diurnal c^uifCiza 



To determine the extent and nature of diurnal changes in the vertical 

 distribution of pilchard larvae, two special pairs of serial hauls were 

 run in June, 19i;l, the first of each pair being occupied around midnight, 

 and the other around noon. Due to unavoidable novements of the vessel the 

 two members of each pair v/ere not located at exactly the same point geograph- 

 ically, but approximately the same position v;as maintained and they were 

 probably in water of similar characteristics. 



One of the chief differences betvreen the night and day hauls was that 

 the former contained' many more large larvae than the latter (fig. k)' This 

 could result from any one of three causes; (1) location of the day hauls 

 in an area of lesser concentration thaji the night hauls, as a result of move- 

 ment .of the ship through the ^vater between the two series; (2) migration 

 of most of the larvae below the levels fished, during the daytime; (3) dodg- 

 ing of the net by the larvae, in the daytime. 



The first explanation is a highly improbable one, since the lai-ger 

 catches at night have been observed many tines previously, for other plank- 

 ton organisms and larvae of other species of fish. Also, our regular sur- 

 veys of the spavming grounds show the night catches of larvae to be con- 

 sistently larger than the day. 



The second explanation has been most \.ldely entc-rtained by others in- 

 vestigating vertical migrations. For instance, it was advanced by E. S. 

 Russel .(1926) to explain the larger catches of clupeoid and gobioid larvae 

 at night, but he v;as later forced to abandon it when subsequent daylight 

 hauls near the bottom (Russel, 1930) did not average larger catches than 

 the ones in the upper layers of irater. For our ovm data, the taking of at 

 least tv;o blank hauls (fig. k) below those which contained any larvae, argues 

 against the probability of a concentration of larvae belov: the levels sampled. 



Elimination of the first tir/o explanations loaves only the third - 

 dodging of the not. in the daytime. FJussel (1926) and Johansen (192.t) admit 

 this possibility, but doubt that fish larvae could swim fast enough for 

 successful dodging. On this point it may be well to consider hovrfast it 

 would be necessary for larvae to svam in order to get out of the way of 

 the net. All nets tov;ed in the conventional manner give warning of their 

 approach by means of the piece of line i.'hich attaches theiii to the towing 

 v/ire. In our gear this line v;as about $ meters long, v;hilo our average speed 

 of towing v;as around 50 cm. per second. The larvae v/ould thus have about 

 10 seconds to sv;im out of the iiray. Since the maximum distance they vrould 

 have to svfin v;ould be the radius of the net, or $0 cm., a sv;imming rate of 

 5 cm. per second would be sufficient if the direction of svdraraing vrere favor- 

 able. This does not seem, an incrcdibl-; rate for pilchard larvae of the size 

 in question (over 8 mm. in length). Likewise it does not seem unreasonable 

 that .they should sv;im away from tho center of disturbance, and therefore 

 in a direction favorable to escape from the oncoming not. 



186 



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