Publisher Summary This chapter discusses that the gonads potentiality to produce steroids, and the regulation of the syntheses. The nature, shape, and intensity of a hormonal signal, ready to be received by a target cell, is the result of an intricate series of positive and negative regulations. In the case of hormonal steroids in fish, only some aspects of this complex have been considered. Once a steroid is secreted, several mechanisms may inactivate it before it reaches its target. Little is known of catabolism of sexual steroids in teleosts. Most available data are concerned with the total radioactivity found in tissues after fish are fed labeled steroid. The biological significance of glucuronidation or sulfonation remains to be explored. Although the conjugated steroids are usually considered to be inactive, recent studies attribute a pheromonal role to glucuronides. In other respects, the binding to plasma proteins may lead to a reversible inactivation, although, in mammals, it has been suggested that steroid secretion may be enhanced by the presence of serum steroid-binding proteins. Finally, the conversion of plasma steroids into biologically active metabolites can occur in some target tissues. The actual physiological role of gonadal steroids in fish is discussed with emphasis on gametogenesis.

7 The Gonadal Steroids

The response of the brown trout to a single, short (c. 1 min) incidence of handling stress was monitored for a period of 1 month post-stress. Significant changes were found in feeding behaviour, in the levels of plasma cortisol, glucose and lactate, in the concentration of circulating lymphocytes and in the degree of epidermal mucification. No changes were detected in the growth rate and coefficient of condition, in the levels of plasma thyroxine, in the concentrations of circulating erythrocytes, neutrophils and thrombocytes or in the thickness of the epidermis. The time-course for recovery of each parameter was examined and from this it was concluded that a minimum of 2 weeks was required for complete recovery and a return to normality.

Recovery of the brown trout, Salmo trutta L., from acute handling stress: a time-course study

Publisher Summary This chapter describes the different aspects of the physiology of smolting salmonids. Many salmonids—the fish of the genera, Oncorhynchus , Salmo , and Salvelinus —are anadromous and undergo a distinct transformation prior to seaward migration. Several morphological and physiological changes occur during smolting. Silvering is because of the synthesis of two purines–guanine and hypoxanthine. The needle-like crystals of these substances are deposited in two distinct skin layers: one directly beneath the scales and the other deep in the dermis adjacent to the muscles. The salmonids, as many other temperate-zone animals, have a seasonally changing physiology that is manifested in the cycles of growth, precisely timed migrations, and the seasons of reproduction geared to the most favorable seasons for the birth and development of the young. There is a circannual rhythm of physiological changes associated with the parr–smolt transformation. After juvenile salmon reach a certain critical size, they become smolts with a capacity to hypoosmoregulate and grow in seawater and behavioral changes result in a downstream migration.

4 The Physiology of Smolting Salmonids

Publisher Summary This chapter discusses the concept of fasting and starvation in fishes. Fish can survive for a long time without food and for many species, a fasting period forms part of the natural life cycle. Winter months, spawning migration, and/or prespawning stage can all be naturally non-feeding periods. Numerous species can starve for many months and then recover fully after refeeding. These species are well adapted to mobilize their metabolic reserves and even body constituents to survive the periods of food deprivation. Specific effects of starvation on metabolism are dependent on multiple variables; including the species under consideration, the preferential tissues needed for metabolic stores, the quantity stored and their availability, and distinct routes of mobilization. In this sense, the pre-fasting diet may exert substantial influence on the metabolic events initiated by fasting. The chapter presents a brief overview of gross metabolic and enzymatic changes observed in fasting and starving fish. It elaborates various parameters of endocrine regulation during fasting and starvation in fishes. The chapter also discusses the influence of starvation on lipolysis and the mobilization of carbohydrate, proteins, and lipids.

Chapter 17 Fasting and starvation

Cortisol and its effects on plasma thyroid hormone and electrolyte concentrations in fresh water and during seawater acclimation in yearling coho salmon, Oncorhynchus kisutch.

Changes in plasma thyroid hormones following administration of exogenous pituitary hormones and steroid hormones to rainbow trout (Salmo gairdneri)

Chronically starved rainbow trout (Salmo gairdneri) showed a significant fall in liver size, total liver glycogen, liver glycogen concentration and plasma glucose levels. Liver lipid concentration did not differ significantly from controls although total liver lipid reserves fell during the first 40 days of starvation but had partly recovered after 65 days of starvation. Plasma cortisol and T3 levels did not show consistent changes concomitant with food deprivation over the 65 day period of the experiment. However, plasma T4 levels in fish starved for 40 or 65 days were significantly lower than comparably fed animals. The involvement of T4 in intermediate metabolic processes in salmonids is discussed.

Robert S. Milne

Papers

Changes in plasma thyroid hormones following administration of exogenous pituitary hormones and steroid hormones to rainbow trout (Salmo gairdneri)

Changes in plasma thyroxine, triiodothyronine and cortisol associated with starvation in rainbow trout (Salmo gairdneri)