LAUSR

The present study aims to describe updated information on the basic reproductive biology of Sphoeroides annulatus females and potential use of gonadosomatic index I G as input data to assess length at 50% maturity ( L 50 ) and identify the breeding season. From April 2014 to December 2015, 767 females were analysed along the northwest coast of the Mexican Pacific Ocean. Both immature and mature females were represented in the total length L T ranged from 81 to 440 mm, the smallest mature female was 92 mm (new record for the species). Gonadal maturity phases were assigned through histological analysis. A I G threshold value I cut was established using the lower whisker value for maturity phase spawning capable (2.06%). The monthly variation of I G supported the annual spawning seasonality, the presence of reproductively active mature females I G ≥ I cut coincide with the periods observed with histological data and the peaks for both seasons 2014 and 2015. Also the presence of reproductively inactive I G < I cut confirmed the non-reproductive season observed from January to April 2015. Estimates of L 50 were based on the cumulative proportion of reproductively active mature females, both datasets drew a 'bumpy trajectory". Considering the functional form of data and models, a second objective was to verify if each dataset presented a simple or double sigmoid trajectory and select the most suitable model through the Akaike information criterion. For both datasets, the best fitted model was the double sigmoidal M DS while the single sigmoidal M SS had low performance and did not have statistical support. The M DS provided a L 50 value ca. 254 mm L T for both data sets. A trade-off between the model's correct fit and reliable L 50 value was found, this fact is not always considered when calculating the maturity, resulting in biased parameters. Correct data fitting may seem trivial; however the consequences on the conservation of immature individuals could be dramatic, affecting the recruitment of the population. In conclusion, the histologically calibrated I cut was suitable to classify females of S. annulatus into reproductively inactive or active types, with an agreement percentage of 99%. The I cut = 2.06% can be useful for subsequent studies in this species.