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The status and conservation of the freshwater pearl mussel Margaritifera margaritifera Linn. in Great Britain
Biological Conservation 25 (1983) 35-52
The Status and Conservation of the Freshwater Pearl Mussel Margaritifera margaritifera Linn. in Great Britain M a r k Y o u n g & J e n n i f e r Williams Department of Zoology, Aberdeen University, Tillydrone Avenue, Aberdeen, AB9 2TN, Great Britain
ABSTRACT All available records of the occurrence of the jreshwater pearl mussel Margaritifera margaritifera Linn. in Great Britain have been collected. Analysis of these records shows that there has been a recent decline in numbers of the species, which is especially marked in England and Wales. The causes of this decline are thought to be primarily pollution and overfishing in England and Wales, and overfishing in Scotland. Apparently even sparse populations remain fertile, but it is concluded that unless conservation measures can reduce overfishing, pearl mussels will become rare and endangered. It is suggested that the best conservation measures will be to restrict pearl fishing to licensed fishermen, perhaps using non-destructive fishing methods, and to prohibit the use of sub-aqua equipment.
INTRODUCTION The freshwater pearl mussel MargaritiJera margaritijera Linn. has been exploited for pearls since at least R o m a n times in Britain. Pliny and Suetonius both refer to Julius Caesar's interest in freshwater pearls, the latter even crediting them as one of the reasons why Caesar invaded Britain in 55 BC ! Since then British freshwater pearls have continued to be harvested, at times intensively, and there are even now several professional and semi-professional fishermen living in Scotland. However, it is clear from the experiences of these fishermen and from the results of our 35 Biol. Conserv. 0006-3207/83/0025-0035/$03.00 ~ Applied Science Publishers Ltd, England, 1983. Printed in Great Britain
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Mark Young, Jennifer Williams
own survey, and those of Kerney (1976) and Jackson (1925), that the pearl mussel has recently declined markedly in both range and abundance. The mussel is long-lived (commonly to 60 years or more), prefers fastrunning, cool waters, low in calcium, and has a parasitic, glochidial stage found primarily on the gills of Salmonidae (Bjork, 1962). These factors restrict its natural range in Britain to the west and north, where it inhabits both large rivers and small streams. These large mussels have always attracted attention, reliable published records of their occurrence being found as far back as 1831, and there are several accounts of their distribution in relation to habitat variables (e.g. Boycott, 1933, 1936) as well as their size and shape (Eagar, 1977) and their long life (Comfort, 1957; Hendelberg, 1960). This paper draws together all the available information on distribution and abundance of the pearl mussel in Britain. These primary data are analysed to determine the extent and possible reasons for the decline of the mussel, and to assess its current status. Further observations on the reproductive capability of various populations were made to obtain the information needed for evaluating potential conservation measures. METHODS Mussel distribution and abundance
Information on the distribution and abundance of the freshwater pearl mussel in Britain was collected from various sources. Published records and data from the Conchological Society were abstracted; many pearl fishermen, fishermen and individuals provided personal observations and information; water authorities and other riparian users were approached for information and field surveys were carried out by ourselves in areas from which little recorded information was available. Whilst this set of information is as complete as possible, some other records must exist and any additional information would be welcomed. The complete set of information, with full references, is lodged at the Institute of Terrestrial Ecology's Biological Records Centre and with Dr Kerney, acting for the Conchological Society, as it is too extensive for inclusion here. It is freely available from either place as a source of distribution data or references. Instead the records are summarised in Fig. 1 and Table 1. For convenience the records are categorised into the following date classes: pre- 1900; 1900-1960; 1960-1970; and post- 1970.
The freshwater pearl mussel in Britain
37
The extent, the date and possible causes of decline are also summarised in the table. Unfortunately, whilst the information has been collected as critically as possible, such a heterogeneous set of references contains many inconsistencies. The limitations of the data are discussed below and data which are unsupported and of doubtful reliability have been omitted.
A non-destructive fishing method Most pearl fishermen kill all the mussels they examine by tearing the valves apart to search for pearls. However, some use an old shell to prise the valves apart slightly so they can see if pearls are present, and if there are non~ they allow the mussel to close again and return it to the river alive. In Germany, until a total ban was placed on pearl fishing, this practice was enforced by requiring fishermen to use special opening 'tongs'. To test the survival of mussels opened in this way we constructed similar 'tongs'. These comprise two thin-flanged jaws which can be inserted between the valves of the mussels and are then opened by pressure on the handles. A small bar maintains the gape for as long as required. In our experiment, 25 mussels, all over 7.0cm long, were selected at random from a small river in Wester Ross in June 1979. They were marked individually by glueing small, numbered pieces of plastic onto cleaned and dried areas of the shell, and then were opened using the tongs to a gape of 1 cm for 30 s. Then they were allowed to close and returned to an area of river which was naturally delimited by large stones. Their survival was monitored approximately monthly until March 1981. Several groups of similarly marked mussels exist in the same river, where they are part of experiments on dispersal rates of mussels, and the survival of these was also monitored over the same period as a control.
The fertility of sparse populations Observations were made to see if populations which had been reduced to very low numbers by overfishing could continue to reproduce successfully. The criterion used for this was the presence of developing glochidia in the gills. If such glochidia are present they appear as a creamy mass within the translucent brown gills and this may be seen easily using opening tongs.
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In 1979 and 1980 two such populations were examined. These were: 1, the Manse Burn, Sutherland (NC 0-2-), where only eight mussels over 7.0cm long could be found in a 100-m stretch; and 2, part of the River Dee, Aberdeenshire at Maryculter (NO 8-9-), where only nine mussels over 7.5cm long could be found in a 50-m stretch. In both cases no mussels were present upstream for at least 100 m. Abundant populations from the Stac Burn, Wester Ross (NC 08-12-) were also sampled, as a control. Sample sizes from sparse populations are necessarily low, and so a further set of data was collected. A series of fertility assessments were made in the Stac Burn at sites upstream to its source in Loch Doire an'Airbhe. Mussels from the upper sites have very few others upstream of them to act as sperm suppliers and so can also be considered as 'sparse populations'. At each of nine sites 30-70 mussels were opened in July 1980 and examined for glochidia (two of the sites had also been assessed in 1979).
Age and size at reproductive maturity Observations were made on the minimum size at which mussels contained glochidia. Since there is a general relationship between length and age for each population (Hendelberg, 1960), this allows an approximate assessment of the age of maturity. These data were collected so that if recommendations are required on a minimum size below which mussels should not be fished, the recommendations can take account of the age/ size at which mussels mature. R a n d o m samples of between 29 and 80 mussels were taken in August 1979 and July 1980 from each of four rivers: 1, Badnaban Burn, Sutherland (NC 08-20-); 2, Rhegreanoch Burn, Wester Ross (NC 08-16-); 3, Stac Burn, Wester Ross (NC 08-12-); and 4, River Dee, Aberdeenshire (NO 7-9-). These sites were chosen to include a range of mussel sizes and types. RESULTS
Mussel distribution and abundance The current British distribution is shown in Fig. 1, which was drawn using the full data now held by the Biological Records Centre. Table 1
The freshwater pearl mussel in Britain
1
Fig. 1.
