The Seasonal Pattern of Otolith Growth and its Application to Back-Calculation Studies in Ammodytes Tobianus L.

Abstract

Otolith growth in a population of Ammodytes tobianus L. in Langstone Harbour, England, has been investigated during 1969 and 1970, as a basis for the estimation of body growth by back-calculation of mean lengths from otolith zone measurements. Most attention has been paid to I-group spring-spawners.

Linear growth of the otoliths was found to take place mainly from April to August, with the deposition of opaque material occurring in April and May. During the initial period of opaque zone deposition, however, there is little or no otolith growth, and from measurements of the zones in successive samples, it appears that the initial deposition of opaque material is by ingrowth into the hyaline material present at the edge. During the period of opaque zone deposition, body growth is at a maximum, and this is followed by a period of slower growth coinciding with hyaline zone formation in the otolith. As a result, the ratio of otolith length to fish length shows a sharp decrease in spring, followed by an increase during summer to a more stable level.

The application of these data to the back-calculation technique has been examined. Because of ingrowth, values of l₁ derived from measurements of the inner boundary of the opaque zone are estimates of body length before the end of the growing season. Estimates of body length at the time of opaque zone initiation in mid-April can be obtained, however, by using measurements at the split in the opaque zone, but no otolith dimension can be used specifically to estimate body length during the October to March period of no-growth. Assumption of a constant relationship between otolith length and body length was found to result in values of l₁ varying systematically with the time of year at which the samples were obtained. A knowledge of the seasonal variation in otolith-body length ratios and in particular the ratios at the times corresponding to the split and inner boundary of the opaque zone were found to be necessary for accurate back-calculation of mean body lengths.