Comparison of a visual method, mass-based and surface-based gonadal indices and gonad histology to assess sexual maturity in the waved whelk, Buccinum undatum

Abstract

To ensure the sustainability of waved whelk, Buccinum undatum, stocks in a context of high environmental variability, it is increasingly needed to assess spatio-temporal variation in size-at-maturity. Validation of more rapid methods, visual assessment, penis length index (PLI) and a surface-based gonadal index (SGI), compared to conventional and more laborious mass-based gonadal index (MGI) and the ‘gold standard’ histological method is needed. These five methods were applied to the same samples of whelks stratified by size (Total shell height: TSH = 50−95 mm) and sex, captured at three sites of the St. Lawrence Estuary in July 2013, during the spawning season. Parasitized and imposex-affected individuals, detected by visual inspection, were excluded from the samples used for maturity assessment. Each whelk was classified as mature or immature using the different methods. The percentages of overall agreement between each method and histological assessment were calculated for each sex and site. In females (N = 140), compared to the MGI and SGI, the visual assessment, the simplest and less costly method, provided the outcome most concordant with histology. The strength of visual grading was that it could classify post-spawning females as mature more accurately than the two biometric indices, more influenced by the timing of sampling in relation to the stage of the reproductive cycle. The TSH at which 50 % of the females were mature was not significantly affected by the method selected for maturity assessment. In males (N = 45) which did not exhibit gonad resorption contrarily to the females, all methods were highly concordant. The PLI combined with visual examination, is recommended for a rapid, low-cost and quantitative male maturity assessment. Overall, this study validates the use of PLI and visual assessment for maturity assessment of whelks, enabling more frequent verification of the pertinence of the current minimum legal size for the protection of whelk stocks in different fishing areas.