BIOLOGY OF THE ATLANTIC MACKEREL 211 



Only one current meter was available, and this was used in the upper of the two 

 nets. When more than one net was on the line, the flow through the lower net was 

 assumed to be the same as that through the upper net except as modified by clogging. 



Four degrees of clogging were recognized according to the following definitions: 

 — When net is hauled to deck, water runs freely out of net and cod-end so that no 

 water is left by the time the net reaches deck. 1 — -Water runs out of net freely but 

 out of cod-end slowly so that some water is left in cod-end when net reaches deck. 

 2 — Water runs out of net so slowly that it remains above level of cod-end coupling 

 when net reaches deck, but falls to level of coupling after a short interval of time. 

 3 — Entire net visibly covered with clogging organisms and water stays above coupling 

 so that special means must be taken for washing down net. 



By the graphical partial correlation method (Ezekiel, 1930, pp. 143-145), it was 

 found how much the relation between the angle of stray and the quantity of water 

 strained was modified by the various degrees of clogging. The amounts by which 

 clogging changed the average rate of flow for given angles of stray was +0.032, —0.03, 

 — 0.073, and —0.108 meters per second for cloggings of 0, 1, 2, and 3, respectively, on 

 the clogging scale as above defined. For the hauls made without current meters in 

 the nets, these values were added to the theoretical flow as estimated from the angle 

 of stray. The magnitude of these corrections is given by their percentage relations 

 to the average rate of flow, which were +8, —1, —18, and —26 for the respective 

 degrees of clogging. These, of course, are averages for each of the 4 degrees of clogging. 

 The extreme individual values were plus 37 percent and minus 29 percent, which 

 indicates that the total flow through an extremely clogged net at times was only half 

 as much as through a very clean net. Since the clogging is progressive during a haul, 

 it is obvious that practically no water is strained toward the end of any haul in which 

 the net becomes badly clogged. The hauling method employed in tbis work, there- 

 fore, would undersample the upper layers relative to the lower layers. This would 

 be a serious difficulty if clogging were often severe, but during 1932 only 4 percent of 

 the hauls were of third degree and 15 percent of second degree clogging; hence uneven 

 vertical distribution of sampling did not often occur. No adjustment was made for 

 trim fifi Apt 



ENUMERATION OF EGGS AND LARVAE 



Eggs and small larvae were so abundant in many of the meter-net catches that a 

 sampling method was necessary to estimate the total numbers caught. The formahn 

 preserved plankton catch was transferred to a wide-mouthed graduated receptacle, 

 enough liquid added to bring the level to a certain mark (often 2,000 cc), the contents 

 stirred vigorously to mix uniformly, and a dipper then plunged into the mixture and 

 withdrawn level full. The dippers were of the type made for dipping cream, each 

 comprised of a small straight-sided cup with a long handle. Several sizes of dipper, 

 each of known capacity, were used and one or several dipperfuls taken, depending on 

 the size of sample desired. All fish eggs and larvae were removed from the sample. 

 From the remainder of the catch, all larvae larger than about 5 mm. in length were 

 removed. From the 2-meter net catches all the larvae were removed. 



Mackerel eggs and larvae were separated from those of other species and further 

 examined, counting the number of eggs at each of three stages of development and 

 the number of larvae at each millimeter of length. Measurements were made with 

 the aid of microscope and eye-piece micrometer for larvae under 7 mm. and with 

 millimeter rule and unaided eye for larger ones. The measurement was from tip of 

 snout to end of notochord in larvae, and to base of caudal rays in post-larvae. Dis- 



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