Abstract
In fish, the lipid resources of the female form a link between the environment and progeny, contributing to the contents of the egg yolk. Variation of the environmental conditions is therefore expected to affect the egg quality via maternal pathways, reflecting the female’s response to the environmental factors before spawning. We investigated the content of lipids and thyroid hormones in the ovary and eggs of the Baltic herring during 1988–2019, when salinity of the Baltic Sea first declined and then stabilized to a lower level, and winters varied between severe and mild. The total lipid resources of spawning females decreased by 40–50% during the study, and the ovarian lipid concentration followed this trend resulting in a decrease of the lipid content of eggs. The concentration of thyroid hormones in the ovary suggested a hormonal response in females to salinity and winter temperature and was observable also in the content of thyroid hormones in the eggs. A change in the egg lipid content took place approximately around 1998 (SE ± 3 years) suggesting that in declining salinity, egg quality was associated with salinity and intrinsic factors in the female, i.e., fecundity and amount of lipids deposited into the ovary. However, during the period of stable salinity, egg quality was mainly limited by the female’s lipid resources. As also the body size of the females decreased over the years, it is suggested that small body size can be a key phenotypic trait to reduce the environmental impact on egg quality in variable environmental conditions.
Highlights
The variability of climate is a major factor contributing to the fluctuation of fish populations and stocks affecting especially the recruitment of small-sized pelagic fish like anchovies, sardines and herring (Lehodey et al, 2006; Peck et al, 2021)
The lipid reserves of the female form a relatively direct link between the environment and progeny (Brosset et al, 2016), since during oogenesis, the female deposits lipids into the ovary, where they form the main constituent of the egg yolk and act as carriers of hormones and vitamins to the eggs and larvae (e.g., Reading et al, 2018)
The mean length of females analyzed for the lipid concentration decreased over the years, somewhat less than in the spawning population, where their average length decreased from 19 cm to about 16 cm during 1988–2019 (Figure 2)
Summary
The variability of climate is a major factor contributing to the fluctuation of fish populations and stocks affecting especially the recruitment of small-sized pelagic fish like anchovies, sardines and herring (Lehodey et al, 2006; Peck et al, 2021). In these species, environmental factors modify the reproductive success when the eggs and sperm are released into the water, but they act through the parent population by affecting the females’ capacities to produce viable offspring. The thyroid hormones (THs) are the mediator of maternal effects in all vertebrates (Ruuskanen and Hsu, 2018), and in fish, they contribute to the development of the embryo and larvae in various ways (Ayson and Lam, 1993; Yoo et al, 2000; Power et al, 2001; Liu and Chan, 2002; Castillo et al, 2015; Deal and Volkoff, 2020)
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