2
3
4
5
6
The distribution of the pearl mussel in Britain. O, post-1970 records; ©, pre1970 records; - , sites now extinct.
summarises these data and itemises the number of localities and rivers where the mussels have apparently changed in their status. Some references and sources quoted actual reasons for observed declines for some sites. These are included, although such data are usually circumstantial and always of limited reliability for the reasons listed below. Consequently declines and increases are only included in Table I where it is certain that they are genuine; in all cases where evidence for a decline is merely suggestive they have been omitted.
40
Mark Young, Jennifer Williams TABLE 1 The Status of the Pearl Mussel in Great Britain
England and Wales Total no. of rivers where mussels occur(red) No. of rivers searched at least twice (so status is accessible)
Scotland
56
Total A
119
Total A
37
Total B
54
Total B
°/ of Total B /o
°J of Total B /o
No. of rivers where mussels now extinct (or not found) No. where decline noted
17
46
11
20
13
35
32
59
Total
30
81
43
79
6
16
13
24
No. of rivers where mussels have not altered in status No. of rivers where mussels have increased No. of rivers where mussels are now common No. of rivers with mussels discovered only since 1970 No. of rivers where decline is suggested as: Overfishing Pollution Physical disturbance Dates of decline
0 /o of Total A
o ¸
' o
0 of Total A
6
11
26
22
6
11
41
35
31 6 2 pre1900 1
1 9 0 0 196~ 1 9 6 0 1970 15 8
post1970 6
pre- 1900- 1960- post1 9 0 0 1 9 6 0 1 9 7 0 1970 0 12 16 15
The freshwater pearl mussel in Britain
41
Regardless of the problems associated with the reliability of the data, and allowing for the fact that many rivers have only been searched recently (so providing many 'recent-only' records), it is clear that a great decline has taken place both in overall distribution (i.e. some rivers now no longer harbour mussels) and within specific rivers. The latter is the main result in Scotland, the former in England and Wales. It is also clear that overfishing is predominantly important in Scotland whereas various causes have operated in England and Wales. Furthermore, declines have been more recent in Scotland than in England and Wales. The distribution of rivers where mussels have apparently become extinct is shown in Fig. 1 by dashes, although several of these are obscured by other existing populations in the same 10km square. In spite of this obvious decline the freshwater pearl mussel remains a widespread animal, and is still abundant in some localities, particularly in Scotland. However, many of the 'post-1970' records refer to only one or 'a few' specimens and the mussel is genuinely threatened in some areas, especially in England. Some published or anecdotal information suffers from one or all of the following faults. These are unavoidable and have been allowed for as far as possible, but nevertheless the distribution results must be viewed cautiously. The problems are that: 1, the species is incorrectly identified; 2, false negatives are included due to inexperience or poor searching conditions; 3, the abundance may be inadequately assessed or reported; 4, the cause of decline noted may be biased or merely hearsay (e.g. some observers clearly overreact to piles of dead mussel shells); 5, the locality may not be quoted precisely.
Non-destructive fishing methods Table 2 summarises the survival of the 25 mussels which were marked and opened using the non-destructive method described above, and of 23 marked control mussels. In both groups only 1 mussel is known to have died in the 21-month period and only a further 2 and 1 respectively are missing. Clearly there is no significant difference in the survival rate of the two groups; opening the mussels has not harmed them. The specimen which was damaged on opening (one adductor muscle was severely torn), apparently recovered and remained alive at the end of the experiment.
Mark Young, Jennifer Williams
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TABLE 2 The Survival of Mussels which were Opened to a Gape of 1cm for 30 s (June 1979-March 1981; 21 months)
Operted mussels
Control mussels
25 a
23
No. of mussels used No. known to be dead No. missing at March 1981
1 2
1 1
Total
3
2
12
8
% dead or missing
a One damaged in opening remained alive until March 1981.
The fertility of sparse populations Table 3 shows the result of the fertility assessment of the populations in the Manse Burn, Sutherland, and a section of the River Dee, Aberdeenshire. These should be compared with the middle sites (e.g. 100 m and 150 m below loch) of the Stac Burn, Wester Ross (Table 4) which act as a control, in that the populations there are fairly dense. As far as the necessarily small amount of data shows there is little difference between any of the estimates. This indicates that sparse populations remain fertile when this is measured as the percentage containing glochidia. However, the number of glochidia produced by each mussel was not assessed and this might vary between mussels in sparse and dense populations. TABLE 3 The Fertility of Mussels from Sparse Populations
Site
Date No. No. and % examined examined Jertile
Manse Burn, Sutherland NC 0-231 July 1979 River Dee, Aberdeenshire (Maryculter) NO 8-925 August 1980
8
3
9
5
Notes
Total population searched--all 38 over 7.0cm long All mussels over 7.5 cm long 56 searched
The freshwater pearl mussel in Britain
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TABLE 4
The Fertility of the Mussel Population at Different Sites Within the Stac Burn, Wester Ross in July 1980 In loch
5m below loch
lOm below loch
~o of mussels either containing glochidia or recently 'spent' 0 43 42 No. of mussels examined 11 30 65 Estimated no. of mussels 0 11 45 above site Equivalent data 70 ~o of for August 30 mussels 1979
lOOm below loch
150m 200m 300m 400m 600m below below below below below loch loch loch loch loch
35
53
14
23
11
25
66
62
65
62
54
65
>10 a > 103 43 % of 30 mussels
>104 >104
>104 >104
All mussels examined or counted were over 7.0 cm long.
Table 4 sets out the fertility estimates of the various sites within the Stac Burn, Wester Ross, and includes an estimate of the minimum number of mussels upstream of each sample. All mussels chosen for examination were over 7.0 cm long. The fertility levels at the extreme upper end of the population are not obviously lower than elsewhere, except in the loch itself, where the lack of a current to distribute sperm might account for the lack of fertile specimens. The low fertility figures noted in the lowest stream sections are almost certainly due to some of those mussels having already shed their glochidia at the time of the sample. This factor means that all the assessments are minimums but does not invalidate the result that the sparse populations remain fertile.
Age and size at maturity Table 5 lists the sizes of mussels examined (as overall length in cm) in each
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TABLE 5 The Size at which Mussels first Mature as Indicated by Presence of Glochidia in July 1980 (west coast rivers) and August 1979 (R. Dee) River
Badnaban Rhegreanoch Stac Burn (mid-section) Dee
No. examined
Size range examined-length in cm
% with glochidia
Size range with glochidia--length in em
75 80 65 29
3.5-9.0 5.7-11.1 4.8-10-6 4.7 11.7
16 41 42 21
6-5-8.8 7-0-11.1 6.9-10.0 7-7-11.7
Badnaban Burn, Sutherland NC 08-20Rhegreanoch Burn, Wester Ross NC 08-16Stac Burn, Wester Ross NO 08-12R. Dee, Aberdeenshire NO 7-9-.
