A study of the effects of size-dependent processes on survival and growth of Atlantic cod (Gadus morhua) larvae

Zhao, Yingming (2001) A study of the effects of size-dependent processes on survival and growth of Atlantic cod (Gadus morhua) larvae. Masters thesis, Memorial University of Newfoundland.

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Abstract

Fish year-class strength can be established at early life stages, such as the egg and larval stage. A small variation in growth and survival during these early life periods can result in a substantial variation in fish recruitment. Therefore, a better understanding of factors influencing growth and survival of fish eggs and larvae can help fisheries scientists better understand the variations in fish population sizes. Based on a literature review and laboratory experiments, this study investigated the size-dependent effects on early life stages (egg and larvae) of Atlantic cod (Gadus morhua). -- Egg size can be influenced by many factors including female size (age, length or weight), fecundity and seasonal temperature. Larval size at hatching is often related to egg size and incubation temperature. Size (stage)-dependent survival has been observed for larvae in many studies. Growth rate, which may be influenced by many factors including temperature and food supply, is one of the key factors determining larval size and mortality rate. -- For Atlantic cod, my study showed that larger eggs yielded larger larvae at hatching, but took longer to hatch. Larval size at hatching and incubation time were negatively correlated with incubation temperature. Although neither egg size nor incubation temperature was found to affect yolk size at hatching, higher accumulated incubation temperature significantly decreased the yolk size at hatching, but increased larval size at hatching. -- The larval survival and growth experiment showed that feeding conditions and larval size at hatching significantly influenced larval survival. Better feeding resulted in higher survival. The study found that the survival rate for small larvae was higher than that for large larvae, which might result from the absence of predators in this study. Higher temperature reduced the time of yolk utilization and thus caused the cod larvae to start exogenous feeding earlier. The growth rate of cod larvae during the exogenous feeding period is higher than that during endogenous feeding period. The first few days of growth mainly resulted in a significant increase in larval weight. Delayed first feeding significantly decreased the growth rate in cod larvae. However, the large larvae showed a higher growth rate compared with small larvae under the delayed first feeding condition. After a 10 to 13-day acclimatization, the larvae under delayed first feeding exhibited the compensatory growth. -- The size effect on cod larval growth was only significant in the delayed feeding condition, which implies that the bigger the better is more evident in cod larvae under unfavourable conditions, such as delayed initial feeding in this study.

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