The limnology of Lake Kariba, especially those aspects that affect sardine biology, are described. The hydrological regime of the lake is seen as a major influence and may be one of the reasons why the fish are very much smaller in Kariba than they are in Lakes Tanganyika or Kivu. Growth was reassessed and there appeared to be a correlation between rates of growth and limnological parameters. In particular, the fish appeared to grow faster, but smaller, in the man-made lakes and this may be a consequence of food shortages brought about by their hydrological characteristics. Mortality rates were also much higher in Kariba and this tended to support the view that the lake is a relatively unsuitable habitat for the sardines. However, estimates of biomass, expressed in terms of lake volume, contradicted this as they were very similar in both Kariba and Tanganyika. The life history strategy of Limnothrissa in Kariba was therefore to reduce its size and increase its productivity in order to maintain the maximum possible biomass and its very high P/B̄ ratio was probably the means by which this was achieved. In terms of lake volume the predicted yields from the pelagic communities of Kariba and Tanganyika were very similar, equivalent to about 20 000 t annually for Kariba. This is close to the yield predicted by several empirical models and the fishery now appears to be entering a phase where increases in effort will not be matched by increased yields. The mechanisms for controlling effort in the fishery appear to be inadequate and particular concern was expressed about the lack of co-ordination between the Zimbabwean and Zambian fishery authorities in planning the development of this shared resource.

The systematics of rattail fishes (Teleostei: Gadiformes, Macrouridae) is reexamined focussing on the Coryphaenoides group of genera, including Albatrossia, Lionurus, Chalinura and Nematonurus. The data matrix consists of 69 osteological characters based on personal observations, 17 characters, generally of the soft anatomy, from various published sources and 34 characters reported from peptide mapping of muscle-type lactate dehydrogenase. An evolutionary systematics of morphology requires, firstly, a historical concept of homology and secondly, a scientific basis for the recognition of patterns. Viewing the organism as a hierarchy of constraint, homology is a relationship of development constraint inherited by parts of organisms. Taxa are types, relationships of constraint inherited by organisms. If, from the morphological perspective, taxa are relationships not groups, conventional concepts of monophyly and related terms cannot apply to them. In practice they describe comparisons between trees. The creation/discovery of patterns is embedded in the practice of systematics and has its basis in the intelligent abilities of human beings. Morphology deals with the linguistic aspect of evolution, rather than with its dynamic genetic aspect. Dynamic and linguistic aspects are complementary yet incompatible. The scientific status of morphology is shown to rest on this principle of complementarity. Through cladistic analysis of a large number of published characters, I investigate the scenarios and relationships of gadiform fishes that have recently been proposed. The results of the rattail analysis are thus placed within the broader context of gadiform ecology and evolution. In cladistics, parsimony plays the role of Popper's-empirical concept of simplicity, as a method of estimating the hypothesis of highest empirical support. Assumptions are made about the likely pathways of evolution in the way the characters are coded. Original classifications of the Gadiformes and the Macrouridae are proposed. Within the gadiforms there is a general trend from jaw precision to jaw protrusion. An index of protrusion/precision shows a negative correlation with depth. Rattails show low values of the index indicating high jaw protrusion. However, within the family the trend is towards higher jaw precision, and the precision/protrusion index is positively correlated with maximum depth. The discovery of cartilage in the exoskeleton of rattail fishes was an unforeseen result of the method of preparation. In rattails alcian blue reveals hyaline cell cartilage at the margins of certain dermal elements where it is gradually replaced by bone.

A continuous-time model, describing the interaction between a "'facultative"' predator and a prey is used to study the consequences of exploitation of either the predator, the prey or both. The model assumes a logistic growth pattern in absence of the interaction. The change in steady-state levels of the associates due to different combinations of fishing intensities are described mathematically and diagrammatically. The effect of density of predation, on the equilibrium populations associated with the yield of either, or both, the predator and the prey are studied. Yield curves were constructed for both the predator and the prey under different fishing intensities of the other associate. The model is given in a discrete-time form, of which the stochastic version is derived to show the effect of intrinsic variability. The stochastic version was simulated on the computor through the use of random normal deviates. Fair agreement between the calculated values of the variances of the steady states, and those empirically found through simulation is listed. This simple structural model reveals that in such an association, fishing the prey population alone gives a lower maximum sustained yield than can be maintained, if the predator is also fished. It is hoped that such a model, which is a preliminary but a necessary step, will lead to a more comprehensive model applicable to natural fish populations. / Science, Faculty of / Zoology, Department of / Graduate

