The production of relaid blue mussels (Mytilus edulis L.) in a Danish fjord

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1 ICES Journal of Marine Science, 54: The production of relaid blue mussels (Mytilus edulis L.) in a Danish fjord Per Sand Kristensen and Hans Lassen Kristensen, P. S. and Lassen, H The production of relaid blue mussels (Mytilus edulis L.) in a Danish fjord. ICES Journal of Marine Science, 54: Mussels (Mytilus edulis L.) smaller than the commercial size caught in Limfjorden, as in other areas, are typically discarded. However, during the period 199 to 1993 these small mussels were returned, after sorting, to mussel beds for later harvest; a process defined as relay. This paper presents data from two commercial culture beds and from two smaller experimental beds established to study growth and mortality of these small mussel discards. The data were analysed by a yield-per-recruit model to calculate yields from such relays. This model was also used to predict the optimal time of harvest. The parameters utilized in the model were: (1) initial mortality due to harvesting, unshipping and sorting; (2) growth and mortality between relay and harvest; and, (3) the drained wet weight of a mussel of a given shell length. The initial mortality was estimated from observations of mussels damaged by harvesting, unshipping, sorting and exposure to desiccation on land in aquaria survival experiments. Growth and mortality rates after relay were estimated from diver investigations of the experimental beds. The relationship between the drained wet weight and the shell length was established based on samples from commercial landings in Limfjorden. The calculated harvest was compared to the observed yield from two commercial culture beds. The productivity in kg m 2 in a bed of relaid mussels was found to be 2 7 times higher than in natural mussel beds in Limfjorden International Council for the Exploration of the Sea Key words: Mytilus edulis, blue mussel, growth, survival, production, relay. Received 3 December 1995; accepted 22 October P. S. Kristensen and H. Lassen: Danish Institute for Fisheries Research, Charlottenlund Slot, DK-292 Charlottenlund, Denmark. Correspondence to P. S. Kristensen: tel: ; fax: ; psk@dfv.min.dk Introduction Limfjorden is the most important blue mussel (Mytilus edulis L.) fishing area in Danish waters with landings in 1994 of 19 kt compared with the total Danish landing of 144 kt. For a detailed description of the mussel fishery in Limfjorden see Kristensen (1996). In 1986, fishing, transportation and landing of mussels in Danish waters with a shell length below 5 cm was banned with a by-catch allowance of 1% (wet weight) of these smaller mussels (Anon., 1986). The minimum landing size in Limfjorden was further lowered in December 1988 to 4.5 cm shell length, and a year later the by-catch limit at these sizes was increased to 3% (weight) of mussels (Anon., 1988). In 1994, this by-catch rule was changed to 27% by weight. The by-catch regulation stipulated that the catches should be sorted and mussels smaller than 4.5 cm in shell length be returned to special selected culture beds or plots. This process of return of harvested undersized mussels to a existing bed is termed relay. Relaying of harvested undersized mussels was initiated in Danish waters in October 1989 (Kristensen, 1993). We present the results of a relay experiment which are used to develop a model for determining the optimal time interval between relay and harvest. Model predictions are then compared to the observed yields from two commercial culture beds. The model includes: (1) initial mortality of the mussels as a result of harvesting, unshipping and exposure to desiccation and relaying; (2) mortality and growth of mussels between relay and harvest; and, (3) conversion of the shell length to drained wet weight. Material and methods All data presented are from Limfjorden in the Northern part of Denmark (Fig. 1). Two types of experiments /97/ $25.//jm International Council for the Exploration of the Sea

2 The production of relaid blue mussels 855 Fae Vig FUR THY Gudnaes Vig MORS Sallingsund Nissum Bredning SALLING Kaas Bredning Lovns 58 N Kattegat 57 NORTH SEA DENMARK E Figure 1. Map of Limfjorden showing the relay sites: ( ) commercial culture beds, ( ) experimental culture beds. were performed during this study: (i) blue mussels were relaid on a commercial scale (2 8 t) between October 1989 and December 199 to two commercial culture beds and (ii) four experimental small beds each with approximately 1 t of mussels were established, in order to investigate the mortality and growth of the relaid mussels. The mortality between time of catch and relay was estimated from samples of landed and sorted mussels, and from aquaria experiments. Growth and mortality parameters were derived from diver observations on the experimental culture beds. The relationship between drained wet weight and the shell length was established based on samples from commercial landings from Limfjorden. Both in March 199 and in October 1991 ten samples were taken from commercial catches, each of approximately 3 kg mussels corresponding to 2 to 3 individuals (representing the size range of mussels in the landings before sorting, and reflecting the size distribution of mussels on the fishing ground). Similarly, ten samples were taken after sorting in two land-based

