COMMON WHELK

Buccinum undatum


Assessment report
Published by

Marine and Freshwater Research Institute, Iceland

Published

12 June 2026

Key signals

  • The stock biomass index (B/BMSY) is above Blim and Btrigger. The index decreased from 2004 to 2012 but has since increased.

  • Fishing pressure (F/FMSY) was highest in 2006 and in 2012 but has been below Flim and FMSY since 2012.

General information

The common whelk (Buccinum undatum) is a large marine gastropod with a conical shell (7–10 cm) found from the intertidal zone to 1,200 m depth, most commonly on muddy, gravelly, or rocky substrates shallower than 50 m. The species is an active predator and scavenger, feeding primarily on bivalves and polychaete worms, as well as amphipods and other small crustaceans. Reproduction takes place in late winter to early spring, with females depositing egg capsules in clusters; only 3–10 juveniles emerge per capsule after roughly two months of development. Age can be determined from growth rings on the operculum, with individuals in Icelandic waters recorded to at least 13 years.

Fishery

Experimental fishing for whelk started in 1996 in the bay of Breiðafjörður where they fished 500 tonnes. The catch peaked the year after, reaching 1,300 tonnes but has fluctuated since, ranging from 0 to 1,000 tonnes (Figure 1). The catch depends highly on economic factors and the number of boats fishing each year. In 2020, no whelk fishery was active, and no whelk was landed. In 2021, the whelk fishery started again and in 2025, 172 tonnes were landed.

Figure 1: Common whelk. Total catch (tonnes) and standardized catch per unit effort in Breiðafjörður

The fishery was mainly in the southern part of the fjord, but the distribution pattern has varied over time. In 2013 and 2014, there was little fishing activity in the whole area, compared to 2011 and 2012 (Figure 2). In 2014 to 2016 the fishing was mainly active in the southern areas of the fjord, but the fishery distributed to the north in the following years. However, since 2021, most of the catch is in the south (Figure 3) and last year, fishing mainly took place in June–September (Figure 4). No fishing was conducted in 2020. Because the fishery is restricted to Breiðafjörður, conclusions drawn from this stock do not apply to whelk populations elsewhere in Icelandic waters.

Figure 2: Common whelk. Distribution of catch in Breiðafjörður from 2007-2025.
Figure 3: Common whelk. Distribution of catch (tonnes) in 2025.
Figure 4: Common whelk. Distribution of catches (tonnes) by months in 2025.

Whelk survey

Two whelk surveys have been conducted in Breiðafjörður: in 1997/1998 during the first years of fishing and in 2012 (Figure 5). The survey index in 1997/1998 was 26.9 compared to 23.7 in 2012. The greatest decline between the surveys was in northwestern part of the fjord where negligible fishing had occurred. In contrast, there was a marked increase in whelk on fishing grounds in eastern area of the fjord between the surveys (Figure 5).

Figure 5: Common whelk. Distribution and abundance in surveys in 1997/1998 (left) and in 2012 (right).

Stock assessment

The common whelk is considered a data limited stock and follows the ICES framework for such (category 3.1, ICES 2021). A stochastic surplus production model in continuous time (SPiCT; Pedersen and Berg, 2017) is one of the official assessment methods for stocks in this category. The model quantifies observation and process errors and estimates stock status and reference levels with associated confidence intervals. SPiCT estimates MSY based reference levels, which can be used to calculate quantities relevant for fisheries management and ICES recommends using the 35th percentile for all quantities (Mildenberger et al., 2021)

Input data

The model synthesizes information from input priors (Woods and Jónasson 2017), catch series and biomass indices (generalized CPUE’s) from 2004-2025. The priors were made by consolidating and utilizing a variety of demographic (age-at-maturity, length-at-age, length-weight keys and length histograms) and environmental data from Breiðafjörður (See Woods and Jónasson 2017 for more details). Priors used for the model were the carrying capacity, K, intrinsic growth r, the medium initial biomass depletion, p, the standard deviation of fishing mortality process, sdf, and the standard deviation of catch observation error, sfd (Table 1). The n is fixed at 2 to resemble the Schaefer production curve.

Table 1: Common whelk. Priors in model
Priors Value Standard.deviation
K log(18500) 0.0925
r log(0.075) 0.0340
p log(0.467) 0.0100
sdf log(0.3) 0.2500
sdc log(0.1) 0.0100

Results

The output from the model is shown below in Table 2 and Table 3. Model diagnostics are shown in Figure 6, the model results in Figure 7 and the analytical retrospective analysis in Figure 8. Following the checklist for the acceptance of SPiCT model (Mildenberger et al., 2021), no issues were found. The assessment converged and all variance parameters of the model parameters are finite. There were no violations of model assumptions based on one-step ahead residuals, the production curve is realistic (B/K = 0.5) (Figure 6) and the patterns in the retrospective analysis are consistent (Figure 8). The Mohn’s rho value for F/FMSY is 0.202 as there is uncertainty in catches due to no fishing in 2020 and therefore, the analytical retro should improve in future stock assessments. BMSY is estimated at 8.2 kt.

