The condition index (ratio of myotome height at anus to standard length) of the larvae was also better in high light intensity and 24 hr photoperiod. Examination of the foraging MAP's indicated that cod larvae reared in higher light intensity captured the prey more efficiently than larvae reared in low light. -- Predator responses (functional, developmental and numerical) of larval cod to different prey concentrations were investigated in an attempt to further study some observations made in my earlier experiments. In this experiment prey consumption rates were investigated in terms of both age and size. Results indicated that the cod larvae exhibited a type II functional response where prey consumption increases with increasing prey concentration asymptotically at a decelerating rate. Developmental response of the cod larvae was closely correlated to the size. Prey consumption rates increased as the larvae grew. During the first two weeks post-hatch, larvae exposed to low prey concentrations (<1000 prey L⁻¹) did not feed enough to sustain sufficient growth and subsequently could not survive beyond three weeks.

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