of the Fishery 7 hard for Scotland. 



301 



When the mackerel were sent to the store the pins which had been 

 used in marking the limits of the dimensions were left in the fish. It 

 was thus possible on re-examination to measure to exactly the same points. 

 Any change found in the dimensions was due then to alteration in 

 the fish. It was found that a shrinkage had taken place in length, in 

 amount about 1 per cent. The fishes had lost in weight, in amount, 

 17, 3-3, 2-5, 2 -5, 3, and 3 per cent, respectively. It might have been 

 inferred that, in consequence of the shrinkage in length, a proportional 

 decrease would have been found in the various measurements. But this 

 was not so ; for while in certain cases exactly the same measurement was 

 obtained, in others the alteration in the dimension was greater than the 

 total shrinkage in length of the fish. In the case of the girth, that in 

 the pectoral region showed a decrease of 2mm. in some cases ; at the 

 second dorsal a decrease of as much as 8 mm. was found in the case of 

 No. 5 — in no example was this exceeded; and in the girth at the root 

 of the tail a shrinkage of as much as 4mm. took place. With the 

 exception of the preceding, and the distances of the dorsal fins, anus, and 

 anal spine from the snout — viz., d 1 D, d 2 D, d A, and d As — none of 

 the differences exceed 2-5mm. In the case of d 1 D and d2D, the greatest 

 differences are 3 - 5 and 3mm. respectively. In d A the difference is as 

 large as 5 '5mm., and so with d As. The latter is the greatest dimension 

 measured on the fish. Those fishes which show the extreme differences — 

 e.g., Nos. 2, 5, and 6 — in this character, also show considerable differ- 

 ences in other characters. Again, it is remarkable that, even although the 

 fish itself has shrunk in length 1 per cent. — e.g., No. 1 — the measure- 

 ment d As. remains at 23*5cm., which it was found to be before freezing. 

 If that measurement had decreased in the same proportion as the total 

 length had lessened, its size should have been 23'3. The fish was no 

 doubt contorted when it was in a fresh condition ; and the disarrangement 

 of its parts may, through its being frozen, have been lessened or 

 increased. It is at least evident that in the varied treatment 

 which a fish may receive in capture, or in packing, an amount of 

 contortion may occur which it is quite impossible to measure or 

 eliminate. This, then, militates against the accuracy of the body- 

 measurements, and must be kept in view in the discussion of the 

 results. The error due to the personal equation of the observer is 

 eliminated in a large series of measurements, when the characters them- 

 selves permit of accurate measurement. In the case of enumeration of 

 vertebras and fin-rays, where the numbers are small, as in the mackerel, 

 the personal error may be left out of consideration. 



It was further found that very often the measurements of certain 

 characters were of different values for the two sides of the fish. Thus, 

 for example, in 14 mackerel, measuring from 31-36cm., the greatest 

 difference between the lengths of the mandibles of the right and left sides 

 was 1mm. The most common difference was ^mm. In the case of the 

 dentary, measured on both sides in 15 specimens, in nine cases equality 

 was found, in one case the difference was 1mm., while in the others 

 it was '5mm. The length of the pectoral fin was found to be the 

 same on both sides in three out of six fish measuring from 34-36cm. ; 

 in two there was a difference of 1mm., and in the sixth a difference 

 of '5mm. As regards the length of snout, the greatest difference was 

 1mm.; this occurred in two specimens. In the lateral length of the 

 head, a difference of 1mm. was found; but this was not exceeded. 

 The distance of the pectoral fin from the snout varied as much as 

 2 -5mm. on the two sides; while in the distance of the ventral fin 

 differences of '5mm. (three specimens), 1mm. (one specimen). l*5mm. 

 (one specimen), 4mm. (one specimen). 



