Reproductive biology of the bullseye puffer Sphoeroides annulatus: Gonadosomatic index and its suitability for estimating length at maturity

Abstract

This study aimed to describe updated information on the basic reproductive biology of Sphoeroides annulatus females and potential use of the gonadosomatic index $\left(I_G\right)$ 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 north-west coast of the Mexican Pacific Ocean. Both immature and mature females were represented in the total length ( $L_T$ ), which ranged from 81 to 440 mm, and 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 $\left(I_{\mathrm{cut}}\right)$ 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, and the presence of reproductively active mature females $\left(I_G\ge I_{\mathrm{cut}}\right)$ coincided with the observed periods with histological data and the peaks for both seasons 2014 and 2015. Also the presence of reproductively inactive $\left(I_G<I_{\mathrm{cut}}\right)$ mature females confirmed the nonreproductive season observed from January to April 2015. Estimates of $L_{50}$ were based on the cumulative proportion of reproductively active mature females: both datasets showed 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 $\left(M_{\mathrm{DS}}\right)$ , while the single sigmoidal $\left(M_{\mathrm{SS}}\right)$ had low performance and did not have statistical support. The $M_{\mathrm{DS}}$ provided a $L_{50}$ value ca. 254 mm $L_T$ for both data sets. A trade-off between the model's correct fit and a reliable $L_{50}$ value was found, but this fact is not always considered when calculating the maturity, resulting in biased parameters. Correct data fitting may seem trivial, but the consequences for the conservation of immature individuals could be dramatic, affecting the population recruitment. In conclusion, the histologically calibrated $I_{\mathrm{cut}}$ was suitable for classifying females of S. annulatus into reproductively inactive or active types, with an agreement percentage of 99%. The $I_{\mathrm{cut}}$ = 2.06% can be useful for subsequent studies for this species.

Keywords: functional form; histological analysis; maturity phases; sigmoidal model; statistical support; threshold value.