The preferred salinities of five Pacific salmon species were studied. Each species was shown to undergo a temporal sequence of preference changes. The sequence began with a preference for fresh water then changed in the direction of increasing seawater concentration, the terminal pattern indicating a preference for water of open ocean concentration. This preference sequence was shown to parallel closely the horizontal salinity gradients typical of estuaries through which juvenile salmon pass on their seaward migration. On the basis of this evidence the following orientation mechanism was postulated: that juvenile salmon are able to use estuarine salinity gradients as one of the directive cues in their seaward migration. Further study of this orientation mechanism showed that the initial part of the preference sequence develops unaffected by seawater exposure. By contrast the latter part of the sequence (corresponding to seaward end of the estuary) was found to depend on a period of exposure to seawater otherwise a regression to a premigratory freshwater preference took place. The sensory stimuli leading to the salinity preference response were shown to depend on a complex interaction of naturally occuring sea salts. Experimentally the simplest salt mixture which would elicit a normal response consisted of two cations (Na⁺ and Ca⁺⁺) and one anion (Cl¯). It was shown further that taste or the common chemical sense was the primary sensory modality underlying the response and that juvenile salmon have an ability approaching absolute salinity discrimination. Speculation concerning the evolution of the salinity preference orientation mechanism was presented. Published evidence favours the view that migratory salmonids evolved from nonmigratory forms with limited osmoregulatory abilities. On this basis it was proposed that originally the ability to orient with respect to seawater concentration was of direct survival value to the stenohaline ancestral salmonid. Later, as diadromous movements expanded along with euryhalinity, salinity preference became integrated into a temporal sequence of changes and thereby an orientation device useful for migration. / Science, Faculty of / Zoology, Department of / Graduate

Treatment of gonadectomized adult male and female sticklebacks as well as normal juvenile, with methyl testosterone, either by immersion in a hormone solution or by implanting hormone pellets, induces the kidney cells to develop into granular and mucous cells, accompanied by an increase in cell height. Increase in cell height, up to a limit, is a function of the length of hormone treatment. Prespawning aggressiveness and territoriality is induced in gonadectomized fish of both sexes by maintaining them under long photoperiod for four to five weeks. Short photoperiod has no effect. Administration of methyl testosterone to the already aggressive fish showed no definite effect on this behaviour. The combined effect of long photoperiod and methyl testosterone treatment induces the nest-building behaviour in gonadectomized males and females with a much higher percentage in the former than in the latter. The component elements of the nest-building movement in the treated gonadectomized male is similar qualitatively and quantitatively to that of the normal reproducing male. Treated gonadectomized females show deviations from the normal male in their nest-building movements. Sexual behaviour is the same in castrates treated with methyl testosterone as in the normal reproducing male, but completely absent in the similarly treated gonadectomized females. / Science, Faculty of / Zoology, Department of / Graduate

The seasonal salinity preference of four species of Pacific salmon was examined. Each species showed a strong preference for hypertonic seawater during the normal period of migration. Pink fry (Oncorhynchus gorbuscha) and coho yearlings (O. kisutch) lost this preference during the summer in contrast to Chum fry (O. keta) and sockeye yearlings (O. nerka). Three other types of behaviour showed seasonal changes consistent with a transitory "migration disposition". A preference for hypertonic seawater was associated with high levels of activity, strong schooling tendencies and depressed aggressive behaviour. Subsequent seasonal changes showed a marked increase in aggressive behaviour accompanied by decreased levels of activity and group behaviour. A long daily photoperiod (16 hours) prolonged the behaviour complex associated with seaward migration. A short daily photoperiod (8 hours) delayed but did not totally inhibit the development of a hypertonic salinity preference and associated behaviour. The preference of chum salmon fry for a series of seawater concentration indicated an all-or-none type response. A consistently strong preference was shown for seawater hypertonic to plasma chloride levels as reported in the literature. No preference was shown for hypotonic seawater. A series of experiments in which the composition of an artificial seawater was altered indicated that under natural conditions the expression of a preference for salt water probably depends on the concentration of sodium chloride. The swiftness of the response (chum and sockeye) indicated stimulation of a peripheral salinity receptor. Coho underyearlings injected with mammalian somatatropin showed an increased although not statistically significant preference for hypertonic sea water. Both activity and aggressive behaviour were depressed in comparison to control fish. / Science, Faculty of / Zoology, Department of / Graduate

