109 



The nine stations used in fig. 29 C were taken by a small shell dredge, 

 one half a meter wide, towed for five minutes at about one quarter of a 

 knot. This method covered about 20 square meters per sample, if the 

 dredge was kept constantly on the bottom. All samples were taken in the 

 Tiburon region in about the same environment as the eight grab samples 

 tabulated above. The overall catch was far greater with the shell dredge 

 than with the Van Veen, being at least ten times more productive in each 

 case, averaging 26 species and 87 individuals per station. Because the area 

 covered by the shell dredge was far greater than that for the grab samples, 

 an entirely different picture is obtained for the kind of animals living 

 there. If the shell dredge samples are equated on a square meter basis, 

 they produced only an average of 4.4 individuals per square meter, roughly 

 15 times less than that computed for the Van Veen samples. It would 

 seem, therefore, that the shell dredge is completely ineffective for quan- 

 titative studies, although it certainly produces a much greater variety 

 of animals, especially from groups seldom taken with bottom grabs (see 

 Thorson, 1957, pp. 473-476). The following table gives the figures for 

 the shell dredge catches. 



Station 

 Number 



178 

 181 

 183 

 184 

 190 

 194 

 195 

 196 

 208 



The figures for the number of identified species in the small dredge 

 samples gives some conception of how prolific this environment can be in 

 diversity of species, living and dead. The total average number of living 

 and dead species for all nine stations is 54 per station, while the greatest 

 number is 129. It should also be mentioned that many of the shells of spe- 

 cies recorded were those taken alive at other stations or in different years. 

 Although the majority of the Van Veen grab samples were taken to the 

 south of the shell dredge samples, two grab samples (Stations 24 and 26) 

 collected within the shell dredge sampling region, produced the same 