of four rivers, and notes the size above which mussels were found to contain glochidia. The mussels from the Badnaban Burn, Sutherland, were generally smaller than those from the other sites and matured at a smaller size, whereas those from the River Dee, Aberdeenshire, were larger in both respects. Since some mussels may have released their glochidia before the samples were taken, these size limits may exclude even smaller mussels which matured earlier in the season, but this seems unlikely. Generally mussels mature at about 6.5-7.0 cm in length. DISCUSSION The status of the freshwater pearl mussel in Britain
It is clear from Table 1 that the freshwater pearl mussel has declined significantly in Great Britain in the last 100 years and that the causes of this decline are overfishing, pollution and to a small extent habitat modification. In spite of this the mussel remains widespread and is obviously not in danger of extinction in Great Britain as a whole, although it may eventually be lost in England and Wales and will almost certainly become extinct in some rivers. These conclusions agree with those of Cranbrook (1976) and Kerney (1975). Presumably there are mussel populations which are so far un-
The freshwater pearl mussel in Britain
45
discovered. Forty-seven new populations have been found since 1970 and of these six are in England and Wales, which have been quite comprehensively searched by naturalists in the past. There are many small rivers in Western Scotland (and to a lesser extent Wales) which probably contain pearl mussels, but their discovery will not alter the general status and distribution patterns of the mussel. A more important point is that it is only where rivers have been searched effectively at least twice that changes in status can be assessed. Table 1 and the following discussion should be judged with this clearly in mind. A larger proportion of English and Welsh populations have declined or become extinct than in Scotland and the various 'pressures' exerted remain greater in the south. (Whilst important politically, this is not relevant biologically as the mussel is primarily a northern and western mollusc in Britain.) The decline in the south was apparently earlier than in Scotland and with limitations the data seem to show that pollution had a significant effect in England and Wales whereas overfishing has been most important in Scotland. Figure 1, in showing the current position, illustrates a classic northern and western calciphuge distribution with few significant gaps apart from the Tweed catchment area and perhaps parts of northern central England and mid-Wales. The absence from the Tweed system is inexplicable but probably genuine and natural. The freshwater pearl mussel is holarctic and in some places common (e.g. South-East Alaska (personal observations), Norway (Okland, 1975) and some parts of Japan (Okada & Koba, 1933)) but has declined greatly in many countries other than Great Britain. In Germany it is nearly extinct (G. Bauer, pets. comm.) and in Finland it has required complete protection since 1955 (Brander, 1957). Consequently the fate of our populations is of more than just local significance. It is not easy to judge why mussel populations have become reduced in countries other than Britain. Certainly in Germany overfishing and pollution have been important, and in Sweden overfishing is considered to be the major cause (Bjork, 1962). The causes o f decline
Overfishing Pearl fishing has been established since at least Roman times and has rarely caused any lasting harm until recently. At some periods enthusiasm for the pearls has been aroused to the extent that mussels became scarce
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Mark Young, Jennifer Williams
locally, as when Moritz Unger came to Scotland in the 1860s and stimulated the trade so much that mussels became rare in the larger rivers. As it became harder to find pearl-bearing mussels the trade declined and the mussel populations recovered. In recent times several factors increased the effect of fishing. One of these is the improved access to the more remote areas, so bringing more visitors to the pearl rivers. National publicity has encouraged these visitors to collect mussels casually and in places (such as near Aviemore) concentrations of visitors have severely reduced mussel populations; this is merely local in its effect. A second factor is the great increase in the membership of sub-aqua clubs. Whilst many clubs act responsibly, others unthinkingly encourage pearl mussel collection as an exciting activity to add interest to a dive. This has had the very serious effect of depleting the populations in deeper pools. Previously these pools have been inaccessible to wading pearl fishermen and the mussels within them acted as a 'reservoir' of reproductive effort. These extra pressures have meant that the activities of the regular pearl fishermen have been imposed on smaller stocks and so assume a greater importance. Furthermore the increased mobility of the professionals has allowed them to exploit populations previously undisturbed. Most pearl fishermen have traditional fishing areas and usually leave 12-15 year intervals between their exploitation. This allows some recovery of the populations. Also the more experienced fishermen take only large specimens with scarred shells, so leaving a proportion of the population untouched. Casual fishermen may take all mussels, including small and unscarred specimens, so reducing greatly the possibility of eventual recovery for that population. This study has shown that mussels do survive the use of opening 'tongs' and if fishermen could be persuaded to use them, then overfishing would cease to be a problem. However, some fishermen regard their use as unnecessary and too time-consuming, and so their adoption would take more than merely making them available--some compulsion would be required. Pollution Evidence for local extinctions caused by pollution in Britain is anecdotal. It is said, probably correctly, that silting and deoxygenation are both
The J?eshwater pearl mussel in Britain
47
inimical to the mussels and some past mussel rivers are undoubtedly now unsuitable with a much altered fauna (e.g. River Almond, Midlothian). Such effects have been locally important but clearly do not threaten the mussel's existence in Britain as a whole. In Germany, however, there is some evidence that pollution has had a serious effect. It is thought that phosphates from sewage have encouraged algal growth to the level that it filis the substrate interstices and smothers the small mussels (G. Bauer, pers. comm.). Furthermore, some ions, including Cr 2 +, may also be directly toxic. In Britain a number of pearl mussel rivers suffer phosphate enrichment from farmland but it is not possible to say whether this has affected the mussels. The River Ythan, Aberdeenshire, for example, has an elevated phosphate level and a reduced mussel population, but overfishing may actually be the cause of the decline. Pollution may act indirectly. If pollutants make a river unsuitable for the salmonid hosts of the mussels then obviously the mussel will eventually disappear.
Loss of host fish Where the salmonid hosts of the mussel glochidia are removed, whether by pollution or physical displacement, the mussel must die out. However, there are no examples of rivers in Britain where this has occurred except where other factors may also have been affecting the mussels directly. Generally salmonids are prized and preserved at the expense of other fish, and so maintenance of the mussel's hosts is assured.
Physical d&turbance In some cases, dredging of the river bed or scouring due to increased flow have removed mussel habitats. Generally this is purely local; for example, salmon pools in the River Polly, Wester Ross, have been created, so displacing mussels; but it may affect a large section of a river. Some rivers are periodically depleted and then scoured below hydro-electric dams and others have been canalised, so altering the substrate. Nationally this is of no real significance, although some pearl fishermen have noted some previously good pearling areas which have been lost (N. McCormick, pers. comm.). Possibilities for natural recovery of mussel populations
Historical evidence suggests that mussel populations can recover
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Mark Young, Jennifer Williams
naturally from low levels; after Moritz Unger's activities in the 1860s the Scottish populations recovered adequately. However, the level to which the populations were reduced is u n k n o w n and the mussels in the deeper pools were certainly untouched. Today, not only are the populations reduced to a lower level than previously, but the fishing pressures are being maintained even on scarce populations. Easier travel, use of subaqua equipment and the casual (rather than strictly economical) nature of the fishing have led to this in places, and in some rivers the remaining mussels are now numbered literally in double figures only (e.g. River Ythan, Aberdeenshire). One of the factors affecting recovery is whether mussels are taken below the size at which they become mature, or whether they are allowed to reproduce. This study shows that mussels generally mature at a length of about 7.0 cm, although in populations of slow-growing specimens this may be reduced to 6.5 cm (Table 5). This corresponds roughly to an age of 12 years, and most mussels then survive to continue reproducing for another 30-50 years (Young & Williams, in prep.). However, there is no evidence to indicate how many years of glochidial production are required before adequate recruitment has taken place and no evidence of density-dependent mortality at any stage in the life cycle. Consequently no rational recommendations can be made as to the number of years of reproduction required to maintain the population, and no real recommendations as to the size below which mussels should be left unfished. What is well established is that pearls take 12-15 years to reach a saleable size and that therefore only reasonably large mussels contain such pearls. Unless mussels are unusually scarred or distorted, experienced fishermen do not bother to examine specimens under about 8-10cm in length. If a length of 10 cm were to be adopted as a lower unit, this would be allowing the mussels at least 10-15 years' reproductive life. However, as said above, this may be insufficient for population maintenance. Our data on the fertility of sparse populations (Tables 3 and 4) indicate that fertility is maintained even where the source of sperm is very reduced. Presumably either a dilute sperm source is adequate or some unfertilised eggs can develop. Most authors (e.g. Hendelberg, 1960; Bauer, 1979) regard the mussel as strictly dioecious, although Wellman (1938) noted one hermaphrodite in a sample of 80 mussels, and so the rather uncritical evidence available suggests that mussels are not normally capable of selffertilisation. These authors note that a sex ratio of approximately 1 :l is normal and that 30-50 ~o of the population carry glochidia each year. Our figures agree with this even for the sparse populations.
