4-9-11 In many of the studies which will be discussed later in this volume a single parameter, such as the GSI, has been chosen for ease of measurement.
4-9-12 Although this informs us that reproduction is most probably inhibited, it is not clear whether the primary site of action is the gonad itself, by blocking receptors or inhibiting steroidogenesis, or whether it is due to primary failure of secretion of the pituitary or blockage of the neurotransmitters in the hypothalamus or the brain itself.
4-9-13 Although gonadotrophin and the gonadal steroids play the major role in reproductive development, their actions may be modulated by a range of other endocrine factors (Figure 4.8).
4-9-14 Stress, for example, has a clear inhibitory effect on reproduction (Kime et al., 1980; Campbell et al., 1992, 1994), probably acting via adrenocorticotrophin (ACTH) or cortisol secreted by the interrenal gland (homologous to the mammalian adrenal gland).
4-9-15 Some of the reported effects of pollution on reproduction could be simply a stress effect.
4-9-16 The thyroid hormones, thyroxine (T4) and triiodothyronine (T3), also affect both reproduction and larval development, and any perturbation of the maternal thyroid gland by pollution could affect both ovarian development and, since maternal thyroid hormones are incorporated into the oocyte, larval development.
4-9-17 Action at the pituitary and hypothalamus might also affect secretion of growth hormone which in turn would affect development of both the exposed fish and its offspring.
4-9-18 In addition to direct effects on the reproductive endocrine system itself, it is also likely that other biochemical processes are affected.
4-9-19 Once the eggs have been spawned, they are under the influence of both external pollutants, and most importantly, of pollutants laid down into the egg itself during vitellogenesis.
4-9-20 Such exposure may affect ontogenesis, resulting in malformed and unviable larvae, or lead to death of either egg or larvae as critical stages of development are blocked.