Bidirectional mass selection for body shape in common carp

Summary - Bidirectional mass selection for the ratio of height/length generated a response in both directions. Estimates of realized heritability were 0.47 f 0.06 and 0.33 ±0.10 for up and down selection respectively and 0.42 t 0.03 for the divergence. This means that body shape of common carp can be changed fairly rapidly by mass selection. Presumably, differences in body shape among different genetic groups of common carp are due to different intensities and number of generations of mass selection for this trait practised in the past. The traditional belief that body shape and growth rate are correlated was not confirmed in this study. The down selection line, possessing the lowest ratio of height/length, showed the fastest growth rate. At face value this implies a negative correlated response of growth to selection for height/length. We suggest treating this finding with some reservation, since growth did not differ significantly among our test lines. These results may be applied to aquaculture by selecting for an increasing ratio of height/length only if high backed fish have some advantage in their own right such as a higher price or improved market acceptance.

finding with some reservation, since growth did not differ significantly among our test lines. These results may be applied to aquaculture by selecting for an increasing ratio of height/length only if high backed fish have some advantage in their own right such as a higher price or improved market acceptance. common carp / body shape / realized heritability / bidirectional mass selection Résumé -Sélection massale divergente pour la conformation chez la carpe commune.

INTRODUCTION
In the traditional carp culture management in central and eastern Europe, body shape and particularly the ratio of height/length is regarded as important. This trait responds to environmental influences (Anwand, 1961;Mann, 1961) and distinguishing carp races by their body shape is considered unreliable. Nevertheless, among common European carp races, Aischgrund and Galician are regarded as relatively high backed, and Lausitz and Frankish as having a relatively elongated body shape (Willer, 1933;Hofmann, 1975). The traditional method of selecting brood stock at the time of harvesting 3-year old fish consisted of picking out the largest individuals with the highest ratio of height/length (Hofmann, 1975). This practice was based on the belief that the ratio of height/length is correlated to growth, and that both growth rate and body shape respond to mass selection. High backed carp are also considered more aesthetic, with a market preference in certain areas.
The paper presented here is based on a thesis of the senior author (Ankorion, 1966). One of the aims of this investigation was to study the inheritance of the ratio body height/body length, and its correlation with growth rate.

MATERIALS AND METHODS
The realized heritability of the ratio height/length was estimated in a bidirectional mass selection experiment. &dquo;Height&dquo; is here defined as the maximum height of the fish, measured from the anterior base of the dorsal fin to the anterior base of the pelvic fin. &dquo;Length&dquo; is standard length, measured from the snout to the base of the caudal fin. Selection was carried out in a commercial production pond at the fish farm of Gan Shmuel, on market sized fish (then about 500 g). The pond was stocked with a sample of crossbred fry. The parents of this cross consisted of 4 females of the Beith Zera isolate, 3 males of the Misgav isolate and a further 3 males of the Aielet isolate (Wohlfarth et al, 1965). The base population thus consisted of a mixture 2 crossbreds, with a common female parent. Selection thresholds were determined from measurements of height and length carried out on a sample of The test was terminated after about 4 months by draining the ponds, removing all the fish, and sorting them according to their group marks. The fish of each group were counted, weighed and the height and length of each was measured individually and noted. This enabled computing survival, growth, height, length and the height/length ratio of these 3 groups of fish. The realized heritability of relative height was estimated from the ratio of selection response to selection differential and the standard error of the estimate was calculated according to Hill (1971). The correlated response of the component traits (height and length) to selection for the height/length ratio was estimated from mean differences in these traits between selected and random samples.

RESULTS
The results of this test, carried out in 3 communal testing ponds are shown in table I. Previously these groups had been nursed in separate nursing ponds to a size sufficient for differential marking (Wohlfarth and Moav, 1985). This resulted in random variation in mean weights among the test groups, that of the control being . somewhat higher (40 g) than those of the up and down select groups (25 and 29 g respectively). Survival of fish was between 75 and 90% with no significant differences between the groups. The apparent survival of >100% in one case (control, Beith Zera), was probably a result of error in identifying fish at harvest. Mean harvest weights at Beith Zera and Nir David were close to market weight (of that time); that at Gan Shmuel was much lower.
Correcting observed weight gains Observed weight gains (final weight -initial weight) of each group were corrected for the deviations of their initial weight, with the aid of a predicted correction factor computed separately for each pond (Wohlfarth and Milstein, 1987). The predicted correction factors were 4.49, 4.24 and 3.86 for the ponds at Beith Zera, Nir David and Gan Shmuel, respectively. Observed weight gains were corrected with the aid of the equation: Y' = Y-b(X -X.), where Y' and Y = corrected and observed weight gains, respectively; X and X. = initial weight of a given genetic group and mean initial weight of all tested groups, respectively; and b = the appropriate correction factor (Wohlfarth and Moav, 1985).

Realized heritability
The results of bidirectional selection for body shape are shown in figure 2. In the parent generation, the selection differentials of the up select and down select groups deviated by 0.049 and 0.030 units respectively around the mean. In the progeny, a selection response was generated in both directions, in each of the 3 replications. Estimates of realized heritabilities are shown in table II. Up selection generated a larger response than down selection, similar to the higher selection differential of the up selection group. Estimates of realized heritability (selection response/selection differential) generated in this study were 0.47 ! 0.06, 0.33 ! 0.10 and 0.42 t 0.03 for up selection, down selection and simultaneous bidirectional selection.
Correlated response of height and length Data on height and length of progenies of up selects, down selects and controls are shown in table III. Selection for the height/length ratio generated a response in both height and length. For each sample and in each pond, fish of the up select line had an absolutely greater mean height and an absolutely smaller mean length than fish of the control group. The opposite is the case for fish of the down select line.
A direct comparison between correlated responses of height and length requires standardization of the results, since mean length of the control group is close to 3 times as high as mean height. Standardization was carried out by dividing response by the absolute value of the control group, in each case (see relative response, last 2 columns in table III). These show that the correlated response of length is about 3.5 as high as that of height, and that the correlated response to up selection is about 2.5 as high as that to down selection. Realized heritabilities of height and length could not be estimated, since absolute data on height and length were not noted for the parent generation (see Materials and Methods).