3 856 P. S. Kristensen and H. Lassen No. of mussels Length (mm) midpoint Figure 2. Pooled length distributions of relaid mussels on the four experimental beds combined. The data were grouped into.5 cm length classes. sorting systems (NL-type and DK-type, see Kristensen, 1991). The number of mussels with visible shell damage from each sorting system was recorded. The possibility that the process of harvesting, unshipping and sorting may weaken mussels without causing visible damage was investigated by observing the survival of harvested mussels in an experimental aquaria system. Two experiments, each of a duration of 7 d, were performed in March and October 199. The system consisted of 15 aquaria (67 l) with running sea water. The water was obtained from the fjord with a submersible pump type (3.6 m 3 of sea water per hour) placed below the daily low water levels. Approximately 1 sorted and unbroken mussels of those to be relaid were placed in each aquarium. Unsorted mussels were used as the control in the survival experiment (the total number of mussels in the survival experiments was 3264). Dead mussels were removed daily and their numbers recorded. Water temperature ( C) and the water flow through each experimental aquarium was recorded three times each day. The flow varied between 1.5 and 3.5 l h 1 mussel 1 throughout the experiment. The temperature fluctuated 1 4 C in daily cycles. Four in situ experiments were performed with sorted mussels on smaller relay culture beds (each of an area of approx. 3 m 2 ). The relays took place in October 199 [bed 2, Sallingsund (GL)], in November 199 [bed 3, Gudnæs Vig (VS)], in May 1991 [bed 4, Kaas Bredning (KS)], and in October 1991 [bed 5, Nissum Bredning (NS)]. Figure 1 shows the sites of the experimental culture beds. On each bed between 71 and 113 t of small mussels sorted on land were relaid at an average density of 3.5 kg per m 2. For each of the four experimental culture beds about 15 samples of 3 kg each were taken of the relaid mussels. The length composition of these mussels is shown combined in Figure 2. Distribution of the mussels in the experimental relay areas was inspected by use of a submersible video three

4 The production of relaid blue mussels 857 Table 1. Relay of blue mussels in Sallingsund and Kaas Bredning in Limfjorden by month and year, together with sampling months and year of diver surveys. Relay localities in Limfjorden Sallingsund Kaas Bredning Time for the relay August 199 May 1991 Relayed amount (tonnes net) Diver sampling Aug. and Nov. 199 Diver sampling March and September 1991 June and September 1991 Diver sampling April, June and September 1992 April 1992 Number of samples 3 12 Number of mussels sampled times during the programme: (1) prior to relay; (2) after relay, and, (3) 6 12 months after relay. The period of observations in Sallingsund (August 199 September 1992, 28 months) was longer than in Kaas Bredning (May 1991 April 1992, 11 months) as the bed in Kaas Bredning was harvested illegally in April May Each experimental bed was monitored by a diver at least three times during the programme (sampling dates are given in Table 1). On each visit, the diver took up to five samples from each culture plot by use of a quadrat of.25 m 2 placed randomly over the culture. All mussels in the quadrat were removed and the number of mussels, individual shell length and individual wet weight were recorded upon return to the laboratory (see Kristensen, 1993). In all, approximately 8 mussels between 1 mm and 8 mm in shell length were examined. These data were then used to estimate individual growth and population mortality. Experimental beds were not harvested and therefore no data on the production and yield of these relaid mussels were available. To establish the relationship between drained wet weight and shell length of the mussels, 4599 mussels were sampled from unsorted commercial catches in Limfjorden , and only mussels larger than 3 cm in shell length were included. The mussels were grouped in.5 cm size groups (3. 