19 



la'ii;h stages of gadoids 



nearly identical in the two bays to the North and South of the Firth; they are both 

 shallow water stations, and resemble one another in general features. The fish from 

 South Bay (Station VI) are on the average a little larger than those from North Bay 

 (Station II), but the mean difference is only a couple of centimetres. On the other hand, 

 there is a considerable difference in the case of Station VIII when compared with the 

 former stations; the median size of the fish is lO or 12 centimetres greater; in other 

 words, there is a distinctly larger proportion of larger fish. 



It is useful to consider that portion of the total catch which lies between the lower 

 and upper quartiles, and which we call the inter-quartile zone. Here we have just one 

 half of the entire body of fish, namely that central block which is left after deducting that 

 quarter of the whole catch which are represented by the smaller fish, and that quarter 

 which are the largest; as will be seen from the curves and from the Tables, this middle 

 50 % is generally more or less homogeneous : there is a comparatively small interval 

 between its lea.st and largest members, that is to say between the two quartiles, while 



Fig. 20. Median sizes of Cod 

 on the stations in tlie Fii-th of Forth. 



Fig. 21. Approximate ages of the 

 predominant classes of Cod in the Firth of Forth. 



beyond the inter-quartile zone the measurements tail off, especially towards the upper limit, 

 the smallest and largest fishes being distributed over a range of very diverse sizes. In 

 the case of Station II the inter-quartile zone lies between 26 and 49 centimetres, or say 

 10 and 20 inches. In the case of Stafion VIII, on the other hand, it lies between 33 and 

 60 centimetres, or say 13 and 24 inches. Now if we compare these sizes with the curve 

 already given for the rate of growth of Cod, wee see that for Station II the interquartile 

 zone corresponds to group II and to part of group I, while the interquartile zone for Sta- 

 tion VIII corresponds to group II and group III. It goes without saying, of course, that 

 both younger and older fishes than these are present upon all these stations, but the above 

 is at least a useful method of discovering and recording the age-groups that predomi- 

 nate upon and are characteristic of each locality. 



The data of Table II may be used in a still simpler way for plotting upon charts; 

 thus in figure 20 we show the median sizes of Cod within the Firth of Forth. It will be 

 seen that at all the stations which lie close inshore and in shallow water, the median 

 sizes are very nearly alike, ranging only from about 34 to 39 centimetres ; but they are 

 distinctly larger, without the difference being very great, at the four stations which lie near 

 and to the outside of the Isle of May. Again we show in figure 21, the predominate ages 



