LENARZ: MESH RETENTION OF LARVAE 



Plots, Figure 1, of the estimates show that there 

 is a very good linear relation between the esti- 

 mates and Lj when 2.50 mm ^ Lj ^ 5.75 mm. 



When Lj > 5.75 mm Sij(2) decreases while 



Sij(3) fluctuates about 1. Thus it appears that 

 a linear relation (4) provides an adequate de- 

 scription of the relation between sn and Lj when 

 2.50 mm ^ Lj < 5.75 mm. When Lj > 5.75 mm 



it is assumed that su = 1. Vannucci (1968) re- 

 viewed papers that used the logistic and normal 

 distribution functions which have sa approach- 

 ing 1 as an asymptote rather than the discon- 

 tinuous equation used in this study. The data 

 used in this study are not accurate enough when 

 Lj > 5.75 mm to warrant use of an asymptotic 

 relation. The very good linear relation be- 

 tween Lj and su when L, ^ 5.75 mm and the high 



values of Sij when Lj = 5.75 mm indicate that 

 any deviation from the equation used in this 



study would cause minor errors when Sij are used 

 to correct catches of anchovy larvae for loss 

 through meshes. 



The estimate of a from (2) and (4) is 

 — 0.1942, b is 0.1945, and the correlation coeffi- 

 cient, r, is 0.9961. When (3) was used a is 

 —0.1075, b is 0.1850, and r is 0.9814. 



Table 2 contains catches of sardine larvae used 

 in the study. Estimates by (2) and (3) of su 

 are as follows: 



The estimates of su for sardine larvae are similar 

 to those for anchovy larvae. Thus the same dis- 

 continuous function is assumed. The estimate 

 of a from (2) and (4) is —0.2382, b is 0.2107, 

 and r is 0.9320. Estimates obtained when (3) 

 was used are a, —0.2286, b, 0.2084, and r, 0.9639. 

 The resulting curves for both anchovy and sar- 

 dine larvae are shown in Figure 2. The curves 

 differ little either within or between species. 

 Since the highest value of r, 0.9961, was esti- 

 mated for anchovy larvae using equation (2), 

 the somewhat arbitrary decision was made to 

 use the results obtained from equation (2). 



ESTIMATION OF PORTION OF 



LARVAE PASSED THROUGH MESHES 



OF TYPE 1 (0.55-MM MESH) NET 



The type 1 net has been the major plankton 

 sampler of the CalCOFI. Estimates of spawn- 

 ing biomass of the northern anchovy population 

 have been based on the ratio of total catches of 

 sardine larvae to total catches of anchovy larvae 

 (Ahlstrom, 1968) . If there are major differen- 

 ces in the percentages of sardine and anchovy 

 larvae retained by the net, then the estimates 

 of anchovy spawning biomass may be seriously 

 in error. Ahlstrom (personal communication) 

 believed that his assumption that the two species 

 were sampled equally well was valid because of 

 the similar forms of larvae of the two species and 

 his interpretations of catch curves of larvae of 

 the two species. 



Catch (average) curves of anchovy and sar- 

 dine larvae were calculated from a series of 

 CalCOFI samples taken during the months of 

 January through July during the decade 1951- 

 1960. More than 10,000 samples are involved. 

 Catch curves were calculated for day and night 

 samples separately because of marked differen- 

 ces between the two. The curves were then cor- 

 rected for nonretention of larvae as follows: 



w'ljfco = nuko(l/si}) 



(5) 



where n'uko 



the corrected catch of larvae of 

 size Lj by net type 1 in the A;th 

 time of day, and oth species. 



843 



