168 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



that surface hauls take more eggs than deeper hauls. The present determination is 

 based on a series of horizontal hauls at different depths in 38 meters of water in the 

 offing of the Fire Island Lightship on May 19, 1929. 



Four series were taken: one at dawn, another at noon, another in the evening, 

 and the final series at midnight. The net was one-half meter in diameter at the mouth 

 and rigged with a closing device actuated by a messenger. It was lowered while 

 open, towed for 20 minutes, then closed and hauled to the surface. Each series 

 included hauls at the surface and at the 5-, 10- , 20-, and 35-meter depths. The 

 courses of the nets were kept as nearly horizontal as possible by periodical estimation 

 of depth based on measuring the towing warp's angle of stray and paying out or 

 hauling in the line as needed to keep the net at the proper level. Since the net was 

 lowered while open, and since the tripping mechanism failed on several occasions, 

 there was some contamination of the haul during its passage through the water over- 

 hang the stratum fished. Correction for this contamination was estimated on the 

 basis of the average concentration of eggs in the overlying water and the time it took 

 the net to pass through the overlying water in an opened condition. An additional 

 correction for variations in speed of towing, based on the angle of stray of the towing 

 warp, was applied to all catches on which data adequate for this purpose were available. 



1 Adjusted for time (20 minutes); speed (to cause stray of 28.5° in towing wire); and for contamination in passing through over- 

 lying strata in paying out and hauling in. 

 ' Not adjusted for speed. 

 ' Adjustment for contamination was large and probably inaccurate. 



As may be seen from figure 4, the numbers decrease rapidly with depth. When 

 the numbers from the several hauls at each level (exclusive of certain unreliable sub- 

 surface hauls designated as questionable in the figure) are averaged, the distribution 

 is as follows: surface, 22,000 per haul; 5 meters, 13,000; 10 meters, 8,000; 20 meters, 

 700; 35 meters, 0. Except for the surface hauls which were not adjusted for towing 

 speed, and certain of the subsurface hauls on which reliable corrections were impossible, 

 the successive hauls at each level yielded nearly the same numbers, indicating at once 

 the reliability of the method of sampling and the stability of the vertical distribution. 



Comparing the distribution of eggs with physical conditions, it is obvious that 

 eggs were abundant from the surface down to a depth of 10 meters, the range in which 

 temperature, salinity, and therefore density were approximately uniform. Between 

 10 and 20 meters the temperature decreased sharply, the salinity increased sharply, 

 and therefore the density increased sharply. In this zone of increasing density, the 

 mackerel eggs rapidly diminished in number so that at 20 meters few were taken and 

 below 20 meters, none. At this station, therefore, the distribution of mackerel eggs 

 was limited to the stratum above the pycnocline (zone of sharp increase in density). 



While this has been demonstrated in detail at only this one station, that it is a 

 general rule is indicated by subsequent experience with oblique hauls, where, with 

 several nets on the line, the deeper nets, when towed entirely below the thermocline, 



