Summary
1. The general structure of the gills of different fishes is compared and it is concluded that, though essentially the same, there are certain differences by which they can be recognized. Possible ways in which they may have evolved from one another are considered.

2. A detailed account is given of the structure of the secondary lamellae, where gaseous exchange takes place, and it is shown that two epithelial sheets are separated by a vascular axis mainly composed of pillar cells overlain by a basement membrane on each side. Blood pathways through the gills are discussed in relation to their respiratory function.

3. The embryonic development of gills is described and evidence regarding homo-logies of different structures, particularly the pillar cells, is reviewed.

4. The gills of fish having different modes of life show variations in (a) the number of arches, (b) the number and length of the gill filaments, and (c) the size and frequency of the secondary lamellae. Ways in which measurements of gill area may be carried out and some of the complications involved are reviewed and a summary given of measurements made for a wide variety of species. Measurements of the thickness of the water-blood barrier are also discussed; the more active fish generally have thinner water-blood barriers and larger gill areas.

5. The different mechanisms of gill ventilation are summarized and characteristics of gill resistance in elasmobranchs and teleosts are compared. Gas exchange is discussed in relation to available techniques and the current terminology and symbols, and to indicate the value of analogies between gill exchangers and systems studied by engineers.

6. It is outlined how studies of the functioning of gills during coughing, parasitic infection, and in polluted waters add to knowledge of their role in respiration.

The structure of fish gills in relation to their respiratory function

Thermal stress is defined as any temperature change that produces a significant disturbance in the normal functions of a freshwater teleost and thus decreases the probability of survival. As stress responses at the population and community level are very complex and poorly understood, thermal stress is consi­ dered only at the level of the individual fish and emphasis is placed on ecological, rather than physiological, effects. The thermal limits, optimum temperature range and preferred temperatures of freshwater teleosts are reviewed, and information on 27 European species is summarized. The concepts of tolerance and resistance to thermal stress are discussed and illustrated by original work on brown trout, Salmo trutta. Some comparisons are made between fish species with examples of cold-water stenotherms, mesotherms, and warm-water eurytherms. The temperature limits for spawning and cgg development are shown to be much narrower than the normal thermal limits for older fish. Some factors that may affect thermal tolerance are briefly discussed. The subtle effects of thermal stress within the normal tolerance range are discussed and illustrated by work on brown trout. Within the lethal limits, temperature can act as a loading stress by affecting functions such as growth and metabolism, especially when food intake is reduced. Temperature can also act as an inhibiting stress by affecting functions such as feeding and spawning. Changes in body temperature and thermoregulation are briefly discussed. As well as combating thermal stress by tolerance, resistance and metabolic adjustments, fish may be able to change their body temperature by physiological regulation in a few species and by behavioural regulation in many species. The latter response is one of the chief factors responsible for fish movements.

http://www.swrcb.ca.gov/water_issues/programs/tmdl/records/region_1/2003/ref1693.pdf

Some aspects of thermal stress on freshwater teleosts

Although rather extensive bibliographies give the impression that there is a vast amount of literature on the effects of temperature on aquatic organisms, when one tries to apply this information to specific interactions, such as the effects of temperature changes on chemical toxicity to aquatic organisms, often very little of the evidence is applicable. Although the most useful information on this relationship has been acquired in laboratory situations even this body of literature (which forms the bulk of this paper) is not adequate to make any scientifically justifiable generalizations. Field data on this relationship is almost non-existent and it is unlikely that much will become available unless specific studies are initiated which are directed toward this end. This is equally true of the laboratory information although it seems more likely that this will be generated as a spinoff from other research activities. In order to develop water quality management programs for steam-electric power plants one should understand the relationship between temperature and the response of aquatic organisms to toxic chemicals. Not only do some power plants discharge chlorine and other similar materials as well as heated waste water but those that discharge the latter only may be located near discharges of toxic chemicals. Since temperature and chemical stress to aquatic organisms are most commonly discussed independently we felt a paper covering this relationship would be useful.

The effects of temperature upon the toxicity of chemicals to aquatic organisms

The head and jaw movements involved in capture, buccal manipulation, ingestion and rejection of prey were investigated using sequential photography of juvenile Atlantic salmon feeding in a simulated stream environment. The results are described and discussed and mouth breadth and gill raker spacing are proposed as morphometric limitations to the range of prey sizes available which remains constant at 0·06 · fish fork length (PFR).



A recirculatory flume tank was used to study prey size selectivity behavior. Simplified downstream-drifting prey items elicited a variety of responses depending on their physical size. One hundred percent of offered prey of PFR 0·025 were ingested, while 90 % of prey at PFR 0·051 and 100% of prey at PFR 0·105 were rejected. It is demonstrated that fish show negative selection for prey sizes smaller than PFR 0·025 and that prey of this size elicits maximum growth response.



The validity of the proposed morphometric limitations on the available prey sizes is demonstrated by reference to selectivity behaviour and prey size related differential growth.

Morphological limitations, prey size selectivity, and growth response of juvenile atlantic salmon, Salmo salar

Biological early warning system based on the responses of aquatic organisms to disturbances: a review.

1. Trout subjected to changes in water temperature (1°/3 min.) between 15° and 30° C. showed a number of responses in their ventilation and cardiac mechanisms.

2. Ventilation rate increased slowly over the range 15°-21° C., but increased more rapidly at higher temperatures (21°-26°, Q 10=2.34; 26°-28°, Q 10=4.02). Cardiac frequency fell markedly about 26° C., and this bradycardia suggests that above this temperature the ventilation mechanism is inadequate to maintain a sufficient level of blood P o . Possibly this insufficiency results from a failure of the pumping mechanism to increase or even maintain a large minute volume at high frequencies.

3. Pressure recordings indicate those parts of the ventilatory mechanism which are mainly involved in these responses. Increases in the buccal, opercular, and mean differential pressures indicated that the volume of water pumped across the gills increased during the initial stages of warming and only at higher temperatures did frequency become involved.

Variability in the balance between the buccal and opercular pumps among individual trout becomes even more apparent under temperature stress, as in some cases the opercular pumps seem to be mainly involved in the increased ventilation but not always.

4. The action of the buccal and opercular pumps seems well co-ordinated, especially at intermediate temperatures, but serious uncouplings occur at higher temperatures as indicated by the shape of the pressure waveforms and especially the appearance of double reversals in the differential pressure curve.

5. The relationship between cardiac and ventilatory cycles was not studied in great detail, but there were certainly indications of changes in coupling. However, it was clear that the two rhythms do not always become phase-locked even at high temperatures.

6. The effect of changing temperature on the physical environment of the fish is discussed in relation to these observed responses.

J. L. Roberts

Papers

A Study of the Effect of Temperature Changes on the Respiratory Pumps of the Rainbow Trout