The phenomenon of apparent change of growth rate in gummy shark (Mustelus antarcticus) harvested off southern Australia Terence I. Walker * , Bruce L. Taylor, Russell J. Hudson, Jason P. Cottier Marine and Freshwater Resources Institute, PO Box 114, Queenscliff, Vic. 3225, Australia Abstract The hypothesis of the `Phenomenon of Apparent Change in Growth Rate' caused by length-selective ®shing mortality is adopted for explaining observed differences in published von Bertalanffy growth curves for gummy shark (Mustelus antarcticus) determined from length-at-age data. Three independent pieces of evidence are presented in support of this hypothesis to explain the differences in the curves between 1973±76 and 1986±87 in Bass Strait and between Bass Strait and South Australia during 1986±87: (1) Mean length of the sharks in each of the ages-classes 3±7 years are shown to be different between the two periods and the two regions, but not different for the 2-year age-class which is affected less than the older age-classes by the ®shing gear deployed in the ®shery. (2) Avoiding the pitfalls of `back-calculation', Rosa Lee's Phenomenon was detected by directly comparing the radii of growth-increment bands visible on the faces of the articular cups of vertebral centra from sharks of various ages caught in the two periods and the two regions. Statistical tests indicated that the degree of Rosa Lee's Phenomenon did not vary signi®cantly between the periods or regions. (3) Through development of an appropriate model, the effects of length-selective ®shing mortality on the mean length of sharks in the wild population for age-classes 2± 16 years were simulated for a range of levels of hook and gillnet ®shing effort, with separate mesh-sizes of 6 and 7 in. for the gillnets. Simulated changes in mean length for sharks older than two years tended to be smaller than the differences observed in the published von Bertalanffy growth curves but they were generally consistent with the observed trends. The simulations demonstrated how the magnitude of the von Bertalanffy growth parameters L 1 and t 0 tend to increase and K tends to decrease as gillnet ®shing effort increases, and hence explain how these types of biases, commonly appearing in the scienti®c literature for gillnet shark ®sheries, can occur. The simulations also demonstrate that hook ®shing effort in a ®shery with legal minimum lengths has to be very high to cause distortion of growth curves. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Shark growth; Gillnet length-selective mortality; Lee's Phenomenon 1. Introduction Moulton et al. (1992) present different von Berta- lanffy growth curves for gummy shark (Mustelus antarcticus) captured from two separate regions in southern Australia and during two separate sampling periods. Three curves for each sex are determined from age estimates made by counting growth-incre- ment bands on the faces of the articular cups of vertebral centra of sharks. By comparing the curves for sharks captured in Bass Strait during 1986±87 with those for sharks captured in Bass Strait during 1973± 76, it appears that growth rates have slowed. Similarly, Fisheries Research 39 (1998) 139±163 *Corresponding author. Tel.: +61 52 580111; fax: +61 52 580270; e-mail: t.walker@msl.oz.au 0165-7836/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0165-7836(98)00180-5