THE GROWTH OF FISH IV. THE EFFECT OF FOOD SUPPLY ON THE SCALES OF SALMO IRRIDEUS BY J. GRAY, F.R.S., AND S. B. SETNA, PH.D.

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1 55 THE GROWTH OF FISH IV. THE EFFECT OF FOOD SUPPLY ON THE SCALES OF SALMO IRRIDEUS BY J. GRAY, F.R.S., AND S. B. SETNA, PH.D. (Zoological Laboratory, Cambridge.) (Received 5th April, 90.) (With Three Text-figures and Two Plates.) CERTAIN scales of Salmonoid fish exhibit a series of concentric ridges or circuli, the number of which increases with the size of the whole fish. Each year of normal growth is recorded on the scale of a salmon by a concentric band which is more or less sharply divided into two regions, (i) in which the concentric ridges are relatively wide apart, (ii) in which the ridges are closer together. It is tolerably certain that the first region is formed whilst the fish is in the sea, and the second region whilst the fish is in estuarine or fresh water; roughly speaking the two regions mark the summer and winter periods respectively. The factor or factors responsible for the difference between winter and summer circuli are not fully understood. There are two possibilities. The variation in width may be dependent on an inherent rhythm upon which external factors may exert little or no control; or the variation may be due to variations in the external environment of the fish. In view of the high economic importance of the facts, curiously little experimental data are available for a solution of the problem. Cutler (98), working with Pleuronectes platessa and P. flesus, altered experimentally the temperature and food supply in the fishes' environment. Fish were kept in "hot" and "cold" tanks, the latter being 5 0 C. cooler than the former, and at each temperature the fish were fed once a day. The effect of food supply was determined by a comparison of fish fed with excess of food twice a day with fish fed sparingly every other day. Cutler's results, as summarised by Graham (99), were as follows: Table I. Tanks Control Hot Cold Abundant Scanty Length increment in cm. Sclerite number Sclerite width in n (average) O"I 9 ± 9±* 04 0 ± ±i 04 9±i 8± 07 ± IO±I 0-8±f 9±i

2 56 J. GRAY and S. B. SETNA These data suggest that high temperature increases the sclerite width without influencing either the growth rate or the number of the sclerites. Abundant feeding, on the other hand, increases the growth rate and the number of sclerites, without marked effect on the width. Cutler's observations are not readily harmonised with those of Thompson (9), Dannevig (95) and Graham (99), since these authors found a marked correlation between sclerite width and growth rate. In a state of nature, there can be little doubtthat the growth rate of fish during the summer is greater than that during the winter. If the sclerite width is associated with growth rate, it follows that wide sclerites will be formed in summer and narrow sclerites in winter; at the same time the controlling factor might either be temperature (Cutler, 98) or food supply (Thompson, 9). The most satisfactory way of attacking the problem is by the observation of fish under controlled conditions of temperature and food supply. For this purpose trout form a satisfactory material, they can readily be reared in captivity and the concentric ridges on the scales are usually well defined. The experiments here described were designed as a direct test of the effect of food supply on the width of the circuli of Salmo irrideus; no attempt was made to keep the temperature constant, although of course it remained the same for each sample of fish under observation. The work was carried out at the Midland Fishery, Nailsworth, Gloucestershire; to the manager of the farm, Mr F. Stevens, we owe our sincere thanks for valuable co-operation and help. It is significant to note, at the outset, that in no case which we have examined do the scales of rainbow trout, which are fed by hand continuously throughout the year, show any well-defined summer or winter zones (see PI. II, figs., ). This fact seems to eliminate the suggestion that the periodicity of circulus width found in other members of the Salmonoid family (when under natural conditions) is due to an inherent rhythm over which the environment has no control. It also suggests that temperature alone is not invariably a decisive factor. For the present experiment, fifty rainbow trout {Salmo irrideus) were taken from the yearling class on July 5th, 98. These fish were approximately cm. long. Twenty-five of these fish were placed in a concrete tank (6 ft. x 6 ft. x 6 ft.), whilst the remainder were kept in a similar tank below the first. The water supply to the two tanks was derived from the surface of a lake measuring about an acre. The amount of water that passed through the experimental tanks was 50 gallons per minute. The top tank represented the starved tank. The fish in this tank were not fed, but maintained themselves on the plankton available in the water. Although the amount of food the fish thus captured was undoubtedly far below their maximum requirements, they showed no symptoms of unhealthiness throughout the period of experiment. Early in December 98, the water entering the scanty tank was made to pass through twofine screens, so that the amount of natural foods that passed into the tank was still further reduced. The fish in the lower tank were fed twice daily with as much food as the fish would eat. They were given a diet of meat (raw and cooked) together with biscuit and fish meals. This tank was well stocked with natural foods (shrimps and snails).