Energy demands for osmotic regulation and the possible osmoregulatory role of the thyroid gland were investigated in the euryhaline starry flounder, Platichthys stellatus. Using a melting-point technique, it was established that flounder could regulate body fluid concentration independent of widely divergent environmental salinities. Small flounder experienced more rapid disturbances of body fluid concentration than large flounder after abrupt salinity alterations. The standard metabolic rate of flounder adapted to fresh water was consistently and significantly less than that of marine flounder. In supernormal salinities standard metabolic rate was significantly greater than in normal sea water. These findings agree with the theory that energy demands for active electrolyte transport are greater in sea water than fresh water. Thyroid activity was studied in flounder adapted to fresh water and salt water. Correlative with the higher metabolic rate of small flounder was the more rapid turnover and excretion of radioiodine and greater thyroid uptake of small than large flounder. Percentage uptake of radioiodine by the thyroid was shown to be an insensitive and inaccurate criterion for evaluating thyroid activity in different salinities because removal rates of radioiodine from the body and blood differed between fresh water and marine flounder. Using thyroid clearance of radioiodine from the blood as a measure of activity, salt water flounder were shown to have much greater thyroid clearance rates and, hence, more active thyroid glands than flounder adapted to fresh water. The greater activity of the thyroid of marine flounder correlates with greater oxygen demands in sea water and suggests a direct or adjunctive osmoregulatory role of the thyroid gland of fish. / Science, Faculty of / Zoology, Department of / Graduate

The relationship between osmoregulatory stress and locomotor performance has been investigated in juvenile salmonids during their adjustment to sea water. Transfer from fresh water to sea water produced a statistically significant depression of the maximum swimming speed of chum salmon fry (Oncorhynchus keta). The effect of sea water was immediate and reached a maximum fourteen hours after transfer. From thirty-six to eighty hours (the duration of the experiment) relatively stable performance levels were recorded. Some recovery from the initial effects of sea water was apparent but the swimming speeds of "recovered" fish were significantly lower than those of fry in fresh water. Statistically significant correlations between swimming speed changes and changes in total body chloride and water concentrations have been demonstrated. Since chum fry were too small to allow separate sampling of plasma and tissue, the Steelhead trout (Salmo gairdneri gairdneri) was used to investigate the sequence of events in the osmoregulatory adjustment of salmonids to sea water. Transfer into sea water was accompanied by increases in plasma concentrations of chloride, sodium and potassium, but not of calcium. Cellular dehydration resulted from transfer of cellular fluids to the extracellular phase, and from loss of water to the environment. Cellular levels of calcium and sodium rose markedly. Smaller increases in cellular potassium and chloride were noted. Changes in cellular sodium and calcium were primarily due to ion uptake. Increases in cellular potassium were the result of cellular dehydration since tissue levels of this cation fell slightly in sea water. The data indicate that impairment of the efficiency of physiological processes sensitive to altered electrolyte concentrations, and the utilization of energy in the restoration of body fluid electrolyte concentrations to regulated levels may account, at least in part, for reduced locomotor ability in salmonid migrants during their adjustment to sea water. Body size was shown to influence the extent and duration of changes in plasma and tissue electrolyte concentrations and distributions. In chum fry the governing factor appeared to be size, larger animals adapting more rapidly, and undergoing less extensive changes than did smaller fish. In Steelhead trout efficiency of osmoregulation was highest at certain stages of growth, and suggested that adjustment to sea water was best achieved during a restricted phase of their life history. This stage appeared to be that of late parr-smolt transformation. The operation of plasma electrolyte homeostatic mechanisms was indicated in the Steelhead trout and the possible roles of the withdrawl of ions from the circulating fluids by complex formation and by active excretion have been considered. / Science, Faculty of / Zoology, Department of / Graduate

