[FONT=Times New Roman, Times, serif]Fish are more susceptible to stress than many other animals because of a greater dependence upon their surrounding environment (Wedemeyer, 1996a). The delicate surfaces of the gills come into direct contact with chemical toxins that cause stress; this is exacerbated in marine fish by the fact they actually drink large quantities of the water.[/FONT]
[FONT=Times New Roman, Times, serif]
[/FONT]
[FONT=Times New Roman, Times, serif]Four important body functions are closely associated with processes in the gills: gas exchange, hydromineral (osmoregulation) control, acid-base balance and nitrogenous waste excretion. These processes are possible because of the close proximity of the blood flowing through the gills to the surrounding water, as well as the differences in the chemical composition of these two fluids (for example, the salt content of fish blood is about 1/3 that of seawater and roughly 100 times that of freshwater).[/FONT]
[FONT=Times New Roman, Times, serif]
[/FONT]
[FONT=Times New Roman, Times, serif]Adrenaline released during the stress response increases blood flow to the gills to provide for the increased oxygen demands of stress. The release of adrenaline into the blood stream elevates the heart rate, blood flow and blood pressure. This increases the volume of blood in vessels contained within the gills, increasing the surface area of the gills while helping fish absorb more oxygen from the water. The elevated blood flow allows increased oxygen uptake for respiration but also increases the permeability of the gills to water and ions. This is what is known as the osmorespiratory compromise (Folmar & Dickhoff, 1980. Mazeaud, et, al., 1977). In saltwater fish, this leads to accelerated ion influxes and water losses. In freshwater fish, the effects are reversed; it increases water influx and ion losses. Small fish are more susceptible to hydromineral disturbances due, in part, to a higher gill-surface-to-body-mass-ratio than their larger counterparts.[/FONT]
[FONT=Times New Roman, Times, serif]
[/FONT]
[FONT=Times New Roman, Times, serif]One of the most characteristic aspects of stress in fish is osmoregulatory disturbance, which is related to the effects of both catecholamine and cortisol hormones. The extent of the disturbance following stress depends upon the ionic and osmotic gradients (difference) between the internal fluids of the fish and its surrounding environment (water). If the stress is persistent and of sufficient intensity, changes in the cellular structure of the gills may occur under the influence of cortisol. In this situation, increased death and turnover rates of branchial epithelial cells leads to accelerated aging of the gills. These degenerating and newly-formed gill cells do not function normally, which further limits the fish's ability to maintain water and ion homeostasis under stressful conditions. Thus, acute stress limits the fish's capacity to osmoregulate, and prolonged periods of extreme stress may result in osmotic shock and death.[/FONT]