A Study of Condition (Weight for Length) of Australian Barracouta, Thyrsites atun (Euphrasen)

Abstract

Regression of weight in the round (w) on length to caudal fork (I) was calculated for 6571 barracouta from southern Victoria and 2711 from south-eastern Tasmania. The equations for the regressions, which differ significantly in position but not in slope, are respectively w = 0.0572 l^2.360588 w = 0.1064 l^2.239524 The values of the exponents are unusually low.

A condition factor K_n was obtained for each fish by dividing the observed weight by the weight estimated from the regression, and geometric mean K_n was calculated for each month for each of the two regions. The mean for southern Victorian fish varied within the year in much the same way in each of three successive years, with three highs and three lows from early spring through early winter, but was consistently higher in 1950-51 than at the corresponding period of the year in 1949-50 or 1951-52. A similar pattern of within-year variation is suggested by the scantier data for other years, and likewise for south-eastern Tasmanian fish. All changes in mean K_n are closely paralleled by those in mean K_n,c the corresponding factor for beheaded eviscerated fish. It is also shown that changes in density of the fish are negligible. Thus the rises and falls in mean K_n represent true increments and decrements of growth in weight and volume of trunk muscle (edible flesh), unassociated with growth in length.

There is a tendency for fish with filling gonads to have better condition than ripe, spent, recovering, or immature individuals, but it does not explain the major part of the variation in condition observed. Neither is there any apparent relationship between mean K_n and percentage of the monthly sample affected by the sporozoan Chloromyxum or the cestode Tetrarhynchus, which cause the conditions known as "milky" and "wormy" and are the only parasites of barracouta that occur in the trunk musculature. It seems most likely that the changes in mean K_n signify a complex interplay between assimilation on the one hand and the energy demands of migratory and food-seeking activity on the other.

Oil content as a percentage of original (wet) weight was determined separately for fillet (edible flesh), viscera, and head of each of 66 south-eastern Tasmanian barracouta. There is a highly significant correlation between the oil content of each portion and the K_n of the fish. Regressions are given, and indicate that oil contents for fish of average condition (K_n = 1.000) are 4.39, 8-08, and 7.98 per cent. for fillet, viscera, and head respectively.