Temporal and spatial variation in size at maturity of the common whelk (Buccinum undatum)

There is little common rationale for the 45 to 75 mmtotal shell length (TSL)minimumlanding sizes (MLS) for Buccinumundatumamong countries in the Northeast Atlantic. Size at maturity and length frequency of B. undatum populations vary over small spatial scales; however, the driving mechanism for this is unknown. Size at maturity research for B. undatum dates back to the late 1980s; since then, there has been little consensus on laboratory methods or in which season to undertake the research. Here, we assess small-scale spatial variation in size at maturity over a year to identify the seasons that increase error in visual maturity estimates. We compare and contrast results frommethods used in the literature to estimatematurity for B. undatum. Monthly, B. undatumsamples were obtained from Welsh fishers betweenMay 2013 andMay 2014 fromeight sites at four locations (n = 5080). All whelks were sexed, weighed, andmeasured, and up to 60 whelks from each location were assessed for maturity (n = 1659). Mature whelks were found in all months, with a peak in reproductive activity through summer and early autumn, followed by the onset of spawning in November. Size at maturity varied between sexes and sites, and ranged from 51 to 76 mm TSL. Whelks caught in shallow waters (0-10 m) matured at a smaller size than those from deeper waters (up to 60 m). Length frequency varied at each site suggesting that a single increase in MLS would impact fishers disproportionately. No methodology trialled was found without inherent errors when used in isolation, though a combination of methods proved ideal. The seasonal variation in maturity estimates suggested that assigning maturity is increasingly difficult outside annual reproductive periods and that prespawning is the ideal time to undertake visual gonad assessments for whelks.