Author(s): Ross Vivian Bulkley
Author(s): Ross Vivian Bulkley
Description: Any method used in aging fish must fulfill certain requirements to be satisfactory. If the method is inaccurate, naturally little reliance pan be placed upon its use. If considerable study of the method and much practice are required before accurate readings can be obtained, the meth od will not come into widespread use. The requirements of an aging meth od, then are that it must be accurate and comparatively easy. The scale method for aging lake trout (Salvelinus nanayoush Walbaum) unfortunately does not fulfill these requirements. It has always been difficult to tell the age of lake trout by the examination of scales--the standard method of aging most fish. Although a few researchers have successfully aged lake trout by the scale method, much training is necessary to be proficient. Older fish are particularly difficult to age by the scales. This lack of an easy and dependable aging method is hampering growth studies of lake trout in Utah lakes. Hence, in 1956, the Department of Wildlife Management at the Utah State Agricultural College and the Utah Department of Fish and Game began a study at Fish Lake, Utah, to deter mine the value of branchiostegal rays as growth indicators of lake trout. The branchiostegal rays are snail semi-transparent bones located in the branchiostegal membrane, a ventral extension of the operoulum. The num ber of these rays varies according to species of fish. Various bony structures have been used in place of scales for deter mining age and growth rates. LeCren (1947) and McConnell (1951) used the operoular bone to age parch and carp. Appelget and Smith (1951). Lewis (1949), and Zarbock (1951) used the vertebrae. Boyko (1946) and Cuerrier (1951) used fin rays to determine age of fish. Otoliths have also been used occasionally. Menon (1949) presented a complete list of the differ ent bones that have been used for age determination in fishes, together with the names of the workers who have used them. Of the bones considered for use in aging lake trout, the branohiostegal rays were considered most promising. Different bones in the head were checked for markings which might be growth indicators. Markings an the branchiostegals were found to have all of the characteristics of annuli and were used,therefore, in this study. branchiostegal rays were also chosen because of the ease of obtain ing specimens.. Most lake trout samples must come from the fisherman's creel, and fishermen do not want their fish defaced by the removal of large or deeply-centered bones in the head or body. This is especially true for fish which the fisherman wishes to mount or photograph. Hence neither the operoular bone nor the vertebrae would be suitable. The small size and surface location of the branohiostegal rays eliminate this dif ficulty in obtaining specimens. One ray on either side of the head can be removed and only a small deformation results. Also many fishermen remove the isthmus and branchiostegal membrane when cleaning the fish. Thus the rays can be obtained easily. Another possible advantage in using the branchiostegal rays is that the careful removal of a single ray apparently does not cause serious in jury to the living fish. This would make it possible to check the age of a fish without killing it--one of the advantages of using scales. Lim ited observation of small fish of several species indicates that fish have a good chance of survival after removal of a single branchiostegal ray. However, this possibility was not explored in the present study. More investigation is necessary to determine the long-range effects of branchiostegal ray removal from lake trout before it is attempted on large numbers of fish.