Life history events of chinook salmon preclude determination of a critical size for this species by established methods. The use of size, rather than age, as a basic correlate of growth rate is discussed and compared to analagous treatment of physiological rates described in literature. Ecological opportunity and physiological opportunity are visualized as the two interacting components that determine growth, both of which are related to size attained. Growth opportunity occurs in stanzas which are entered at "threshold" sizes. The function, dw/dt = kw(x) is developed into a growth equation for linear dimentions, 1(z/t+1) = ɑ + (1z/t) and three methods of fitting this equation to growth data are demonstrated. Application is explored and discussed using steel-head trout and chinook salmon as examples. Significant differences in growth rate were found between life history types and sexes. The chinook data were then treated on a 1(t+1), 1(t) plot and it was shown how an apparent fit of the von Bertalanffy type growth equation can result from selectively fishing for the larger fish of any brood year. Accordingly, life history subgroups of a year class must either be treated separately or weighted according to relative abundance in determining critical size. The former alternative is followed in lieu of necessary weighting data. Natural mortality of a chinook population is estimated from the pattern of tag recoveries, taking advantage of the fact that maturity occurs at different ages for individuals of a year class and that the fishery operated mainly on maturing individuals. Annual instantaneous natural mortality was estimated to lie in the range 0.3 to 0.4. The growth equation was then transformed to a length-specific average annual instantaneous growth (weight) rate and critical size was observed to occur at maturity for each life history type. Since fishing is presently allowed on the immature stock, a size limit protecting the older life history types causes a loss in yield from the younger life history types. This loss might be offset, depending on the relative abundance of life history types in the stock, providing mortality due to hooking and releasing is negligible. Capture by trolling was found to subject feeding coho and chinook salmon to hyperactivity which may lead to a distressed condition or death, and death cannot be predicted from examination of individual fish at time of capture. Mortality of coho was estimated to be in the 0.95 confidence interval of 34 percent and 52 percent; of chinook in the 0.95 confidence interval of 40 percent and 71 percent. Time of maximum death rate is shown to coincide with the period of maximum blood lactate response. Survival occurred either when blood lactate did not reach critical levels (above 125 mg%) or reached critical levels and subsequently subsided. Holding salmon in a live box for 8-14 hours before release did not improve tag recovery, suggesting additional indiscriminant stress was caused at release. Adult coho in freshwater did not appear capable of lethal hyperactivity. This led to the hypothesis that cessation of feeding during spawning migration has adaptive significance for survival of Pacific salmon. The combination of natural mortality, mortality from hooking injury and delayed mortality from fatigue gave a total instantaneous first year mortality rate (exclusive of fishing) greater than 1.0 and possibly as high as 2.5. This mortality rate results in a critical size of not more than 22.5 inches and most likely about 15.0 inches fork length. It is thus concluded that for maximum yield in pounds (1) fishing for chinook should be restricted to their ultimate year (maturity) and (2) the use of non-selective gear should be encouraged. These recommendations are opposite to present practices. If fishing is to be allowed on the immature stock, size limits should be abolished. / Science, Faculty of / Zoology, Department of / Graduate

