13 



It is clear that the value of the variable characters increases 

 the more the species to be compared diverge from one another. 

 In other words the variable characters will be of more use in 

 comparing the brown trout, which is not migratory, with a 

 migratory form such as the salmon, than they would be in 

 comparing two closely allied migratory species, such as salmon 

 and salmon trout. 



The determination of the species of any given mature fish 

 of the genus sal/mo in this colony is divested of much of the 

 complication which has arisen as to European specimens 

 because three species only have been brought to Tasmania, 

 (that is to say), the salmon (Sal mo salar), the salmon trout 

 (Salmo trutta), and the common, or brown, trout (Salmo farid). 



In ascertaining the species of the fish caught at Bridge- 

 water, which I shall hereafter call " the Derwent Grilse," a 

 very cursory examination is enough to demonstrate that it is 

 not a common, or brown, trout (Salmo fario), as it differs 

 materially from that species in both variable and constant 

 characters. The Derwent Grilse is nearly 20 inches long 

 without the caudle fin, and weighs two pounds fourteen ounces. 

 It is a healthy well-fed fish, as is abundantly proved by its 

 small short head and brilliant silvery sides. It is a female 

 with the ova so far advanced towards maturity as to be as 

 large as No. 5 shot. A well conditioned brown trout of 

 the same length would weigh far more, as its depth and thick- 

 ness would be greater in proportion to its length. A specimen 

 of brown trout now in the museum, which was taken immedi- 

 ately after spawning, and is in the most wretched condition, 

 measures 19 inches in length and weighs nearly four pounds. 

 Another specimen of which we have a plaster cast, measured 

 26 inches in length and weighed nearly nine pounds. 



The difference in colour is most striking, the Derwent Grilse 

 being bluish black on the back, passing gradually into brilliant 

 silver on the sides, which are marked with numerous irregular 

 spots above, and a few similar spots below the lateral line — 

 the spots are none of them ocellated as in the brown trout, 

 but appear to be below the scales. Although many of the 

 large female brown trout caught in the tideway about .New 

 Norfolk, are remarkably silvery when taken from the water, 

 they acquire a coppery hue, when dead, utterly unlike the 

 brilliant silver retained by the Derwent Grilse and which is 

 only to be found in perfection on specimens resident in, or 

 fresh run from, the open sea, in which the brown trout could 

 not live an hour. Again the fin rays of the Derwent Grilse 

 agree with Dr. Giiuther's formula for salmon and differ from 

 that for brown trout. 



Turning to the constant characters it will be found that in 



