No. 2] 
OF THE NORTH SEA. 
13 
The I-group. 
The material of the I-group is more abundant than that of any 
other of the age-groups treated in this paper. Especially some sum¬ 
mer samples display a lot of individuals, up to 206 and 156 in two 
samples from the Firth of Forth and more than 50 in two samples from 
the west coast of Denmark. 
As this group has a length of more than 10 cm., the lower limit 
of the capacity of the trawl, it would thus seem as if the material 
might give good observations as to the length-frequency of the age- 
# 
group at the time of the cruises. As shown below the conditions do 
not, however, fall quite as simply. 
In fig. 5 I have made a comparison of the length-frequency of 
the I-group of the principal samples taken during the summer cruises. 
As seen I have grouped them according to locality and depth, where 
the samples are taken. By far the larger part of these samples are 
taken by trawl, which as stated above of this age-group is supposed 
to take samples representative of the composition of the shoals. 
The first frequency-curve shows the length of the individuals of 
of the I-group of a sample taken by trawl in the Firth of Forth at a 
depth of about 21 m. The number of individuals is 206. The average 
length is 12,1 and the mode of the curve lies by 12 cm., but some few 
individuals are up to 15 cm. long. 
In the next part of the figure the length-composition of the I-group 
of the samples 2, 3 and 6, also from the Firth of Forth, are found 
represented. These samples are taken at a depth of about 50 — 53 m. 
The samples 3 and 6 show very marked modes at 14 cm. and the 
average length is 14,1 and 14,7 cm. respectively. The sample 2 display 
an intermediate length-composition, with the mode by 13 cm., but the 
14 cm. group is also well represented. The continuously-drawn line 
represents the average length composition of the 3 samples from 
deeper water. As seen at once there is a difference in the mean length 
of the two groups of about 2 cm., and remembering what is stated 
above of the capacity of the trawl as regards the catch of this special 
age-group, we come to conclusion that a pronounced difference in 
length-composition is actually to be found between the shoals in the 
deeper and those in the shallower parts of the Firth of Forth. 
