haulingo In Hawaiian waters the volumes of night hauls ^ usirtg a 200- 

 meter oblique towj have averaged about 1-g^ times the volumes of day- 

 hauls „ This is sufficient variation to obscure the geographical and 

 seasonal features of distribution which are of primary interest in this 

 studjTo An adjustment to remove the effect of diurnal change in volujrs 

 is the obvious solution to this difficultyo A suitable adjustment muat 

 not only take into account the contrast betvreen full daylight and night- 

 time conditions but also the intermediate dawn and twilight effects on 

 plankton volume o 



Presumably the difference bet^veen day and night hauls is dvie 

 either to an augmentation in the upper strata of water by upward migra- 

 tion of the plankton at night or to a reduction in catch in the daytime 

 owing to greater ability of the plankton to dodge the net vfhen there is 

 light, or to a combination of these twoo In any case, one would ex- 

 pect the amount caught to depend basically on the amount of plankton 

 generally present at the time and place of hauling and the diurnal change 

 to be a percentage of that amount » This being true^, the plankton volmnes 

 should either be expressed as ratios to the basic population level or as 

 logarithms o The logarithmic transformation is by far the most convenient 

 and has the additional advantage of correcting for the natural ske-'.vness 

 in the zooplankton volumes ivhen arranged according to frequency of occur- 

 rencej after transformation the frequency distribution more closely 

 approxinates a normal distribution » 



The authors are indebted to 0, Eo Sette for suggesting the 

 method, v;hich vie present here, for adjusting the zooplanktion data for 

 this diurnal variation- As we are net aware of any previous references 

 to this method in plankton literature ^ we will describe it in some de- 

 tails 



Method of Adjusting for Diurnal Variation 



To study the nature of the diurnal cycle it vjould be desirable 

 to have zooplankton hauls made throughout the day and night in the same 

 place or in the same water mass^ so that geographical ^ ecologicalj, or 

 faunal differences ^vould not obscure the diurnal cycle o Among the hauls 

 available to us the group resulting from I&nning cruise 8 most nearly 

 approaches this conditiono On this cruise a set of 9 stations on a 

 160-mile section on 158 25' Wo longitude, lying just west cf the island 

 of Oahu, was visited weekly for 4 weeks, 'wdth hauls made around the clock 

 as the stations were reached in consecutive orders 



If the logarithms of the plankton volumes are plotted as ordin- 

 ates against time of day as the abscissa, mthout regard to date or 

 locality and rdth midnight at the center of the abscissa, as in fig\ire 6, 

 it is seen that there is a period from about 1900 to 0600 hours v/hen 

 catches are high and a period from about 0800 to 1800 hours when they are 

 low« The hours from 0400 to 0800 and from 1600 to 2000 appear to in- 

 clude the periods of maximura change o A mathematical curve approximately 

 describing this type of change is the sine function, when midnight is 

 equated to the angle whose sine is / loO (fig- 7) o To fit this curve to 



15 