8. cm, Fig. 3). From October 1989 to December 199, two companies relaid blue mussels, the majority of which were smaller than 4.5 cm, in two areas in Limfjorden, Sallingsund and Fæ Vig (see Fig. 1). No data on the length compositions were available. The relaid mussels were harvested between 1 October 1992 and 1 February 1993 after 2 3 months of culture with daily landings reported by both the fishermen and the industry. Results Video records of the commercial culture beds showed that the mussels were deposited in a striated pattern (ribbons) with a width of around 1 m. The area between the ribbons was free of mussels and of approximately the same width. The abundance of the relaid mussels in the ribbons was about twice as high as the average density in the beds (3.5 kg m 2 ). Nissum Bredning (bed 5) was attacked by sea stars (A. rubens L.) within 3 months of relaying, resulting in total mortality of the experimental relaid mussels after only 3 months (video recorded). Gudnæs Vig (bed 3) was influenced by settling of mussel spat in spring 1991 which made it impossible to distinguish between the natural settled spat and the relaid mussels for a substantial number of the size ranges. Data from these two experimental culture beds were therefore discarded. The fractions of mussels that were damaged (i.e. with broken shells) during harvesting, unshipping and sorting on land are presented in Table 2a. These damage data were based on ten samples of approximately 3 kg each for a total of between 2 and 3 mussels. On average, 6% of the mussels were damaged with a maximum of 9% observed. Further, an average mortality of 2% due to desiccation was observed in the aquaria experiment for those undamaged mussels that were harvested, unshipped and sorted on land (Table 2b). Based on these observations, a total mortality estimate of 8 to 1% was used in the calculations (Table 3). Figure 4 shows the average shell length from diver observations vs. time (month) for Kaas Bredning and Sallingsund experimental culture beds. Growth appears to be linear over time (Fig. 4), although the data are highly variable. A linear regression was used to model the growth. The growth in shell length for relaid mussels in Kaas and Sallingsund were 2.1 mm per month and.43 mm per month respectively. A pooled Kaas Sallingsund growth estimate of.52 mm per month was obtained by linear regression (R 2 =.44) (Table 3). The data from Kaas Bredning show much higher variability around the average growth than data from the Sallingsund bed. The linear growth model is obviously only applicable in the limited size range of

5 858 P. S. Kristensen and H. Lassen Drained wet weight (g) Shell length (cm) Figure 3. Drained wet weight plotted against shell length in samples taken from the commercial landings from two areas in Limfjorden between February 1989 and December 1991 together with the regression curve. The plot includes 4599 mussels grouped into.5 cm classes. Only mussels larger than 3 cm are included cm in shell length, hence the calculations simply included a cut-off at the 8 cm shell length. Figure 5 shows the logarithm of the mean number of mussels per m 2 vs. months after relay with a regression of the data from the Kaas and Sallingsund experimental culture beds. The estimated total instantaneous mortalities for each bed together with the correlation coefficients are given in Table 3. The observations from Kaas Bredning and Sallingsund beds were pooled and an estimated total mortality of.27 per month (R 2 =.25) was obtained. The data showed large variation and the estimated total mortality is not significantly different from zero based on a t-test with a 95% significance level. Since there was no harvesting during the period, the total mortality is attributed to natural mortality. Hence for subsequent calculations the value of.27 per month is applied. From October 1989 until December 199 two companies relaid a total of 7214 t of mussels in two areas in Limfjorden (Sallingsund and Fæ Vig, Fig. 1). Figure 6 shows the monthly relays in Sallingsund and in Fæ Vig. The monthly yields from October 1992 to February 1993 are presented in Figure 7. The yield in Sallingsund was 4585 t, while the yield in Fæ Vig was only 376 t wet weight of mussels. According to information from fishermen, Fæ Vig was exposed to heavy attacks from sea stars (A. rubens L.). The average yield on the two commercial culture beds was 69% of the relaid amount (Figs 6 and 7). The linear regression of the average individual drained wet weight vs. shell length for both experimental commercial beds is shown in Figure 3 together with the regression line for data from commercial samples. The estimated relationship is given in Table 3. The projection model developed includes mortality due to harvesting, unshipping and sorting, individual growth and mortality. However, losses due to sea star attack and other external agents were not included. The