The freshwater pearl mussel in Britain
49
More important than the fertility of sparse populations is the fact that they would produce a low total number of glochidia and therefore few young mussels. Our other work (Young & Williams, in prep.) suggests that key mortality occurs at the young mussel stage, just after leaving the fish host, and that it is density independent, so the absolute scarcity of the young mussels may be important even if the fertility of the few remaining adults is maintained. Our view is that only time will tell whether sparse populations will recover and so for safety populations should not be permitted to become very scarce. So far no recovery has taken place in areas where the decline occurred many years ago (e.g. River Conway, North Wales) but perhaps the factors causing the decline have not been alleviated. Clearly no recovery can take place unless the cause of the decline is removed. Restocking
No one has yet succeeded in rearing young mussels to a size where mortality on release is reasonably low and so restocking would have to be with adults. It could only succeed where conditions were suitable and the cause of the previous decline was known and removed; such circumstances are rare. Consequently restocking is largely impractical at present and the few known attempts are unmonitored (W. Abernethy, pers. comm., N. McCormick, pers. comm.). Conservation measures
If the pearl mussel is to remain widespread, perhaps even to survive in England and Wales, conservation measures are necessary. Possible measures are discussed below. 1. Pollution and physical disturbance must be controlled. Existing legislation seems adequate but needs rigorous enforcement. 2. Salmonid host stocks must be maintained, and as they are usually highly prized, this measure is easy to ensure. 3. Pearl fishing must be controlled in some way. This option is hampered by the difficulty of policing any recommendations but could take various forms: (a)
To do nothing and to allow the increasing difficulty of finding mussels to lead to a reduced fishing effort. For the reasons discussed above this control may not operate soon enough.
50
Mark Young, Jennifer Williams
(b) To stop all exploitation of the mussel by making it a protected species under an appropriate parliamentary Act. This is the easiest legal option and would be the most easily enforced, however it is probably unnecessarily restrictive and may be too unpopular for acceptance. Pearl fishing has continued since Roman times and its cessation now may be an over-reaction. (c) To apply some of a number of controls to the fishing. Various strategies are possible, some specific and some general. Specifically certain rivers or parts of rivers could be declared as mussel reserves with no fishing allowed. These rivers could either be chosen from those still harbouring abundant mussels or could be those most threatened, allowing the depleted stocks to recover. Alternatively fishermen could be licensed and required to follow a code of operation. This would restrict fishing to a small number of professionals whose livelihood would be assured by adherence to sound practices. Another policy would be to place a lower limit on the size of shells examined, say a length of 10 cm, so as to give the mussels a guaranteed reproductive life. However, the difficulties in deciding the validity of this limit are considered above, and render this suggestion impracticable. Lastly if fishermen were licensed it would be possible, if considered necessary, to enforce the use of 'tongs'. All such measures would be expensive and/or difficult to enforce. A simpler (but perhaps inadequate) measure would be to ban the exploitation of mussels by skin divers and snorkelers, achieving this by agreement with the appropriate national association. This would allow deep-water populations to remain undisturbed, so acting as 'reservoirs' of reproducing animals, but no one can know whether this measure would be sufficient, and it certainly would not be in many shallow pearl mussel rivers where there are few deep pools. Our conclusion is that the best conservation measure would be to limit pearl fishing to serious fishermen, who would be required to pay a realistic licence fee and who might also be required to use opening 'tongs'. CONCLUSIONS The freshwater pearl mussel has declined greatly in Great Britain recently, and, although still c o m m o n in parts of northern Scotland, is now in danger of local extinction and overall reduction to the status of a rare
The freshwater pearl mussel in Britain
51
animal. This decline seems to be due mainly to overfishing, but pollution and habitat destruction are also implicated, especially in England. It may be that some recovery will occur, if fishing pressure relaxes, as sparse populations continue to produce glochidia. However, this recovery is not certain, and some conservation policy seems necessary. The best measures may be licensing fishermen and perhaps enforcing the use of non-destructive 'tongs' for examining the mussels. Other important measures would be prohibiting the use of sub-aqua equipment for pearl fishing and perhaps introducing a generous minimum size limit, so as to allow part of the reproducing population to remain unexploited. Policing any conservation measures would be difficult, but if no controls are imposed the mussel may soon decline to the extent that it needs complete protection.
ACKNOWLEDGEMENTS We are grateful to the many people who have provided help and encouragement, but especially to Dr M. Kerney for allowing us access to his records, Neil McCormick and his associates and Bill Abernethy for much useful data and comment, and Gerhardt Bauer for continental information. We are also grateful to the Nature Conservancy Council and the various landowners on whose property we carried out our fieldwork. The work was supported by a grant from the Natural Environment Research Council.
REFERENCES Bauer, G. (1979). Untersuchungen zur Fortpflanzungsbiologie der Flussperlmuschel ( Margaritana margaritifera) im Fichtelgebirge. Arch. Hydrobiol., 85, 152 65. Bjork, S. (1962). Investigations on Margaritifera margaritifera and Unio crassus. Acta limnol., 4, i 109. Boycott, A. E. (1933). The pearl mussel (Margaritana margaritifera) in hard and soft water. Fasculum, 19, 47-51. Boycott, A. E. (1936). The habitats of freshwater mollusca in Britain. J. Anita. Ecol., 5, 116-86. Brander, T. (1957). Aktuelles fiber die Flussperlmuschel, Margaritana margaritifera (L.) in Finnland. Acta Soc. Fauna Flora Jenn., 74, 1-29.
