HOBSON and CHESS: MOVEMENTS OF PRANESUS PINGUIS 



providing a comi)arison of the plankton between 

 day and night. These data (Table 2) demonstrate 

 the importance of sampling the plankton right 

 along with the fish. The methods of collecting 

 were as follows: Seven tows were made over 

 one 500-m course (duration approximately 15 

 min), about 600 m offshore from Arniel Island 

 using a 1-m net with a 0.333-mm mesh. Night 

 tows were made on 23 and 24 March at the same 

 time that the fish were collected for gut-content 

 analysis (Table 1). Day tows were made at 1600 

 h on 22 March and at 1200 h on 24 March. Six 

 of the tows were paired: on both nights and 

 during the day of 24 March one tow at 4 to 5 m 

 deej) was immediately followed by a second tow 

 with the net breaking the water's surface. On 

 the day of 22 March only the 4- to 5-m depth 

 was sampled. The analysis is of 10% subsamples, 

 one taken from each collection. Significantly, 

 except for a lone alphaeid shrimp in one gut, 

 every prey species found in the silverside occur- 

 red in the night plankton samples. 



Clearly there was a great difference in the 

 composition of the plankton between day and 

 night, and a lesser though significant difference 

 from one day or night to the next. Furthermore, 

 these differences are reflected in the feeding 

 habits of the silverside. 



The daytime samples had greater volume 

 (measurements by displacement: day volume — 

 mean 16 ml, range 11-26 ml; night volume — 

 mean 13 ml, range 10-15 ml), but this was due 

 mostly to there being more chaetognaths in 

 those collections. As shown in Table 2, there 

 were many more species in the plankton samples 

 taken at night, and most of the species present 

 in both day and night collections were more 

 numerous in the night collections. This is 

 especially true of forms taken as prey by the 

 silverside. Nevertheless, although many forms 

 taken by silverside were relatively numerous, 

 they contributed relatively little to the total 

 volume of the plankton samples, even at night. 



Major Prey Organisms 



The habits of the silverside are better under- 

 stood after a closer look at some of its major 

 prey. 



Hyp er/ii h c ni^cileiis is 



This amphipod was the major prey of the 

 silverside at the time of this study. Our collec- 

 tions indicate that it is present at the surface in 

 greater numbers at night. This widespread 

 species has been found elsewhere to move toward 

 the surface after dark, and retreat to deeper 

 water in daylight (G. J. Bi-usca, Humboldt 

 State College, Calif., pers. comm.). It was 

 equally numerous at the surface and at the 4- 

 to 5-m depth in all night collections, but 

 occurred in only 1 day collection, here in lesser 

 numbers (Table 2). 



Although H. bengalensis was the top-ranked 

 prey on both nights, it was considerably more 

 numerous in the gut samples on 23 March than 

 on 24 March. Significantly, it was also more 

 numerous in the plankton samples taken on the 

 23d (Table 2). 



Tretomphalus Stage of Foraminiferans 



Perhaps several species are represented here, 

 but all are characterized by a calcareous, gas- 

 filled float that buoys them at the surface 

 (Myers, 1943). Undoubtedly, this habit accounts 

 for this form's high incidence among prey of 

 the surface-feeding silverside. One would not 

 expect the tretomphalus to migrate vertically 

 between day and night, and the data indicate 

 they do not (Table 2). 



Although the tretomphalus were well repre- 

 sented in the plankton collections on both nights, 

 they were more numerous on 24 March; never- 

 theless, although their importance as prey on 

 the two nights was similar, they were the third- 

 ranked prey on the 24th and second-ranked prey 

 on the 23d. 



Myodocopid Ostracods 



Being relatively numerous in the night plank- 

 ton collections, but absent from the day plankton 

 collections, these forms, like the hyperiid, 

 apparently rise toward the surface in darkness, 

 and descend to deeper water in daylight. Myo- 

 docopids were prominent among the gut con- 

 tents on both nights, but whereas they ranked 

 only fourth on the 23d, they ranked second on 



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