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Part III. — Seventeenth Annital Report 



the temperature has risen about 0-5 deg. F. above the minimum mean 

 temperature that the gurnards begin to make their appearance at the 

 outer stations. 



An examination of the details for each station at which gurnards were 

 caught in March shows that as a general rule they were caught in 

 years when the bottom temperature was highest, and that when the 

 bottom temperature was lowest no gurnards were obtained. 



In April, as we have already seen, gurnards were caught at all stations, 

 the general average per haul being 11*1. The mean bottom temperature 

 in this month is 42*2 deg. F., or 2*2 deg. F. above the mean for March. 

 An examination of the temperatures at each station in each year shows 

 the operation of the general law, most of the gurnards being caught when 

 the temperature was highest, and few or none when it was lowest. This 

 applies without exception to the inner and intermediate groups of 

 stations, except that in 1890, when the temperature was highest, no 

 gurnards were caught at Station I. or Station V. 



The migrating gurnards move into the Forth with considerable rapidity, 

 and it is possible that this movement takes place chiefly at night. At all 

 events, by far the most gurnards are got on the bottom in daylight. 

 Thus, in a number of hauls which were made by night and by day at the 

 same stations in the Firth of Clyde, at intervals of usually a few days 

 between the day and night hauls, the proportion of gurnards captured 

 was very different. In 20 hauls in daylight 513 gurnards were caught, 

 an average of 26, and no haul was blank ; 20 hauls at night caught 95 

 gurnards, an average of 5, and thirteen hauls were blank. 



In May, when the bottom mean temperature is increased by 3 '4 deg. 

 F. over the mean of April, the abundance of gurnards is greatest (Table 

 I.) ; but from this period, although the temperature of the water goes on 

 increasing until it reaches its maximum in August, the number of 

 gurnards diminishes. It is noteworthy, however, that in August, and in 

 some places in September, there is again an increase in their numbers. 

 The reason of this will be indicated later. It occurs in the inner parts 

 of the Firth of Forth, in St. Andrews Bay, the Moray Firth, and the 

 Firth of Clyde. About the middle of August — in some years a little 

 earlier, and in some years a little later — the temperature of the bottom 

 water in the Firth of Forth and St. Andrews Bay, and in the inshore 

 waters on the East Coast generally, begins to decline. The fall is at first 

 very gradual, so that the temperature lor September is not very much 

 below the temperature for August, but in October the fall is very marked, 

 amounting to about 2 deg. F. under that for September. It is at this 

 time that the gurnards begin to leave the inshore waters for the open sea. 

 Just as their inshore migration in spring is associated with the initiation 

 of the rise in temperature, so is their offshore migration associated with 

 the fall in the temperature. At first, in September, their uumbers are 

 not greatly diminished, but in October the exodus is marked, and the 

 mean temperature is about 2 deg. F. below that of the previous month. 

 In November, when the temperature has sunk about 5'5 deg. F. below the 

 maximum, comparatively few gurnards remain. In December, a few still 

 linger in the inshore waters, with the mean temperature nearly 9 deg. F. 

 below the maximum. In January, when the mean temperature is about 

 1 1 deg. F. below the maximum, they may be regarded as absent from the 

 Firth of Forth and St. Andrew's Bay. Only one small specimen was 

 caught in the Firth of Forth (see p. 211) ; another was got at Liston 

 Bank in the open sea, 22 miles east from the Isle of May. 



The close connection between the changes in the temperature in the 

 Firth of Forth area and the abundance and migratory movements of the 

 gurnards is shown in the adjoining diagram (Fig. 2), which also shows the 

 spawning period (see p. 222). 