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Comfort, A. (1957). The duration of life in molluscs. Proc. Malac. Soc. Lond., 32, 219-41. Earl of Cranbrook. (1976). The commercial exploitation of the freshwater pearl mussel, Margaritifera margaritifera L. (Bivalvia: Margaritiferidae) in Great Britain. J. Conch., Lond,, 29, 87-91. Eagar, R. M. C. (1977). Shape of shell in relation to weight of Margaritijera margaritifera (L.) J. Conch., Lond., 29, 207 18. Hendelberg, J. (1960). The freshwater pearl mussel Margaritijera margaritijera. On the localisation, age and growth of the individual and on the composition of the population according to an investigation in Parlalven in Arctic Sweden. Rep. Inst. Freshwat. Res. Drottningholm, 41, 149-71. Jackson, J. W. (1925). The distribution of Margaritana margaritijera in the British Isles. J. Conch., Lond., 17, 195-211,270, 278. Kerney, M. P. (1975). European distribution maps of Pomatias elegans, Discus ruderatus, Eobania vermiculata, and Margaritifera margaritifera. Arch. Molluskenk., 106, 243 9. Kerney, M. P. (ed.) (1976). Atlas of the non-marine Mollusca of the British Isles. Cambridge, Institute of Terrestrial Ecology. Okada, Y. & Koba, K. (1933). Notes on the distribution of the freshwater pearl mussel Margaritifera margaritifera in Japan. Proc. imp. Acad. Japan, 9, 337-9. Okland, Jan. (1975). Utbredelsen av noen ferskvannsmuslinger i Norge, og litt om European Invertebrate Survey. Fauna, Oslo, 28, 61-70. Pliny, Naturalis historia, 9, 35. Suetonius. De vita Caesarum. Divus Julius, 47. Wellman, G. (1938). Untersuchungen fiber die Flussperlmuschel (Margaritana margaritifera (L.)) und ihren Lebensraum in Bachen der Luneburger Heide. Z. Fisch., 36, 489-603.
The Status and Conservation of the Freshwater Pearl Mussel Margaritifera margaritifera Linn. in Great Britain M a r k Y o u n g & J e n n i f e r Williams Department of Zoology, Aberdeen University, Tillydrone Avenue, Aberdeen, AB9 2TN, Great Britain
ABSTRACT All available records of the occurrence of the jreshwater pearl mussel Margaritifera margaritifera Linn. in Great Britain have been collected. Analysis of these records shows that there has been a recent decline in numbers of the species, which is especially marked in England and Wales. The causes of this decline are thought to be primarily pollution and overfishing in England and Wales, and overfishing in Scotland. Apparently even sparse populations remain fertile, but it is concluded that unless conservation measures can reduce overfishing, pearl mussels will become rare and endangered. It is suggested that the best conservation measures will be to restrict pearl fishing to licensed fishermen, perhaps using non-destructive fishing methods, and to prohibit the use of sub-aqua equipment.
INTRODUCTION The freshwater pearl mussel MargaritiJera margaritijera Linn. has been exploited for pearls since at least R o m a n times in Britain. Pliny and Suetonius both refer to Julius Caesar's interest in freshwater pearls, the latter even crediting them as one of the reasons why Caesar invaded Britain in 55 BC ! Since then British freshwater pearls have continued to be harvested, at times intensively, and there are even now several professional and semi-professional fishermen living in Scotland. However, it is clear from the experiences of these fishermen and from the results of our 35 Biol. Conserv. 0006-3207/83/0025-0035/$03.00 ~ Applied Science Publishers Ltd, England, 1983. Printed in Great Britain
36
Mark Young, Jennifer Williams
own survey, and those of Kerney (1976) and Jackson (1925), that the pearl mussel has recently declined markedly in both range and abundance. The mussel is long-lived (commonly to 60 years or more), prefers fastrunning, cool waters, low in calcium, and has a parasitic, glochidial stage found primarily on the gills of Salmonidae (Bjork, 1962). These factors restrict its natural range in Britain to the west and north, where it inhabits both large rivers and small streams. These large mussels have always attracted attention, reliable published records of their occurrence being found as far back as 1831, and there are several accounts of their distribution in relation to habitat variables (e.g. Boycott, 1933, 1936) as well as their size and shape (Eagar, 1977) and their long life (Comfort, 1957; Hendelberg, 1960). This paper draws together all the available information on distribution and abundance of the pearl mussel in Britain. These primary data are analysed to determine the extent and possible reasons for the decline of the mussel, and to assess its current status. Further observations on the reproductive capability of various populations were made to obtain the information needed for evaluating potential conservation measures. METHODS Mussel distribution and abundance
Information on the distribution and abundance of the freshwater pearl mussel in Britain was collected from various sources. Published records and data from the Conchological Society were abstracted; many pearl fishermen, fishermen and individuals provided personal observations and information; water authorities and other riparian users were approached for information and field surveys were carried out by ourselves in areas from which little recorded information was available. Whilst this set of information is as complete as possible, some other records must exist and any additional information would be welcomed. The complete set of information, with full references, is lodged at the Institute of Terrestrial Ecology's Biological Records Centre and with Dr Kerney, acting for the Conchological Society, as it is too extensive for inclusion here. It is freely available from either place as a source of distribution data or references. Instead the records are summarised in Fig. 1 and Table 1. For convenience the records are categorised into the following date classes: pre- 1900; 1900-1960; 1960-1970; and post- 1970.
The freshwater pearl mussel in Britain
37
The extent, the date and possible causes of decline are also summarised in the table. Unfortunately, whilst the information has been collected as critically as possible, such a heterogeneous set of references contains many inconsistencies. The limitations of the data are discussed below and data which are unsupported and of doubtful reliability have been omitted.
A non-destructive fishing method Most pearl fishermen kill all the mussels they examine by tearing the valves apart to search for pearls. However, some use an old shell to prise the valves apart slightly so they can see if pearls are present, and if there are non~ they allow the mussel to close again and return it to the river alive. In Germany, until a total ban was placed on pearl fishing, this practice was enforced by requiring fishermen to use special opening 'tongs'. To test the survival of mussels opened in this way we constructed similar 'tongs'. These comprise two thin-flanged jaws which can be inserted between the valves of the mussels and are then opened by pressure on the handles. A small bar maintains the gape for as long as required. In our experiment, 25 mussels, all over 7.0cm long, were selected at random from a small river in Wester Ross in June 1979. They were marked individually by glueing small, numbered pieces of plastic onto cleaned and dried areas of the shell, and then were opened using the tongs to a gape of 1 cm for 30 s. Then they were allowed to close and returned to an area of river which was naturally delimited by large stones. Their survival was monitored approximately monthly until March 1981. Several groups of similarly marked mussels exist in the same river, where they are part of experiments on dispersal rates of mussels, and the survival of these was also monitored over the same period as a control.
The fertility of sparse populations Observations were made to see if populations which had been reduced to very low numbers by overfishing could continue to reproduce successfully. The criterion used for this was the presence of developing glochidia in the gills. If such glochidia are present they appear as a creamy mass within the translucent brown gills and this may be seen easily using opening tongs.
38
Mark Young, Jennifer Williams
In 1979 and 1980 two such populations were examined. These were: 1, the Manse Burn, Sutherland (NC 0-2-), where only eight mussels over 7.0cm long could be found in a 100-m stretch; and 2, part of the River Dee, Aberdeenshire at Maryculter (NO 8-9-), where only nine mussels over 7.5cm long could be found in a 50-m stretch. In both cases no mussels were present upstream for at least 100 m. Abundant populations from the Stac Burn, Wester Ross (NC 08-12-) were also sampled, as a control. Sample sizes from sparse populations are necessarily low, and so a further set of data was collected. A series of fertility assessments were made in the Stac Burn at sites upstream to its source in Loch Doire an'Airbhe. Mussels from the upper sites have very few others upstream of them to act as sperm suppliers and so can also be considered as 'sparse populations'. At each of nine sites 30-70 mussels were opened in July 1980 and examined for glochidia (two of the sites had also been assessed in 1979).
