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Determination of the Size of Maturity of the Whelk Buccinum undatum in English Waters – Defra project MF0231 Andy Lawler Funded by Defra Contents Determination of the Size of Maturity of the Whelk Buccinum undatum in English Waters – Defra project MF0231 ....................................................................................................................................... 0 1. Executive Summary ......................................................................................................................... 2 2. Introduction .................................................................................................................................... 3 3. Objectives........................................................................................................................................ 4 4. Methods .......................................................................................................................................... 4 4.1 Size of maturity ....................................................................................................................... 4 4.2 Ageing and growth .................................................................................................................. 8 4.3 Parasitological ......................................................................................................................... 9 5. Results ............................................................................................................................................. 9 5.1 Size of maturity ....................................................................................................................... 9 5.1.2. Alternative method of male maturity determination ................................................... 15 5.2 Ageing and growth ................................................................................................................ 16 5.2.1 Reliability of opercula age determination .................................................................... 16 5.2.2 Growth Model fitting .................................................................................................... 19 5.2.3. Alternative ageing method ........................................................................................... 24 5.3 Parasitological ....................................................................................................................... 25 6. Discussion ...................................................................................................................................... 26 6.1. Size of maturity ................................................................................................................. 26 6.2. Ageing and growth ............................................................................................................ 28 6.3. Parasitological ................................................................................................................... 29 7. Conclusions ................................................................................................................................... 29 7.1. Implications ....................................................................................................................... 30 7.2. Recommendations for future work .................................................................................. 30 8. References .................................................................................................................................... 31 9. Acknowledgements ....................................................................................................................... 33 Appendix 1 ............................................................................................................................................ 34 1 1. Executive Summary This study has provided estimates of size of maturity (SOM) for whelks in ten important English fisheries using visual observation of the gonad for maturity determination. The potential of using opercula and statolith ring counting methods to age whelks was investigated and variability between alternative readers for the widely used opercula counting (OC) method was summarised. Von Bertalanffy growth models (VBGM) were fitted to length and age data derived by the OC method to provide provisional and plausible growth estimates for each of the ten areas. Estimates of SOM and growth parameters were compared with two sites sampled using identical methodology in a previous study. Samples of catch were sourced from ten English ports chosen in consideration of the economic value of recent and historic reported landings of whelks. Fishers provided samples from sites typically exploited by the local fishery comprising suitable numbers and size ranges of whelks, enabling precise estimates of SOM. Because of the time consuming nature of the sample processing, only one sample was acquired and analysed from each site. Individual whelks from size and sex stratified sub-samples from each site were measured (shell height), removed from their shell and the maturity status and gender recorded. The operculum (trap door) of each whelk was removed for age analysis. Occurrences of host whelks with atypical reproductive development, or sterility, caused by a parasite were recorded. Statoliths from a small number of whelks were removed after digestion of the body in a caustic solution, polished and their rings counted as an alternative approach to OC. The probability of a whelk being mature with size was estimated using logistic regression analysis, and the size at which the probability was 0.5 (the definition of SOM) was estimated for both sexes at each site. Results were generally consistent with previous work where corresponding sites had been sampled. However, earlier studies often used different methodologies and differences between results were not necessarily explained by regional variation alone. Estimates of SOM from this study ranged between 44.8 mm and 46.4 mm shell height (female and male, respectively) for a site in the Solent (Portsmouth) and 77.8 mm and 76.2 mm (female and male, respectively) for a site in the North Sea (Bridlington). Estimates of SOM generally fell into three groups; those around 70mm and above (Bridlington-North Sea, Exmouth-Western English Channel, Ilfracombe-Celtic Sea and Whitehaven-Irish Sea), those between about 50 to 65mm (Whitstable, Poole, Selsey, Ramsgate and Weymouth in the English Channel and Wells-next-the-Sea – Southern North Sea), and that around 45mm (Portsmouth-Solent). In general estimates of SOM by gender were similar (<5% difference), but at some sites differences were about 10% (e.g. Eastbourne where females had a higher SOM than males and Selsey and Weymouth where SOM for females was lower than for males. Four scientists independently counted the rings on a sample of approximately 500 whelk opercula, stratified by size group, sex and site. Variability between readers as deviations from the mode were summarised and percentage agreement for each reader calculated using a similar method to that used by colleagues investigating variability in age determination in fin-fish using otoliths. The highest percentage agreement achieved by one of the readers was close to that typically observed for difficult to age fin-fish species such as whiting Merlangius merlangus using the otoliths (79.8% c.f. ~ 80%). The other readers achieved lower percentage agreement scores (63.5-72.8%) highlighting a need for improved methodology or higher levels of expertise. Statoliths isolated from a small sample of whelks already aged by the OC method were polished and rings counted. Despite lack of agreement with the counts from the OC method, rings in the statoliths were on occasion clearly defined and this and uncertainty with the OC method suggests that the statoliths warrant further investigation using improved procedures. VBGMs fitted to the age and size data by least squares methods provided parameter estimates of L and K generally consistent with earlier work at corresponding sites. Growth models fitted to the data suggest growth for whelks did not vary significantly by gender but varied considerably between sites. Fitted models for whelks 2 from the Portsmouth site exhibited a lower growth rate than other sites (L , 62.62mm and K, 0.41) whilst that for whelks from the Bridlington site was the highest (L , 130.73mm and K, 0.23). These provisional growth estimates suggest that whelks are likely to attain the EU Minimum Landing Size (MLS) of 45mm in as little as 2 years except in the Solent (Portsmouth) where this may take 3 years. When combined with estimates of SOM this suggests that the whelks from Eastbourne and Ramsgate reach sexual maturity in 2 years, those from Portsmouth, Poole, Selsey, Wells-next-the-Sea, Whitstable and Weymouth in about 3 years, whilst those from the Bridlington, Exmouth, Whitehaven and Ilfracombe sites will take about 4 years. The incidence of observations of atypical gonad development in host whelks caused by digenean parasites was reported as low (<0.5%), but paucity of infected animals and problems with sample fixation prevented positive identification of the species responsible. The estimates of SOM for all sampled sites except the Solent (Portsmouth) indicated that the current EU MLS of 45mm does virtually nothing to protect spawning stocks. The MLS has a modest conservation value for Eastbourne and Ramsgate and a more

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