FISHERY BULLETIN: VOL. 72. NO. 1 



degree of variation in tlie precise time that activ- 

 ity began or ceased relative to morning and 

 evening civil twilight (Table 2). Activity began 

 from 10 min before to 69 min after the start of 

 morning twilight. Cessation of activity, however, 

 was more variable, ranging from 222 min before 

 to 69 min after the end of evening twilight. Al- 

 though we were unable to arrive at the cause for 

 this variation, there were indications that cloud 

 cover and water transparency, both affecting 

 light penetration, might play a role. Our direct 

 scuba observations (135 h of observation) on 

 untagged tautog showed that the fish were active 

 during the day and inactive at night. Activity 

 as well as responsiveness at night were at such 

 a low level that we were able to touch individual 

 fish or catch them easily with a hand-held net. 

 Five fish (No. 1-5, Table 1), tracked at dif- 

 ferent times from July through September 1971 

 and 1972, exhibited similar j^atterns in their 

 daily movements. Each morning at the onset of 

 activity or soon after, the fish moved out and 

 usually remained within 500 m of the basin. 

 They spent most of each day at locations in 



which there were large concentrations of blue 

 mussel {Mijfihis cdnlis) (areas A2-A9, Figure 

 2; Table 1). Towards late afternoon or early 

 evening, the fish returned to the basin and with- 

 in 1 to 198 min (x= 55.7), settled in one location 

 and remained throughout the night in an inac- 

 tive state. 



Another fish (No. 6, Table 1) tracked during 

 this period returned to the basin the first night 

 after being released, following the same i)attern 

 as fishes 1 to 5. However, after s})ending most of 

 the second day in close proximity to the basin, 

 it did not return but rather, at 172 min prior to 

 the end of evening twilight, swam 6.2 km in 

 a direct easterly course to an artificial reef 

 (consisting of sunken barges and tires) where it 

 si)ent the night (area G, Figure 1). 



Underwater observations made during July 

 through mid-October showed that the number of 

 fish, measuring about 30 to 50 cm, in close prox- 

 imity to the basin increased just prior to and 

 immediately after the beginning of evening 

 twilight in comparison to the number that were 

 present during the day. Comparing these obser- 



Table 2. — Onset and end of the daily activity of individual tautog relative to morning and evening 



civil twilight (MCT and ECT'). 



' MCT: start of morning civil twilight. 

 ECT: end of evening civil twilight. 



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