Table 2: Common whelk. Summary of model results
Parameter Estimate 95% lower CI 95% upper CI Log estimate
alpha 1.653 1.101 2.482 0.502
beta 0.112 0.066 0.190 −2.192
r 0.076 0.054 0.107 −2.580
rc 0.076 0.054 0.107 −2.580
rold 0.076 0.054 0.107 −2.580
m 334.733 194.464 576.179 5.813
K 17676.480 10794.860 28945.070 9.780
q 0.000 0.000 0.000 −9.410
sdf 0.887 0.623 1.262 −0.120
sdi 0.165 0.110 0.248 −1.800
sdc 0.099 0.067 0.146 −2.312
Table 3: Common whelk. Summary of model results. Estimates for deterministic reference points and stochastic reference points.
Parameter Estimate 95% lower CI 95% upper CI
Bmsys 8232.092 5048.550 13423.130
Fmsys 0.035 0.024 0.051
MSYs 289.689 163.999 511.708
Figure 6: Common whelk. SPiCT-model diagnostics.
Figure 7: Common whelk. SPiCT-model results.
Figure 8: Common whelk. Analytical retrospective analysis from SPiCT-model.
Table 4: Common whelk. Estimates of B/Bmsy and F/Fmsy with 95% confidence intervals from the SPiCT model.
Year 95% lower CI B/Bmsy 95% upper CI 95% lower CI F/Fmsy 95% upper CI
2004 0.820 1.001 1.221 0.941 2.841 8.580
2005 0.753 0.990 1.301 1.305 3.286 8.274
2006 0.660 0.907 1.247 1.435 3.647 9.269
2007 0.589 0.835 1.182 1.035 2.693 7.008
2008 0.540 0.774 1.111 0.890 2.307 5.978
2009 0.531 0.770 1.116 0.311 0.811 2.113
2010 0.535 0.771 1.111 0.167 0.453 1.228
2011 0.506 0.720 1.025 0.496 1.297 3.392
2012 0.433 0.624 0.900 1.294 3.274 8.284
2013 0.388 0.565 0.823 0.324 0.848 2.219
2014 0.420 0.606 0.873 0.163 0.440 1.186
2015 0.425 0.607 0.869 0.261 0.692 1.835
2016 0.441 0.630 0.900 0.617 1.592 4.104
2017 0.452 0.650 0.936 0.443 1.148 2.979
2018 0.496 0.714 1.028 0.297 0.795 2.126
2019 0.526 0.761 1.100 0.319 0.904 2.563
2020 0.552 0.810 1.188 0.272 0.734 1.977
2021 0.595 0.865 1.257 0.219 0.620 1.756
2022 0.593 0.853 1.226 0.368 0.971 2.564
2023 0.576 0.830 1.197 0.369 0.965 2.529
2024 0.569 0.823 1.190 0.317 0.849 2.273
2025 0.573 0.842 1.240 0.304 0.826 2.242
2026 0.554 0.853 1.315 0.179 0.639 2.276
2027 0.538 0.865 1.391 0.074 0.639 5.500
2028 0.522 0.877 1.471 0.040 0.639 10.161

Diagnostics of indices

Table 5: Common whelk. Analysis of deviance table. Response variable is square root of catch per unit effort (CPUE).
Factor Df Deviance Resid. Df Resid. Dev F Pr(>F)
NULL - - 6,994 861.030 - -
factor(ar) 21 255.818 6,973 605.212 146 0.000000e+00
factor(man) 11 9.172 6,962 596.040 10 3.075263e-18
factor(sreitur) 6 14.222 6,956 581.817 28 1.171078e-33
Figure 9: Common whelk. Diagnostics plot from Generalilzed linear model (glm).

Management

Since 2005, whelk has only been fished using whelk pots, and in recent years only a single vessel has been active in the fishery (MFRI, 2024a). The minimum landing size (MLS) is 50 mm, and discard mortality is considered minimal as whelks are sorted using hydraulic drums; the selection in one of the drums used has been estimated, where L50 was 53.54 mm and selection range 4.2 mm. Landing of undersized whelk (<50 mm) is infrequent. The fishery is classified as data-poor, which limits the precision of reference points and harvest advice. CPUE (kg per pot) has fluctuated between years and does not directly reflect abundance or changes in stock size, as it does not account for differences in fishing effort across space and time; CPUE has therefore been standardised using a Generalised Linear Model (GLM) incorporating month and area factors. The stock assessment is currently based on a surplus production model (SPiCT), with total catch and standardised CPUE from Breiðafjörður as input data. There is no formal management plan for this stock and MFRI provides advice in accordance with the MSY approach.

