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Part III. — Eighteenth Annual Report 



Previous Work on the Races of Mackerel. 



The mackerel caught off the southern coasts of Ireland and England 

 have been investigated by Garstang. Owing to the fact that the present 

 research has been conducted on different Hues from his work, it is not 

 possible to institute a very close comparison. It may not, however, be 

 unprofitable to review the results obtained by that author, more especially 

 with regard to the correlation of the variations of the numbers of finlets 

 and dorsal fin-rays. 



Garstang established among the European mackerel which he examined, 

 two races, viz., the Irish (Kerry and Kinsale) mackerel and the North 

 Sea and English Channel variety. The characters upon which he 

 founded his comparison were — (1) the number of transverse bars ; (2) 

 the number of transverse bars which cross the lateral line ; (3) the 

 number of dorsolateral intermediate spots ; (4) and (5) the number of fin- 

 rays in the first and second dorsal fins ; (6) the number of dorsal finlets. 

 With regard to the first of these characters the author says : — " The bars 

 are not always distinct and parallel, but are frequently branched and 

 anastomosed with one another, broken or otherwise irregular, rendering 

 the task of enumeration not always easy, and so introducing a certain 

 subjective element into the records." In the case of the second character, 

 viz., the number of bars that cross the lateral line, another interfering 

 factor is present, namely, the irregularity of the lateral line in its course. 

 By means of the third character — the " spottiness " of the fish — he was 

 enabled to separate the American examples from the European specimens. 

 I have not adopted these characters for the present research, because in 

 such a case, where the colour-markings are so irregularly disposed, that 

 the two sides of the one fish generally differ in the arrangement of the 

 bars, too much will depend upon the observer, and too little upon any 

 rule which can be enunciated for guidance. Owing to this a comparison 

 between the results of different workers would be difficult, if not 

 impossible. As regards the number of rays in the first dorsal fin, I have 

 found a considerably higher average than was found by Garstang in 

 his specimens. The highest average which he found was 12*5 rays (in 

 the Scilly mackerel), while for Plymouth the average was 12*18. The 

 lowest average which I obtained was 13*4 (for the Clyde fish), while the 

 Aberdeen specimens gave 13*5, and the Barra and Stornoway, 13*76. 

 Moreover, the evidence accumulated by his investigation of this character 

 indicated that a reduction in the number of recognisable fin-rays accom- 

 panied the increase in length of the fish. This reduction, he is inclined 

 to believe, is due to the concealment of the minute posterior rays by the 

 encroachment of the surrounding tissues on the lower part of the fin. 

 Their concealment is not a sufficient description of the reduction in 

 number of the recognisable rays. The mackerel which I have examined 

 do certainly afford a slight indication of a reduction due to growth, as 

 the following Table shows ; but the evidence here furnished is not 

 conclusive. I have arranged all the mackerel of which I have noted 

 the number of first dorsal rays into two-centimetre groups. The number 

 of specimens in each, and the average for each group, are given alongside. 



[Table. 



