The Determination of the Age and Growth of the Perch (Perca fluviatilis) from the Opercular Bone

Abstract

The fact that many fish show rings on such skeletal structures as scales, bones, otoliths and fin rays, has, been known for a long time. In many cases these rings have been shown to be annual in formation and to be probably due to seasonal fluctuations in growth. They thus provide a very useful method for assessing the ages of fish, and as such have been extensively used by many workers. In fact, age determination, usually from the rings on the scales, has become one of the standard methods in fishery research. Age determinations are very useful in practical fishery management as they can lead to assessments of mortality rates and by calculations of the mean sizes of the age groups to a knowledge of the growth rates of the fish. Additional information on the growth of fish can also be ascertained from the scales and similar skeletal structures by calculating the length of a fish at each year of its life from the proportional width of the rings laid down each year. These methods have been very widely used and a large proportion of the extensive modern research on fish ecology and fishery management is based on age and growth studies using the 'scale method'. There is also a large literature on the validity of the use of these methods, which discusses whether the rings are true annual formations and whether they provide accurate estimates of past growth. Although there is relatively little substantial direct evidence of the soundness of age and growth determinations from scales, the sum of indirect evidence leaves no doubt that in general their use is justified. At the same time the practice of many workers, who have published accounts of age and growth based on the scale method without attempting to substantiate the validity of its application to the species or problem studied, is to be deprecated. The present paper is a preliminary to an account of the study of perch (Perca flutviatilis L.) in Windermere, and its aim is to demonstrate that the methods used to determine age and growth are valid. The literature on the method and the investigations towards proving its correctness has been discussed by several workers, and the papers of Graham (I 929) and Van Oosten (I929, 1941) give detailed reviews. It is not intended here to enter into a general discussion on the scale method and its validity. In the perch, some age and growth studies have been made by Scandinavian and German workers, and recently the ecology of the closely related yellow perch (P. flavescens Mitchill) has been investigated in many North American waters. In nearly all cases the scales have been used, but Nilsson (I92I) concluded that in practice the opercular bone was more accurate and easier to use than the scales. Roper (1936) used scales and operculars in a study of perch in Germany, and found that infra-red photography helped to reveal the obscure inner annual rings on the operculars. The opercular bone has also been used by Nikolsky & Evtiukhov (I940) to determine the age of P. shrenki Kessler. Segerstrale (I933) has studied the use of the scales of the perch in detail, has shown the need for correcting back-calculated readings of growth made by direct proportion, and has constructed a curve for this purpose. When data were collected from the perch in Windermere in connexion with the current trap fishery experiment (Worthington, I94), Mr H. J. Buchanan-Wollaston tried using the opercular bones and compared them with scales from various parts of the perch. He discovered that the use of polarized light often clarified the rings on the operculars, and that by projection the width of these rings could easily be measured. He found that in general the