THE RELATIVE GROWTH AND MORTALITY OF THE PACIFIC RAZOR 
CLAM ( SILIQUA PATULA, DIXON), AND THEIR BEARING ON THE 
COMMERCIAL FISHERY 1 
F. W. WEYMOUTH, Ph. D., Professor of Physiology, Stanford University, and H. C. McMILLIN, 
Junior Aquatic Biologist, United States Bureau of Fisheries 
CONTENTS 
Introduction 
The genus Siliqua . 
Differential growth ratios in Siliqua 
patula and S. alta 
Width 
Position of umbo 
Direction of rib 
Siliqua patula 
Variability 
Sexual differences 
Mortality 
Growth 
Length as measure of size 
Localities studied 
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552 
Siliqua patula — -Continued. 
Growth — Continued. 
Quantitative treatment of data. 
A growth formula based on rela- 
tive rate 
Differences in growth in different 
localities 
Biological findings and their bearing on 
fishery regulations 
Biological findings 
Razor clam fishery and its regulation . 
Summary 
Bibliography 
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INTRODUCTION 
The present paper is a continuation of previous work on the economically impor- 
tant mollusks of the Pacific coast of North America. The earlier studies of the senior 
author on the bivalves of California led to the development of a method of determining 
age and thus made possible an accurate study of growth. The seasonal growth 
shown for the Pismo clam has since been substantiated in other species, for example, 
in Anodonta, Thiel (1928) found practically no increase in length during the entire 
winter. This method of age determination based on the annual rings of the shell 
was later successfully applied to the Alaska razor clam (fig. 1) which was showing- 
signs of depletion (Weymouth, McMillin, and Holmes, 1925). In the present report 
the accumulated data on the growth of this species over a wide range of latitude has 
been analyzed in greater detail. It is clear that in all parts of the coast, where the 
razor clam is fished commercially, supervision is necessary to maintain this valuable 
resource. It has become equally clear that the course of growth, breeding habits, 
and the like vary so widely in different parts of the coast that the regulations must be 
adapted to the district. Thus in Alaska the set of the young clams seems never to 
be more than a fraction of that on the Washington beaches, and the growth is much 
slower. For example, the clams reach the breeding size in two years at Copalis on 
the Washington coast but require four years at Swickshak, Alaska. The smaller set 
and slower growth of the clams make the northern beds less resistant to heavy fishing 
i Submitted for publication Sept. 18, 1930 
543 