6 The production of relaid blue mussels 859 Table 2. (a) Proportion of damaged mussels due to handling and sorting between catch and relay. Sorting system Damage by unshipping % (average) Damage by sorting % (average) Total physical damage in % by handling on land (mean) NL-type DK-type Table 2. (b) Proportions of sorted mussels surviving in aquaria experiments. Experimental period Experimental type Initial number of mussels Numbers of survivors after 7 days Survival % after 7 days Water temperature C March 199 Sorted Control October 199 Sorted Control Table 3. Estimated growth, mortality and the relation between drained wet weight (g) and shell length (cm). Initial handling survival 9%* Growth and mortality on the culture beds Relay localities in Limfjorden Kaas Bredning Sallingsund Pool* Growth month 1 (mm) Std. dev R Z (mortality month 1 ) Std. dev R Drained wet weight (DWW) shell length relation Relation R 2 Number obs. Dww(g)=.1533*L(cm) *Parameters used in the model. projection was initiated with the length distribution of the relaid mussels, the initial size composition is the combined length frequency (Fig. 2) of relaid mussels after sorting from the four experimental culture beds. The length-frequency is converted to biomass with the relation between the drained wet weight and shell length. Results are expressed as the ratio between the relaid biomass and the biomass of mussels >4.5 cm when harvested. Calculations of the expected yield from the two commercial beds (Sallingsund and Fæ Vig) are presented in Figure 8. The yield in drained wet weight is presented as a function of the growth period allowed between relay and harvest. The calculations indicate that the yield reaches a plateau after about 24 months of culture. The average time between relay and harvest at 27 months (relaid mussels cultured between months) only allowed relaid mussels larger than 2.7 cm in shell length to reach commercial size, mussels below this size need more time. Trial calculations indicate that the initial size distribution of the relaid mussels is crucial when determining the appropriate time of harvest. The projections suggested that for a 27 month growth period the ratio between relaid biomass and harvested biomass should be around 1:.92 provided that the culture bed is not attacked by sea stars. Sea star attacks may reduce yield to only 3% of the relaid biomass, as observed in the Fæ Vig relay (see also later). The relays in Sallingsund on the commercial culture bed were initiated in October 1989 and ended in December 199, by then a total of 5947 t were relaid. The mussels on the culture plots were harvested by the fishing fleet (43 vessels) over a four month period between October 1992 and February The total

7 86 P. S. Kristensen and H. Lassen Mean shell length (mm) Months after relay Figure 4. Mean shell length (mm) of mussels for Sallingsund ( ) and Kaas ( ) experimental culture beds plotted against sampling date, pooled regression shown by solid line. Data obtained by diver sampling from plots of.25 m 2. Data for Kaas Bredning have been corrected by subtracting mm to account for the difference in mean size between the two beds. 3 yield was 4585 t of commercial mussels. The ratio between relaid and harvested biomass was 1:.77. This result is slightly lower than what was suggested by the model, however, the bed was not completely empty after harvest as demonstrated by a later fishery of mussels on the beds. Relays in Fæ Vig began in October 199 and were concluded late in 1991, a total of 1267 t were relaid. The harvest was only 376 t in wet weight of mussels or a yield ratio of 1:.3. The bed was attacked by sea stars according to information from fishermen. Although the data do not allow any precise estimate of the losses from attacks by sea stars these losses seem significant. Four out of six beds were not attacked during the 34 month period of observation. This provides an estimate of the average attack risk over the two year experimental period of 1.2% per month. As an illustration of how these attacks may affect our conclusions we assumed a constant risk of sea star attack of 1% per month and applied that in our model to predict the harvest result. The result is shown in Figure 8. Given a constant risk of an attack of sea stars it would be advisable to harvest as early as possible. Discussion Mortality due to sorting and handling was found to be insignificant, although data from survival experiments in aquaria (Kristensen, 1991) suggest that increased mortality rates could occur during the summer months, possibly resulting in as little as 4% of the mussels being relaid. At present, the fishery for mussels is normally closed in June, July, August and September each year and therefore this problem was not considered any further. In our model, growth in shell length was assumed to be constant over seasons. The winters in which observations were made were unusually mild and although it cannot be demonstrated with the material available, reduced growth in the winter months is most likely to take place (Theisen, 1968; Dare and Edwards, 1976). Theisen (1968) found seasonal changes in growth rate for intertidal mussels in the Danish Wadden Sea over a four-year period. Dolmer (1996) observed that a growth in shell length of blue mussels in the summer in the natural beds in Limfjorden varied between mm per two month period depending on the size of the mussels. He also observed differences in growth between

