SEA-FISHERIES LABORATORY. 221 
to a maximum in July-September. It also varies from 
ground to ground, and if its value is estimated for any 
particular ground from year to year it will be found to 
vary for corresponding months. It could be made use of 
to estimate the weights of plaice without weighing, but 
obviously the result so obtained would be approximate 
only. It is valuable from the point of view of affording 
‘* condition ”’ 
of the plaice inhabiting a fishing ground at a certain 
perhaps the best general expression for the 
time of year. It will vary according to the reproductive 
phase if mature fish are dealt with, and according to the 
seasonal nutrition-phase if immature fish are dealt with. 
The formula may also be used as a means of 
‘“smoothing’’ a curve representing the variation of 
weight with length in the case of a catch of plaice, but 
it is not very suitable for this purpose; for if one plots 
a series of points representing average weights and draws 
a curve as smoothly as possible through them, and then 
superposes a curve drawn from the formula, close 
agreement will not always be found. Generally 
speaking, the formula expresses with considerable 
accuracy the variation in weight within narrow limits 
(say 20 to 30 cms. of length), but for a catch of fish 
varying, say, from 15 to 35 cms. it will often fail. 
One can easily conceive of many purposes for which 
length-frequency and length-weight curves and equations 
may be used by a fishery authority of the future, seeking | 
for the most profitable continued yield of any species of 
edible fish from the grounds under its control. Such an 
authority would keep itself acquainted with the distribu- 
tion of plaice on the grounds from season to season, by 
means of samples, and would have an accurate knowledge 
of the weights of fish landed from each ground for short 
periods of time. Even at the present time it has been 
