4-9-34 （4 沼口君）Controlled fisheries taking a limited number only of those fish that have had the opportunity to reproduce at least once may have a relatively small effect on population numbers.
4-9-35 （4藤田さん）Uncontrolled fishery with use of smaller mesh sizes for capture of immature fish, together with larger numbers of unwanted non-commercial species can have a more serious impact.
4-9-36 （4藤田さん）If this is combined with high pollution levels which both decrease the fecundity of the remaining fish and shorten their lifespan and therefore their number of spawnings, recruitment can no longer match losses and stocks will rapidly decline.
4-10-1 （4藤田さん）Pollutants can act at a range of sites within the reproductive system (Figure 4.10).
4-10-2 （4松島君）Pesticides, in particular, can damage the nervous system and so might be expected to damage the neurons involved in reproductive behavior, transmission of retinal or pineal signals of seasonal or diurnal cyclicity or in regulation of the hypothalamus.
4-10-3 （4松島君）There has been, as yet, no studies to determine whether these neural functions are affected to any greater extent than any other area of the brain.
4-10-4 （4松島君）Pollutants may also inhibit the enzymes involved in brain aromatization of testosterone which may be important in feedback signaling from the gonads.
4-10-5 （5八巻さん）They can affect the pituitary by altering its ability to synthesize and release gonadotrophin (GtH), thyrotrophin (TSH), adrenocorticotrophin (ACTH) and growth hormone (GH) which in turn will lead to decreased stimulation of the gonads, the thyroid and interrenal glands and general growth.
4-10-6 （5八巻さん）Such effects might be exerted by causing morphological damage to the pituitary, by affecting the hormone synthesizing mechanism or by inhibiting the feedback regulation by gonadal and thyroid hormones.
4-10-7 （5古川さん）Within the gonad, thyroid, interrenal and liver, pollutants can affect the activities of biosynthetic enzymes.
4-10-8 （5古川さん）Cytochrome P450 enzymes, which are involved in many steps of both gonadal synthesis and hepatic catabolism, are sensitive to a range of pollutants (see Chapter 9).
4-10-9（5古川さん） Morphological changes may also result from pollutant exposure, but it is necessary to distinguish between primary causes in which the pollutant directly affects the cellular integrity of the gonad, and secondary causes which result from under or over secretion of pituitary gonadotrophin.
4-10-10 （5本田君）Pollutants can also affect a range of developmental processes within the embryo and larvae.
4-10-11 （5本田君）In addition to their toxic effects, some pollutants may act as hormone agonists or antagonists
4-10-12 （5本田君）To act, a hormone must bind to a receptor.
4-10-13 （5本田君）In the case of large peptide molecules such as gonadotrophin, these are located in the cell membrane, but cause concentration changes within the cell of second messengers such as c-AMP or calcium.
4-10-14 （5山口さん）Small molecules such as steroids pass through the membrane and bind to receptors within the cell, which then initiate translation of relevant genes within the DNA of the nucleus.
4-9-15 （5山口さん）Such receptors may recognize a pollutant, such as an environmental estrogen, bind it and act in the same way as if they had bound the natural hormone.
4-10-16 （5山口さん）Such pollutant will act as hormone mimics.
4-10-17 （5山口さん）Alternatively the receptor may bind a pollutant, but fail to elicit the same changes that occur when it binds to the natural hormone, or it may be bound so strongly that it cannot be displaced by the natural hormone (Figure 4.11).
4-10-18 （5大川君） The anti-androgenic action of the DDT metabolite DDE in mammals (Kelce et al., 1994, 1995) may result from such mechanism where the receptor site is blocked and cannot bind the natural hormone.
4-10-19 （5大川君） In both cases, the relative affinity of the pollutant and the natural hormone for the receptor site will determine the activity.
4-10-20 （5大川君） In the case of membrane receptors for peptide hormones such as gonadotrophin, pollutants could affect either binding of the hormone to its receptor or the production and action of the second messenger.