Aspects of the physiology of North Sea cod (Gadus morhua, L.)
A year-round survey of an inshore cod population from Aberdeen waters showed that white muscle water content, endolymph protein concentration, Brockman body insulin content and the RNA/DNA ratio in both dark and white muscle, all show biannual variation. The principal phase occurs after the spawning period, in spring, the second phase occurs before it, in the autumn. It is suggested that, as well as growth, the restructuring necessary following spawning depletion in mature animals, takes place in spring. Starvation and refeeding experiments indicate the possibility that feeding rate could be the dominant influence on condition of muscle and liver during the summer. Sexually mature fish differ from immature fish in respect of seasonal variation in muscle water content, p.m. pH and relative liver weight. Fish of mature size, starved in an aquarium up to the spawning period, did not show significant differences in condition according to the presence or absence of a maturing gonad. From this experiment, there is no evidence that developing gonads are directly responsible for differences in the condition of mature fish, or for the autumn phase of changes, although such effects may only be seen when the fish are free to feed. The use of endolympla protein, insulin secretion and white muscle post-mortem pH as indexes of nutritional condition are discussed; all show potential but the limitations on use are not yet fully defined. In order to assess these methods, and the causes of variation in condition, the influence of exercise must be known. Difficulties were encountered for two reasons; (a) there is at present no way of measuring the amount of exercise wild fish may take over a period of weeks, and (b) several aspects of the physiology of exercise are still obscure 3 Three weeks of cruising at 0,8 L/s in a Flume increased the total haem pigment concentration in dark muscle by 76% compared with controls. The inshore Aberdeen cod population which was monitored throughout the year showed minimum haem values in summer, which were only slightly below those of the experimentally exercised fish. The possibility of developing these observations to assess swimming effort is discussed. The same exercised fish showed decreased SDH and LDH activities in dark and white muscle However fish exercised at .lower speeds, contained in a pen anchored in tidal currents, showed in June, reduced LDH activity; in November, increased aerobic-type and total LDH activity. Some responses were significant in white muscle, despite the supposed non-involvement of white muscle at such low swimming speeds0 The RNA/DNA ratio, for example, increased by 5 times in white muscle but by 2--4 times in dark muscle in response to exercise in the flume at 0.8 L/s. Electromyography confirmed the active contraction of white muscle at swimming speeds less than 0.8 L/s, The role of white muscle in cruising swimming is discussed. Control fish of the tidal current experiment showed livers which, relative to the gutted body weight, were half as heavy as those of exercised fish. There was no significant difference in the IC factor of exercised or control fish in June or November. Migrating cod from the Arctic shwoed significantly higher dark muscle haem pigment concentration when compared with non-migratory cod from Arctic and North Seas. SDH activity in dark muscle varied as haem in September, but not in June. Condition of cod from different grounds from the Minch to Bear Island was assessed in June, and it was found that the relative weights of Brockman body and liver, white muscle water content and pH in a single month, could vary between limits as great as the annual variation monitored in the population off Aberdeen. Possible causes are discussed. During two months without food, dark muscle water content rises 0.33% for every 1% rise in white muscle water. This infers the preferential conservation of dark muscle over white, and the greater rise in RNA/DNA ratio shown by dark muscle on refeeding indicates preferential restoration. Starvation for 160 days causes significant reduction of SDH activity in dark muscle but not in white, and a reduction of LDH activity which is more significant in white, than in dark muscle. The distribution of SDH activity in the muscle from anterior to posterior is consistent with the importance of dark muscle as a postural, and therefore essential muscle. Electromyography at swimming speeds from 0.5 L/s shows active contractions in dark muscle which continues well into speeds at which white muscle activity was recorded (around 0;8 L/s and above).