8 The production of relaid blue mussels Ln (no. of mussels/.25 m 2 ) Months after relay Figure 5. Number of mussels for Sallingsund ( ) and Kaas ( ) experimental culture bed plotted against sampling month, pooled regression shown by solid line. Data obtained by diver sampling from plots of.25 m 2. Data for Kaas Bredning have been corrected by subtracting exp(.7618) to account for the difference in mean density between the two beds. The Y-axis is in logarithm scale. 3 localities. Therefore, the projections given here may be useful to discuss whether the relay beds should be allowed to grow for one, two or more years, while conclusions for a finer time scale may be more uncertain. Geographical differences in growth were observed in our study, with faster growth in shell length observed for mussels relaid in Kaas Bredning (2.1 mm per month in shell length). Dolmer (1996) observed higher growth rates of mussels in natural beds in Kaas Bredning ( mm per two month period) compared to the growth in Sallingsund ( mm per month). In Kaas Bredning the water currents are quite strong which provides a larger volume of water with food for the mussels, and may minimize recirculation of water which have been filtrated by other mussels in the bed. Hence, a strong current may improve the growth conditions for the individual mussel in the bed. The growth estimate for Kaas is based on a fairly short time period and compared to the values cited from other authors seems to be quite high. The Sallingsund estimate was from a longer time period and was therefore considered to reflect better the growth possibilities. The pooled estimate (.52 mm per month) place the main emphasis on the data with the longer growth period and is not statistically different (t-test at 5% significance level) from the Sallingsund estimate. This value was used in the light of the literature values cited above which are generally higher. Application of this low growth rate partly safeguard against overestimation of the production. Growth in our study was assumed to be linear in time. Seed (1969) and Dare (1974) have both recorded changes in growth rates over a growth period of 2 to 3 years for mussels living in the intertidal zone and in suspended cultures in the shell length range between 1 and 2 mm, with a lower growth rate during the winter month. They observed linear growth between May and September and a slowing down of the growth between December and March. Dolmer s (1996) data showed accordingly decreasing growth rates with increasing shell length and age. The estimate of mortality is dominated by data from the Sallingsund bed due to the length of the observation period. While this estimate is uncertain, mortality rates do seem to be low. Attack by sea stars (A. rubens L.) can virtually wipe out the entire mussel population on a culture bed as observed in this study. Similar observations have been made for mussels in intertidal beds (Theisen, 1968; Seed,

9 862 P. S. Kristensen and H. Lassen Amount relayed (t) (a) Months and years Amount relayed (t) 14 (b) Months and years Figure 6. Relaid amounts (t) of blue mussels (t) by month on the two commercial culture beds in (a) Sallingsund and in (b) Fæ Vig. 1969). When projecting the potential yield from relays on a large scale, the calculated yield should be reduced to account for those beds which will be destroyed by sea stars. The present material includes too few culture beds to allow a reliable estimate of this reduction in yield and no other information seems available on the mortality due to sea star predation. There are also other agents that present a risk to the survival of mussels. For example, in the autumn of 1994 anoxic conditions and the presence of H 2 S in the water around the mussels leaking from the sediments were observed to kill approximately 13 t of mussels in Lovns Bredning and Skive Fjord both areas in Limfjorden (P. S. Kristensen, pers. obs.). Settling may increase the productivity of a bed. However, the settling mussels are small and these will not, within the time frame considered (around 2 years), grow to commercial size. These