Age and size at reproductive maturity Observations were made on the minimum size at which mussels contained glochidia. Since there is a general relationship between length and age for each population (Hendelberg, 1960), this allows an approximate assessment of the age of maturity. These data were collected so that if recommendations are required on a minimum size below which mussels should not be fished, the recommendations can take account of the age/ size at which mussels mature. R a n d o m samples of between 29 and 80 mussels were taken in August 1979 and July 1980 from each of four rivers: 1, Badnaban Burn, Sutherland (NC 08-20-); 2, Rhegreanoch Burn, Wester Ross (NC 08-16-); 3, Stac Burn, Wester Ross (NC 08-12-); and 4, River Dee, Aberdeenshire (NO 7-9-). These sites were chosen to include a range of mussel sizes and types. RESULTS
Mussel distribution and abundance The current British distribution is shown in Fig. 1, which was drawn using the full data now held by the Biological Records Centre. Table 1
The freshwater pearl mussel in Britain
1
Fig. 1.
2
3
4
5
6
The distribution of the pearl mussel in Britain. O, post-1970 records; ©, pre1970 records; - , sites now extinct.
summarises these data and itemises the number of localities and rivers where the mussels have apparently changed in their status. Some references and sources quoted actual reasons for observed declines for some sites. These are included, although such data are usually circumstantial and always of limited reliability for the reasons listed below. Consequently declines and increases are only included in Table I where it is certain that they are genuine; in all cases where evidence for a decline is merely suggestive they have been omitted.
40
Mark Young, Jennifer Williams TABLE 1 The Status of the Pearl Mussel in Great Britain
England and Wales Total no. of rivers where mussels occur(red) No. of rivers searched at least twice (so status is accessible)
Scotland
56
Total A
119
Total A
37
Total B
54
Total B
°/ of Total B /o
°J of Total B /o
No. of rivers where mussels now extinct (or not found) No. where decline noted
17
46
11
20
13
35
32
59
Total
30
81
43
79
6
16
13
24
No. of rivers where mussels have not altered in status No. of rivers where mussels have increased No. of rivers where mussels are now common No. of rivers with mussels discovered only since 1970 No. of rivers where decline is suggested as: Overfishing Pollution Physical disturbance Dates of decline
0 /o of Total A
o ¸
' o
0 of Total A
6
11
26
22
6
11
41
35
31 6 2 pre1900 1
1 9 0 0 196~ 1 9 6 0 1970 15 8
post1970 6
pre- 1900- 1960- post1 9 0 0 1 9 6 0 1 9 7 0 1970 0 12 16 15
The freshwater pearl mussel in Britain
41
Regardless of the problems associated with the reliability of the data, and allowing for the fact that many rivers have only been searched recently (so providing many 'recent-only' records), it is clear that a great decline has taken place both in overall distribution (i.e. some rivers now no longer harbour mussels) and within specific rivers. The latter is the main result in Scotland, the former in England and Wales. It is also clear that overfishing is predominantly important in Scotland whereas various causes have operated in England and Wales. Furthermore, declines have been more recent in Scotland than in England and Wales. The distribution of rivers where mussels have apparently become extinct is shown in Fig. 1 by dashes, although several of these are obscured by other existing populations in the same 10km square. In spite of this obvious decline the freshwater pearl mussel remains a widespread animal, and is still abundant in some localities, particularly in Scotland. However, many of the 'post-1970' records refer to only one or 'a few' specimens and the mussel is genuinely threatened in some areas, especially in England. Some published or anecdotal information suffers from one or all of the following faults. These are unavoidable and have been allowed for as far as possible, but nevertheless the distribution results must be viewed cautiously. The problems are that: 1, the species is incorrectly identified; 2, false negatives are included due to inexperience or poor searching conditions; 3, the abundance may be inadequately assessed or reported; 4, the cause of decline noted may be biased or merely hearsay (e.g. some observers clearly overreact to piles of dead mussel shells); 5, the locality may not be quoted precisely.
Non-destructive fishing methods Table 2 summarises the survival of the 25 mussels which were marked and opened using the non-destructive method described above, and of 23 marked control mussels. In both groups only 1 mussel is known to have died in the 21-month period and only a further 2 and 1 respectively are missing. Clearly there is no significant difference in the survival rate of the two groups; opening the mussels has not harmed them. The specimen which was damaged on opening (one adductor muscle was severely torn), apparently recovered and remained alive at the end of the experiment.
Mark Young, Jennifer Williams
42
TABLE 2 The Survival of Mussels which were Opened to a Gape of 1cm for 30 s (June 1979-March 1981; 21 months)
Operted mussels
Control mussels
25 a
23
No. of mussels used No. known to be dead No. missing at March 1981
1 2
1 1
Total
3
2
12
8
% dead or missing
a One damaged in opening remained alive until March 1981.
The fertility of sparse populations Table 3 shows the result of the fertility assessment of the populations in the Manse Burn, Sutherland, and a section of the River Dee, Aberdeenshire. These should be compared with the middle sites (e.g. 100 m and 150 m below loch) of the Stac Burn, Wester Ross (Table 4) which act as a control, in that the populations there are fairly dense. As far as the necessarily small amount of data shows there is little difference between any of the estimates. This indicates that sparse populations remain fertile when this is measured as the percentage containing glochidia. However, the number of glochidia produced by each mussel was not assessed and this might vary between mussels in sparse and dense populations. TABLE 3 The Fertility of Mussels from Sparse Populations
Site
Date No. No. and % examined examined Jertile
Manse Burn, Sutherland NC 0-231 July 1979 River Dee, Aberdeenshire (Maryculter) NO 8-925 August 1980
8
3
9
5
Notes
Total population searched--all 38 over 7.0cm long All mussels over 7.5 cm long 56 searched
The freshwater pearl mussel in Britain
43
TABLE 4
The Fertility of the Mussel Population at Different Sites Within the Stac Burn, Wester Ross in July 1980 In loch
5m below loch
lOm below loch
~o of mussels either containing glochidia or recently 'spent' 0 43 42 No. of mussels examined 11 30 65 Estimated no. of mussels 0 11 45 above site Equivalent data 70 ~o of for August 30 mussels 1979
lOOm below loch
150m 200m 300m 400m 600m below below below below below loch loch loch loch loch
35
53
14
23
11
25
66
62
65
62
54
65
>10 a > 103 43 % of 30 mussels
>104 >104
>104 >104
All mussels examined or counted were over 7.0 cm long.
Table 4 sets out the fertility estimates of the various sites within the Stac Burn, Wester Ross, and includes an estimate of the minimum number of mussels upstream of each sample. All mussels chosen for examination were over 7.0 cm long. The fertility levels at the extreme upper end of the population are not obviously lower than elsewhere, except in the loch itself, where the lack of a current to distribute sperm might account for the lack of fertile specimens. The low fertility figures noted in the lowest stream sections are almost certainly due to some of those mussels having already shed their glochidia at the time of the sample. This factor means that all the assessments are minimums but does not invalidate the result that the sparse populations remain fertile.