3 The Growth of Fish 57 The water flowing through both experimental tanks varied in temperature according to the season of the year, although the extremes for the monthly averages (Table II) were not as great as those to which fish are often exposed under natural conditions. The daily temperatures were recorded; the maximum being 6 0 F. and the minimum 4 F. The monthly averages were as follows: Table II. 98 F. 99 F. July August September October November December January February March April May For an examination of the scales, Winge's (95) method was employed, the scale being mounted, on a microscope slide, in glycerine. An eyepiece micrometer was focussed on the centre of the scale and the number of ridges in the direction of the anterior longitudinal radius read off, together with the distances apart of successive ridges. The chart shows the total number of ridges and the variations in spacing of the ridges in the anterior quadrant. The scales for examination were removed in each case from the same region (shoulder), and the extent to which they were magnified was the same throughout. The fish were placed in the tanks on July 5th, 98; the first scales were taken on September 8th, 98. Scales were removed from fish selected at random from each tank. Charts were prepared and representative scales were preserved in 0 per cent, formalin. Material of this type was collected at monthly intervals, care being taken to avoid undue disturbance to the other fish in the tanks. Individuals used for scale examination were not replaced in the tanks. In order that an experiment of this type should yield useful results it is essential that growth should occur at a measurable rate in each tank. If the amount of food present in the scanty tank falls below a critical level, the fish will not grow since the available food is all required for maintenance: on the other hand, the food supply must be definitely lower than in the tank in which food is abundant. However carefully the conditions are controlled, there is always a marked variation in the ability of individual fish to grow on artificial or natural diets, and this variation is accentuated when food is scarce. The twenty-five fish placed in the abundant tank increased in length from approximately cm. in July 98 to 0 cm. in November 98, and to cm. in March 99. These fish under normal hatchery conditions would have reached an average length of 9 cm., although a significant amount of variation might be expected. It is, however, quite clear that an abundance of food in the experimental tank resulted in a very rapid growth rate. In the scanty tank, on the other hand, it is impossible to give even approximately comparable figures. The fish under these conditions are best divided into three categories: (i) those which grew comparatively steadily and attained in February 99 a length of 6-5 cm.;

4 58 J. GRAY and S. B. SETNA (ii) fish which grew very much more slowly; (iii) fish which showed little or no increase in size, particularly during the latter months of the experiment. Marked differences of this type might well be expected, and probably indicate a variation in the ability of the fish to obtain the food available. An examination of selected scales revealed quite definite data. PI. II, fig. i, shows a scale typical of thefishas placed in the tanks in July 98; PI. II,fig., shows a scale from a fish reared under hatchery conditions precisely similar to those of fig., but killed on January 5th, 99. These two scales may be regarded as typical of fish reared under standard conditions of feeding. It will be noted that, although the latter fish was two years old, the scale exhibits no well-marked seasonal variation in the spacing of its ridges; the spacing tends to increase towards the periphery of the scale, in spite of the fact that these rings were formed during the winter months. The contrast between these scales and some of those obtained from fish fed with abundant food is quite definite. A definite percentage of scales removed from the abundantly fed fish at approximately monthly intervals from September 98 to March 99 show that the peripheral rings are markedly wider apart than those nearer to the centre of the scale, and are clearly wider than those characteristic of standard hatchery conditions. PI. Ill,figs. 6,7,8, and Text-fig. are typical records of such scales from the hand-fed fish. It is, of course, difficult to determine precisely those ridges which were laid down after the period of abundant feeding began, but the change from normal width to wider widths is usually fairly abrupt and can be safely assumed to represent the period of rapid growth under the experimental conditions. Table III. Distribution of circulus width for peripheral circuli. Number of circuli measured on each scale: width in n A Abundance of food B c D E F Total A Scarcity of food B C, Fi Total Total 5 I. 8 b b 4 4 b ' l 8 6 A.M.= = 5 ±8 A.M. = =6 ±6 Taking the outer twenty ridges on each selected scale as revealed by the Winge charts, the variation can be expressed by the graph in Text-fig.. It will be noted that, although the total number of fish observed was small, the effect of the food supply upon the spacing of the ridges is sufficiently clearly marked to leave little or no doubt of the conclusion to be drawn, or of the fact that wide rings can be laid down in winter months. In order to compare these results with those obtained

5 The Growth of Fish 59 i i i I i i i i B C D WV \ G ir Text-fig, i. Curves of scales showing the successive widths of circulus from measurements of individuals kept in the "abundant" tank. Each minor subdivision of the ordinates represents 5/; each subdivision of the base line indicates a circulus.