mussels do not contribute to the yield of the beds but may be sorted and relaid for another 2 years of culture. New settlements of mussels on the relaid mussels may cause postponement of the harvest, due to a too high number of under-sized mussels after 1 2 years of culture (more than 3% of mussels with a shell length <4.5 cm makes fishing illegal). This postponement is a result of increased competition for food and space between the relaid mussels and the new settled mussels which may slow down the growth rate (Kristensen, 1991). The first Danish experiments with transplantation of mussels were carried out in the Danish Wadden Sea in by Theisen (1968). He estimated an average growth rate of 1.38 mm per month for transplanted mussels, which is higher than the growth in shell length of.52 mm per month used in our projections. Theisen (1968) estimated a survival of 26 51% over 2 years for the transplanted mussels in the Danish Wadden Sea, a survival rate comparable to that observed in our study. In Germany and in The Netherlands blue mussels have been transplanted for culture use in the Wadden Sea for many years (Dankers, 1987; Meixner, 1987). They found ratios between the relaid amount and the yield in the German and the Dutch mussel culture to be between 1:1 to 1:3 4 depending on food supply for the mussels in the culture and the size of mussels in the culture. We found that the ratio between biomass of relaid and harvested mussels in our study was slightly lower at about 1:.8. Is relaying a good idea? Whether relaying of the small sorted non-commercial mussels is a rational use of the available culture beds can

10 The production of relaid blue mussels 863 Landings (t) (a) Months and years Landings (t) (b) Months and years Figure 7. The harvest (t) of relaid mussels by month from the commercial culture beds in (a) Sallingsund and (b) Fæ Vig. be discussed from two points of view: (1) whether the production in a relaid culture bed per area is comparable Months after relay Figure 8. Simulated biomass of mussels above 4 5 cm plotted against months after relay. Initial relay shell length distribution equal to the pooled length composition of mussels relaid on the four experimental beds given in Figure 3. No sea star attack, sea star attack. Harvest to relayed biomass ratio to the production in a natural bed; and, (2) the ethical argument, that we must use not just discard the production of the smaller non-commercial mussels. The latter argument was considered when the relay project was planned. The average density of whole life blue mussels in all relays was 3.5 kg m 2. Natural mean densities of mussels in Limfjorden in were between.21 and 3.86 kg m 2 (unpubl. data). The harvest ratio was approximately 1:.8 for the mussels relaid in Sallingsund after 2 years of culture. Therefore, on the culture bed not heavily attacked by sea stars the yield was approximately 3.5.8/2 or 1.4 kg m 2 year 1. The production loss by sea stars in Fæ Vig was approximately half of the harvest compared to Sallingsund. Two out of six culture beds were attacked, therefore we corrected the estimated annual production of 1.4 kg m 2 year 1 ( =.83) or an estimated yield of approximately 1.16 kg m 2 year 1. This could be compared with the average harvest from a highly productive and heavily exploited area such as

11 864 P. S. Kristensen and H. Lassen Lovns Bredning in Limfjorden (see Fig. 1) where the annual phytoplankton production is around 4 g C m 2, (Anon., 1995). The harvest from this area was 8463 t in , and t in , in an area of 28 km 2. The production/yield is therefore, respectively,.3 kg (1993/1994) and.42 kg (1994/ 1995) m 2 year 1. The total harvest of mussels from the natural beds in Limfjorden in 1993/1994 and in 1994/1995 was, respectively, 99 1 and t, and the total area with mussels in Limfjorden is approximately 682 km 2. The production/yield (kg mussels m 2 year 1 ) in the Limfjorden as a whole is on average only.14 kg m 2 year 1 when the mussels are 3 5 years old. The annual production (kg relaid mussels m 2 year 1 ) of mussels in the culture beds for relaid mussels is twice or at best seven times higher than the production of mussels in the natural beds. The cost of relaying l t of mussels is today approximately DKK 8 (P. Willadsen, Copenhagen, pers. comm.). From our findings, l t of relaid mussels may give a yield after 2 years of culture of around 8 kg of commercial mussels at a value of approximately DKK 4, which gives an economical yield of around 25% annually. Optimal yield from relays Figure 8 shows the simulated ratio between the relaid biomass and the biomass of mussels larger than 4.5 cm after a growth period. The projection is based on a length distribution of the relaid mussels as was observed for the combined experimental beds (Fig. 2). We assumed that it is possible to harvest the entire biomass. Reduction of the biomass by sea star attacks was included in the graph assuming that there is a constant risk of losing biomass with time due to predation. Harvesting 12 months after relay gives a relay biomass to yield ratio of approximately 1:.69 increasing with time to a maximum of 1:.94. Projections indicated that the relay biomass to yield ratio of 1:.9 is only possible for relaid mussels of a shell length above cm. Smaller mussels, which will need an extra year to grow to 4.5 cm, will also be exposed to one extra year of mortality, and the relaid biomass to yield ratio will be less. The general rule appears to be harvest as soon as possible when a significant proportion of the mussels reach minimum commercial size. The regulation which is applied in the mussel fishery in Limfjorden today stipulates that the bed should be harvested when 7% of the relaid mussels reaches the minimum commercial size of 4.5 cm in shell length. Based on the assumptions of linear growth and constant total mortality the length for optimal yield (Lopt) would depend on the exponent in the Drained wet weight shell length relation (b), the linear growth coefficient (K) and the mortality (Z). The rate of change of biomass is: and therefore the optimal length for harvest Lopt would be Using the estimates presented in Table 3 this would be about 53 mm. This is in general agreement with the harvest rule noted above. However, this calculation does not include a cost benefit estimation required to account for the size distribution some mussels will be too small others too large when deciding whether or not to harvest a relaid mussel bed. Acknowledgements The study was financed by the Danish Ministry for Fisheries. Vejle Mussel Industry. We would also like to thank the fishermen Gerdt Sørensen, Børge Sørensen and Jørgen Back who provided us with data on the drained wet weight and the meat yield of commercial mussels, and Michael St John for his critical reading of the manuscript. References Anon Bekendtgørelse om mindstemål for blåmuslinger. FM bek. nr. 656 af 1. oktober (Order on minimum landing size for blue mussels). Fiskeriministeriet (1986). Anon Bekendtgørelse om betingelserne for udøvelse af fiskeri efter og landing af blåmuslinger. Bek. nr. 872 af 22. december 1988 (Order on the conditions for fishing and landing of blue mussels). Fiskeriministeriet (1988). Anon Vandmiljø i Limfjorden pp. 114 (mimeo). (The Water Quality in Limfjorden Report from the Environmental Authorities in Ringkjøbing amt, Viborg amt and Nordjyllandsamt to the Municipalities around Limfjorden). Dankers, N Some ecological effects of the mussel culture in the Dutch Wadden Sea. pp In Proceedings of the 5th International Wadden Sea Symposium, 29 September 3 October 1986, pp Ed. by S. Tougaard and S. Asbirk. The National Forest and Nature Agency and The Museum of Fisheries and Shipping, Esbjerg. Dare, P. J Settlement, growth and production of the mussel, Mytilus edulis L., in Morecambe Bay, England. MAFF, Fisheries Experiment Station, Conway, North Wales. London, Her Majesty s Stationery Office. Series II, 28: 25. Dare, P. J. and Edwards, D. B Experiments on survival, growth and yield of relaid seed mussels (Mytilus edulis L.) in the Menai Straits, North Wales. Journal du Conseil International pour l Exploration de la Mer, 37:

12 The production of relaid blue mussels 865 Dolmer, P Blåmuslingers vækst og dødelighed i Limfjorden. DFU-Rapport nr , pp. 66. Kristensen, P. S Sortering og genudlægning af blåmuslinger i Limfjorden. (Sorting and relaying of Blue Mussels in Limfjorden). DFH-rapport nr To: Foreningen Muslinge-erhvervet. pp. 55 (mimeo). Danish Institute for Fisheries and Marine Research, Charlottenlund, Denmark. Kristensen, P. S Growth and production of small mussels (Mytilus edulis L.) sorted and relaid from commercial catches in Limfjorden. ICES Shellfish Committee, CM 1993/K: 12. p. 25. Kristensen, P. S Oyster and Mussel fisheries in Denmark. In The history, present condition and future of the molluscan fisheries of North America and Europe, pp Ed. by C. L. Mackenzie, Jr., V. Burrel and A. Rosenfield. US Department of Commerce, NOAA Technical Report. Meixner, R The importance of Mussel fishery and farming in the German Wadden Sea. In Proceedings of the 5th International Wadden Sea Symposium, 29 September 3 October 1986, Esbjerg, pp Ed. by S. Tougaard and S. Asbirk. The National Forest and Nature Agency and The Museum of Fisheries and Shipping, Esbjerg. Seed, R The ecology of Mytilus edulis L. (Lamellibranchiata) on exposed rocky shores. II. Growth and mortality. Oecologia (Berl.), 3: Theisen, B. F Growth and mortality of culture mussels in the Danish Wadden Sea. Medd. Danm. Fiskeri og Havunders. N.S. 6: