Effects of mortality and gear selectivity on the fish otolith radius-totallength relation

The effects of mortality and gear selectivity on the otolith radius-total l ength relation (OR-TL), an important component of the back-calculation of l ength-at-age process, were simulated using a somatic-otolith bioenergetics population model. Four gear selectivities were examined and used to produce fishery-dependent samples for statistical analysis. Samples taken with sel ective gears produced OR-TLs with significantly different gamma-intercepts than those from an unfished population; significant differences were also f ound between gear selectivities. The OR-TL gamma-intercept of a non-selecti vely fished population was found to be systematically influenced downward b y total mortality as a result of a decrease in the number of larger fish. T his influence was removed using the mean otolith radius-at-age and mean len gth-at-age in the OR-TL regression, which resulted in more accurate estimat es of back-calculated size-at-age; the best estimates were made using only the last annulus in the calculation. The results of this study imply that, rather than a fixed value, the gamma-intercept of the OR-TL and the underly ing population growth rate are more accurately considered dynamic parameter s that changes as a cohort ages and experiences mortality. Use of the popul ation growth rate and gamma-intercept in the back-calculation of size-at-ag e needs to account for the degree and selectivity of recent fishing mortali ty of the population under investigation. (C) 2000 Elsevier Science B.V. Al l rights reserved.