A re-evaluation of the spawning fraction estimation procedures for Bay of Biscay anchovy, a species with short interspawning intervals

The estimation procedures of the spawning fraction (S) of the Bay of Biscay anchovy (Engraulis encrasicolus), a species with short spawning intervals, have been revised using the samples collected in 14 surveys for the application of the daily egg production method in the period 1990-2007. Towards this aim, the procedure to assign mature females to daily spawning classes was first improved by incorporating all the available knowledge on final oocyte maturation and degeneration of post-ovulatory follicles (POFs) in a matrix system which defines the probabilities of females with either of these histological indicators belonging to pre- and post-spawning daily classes according to the time of capture. Subsequently several S estimators and their biases were evaluated: The proportion of day_0 and day_1 spawners showed a high consistency over a 24-hour cycle around the mean S value of 0.395 (CV = 18%), although some oversampling of day_0 females, up to 0.48, was observed around peak spawning time (at 23:00 hours). Individual daily spawning classes had a clumped distribution, resulting in strong negative correlation (-0.73) between day_0 and day_1 spawners. For all these features the mean proportion of day_0 and day_1 S(0 + 1), was practically unbiased and slightly more precise than the traditional S(1) corrected estimator (based upon the proportion of day_1 females, corrected for oversampling of day_0). The new estimates were very similar throughout the study period and were independent of the fishing gear or sampling time. Female size had a small but significant influence on S. In support of the high spawning fraction recorded, it is shown that many females received a double allocation to a pre and a post spawning daily class; this allowed an alternative estimation of S based upon the reciprocal of the individual spawning frequencies of females which produced a result very consistent with the other S estimators. (C) 2011 Elsevier B.V. All rights reserved.