FISHERY BULLETIN; VOL. 87, NO. 3, 1989 



1.00 



^ 3.00 

 E 



fo 2.00 



1,1 .00 



in 

 tn 



0.00 



• OBSERVED 



rs 

 OBSERVED 



2.0 4.0 6.0 



8.0 10.0 12.0 14.0 

 LENGTH (mm) 



16.0 18.0 20.0 



Figure 2. — Estimated values of Pes and the day to night catch ratio by 0.5 mm 

 length intervals obtained in the avoidance experiment (upper panel). Estimates of 

 Prs and the 0.335 mm standard net to the 0.183 mm test net catch ratio by 1.0 mm 

 length intervals obtained in the extrusion experiment (lower panel). 



simultaneously from a small boat. For the entry 

 experiment, however, deployment of one of the 

 nets occurred approximately 10 hours after the 

 other, and any patchiness in the larval distribu- 

 tion coupled with advective movement could 

 have substantially altered the characteristics of 

 the population sampled. We attempted to min- 

 imize this problem by increasing the sample size, 

 relative to the retention experiment, and by par- 

 titioning the sampling among three locations 

 rather than by concentrating it at one. Sampl- 

 ing variability, however, may still have been 

 responsible for some of the differences between 

 the day and night size distributions. This prob- 

 lem has been encountered in other studies using 

 day and night comparisons to estimate entry 

 probabilities (Murphy and Clutter 1972), and 

 the only effective solution is increased sample 

 sizes. 



Since the method of estimating entry and re- 

 tention probabilities proposed here i-equires 

 more effort than that using simple catch ratios, it 

 is important to determine at the outset whether 

 the assumptions that P,-s = P,-,, and Pp., = P^,- 

 have been violated so that the appropriate 

 method of analysis can be chosen. Some indica- 



tion of the validity of these assumptions can be 

 obtained by examining plots of catch ratios as a 

 function of larval length (Fig. 2). Two cases are 

 evident in our data. First, if the assumptions are 

 met, catch ratio should be a monotonically in- 

 creasing or decreasing function of larval length 

 because extrusion and avoidance are monotonic 

 functions of larval length. This is not true in the 

 avoidance experiment where the catch ratios in- 

 creased for lengths <4 mm and decreased there- 

 after. Second, if the assumptions are met, catch 

 ratios cannot be >1.0, because, except by chance 

 alone, catch in the standard net is less than the 

 catch in the test net. This is not true for the 

 extrusion experiment where the catch ratios in 

 some length intervals are >2.0. If either of these 

 conditions are evident in plots of catch ratios, the 

 method of estimating entry and retention prob- 

 abihties proposed here is preferable to simple 

 catch ratios. 



Although we considered the problem in which 

 both Prs =?*= Pre and Pes =^ P er, thls Is the most 

 general of several related problems that could be 

 approached with slight variations in our method- 

 ology. One example occurs when either Prs +Pre 

 or Pfs "^ Per but not both. In this case, either 



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