Maternal Effects in Farmed Salmon

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Stress in sexually mature females affects mortality, growth, morphology and behaviour of offspring. Could the environment in which a ‘fish mother’ lives prior to spawning influence the traits and development of her offspring?

Prenatal stress can be defined as stress experienced by a sexually mature/pregnant female that can affect the development of the offspring. In mammals it has been shown that prenatally stressed offspring have higher rates of mortality, reduced growth and weakened immune systems. Such offspring also exhibit reduced learning ability, curiosity and playfulness, as well as sub-optimal sexual and maternal behaviour. The animals’ ability to deal with stressful situations or other strain can also be affected. Furthermore, morphological parameters were also shown to be affected by prenatal stress; increased occurrence of morphological deviations has been documented in prenatally stressed offspring. We have a fair amount of knowledge of how stress influences specific reproductive mechanisms in fish, but there are relatively few studies of how stress experienced by sexually mature female fish affects the traits of their offspring.

Most organisms, both plants and animals, are relatively symmetrical. Developmental stability reflects the ability of an organism to maintain stable development under given environmental and genetic conditions. Developmental stability is measured as the degree of fluctuating asymmetry (FA) and the frequency of phenodeviants. Fluctuating asymmetry (FA) is defined as small, random deviations from physical symmetry in traits that otherwise are bilaterally symmetrical. Phenodeviants are defined as relatively large deviations from the morphological norm, and these are rather seldom. Nevertheless, phenodeviants can occur to a considerable extent in certain environments, e.g., in commercial fish farming. The increasing degree of malformations, e.g., of the bones, head, scales, lateral line and inner organs, is a considerable problem in the fish farming industry. Asymmetry or malformations in an organism can be the result of stress exposure during ontogenesis, which caused the energy that was meant to be used in the formation of morphological symmetry to be diverted to cope with the stress instead. In general, one could say that “all weaknesses” accompany asymmetry. Asymmetrical and malformed individuals have reduced survival, poorer growth and fertility, reduced immunodefence and competitive ability, increased fearfulness and higher rates of behavioural anomalies. Morphological deviations thus imply reduced welfare, in addition to the reduced economic value of deformed fish. It is therefore desirable to limit the occurrence of such deformations. Several aspects of the fish farming industry have been shown to cause morphological deviations, but there has been very little focus on the issue of maternal effects.

In commercial fish farming, fish are exposed to numerous acute and chronic stressors throughout their entire life cycle. In addition, broodstock is regularly handled in connection with sexual maturity and stripping. Stressed females could have increased levels of stress hormones in their blood, which in turn could be reflected in their eggs. The stress hormone cortisol is assumed to affect the traits of offspring. It is already known that there is a correlation between the level of stress hormones in the blood and gametes of parent animals, but the question is to what degree and in what way do stress hormones in females affect the development of their offspring? Our studies are part of a larger joint project on farmed fish welfare. The project is being conducted in cooperation between the Norwegian Institute of Fisheries and Aquaculture Research in Tromsø, the Institute of Marine Research in Bergen and the Norwegian University of Life Sciences. We wished to study the effects of prenatal stress in farmed salmon, including the study of how additional environmental stress, modelled as an early incidence of temperature stress, affects the influence of prenatal stress. One of the main objectives was to examine if prenatal stress affects the occurrence of malformations in the offspring, in addition to studies of such parameters as mortality, growth and behaviour.

To stimulate prenatal stress, cortisol was implanted in female salmons six days before stripping. Their offspring were then monitored for one year. The results showed that offspring from the ‘stressed’ mothers had high mortality, reduced body length and small yolk sacs at hatching. They showed reduced growth early in life, but this was considerably compensated for later in their lives. Furthermore, offspring from the cortisol-treated mothers showed reduced developmental stability throughout the entire period, observed as increased occurrence of various malformations and increased levels of fluctuating asymmetry. These offspring also had a reduced ability to cope with an additional stressor. Behavioural studies showed that prenatally stressed offspring were less active when moved to an unfamiliar environment, which could be an indication of increased fearfulness among these fish. The results show that maternal hormones could have a significant impact on the traits and the development of farmed salmon offspring. These findings are interesting both academically and with regard to application in intensive fish farm operations.

Published: 07.04.06
Updated: 14.05.09
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