6 6o J. GRAY and S. B. SETNA from the "scanty" tank it is necessary to select from the latter each of the specific types mentioned on p. 58. Firstly, we may consider the scales from a fish which grew steadily on the reduced food supply. PI. Ill,fig.9, is a typical scale of such a fish killed on February th, 99, when it had attained a length of 6-5 cm. With one exception the distance between successive ridges are all less than 40 ft, the average width being approximately /*. It will be noted that the peripheral rings, on the right side of the scale, are markedly narrower than the average. The scales of fish (from the scanty tank) which grew more slowly exhibit the same phenomenon, viz. the peripheral rings are narrow, and are of the order of /x or less (see Textfigs. and and PI. II, figs., 4, 5). It will also be noted that the outer rings on the scales of slowly growing fish are often incomplete, and are more irregular in form than those characteristic of well-fed individuals Scanty food U-Abundant food Width of ring in /t Text-fig.. Showing the distribution of concentric rings of varying widths at the periphery of fish fed with abundant and scanty food respectively. Although the population concerned is not as large as might be desired, the facts seem fairly clear, (i) Abundant food can effect an increased growth rate, and during this period the ring width in some scales is increased, (ii) The formation of wide rings can occur during the winter, (iii) Scarcity of food entails a reduction of ring width. It is also noticeable that several of the fish in the abundant tank spawned during the winter, but the fact is not recorded by any disturbance of ring formation. The results of the present experiments obviously differ from those obtained by Cutler (98), but are in closer harmony with those of Thompson (9), Dannevig (95) and Graham (99), all of whom recorded a correlation between growth rate and sclerite width. It should not be inferred, however, that temperature is without its effect. Hathaway (97) has shown that the ability of fish to feed at low tempera-

7 The Growth of Fish 6 "ures is markedly less than at higher temperatures; in other words, although food may be present in abundance, the amount actually absorbed may be greatly reduced at a low temperature, and in this way the growth rate will be affected. In addition, temperature may affect the growth rate in another way. The food which is actually J I I i I I I I I I I I I I I I I I I I I I I I I I I rrr TTT i i i MIL G Text-fig.. Curves of scales showing the successive widths of the concentric rings from measurements of individuals kept in the " starved " tank. Each subdivision of the ordinates represents 5/x. absorbed by the fish is utilised in two ways, firstly for the maintenance of existing tissues, and secondly for the formation of new tissue. If a fish absorbs a unit quantity of food (a) at a low temperature, we may suppose that a definite fraction (xj) is required for maintenance, leaving the remainder (a x x ) for the production of new tissue. If the temperature be raised, the rate of metabolism increases so that Xi is i i i

8 6 J. GRAY and S. B. SETNA increased to x leaving a smaller proportion (a x ) for the purposes of growth. As shown by Gray (98) for the embryonic cycle of the trout, the effect of high temperatures is to produce larger organisms for each standard diet, although the time required for their growth is increased. As far as one can see, under natural conditions, the growth of post-embryonic trout will be influenced by temperature in several ways: (i) a higher temperature may induce a more rapid formation of nutritive organisms and hence increase the quantity of food available; (ii) it will increase the capacity of the fish to capture the food; (iii) it will increase the rate at which the food is converted into new tissue; (iv) it will decrease the proportion of acquired food which is available for growth. The net result will depend on the equilibrium which exists between these various effects. SUMMARY.. Salmo irrideus fed continuously throughout the year exhibit on their scales no seasonal periodicity in the distance apart of their concentric ridges or rings.. Similar fish fed with abundant food form on a proportion of their scales abnormally wide rings even during the winter months.. Fish fed with limited diet develop abnormally narrow rings. 4. The width of the rings is probably ciosely associated with growth rate. BIBLIOGRAPHY. CUTLER, D. W. (98). Journ. Mar. Biol. Assoc., 470. DANNEVIG, A. (95). Rep. Norwegian Fish. Invest., 6. GRAHAM, M. (99). Fishery Investigations, Ser.,. GRAY, J. (98). Brit. Journ. Exp. Biol. 6, 5. HATHAWAY, E. S. (97). Ecology, 8. THOMPSON, H. (9). Fisheries, Scotland, Set. Invest. 5. WINGE (95). Medd. Komm. Havunsdersegelser Fisk. 4, 8. EXPLANATION OF PLATES. PLATE II. FIG.. Scale characteristic of a fish placed in the experimental tanks in July 98. Length of fish, cm. FIG.. Scale of a fish of the same age as experimentalfish but fed under standard hatchery conditions. Fish killed on January 5th, 99. Length of fish, 9 cm. FIG.. Scale of a fish removed from scanty food conditions on September 8th, 98. Length of fish, cm. FIG. 4. Scale of a fish removed from scanty food conditions on December 5th, 98. Length offish, 5-8 cm. FIG. 5- Similar to the above, but killed on February th, 99. Length of fish, cm. PLATE III. FIG. 6. Scale of a fish fed on abundant food from July 98 until September 8th, 98. Length of fish, 0 cm. FIG. 7. Similar to Fig. 6, but killed on November 7th, 98. Length of fish, 5 cm. FIG. 8. Similar to Figs. 6 and 7, but killed on March th, 99. Length of fish, -5 cm. FIG. 9. Scale of a fish which grew steadily on scanty diet from July 98 to February th, 99. Length of fish, 6-5 cm.

9 JOURNAL OF EXPERIMENTAL BIOLOGY. VOL. VIII, PLATE II. Fig. i. Fig.. Fig- - Fig- 5- J. GRAY AND S. B. SETNA THE GROWTH OF FISH (pp.55-6).

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11 JOURNAL OF EXPERIMENTAL BIOLOGY. VOL. VIII, PLATE III. Fig. 6. Fig. 7. Fig. 9. J. GRAY AND S. B. SETNA. THE GROWTH OF FISH (pp. 55-6).

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