Bias associated with statolith-based methodologies for ageing squid; a comparative study on illex illecebrosus

Estimates of ages for short-finned squid (Illex illecebrosus LeSueur) were derived from two different methods for preparing statoliths and enumerating daily increments. Each pair of statoliths was divided between two experienced agers. One (Ager B) ground the statoliths on one side in the frontal plane for examination by light microscopy with camera lucida. The other (Ager G) ground statoliths on two sides in the sagittal plane for examination by light microscopy with an image analysis system. Mean counts of increments did not differ between the two agers reflecting similar peak hatching periods and patterns of migration. Systematic differences were, however, evident in that counts by Ager G were larger in young squid and smaller in old squid than those by Ager B. Crossing of methods suggested that estimated age was affected by the method of preparation of statoliths, but bias between agers was primarily due to differences in methods of interpreting and enumerating increments. This bias greatly affected estimates of growth rates. Rates of growth of groups obtained by the two agers based on the estimated month of hatching were compared with those of individual gladii. Such comparisons failed to identify which method was the more reliable because of small sample sizes and poor fit of the growth model, as well as uncertainty regarding the effects of time of hatching on growth rate. This uncertainty was related to unknown error in the estimation, from statoliths, of age and time of hatching. Broad-scale collaborative studies are recommended, involving exchange of unprocessed and processed statoliths among agers, to evaluate precision and maximize consistency among investigators.  2000 Elsevier Science B.V. All rights reserved.