MESH RETENTION OF LARVAE OF SARDINOPS CAERULEA 

 AND ENGRAULIS MORDAX BY PLANKTON NETS 



William H. Lenarz' 



ABSTRACT 



Mesh retention of the standard plankton sampling gear used by the California Cooperative 

 Oceanic Fisheries Investigations between 1949 and 1968 for larvae of the Pacific sardine 

 (Sardinops caerulea) and northern anchovy (Engraulis mordax) was estimated by com- 

 paring catches made by the standard gear with a gear that retains larvae of all sizes. 

 The results indicate that 67% of sardine larvae and 60% of anchovy larvae are retained 

 by the meshes of the standard gear. The standard gear was replaced by a similar gear 

 in 1969. Apparently all anchovy larvae are retained by the new gear. The new gear 

 is the same as the old gear except for the netting. The netting of the new gear is con- 

 structed from 0.505-mm mesh width nylon while the netting of the standard gear was 

 constructed from 0.55-mm mesh width silk. Catch curves of anchovy and sardine larvae 

 corrected for escape through meshes revealed no evidence of a critical period. 



Although fisheries literature contains numerout^ 

 examples of estimates of mesh retention of fish 

 by commercial fishing gear, there is little quan- 

 titative work on mesh retention of fish larvae 

 by plankton gear. A review of such studies by 

 Vannucci (1968) revealed only three papers con- 

 taining quantitative estimates of mesh retention 

 by plankton gear. The lack of knowledge on this 

 subject can cause serious errors when comparing 

 the estimates of abundance of fish larvae made 

 from samples taken by diff"erent sampling gears 

 or of two or more species of fish larvae by the 

 same gear. 



This paper presents estimates of mesh reten- 

 tion of larvae of the Pacific sardine (Sardinops 

 caerulea) and northern anchovy {Engraidis 

 mordax) by plankton nets that have been used 

 by the CalCOFI (California Cooperative Oceanic 

 Fisheries Investigations) . Estimates are made 

 of the errors that result because mesh retention 

 is ignored when the relative abundance of the 

 two species is calculated and when abundance 

 of the anchovy is computed using data from two 

 quite similar types of sampling gear. Finally the 

 catch curves of the two species are examined for 

 evidence of the critical period proposed by 

 Hjort (1926). 



^ National Marine Fisheries Service, Southwest Fish- 

 eries Center, La Jolla, CA 92037. 



Several authors have noted that small larvae 

 of the Pacific sardine and northern anchovy are 

 not completely retained by the meshes of the 

 plankton nets used by the CalCOFI, but did not 

 have the necessary data to make quantitative 

 estimates of the retention rates. Ahlstrom 

 (1954) noted that small sardine larvae were not 

 fully retained by the meshes of the net in use 

 at that time. Ahlstrom (personal communica- 

 tion) attempted to obtain a measure of the re- 

 tention rates with a series of paired plankton 

 samples made with a regular CalCOFI net and 

 a net with a finer mesh, but failed to capture 

 an adequate number of sardine larvae. Isaacs 

 (1965) concluded that anchovies up to 7.75 mm 

 were not fully retained by the meshes. He at- 

 tributed differences in the form of catch curves 

 of anchovy larvae in different years to changes 

 in the minimum size of complete retention. 

 Murphy (1966) stated that anchovy larvae are 

 undersampled ". . . by a factor of about 2, rel- 

 ative to sardines, i.e., they tend to pass through 

 the mesh of the net to a marked extent." He 

 did not describe his method of obtaining the es- 

 timate but later informed me (Murphy, personal 

 communication) that he based his conclusion on 

 comparison of catch curves with hypothetical 

 curves based on the assumption of exponential 

 mortality. Lenarz (in press) estimated that 



Manuscript accepted March 1972. 



FISHERY BULLETIN: VOL. 70, NO. 3, 1972. 



839 