Age and size at maturity Table 5 lists the sizes of mussels examined (as overall length in cm) in each
44
Mark Young, Jennifer Williams
TABLE 5 The Size at which Mussels first Mature as Indicated by Presence of Glochidia in July 1980 (west coast rivers) and August 1979 (R. Dee) River
Badnaban Rhegreanoch Stac Burn (mid-section) Dee
No. examined
Size range examined-length in cm
% with glochidia
Size range with glochidia--length in em
75 80 65 29
3.5-9.0 5.7-11.1 4.8-10-6 4.7 11.7
16 41 42 21
6-5-8.8 7-0-11.1 6.9-10.0 7-7-11.7
Badnaban Burn, Sutherland NC 08-20Rhegreanoch Burn, Wester Ross NC 08-16Stac Burn, Wester Ross NO 08-12R. Dee, Aberdeenshire NO 7-9-.
of four rivers, and notes the size above which mussels were found to contain glochidia. The mussels from the Badnaban Burn, Sutherland, were generally smaller than those from the other sites and matured at a smaller size, whereas those from the River Dee, Aberdeenshire, were larger in both respects. Since some mussels may have released their glochidia before the samples were taken, these size limits may exclude even smaller mussels which matured earlier in the season, but this seems unlikely. Generally mussels mature at about 6.5-7.0 cm in length. DISCUSSION The status of the freshwater pearl mussel in Britain
It is clear from Table 1 that the freshwater pearl mussel has declined significantly in Great Britain in the last 100 years and that the causes of this decline are overfishing, pollution and to a small extent habitat modification. In spite of this the mussel remains widespread and is obviously not in danger of extinction in Great Britain as a whole, although it may eventually be lost in England and Wales and will almost certainly become extinct in some rivers. These conclusions agree with those of Cranbrook (1976) and Kerney (1975). Presumably there are mussel populations which are so far un-
The freshwater pearl mussel in Britain
45
discovered. Forty-seven new populations have been found since 1970 and of these six are in England and Wales, which have been quite comprehensively searched by naturalists in the past. There are many small rivers in Western Scotland (and to a lesser extent Wales) which probably contain pearl mussels, but their discovery will not alter the general status and distribution patterns of the mussel. A more important point is that it is only where rivers have been searched effectively at least twice that changes in status can be assessed. Table 1 and the following discussion should be judged with this clearly in mind. A larger proportion of English and Welsh populations have declined or become extinct than in Scotland and the various 'pressures' exerted remain greater in the south. (Whilst important politically, this is not relevant biologically as the mussel is primarily a northern and western mollusc in Britain.) The decline in the south was apparently earlier than in Scotland and with limitations the data seem to show that pollution had a significant effect in England and Wales whereas overfishing has been most important in Scotland. Figure 1, in showing the current position, illustrates a classic northern and western calciphuge distribution with few significant gaps apart from the Tweed catchment area and perhaps parts of northern central England and mid-Wales. The absence from the Tweed system is inexplicable but probably genuine and natural. The freshwater pearl mussel is holarctic and in some places common (e.g. South-East Alaska (personal observations), Norway (Okland, 1975) and some parts of Japan (Okada & Koba, 1933)) but has declined greatly in many countries other than Great Britain. In Germany it is nearly extinct (G. Bauer, pets. comm.) and in Finland it has required complete protection since 1955 (Brander, 1957). Consequently the fate of our populations is of more than just local significance. It is not easy to judge why mussel populations have become reduced in countries other than Britain. Certainly in Germany overfishing and pollution have been important, and in Sweden overfishing is considered to be the major cause (Bjork, 1962). The causes o f decline
Overfishing Pearl fishing has been established since at least Roman times and has rarely caused any lasting harm until recently. At some periods enthusiasm for the pearls has been aroused to the extent that mussels became scarce
46
Mark Young, Jennifer Williams
locally, as when Moritz Unger came to Scotland in the 1860s and stimulated the trade so much that mussels became rare in the larger rivers. As it became harder to find pearl-bearing mussels the trade declined and the mussel populations recovered. In recent times several factors increased the effect of fishing. One of these is the improved access to the more remote areas, so bringing more visitors to the pearl rivers. National publicity has encouraged these visitors to collect mussels casually and in places (such as near Aviemore) concentrations of visitors have severely reduced mussel populations; this is merely local in its effect. A second factor is the great increase in the membership of sub-aqua clubs. Whilst many clubs act responsibly, others unthinkingly encourage pearl mussel collection as an exciting activity to add interest to a dive. This has had the very serious effect of depleting the populations in deeper pools. Previously these pools have been inaccessible to wading pearl fishermen and the mussels within them acted as a 'reservoir' of reproductive effort. These extra pressures have meant that the activities of the regular pearl fishermen have been imposed on smaller stocks and so assume a greater importance. Furthermore the increased mobility of the professionals has allowed them to exploit populations previously undisturbed. Most pearl fishermen have traditional fishing areas and usually leave 12-15 year intervals between their exploitation. This allows some recovery of the populations. Also the more experienced fishermen take only large specimens with scarred shells, so leaving a proportion of the population untouched. Casual fishermen may take all mussels, including small and unscarred specimens, so reducing greatly the possibility of eventual recovery for that population. This study has shown that mussels do survive the use of opening 'tongs' and if fishermen could be persuaded to use them, then overfishing would cease to be a problem. However, some fishermen regard their use as unnecessary and too time-consuming, and so their adoption would take more than merely making them available--some compulsion would be required. Pollution Evidence for local extinctions caused by pollution in Britain is anecdotal. It is said, probably correctly, that silting and deoxygenation are both
The J?eshwater pearl mussel in Britain
47
inimical to the mussels and some past mussel rivers are undoubtedly now unsuitable with a much altered fauna (e.g. River Almond, Midlothian). Such effects have been locally important but clearly do not threaten the mussel's existence in Britain as a whole. In Germany, however, there is some evidence that pollution has had a serious effect. It is thought that phosphates from sewage have encouraged algal growth to the level that it filis the substrate interstices and smothers the small mussels (G. Bauer, pers. comm.). Furthermore, some ions, including Cr 2 +, may also be directly toxic. In Britain a number of pearl mussel rivers suffer phosphate enrichment from farmland but it is not possible to say whether this has affected the mussels. The River Ythan, Aberdeenshire, for example, has an elevated phosphate level and a reduced mussel population, but overfishing may actually be the cause of the decline. Pollution may act indirectly. If pollutants make a river unsuitable for the salmonid hosts of the mussels then obviously the mussel will eventually disappear.
Loss of host fish Where the salmonid hosts of the mussel glochidia are removed, whether by pollution or physical displacement, the mussel must die out. However, there are no examples of rivers in Britain where this has occurred except where other factors may also have been affecting the mussels directly. Generally salmonids are prized and preserved at the expense of other fish, and so maintenance of the mussel's hosts is assured.