Aspects of ventilation and ventilatory control were investigated in an air-breathing fish, Amia calva, to determine the extent to which Amia is similar to water-breathing fish. The possibility that Amia uses the air-breathing organ to maintain gas-exchange during periods of aestivation was tested. During gradual air-exposure, Amia showed no reduction in oxygen consumption, no increase in plasma urea levels or in urea excretion. Arterial blood pH (pHa) remained constant, and arterial plasma total carbon dioxide (TaCO₂) and carbon dioxide partial pressure (PaCO₂) increased. Arterial plasma total ammonia (Tamm) and NH₃ concentrations rose significantly. Exposure to elevated total ammonia concentrations in the water did not elicit an increase in urea production or air-breathing. Aquatic hypoxia without access to air did not cause a reduction in aerobic metabolism and moderate levels were fatal. These results indicate that Amia are incapable of aestivation, due to an inability to reduce metabolism and detoxify ammonia to urea, and die following three to five days of air-exposure. The air-breathing organ is used to maintain aerobic metabolism under aquatic conditions of hypoxia or raised temperature. The characteristics of air-breathing and gill ventilatory responses to internal acid-base disturbances were investigated in Amia. Acid infusions lowered pHa and arterial blood oxygen content (CaO₂), raised PaCO₂, and stimulated air-breathing and gill ventilation. Ammonium bicarbonate infusions did not change pHa or CaO₂, raised PaCO₂, and did not stimulate any ventilatory responses. Acid infusions during aquatic hyperoxia lowered pHa and raised PaCO₂. Arterial blood O₂ content declined but remained above normoxic levels. There were no ventilatory responses. These results indicate that air-breathing and gill ventilation responses in Amia are most closely correlated with blood O₂ status, not pHa or PaCO₂. Air-breathing and gill ventilation responses following acid infusion were associated with a release of catecholamines into circulation. Catecholamine infusion stimulated gill ventilation but not air-breathing in Amia, suggesting that endogenous catecholamine release may have mediated gill ventilatory responses to hypoxaemia. These ventilatory reflex responses to acid-base disturbance, and the correlation between gill ventilation responses and catecholamine release are similar to observations made on water-breathing fish. Ventilatory responses to increases in TaCO₂ and Tamm were investigated in rainbow trout, and compared with responses by Amia. In trout, infusion of NaHCO₃ raised pHa and TaCO₂, did not change PaCO₂ or CaO₂, and stimulated ventilation. Infusion of NH₄HCO₃ did not change pHa or CaO₂, raised TaCO₂, PaCO₂ and Tamm, and stimulated ventilation. Infusion of NH₄Cl lowered pHa, raised Tamm, and stimulated ventilation. Infusion of HCl lowered pHa, TaCO₂ and CaO₂, and stimulated ventilation. Infusion of NaOH raised pHa but did not stimulate ventilation until twenty minutes post-infusion. Infusion of NaCl had little or no effect on pHa, CaO₂, TaCO₂ or Tamm, and no effect on ventilation. These results indicate that trout show a ventilatory response to increases in TaCO₂, increases in Tamm and decreases in pHa and CaO₂, but not to increases in pHa. Following HCl and NaHCO₃ infusion, there was a significant increase in the level of circulating catecholamines, indicating that the ventilatory responses to reductions in pHa and CaO₂ and increases in TaCO₂ may be Immorally mediated by catecholamine release. The ventilatory responses to increases in Tamm were not associated with a catecholamine release. Unlike trout, Amia do not show a ventilatory response to infusion of NH₄HCO₃, i.e. to increases in TaCO₂ and Tamm. Sites and afferent pathways for ventilatory reflex responses to blood and water O₂ status were determined in Amia. Air-breathing and gill ventilatory reflex responses to hypoxia, sodium cyanide (NaCN), hypoxaemia and catecholamines were investigated in intact Amia, and compared with responses in animals following section of branchial branches of cranial nerves IX and X, and extirpation of the pseudobranch. In intact, sham-operated animals, hypoxia stimulated an increase in air-breathing and gill ventilation. Following denervation, the air-breathing response was abolished, and the gill ventilation response significantly attenuated. In sham-operated animals, NaCN in the water flowing over the gills stimulated air-breathing and gill ventilation, and NaCN given in the dorsal aorta stimulated gill ventilation. These responses were abolished following denervation. In intact animals, HC1 infusion stimulated air-breathing and gill ventilation, but following denervation, the air-breathing response was abolished. The ventilatory response to catecholamines was significantly attenuated in denervated animals as compared with shams. These results indicate that air-breathing and gill ventilation reflex responses are controlled by oxygen-sensitive receptors in the gills and pseudobranch, innervated by cranial nerves VII, IX and X. These sites and afferent pathways are similar to receptors controlling hypoxic reflex responses in water-breathing fish. The effects of catecholamines on gill ventilation are mainly exerted via stimulation of receptors in the gills, which are separate from those controlling air-breathing. The gill ventilatory responses to hypoxia, hypoxaemia and acidosis following denervation may be mediated by central effects of circulating catecholamines, or by an extrabranchial oxygen or pH receptor. In conclusion, Amia is an entirely aquatic animal with the primary ventilatory control mechanisms of water-breathing fish intact, but with the added ability to breathe air at the surface. / Science, Faculty of / Zoology, Department of / Graduate