Table 6: Common whelk. Recommended TAC in Breiðafjörður, total landings, a standardized CPUE index and CPUE (kg per pot)
Fishing Year Advice Total Year Index CPUE
1996 - 524 1 996 - -
1997 - 1,284 1 997 - -
1998 - 10 1 998 - -
1999 - 417 1 999 - -
2000 - 824 2 000 - -
2001 - 709 2 001 - -
2002 - 0 2 002 - -
2003 - 248 2 003 - 4.843
2004 - 863 2 004 0.570 3.032
2005 - 991 2 005 0.774 3.975
2006 - 839 2 006 0.618 2.935
2007 - 554 2 007 0.609 2.933
2008 - 398 2 008 0.448 1.869
2009 - 116 2 009 0.580 2.644
2010 - 142 2 010 0.655 3.219
2011 - 512 2 011 0.502 2.564
2012/2013 750 89 2 012 0.370 1.741
2013/2014 750 0 2 013 0.288 1.059
2014/2015 750 166 2 014 0.509 2.282
2015/2016 750 332 2 015 0.359 1.370
2016/2017 750 186 2 016 0.386 1.582
2017/2018 500 171 2 017 0.398 1.990
2018/2019 500 324 2 018 0.527 2.630
2019/2020 220 133 2 019 0.509 2.323
2020/2021 264 88 2 020 0.624 -
2021/2022 264 239 2 021 0.739 3.869
2022/2023 254 268 2 022 0.561 2.798
2023/2024 196 211 2 023 0.549 2.704
2024/2025 190 239 2 024 0.505 2.380
2025/2026 178 - 2 025 0.591 2.845

Management considerations

The MRI advice for whelk in 1999 to 2001 was that effort should be no more than the effort in 1997 when landings were 1,284 tonnes. In the advice in 2007 it was stated that the sustainable catch level for whelk was between 800 and 1,000 tonnes but there was great uncertainty about this estimate. In the advice in 2008 to 2011 there was no mention of possible magnitude of sustainable catches. In 2012 MRI advised catches of 750 tonnes of whelk from Breiðafjörður, which was unchanged until 2017. The basis of the advice was the average of annual catches during the last decade in the southern part of Breiðafjörður of 450 tonnes and additionally 300 tonnes in the northern part. In 2017 the advice was lowered to 500 tonnes for the whole of Breiðafjörður and the same advice was released in 2018. In this period (2012–2018) landed catches never exceeded 350 tonnes, therefore the advice was always more than realised catches and fishing was, in effect, not constrained by the advice. The advice in 2019 was based on changes in commercial CPUE (unstandardized). CPUE gradually decreased from 2003 to 2013, when it was at its lowest point in the time series. During the decline, catches of whelk were on average around 430 tonnes. It is therefore clear that fishing mortality was higher than could be sustained by whelk in Breiðafjörður. In the period between 2013 and 2021, CPUE increased again and at the same time catches were on average around 180 tonnes. It can therefore be concluded that previous advice between 500 and 750 tonnes was more than the stock could sustain. Since 2021, CPUE has increased again and in 2025, CPUE was just above the average (Table 6). From 2020, standardised CPUE has been used as the stock index in place of unstandardised CPUE. The index has shown a broadly positive trend; it increased from 0.624 in 2020 to 0.739 in 2021, the highest value in the time series, before declining somewhat to 0.505 in 2024. In 2025, the index rose again to 0.591, which is above the time series mean of 0.527 (Table 6).

References

ICES. 2021. Benchmark Workshop on the development of MSY advice for category 3 stocks using Surplus Production Model in Continuous Time; SPiCT (WKMSYSPiCT). ICES Scientific Reports. 3:20. 317 pp. https://doi.org/10.17895/ices.pub.7919

Pamela Woods, Jónas Páll Jonasson. 2017. Bayesian hierarchical surplus production model of the common whelk Buccinum undatum in Icelandic waters, Fisheries Research, Volume 194, 2017, Pages 117-128, https://doi.org/10.1016/j.fishres.2017.05.011.

Pedersen, Martin W., and Casper W. Berg. 2017. “A stochastic surplus production model in continuous time.” Fish and Fisheries 18 (2): 226–43. https://doi.org/10.1111/faf.12174

Mildenberger, T. K., Kokkalis, A., & Berg, C. W. (2021). Guidelines for the stochastic production model in continuous time (SPiCT).