Maternal influences on egg quality and larval morphology, survival and growth of the batch-spawning atlantic cod (gadus morhua)

Bachan, Michelle (2011) Maternal influences on egg quality and larval morphology, survival and growth of the batch-spawning atlantic cod (gadus morhua). Masters thesis, Memorial University of Newfoundland.

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Abstract

Recruitment variability in fish populations is considered to be associated with the number and quality of eggs extruded by the female segment of the population, the size of larvae at hatch, and prey type, size and quantity at the start of exogenous feeding. Although a great number of studies have been undertaken to examine these aspects, very little information exists on batch-specific lipid allocation (i.e. lipid classes and fatty acids) in eggs of spawners of wild origin. Moreover, the associations of maternal attributes (e.g., egg size, batch sequence) with early life history traits of larvae under unfed and fed conditions have received little attention though it is perceived to be an important suite of recruitment processes during a critical life period. The purpose of this thesis was to quantify maternal patterns of lipid allocation to egg production and to assess the influence of female attributes on larval morphology, survival, growth and condition in a batch spawning fish, the Atlantic cod (Gadus morhua). Eight pairs of Atlantic cod of wild origin (held in captivity four months prior to the onset of the experiment) were allowed to spawn naturally in outdoor holding tanks at the St. Andrews Biological Station (St. Andrews, New Brunswick). A total of forty three egg batches were collected from all females and used to address the following two objectives and related experiments. -- My first objective was to determine the change in selected lipid classes and fatty acids of each egg batch over the spawning season for the eight females. Phospholipids were the predominant lipid class (40-86%) within eggs, with polar lipids accounting for 47-87% of total lipids and neutral lipids 15-52% of total lipids. Polyunsaturated fatty acids (PUFAs) made up 16-50% of total fatty acids, where the lower values were representative of samples with lower docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) content. It was shown that no common pattern emerged in the deposition of lipids over the course of the spawning season; three females consistently showed declines in lipid deposition parameters (µg/egg) with both batch number and egg dry weight, while one female showed consistent increases. These disparate trends were interpreted to reflect differences among individual females in relation to environmental foraging history, age, condition and years of reproductive experience. -- My second objective was to determine the effects of female and inter-batch differences on larval traits at 0 and 5 days post hatch (dph) and on larval performance under two feeding regimes. Inter-batch and female differences were evident; these differences were manifested in larval traits of unfed larvae at 0 and 5 dph and for larvae exposed to two feeding regimes. Egg size was positively correlated with larval size and other body measurements. Larval survival rates among two feeding regimes (1,500 and 4,000 rotifers/L; 2.7-fold difference) were highly variable ranging from 0 to 60% with mean survivorship near 15% in each treatment and did not differ significantly. Myotome condition index differed significantly between the two regimes and was greater for the better fed group. Both studies detected the presence of female and batch effects, indicating the important role female and batch number play in egg quality as well as larval morphology, growth and survival. -- In summary, a number of new findings were made of the reproductive biology and early life history of Atlantic cod that are of relevance to our understanding of recruitment processes of this important demersal species of the Northwest Atlantic.

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