Physical d&turbance In some cases, dredging of the river bed or scouring due to increased flow have removed mussel habitats. Generally this is purely local; for example, salmon pools in the River Polly, Wester Ross, have been created, so displacing mussels; but it may affect a large section of a river. Some rivers are periodically depleted and then scoured below hydro-electric dams and others have been canalised, so altering the substrate. Nationally this is of no real significance, although some pearl fishermen have noted some previously good pearling areas which have been lost (N. McCormick, pers. comm.). Possibilities for natural recovery of mussel populations
Historical evidence suggests that mussel populations can recover
48
Mark Young, Jennifer Williams
naturally from low levels; after Moritz Unger's activities in the 1860s the Scottish populations recovered adequately. However, the level to which the populations were reduced is u n k n o w n and the mussels in the deeper pools were certainly untouched. Today, not only are the populations reduced to a lower level than previously, but the fishing pressures are being maintained even on scarce populations. Easier travel, use of subaqua equipment and the casual (rather than strictly economical) nature of the fishing have led to this in places, and in some rivers the remaining mussels are now numbered literally in double figures only (e.g. River Ythan, Aberdeenshire). One of the factors affecting recovery is whether mussels are taken below the size at which they become mature, or whether they are allowed to reproduce. This study shows that mussels generally mature at a length of about 7.0 cm, although in populations of slow-growing specimens this may be reduced to 6.5 cm (Table 5). This corresponds roughly to an age of 12 years, and most mussels then survive to continue reproducing for another 30-50 years (Young & Williams, in prep.). However, there is no evidence to indicate how many years of glochidial production are required before adequate recruitment has taken place and no evidence of density-dependent mortality at any stage in the life cycle. Consequently no rational recommendations can be made as to the number of years of reproduction required to maintain the population, and no real recommendations as to the size below which mussels should be left unfished. What is well established is that pearls take 12-15 years to reach a saleable size and that therefore only reasonably large mussels contain such pearls. Unless mussels are unusually scarred or distorted, experienced fishermen do not bother to examine specimens under about 8-10cm in length. If a length of 10 cm were to be adopted as a lower unit, this would be allowing the mussels at least 10-15 years' reproductive life. However, as said above, this may be insufficient for population maintenance. Our data on the fertility of sparse populations (Tables 3 and 4) indicate that fertility is maintained even where the source of sperm is very reduced. Presumably either a dilute sperm source is adequate or some unfertilised eggs can develop. Most authors (e.g. Hendelberg, 1960; Bauer, 1979) regard the mussel as strictly dioecious, although Wellman (1938) noted one hermaphrodite in a sample of 80 mussels, and so the rather uncritical evidence available suggests that mussels are not normally capable of selffertilisation. These authors note that a sex ratio of approximately 1 :l is normal and that 30-50 ~o of the population carry glochidia each year. Our figures agree with this even for the sparse populations.
The freshwater pearl mussel in Britain
49
More important than the fertility of sparse populations is the fact that they would produce a low total number of glochidia and therefore few young mussels. Our other work (Young & Williams, in prep.) suggests that key mortality occurs at the young mussel stage, just after leaving the fish host, and that it is density independent, so the absolute scarcity of the young mussels may be important even if the fertility of the few remaining adults is maintained. Our view is that only time will tell whether sparse populations will recover and so for safety populations should not be permitted to become very scarce. So far no recovery has taken place in areas where the decline occurred many years ago (e.g. River Conway, North Wales) but perhaps the factors causing the decline have not been alleviated. Clearly no recovery can take place unless the cause of the decline is removed. Restocking
No one has yet succeeded in rearing young mussels to a size where mortality on release is reasonably low and so restocking would have to be with adults. It could only succeed where conditions were suitable and the cause of the previous decline was known and removed; such circumstances are rare. Consequently restocking is largely impractical at present and the few known attempts are unmonitored (W. Abernethy, pers. comm., N. McCormick, pers. comm.). Conservation measures
If the pearl mussel is to remain widespread, perhaps even to survive in England and Wales, conservation measures are necessary. Possible measures are discussed below. 1. Pollution and physical disturbance must be controlled. Existing legislation seems adequate but needs rigorous enforcement. 2. Salmonid host stocks must be maintained, and as they are usually highly prized, this measure is easy to ensure. 3. Pearl fishing must be controlled in some way. This option is hampered by the difficulty of policing any recommendations but could take various forms: (a)
To do nothing and to allow the increasing difficulty of finding mussels to lead to a reduced fishing effort. For the reasons discussed above this control may not operate soon enough.
50
Mark Young, Jennifer Williams
(b) To stop all exploitation of the mussel by making it a protected species under an appropriate parliamentary Act. This is the easiest legal option and would be the most easily enforced, however it is probably unnecessarily restrictive and may be too unpopular for acceptance. Pearl fishing has continued since Roman times and its cessation now may be an over-reaction. (c) To apply some of a number of controls to the fishing. Various strategies are possible, some specific and some general. Specifically certain rivers or parts of rivers could be declared as mussel reserves with no fishing allowed. These rivers could either be chosen from those still harbouring abundant mussels or could be those most threatened, allowing the depleted stocks to recover. Alternatively fishermen could be licensed and required to follow a code of operation. This would restrict fishing to a small number of professionals whose livelihood would be assured by adherence to sound practices. Another policy would be to place a lower limit on the size of shells examined, say a length of 10 cm, so as to give the mussels a guaranteed reproductive life. However, the difficulties in deciding the validity of this limit are considered above, and render this suggestion impracticable. Lastly if fishermen were licensed it would be possible, if considered necessary, to enforce the use of 'tongs'. All such measures would be expensive and/or difficult to enforce. A simpler (but perhaps inadequate) measure would be to ban the exploitation of mussels by skin divers and snorkelers, achieving this by agreement with the appropriate national association. This would allow deep-water populations to remain undisturbed, so acting as 'reservoirs' of reproducing animals, but no one can know whether this measure would be sufficient, and it certainly would not be in many shallow pearl mussel rivers where there are few deep pools. Our conclusion is that the best conservation measure would be to limit pearl fishing to serious fishermen, who would be required to pay a realistic licence fee and who might also be required to use opening 'tongs'. CONCLUSIONS The freshwater pearl mussel has declined greatly in Great Britain recently, and, although still c o m m o n in parts of northern Scotland, is now in danger of local extinction and overall reduction to the status of a rare
The freshwater pearl mussel in Britain
51
animal. This decline seems to be due mainly to overfishing, but pollution and habitat destruction are also implicated, especially in England. It may be that some recovery will occur, if fishing pressure relaxes, as sparse populations continue to produce glochidia. However, this recovery is not certain, and some conservation policy seems necessary. The best measures may be licensing fishermen and perhaps enforcing the use of non-destructive 'tongs' for examining the mussels. Other important measures would be prohibiting the use of sub-aqua equipment for pearl fishing and perhaps introducing a generous minimum size limit, so as to allow part of the reproducing population to remain unexploited. Policing any conservation measures would be difficult, but if no controls are imposed the mussel may soon decline to the extent that it needs complete protection.
ACKNOWLEDGEMENTS We are grateful to the many people who have provided help and encouragement, but especially to Dr M. Kerney for allowing us access to his records, Neil McCormick and his associates and Bill Abernethy for much useful data and comment, and Gerhardt Bauer for continental information. We are also grateful to the Nature Conservancy Council and the various landowners on whose property we carried out our fieldwork. The work was supported by a grant from the Natural Environment Research Council.
REFERENCES Bauer, G. (1979). Untersuchungen zur Fortpflanzungsbiologie der Flussperlmuschel ( Margaritana margaritifera) im Fichtelgebirge. Arch. Hydrobiol., 85, 152 65. Bjork, S. (1962). Investigations on Margaritifera margaritifera and Unio crassus. Acta limnol., 4, i 109. Boycott, A. E. (1933). The pearl mussel (Margaritana margaritifera) in hard and soft water. Fasculum, 19, 47-51. Boycott, A. E. (1936). The habitats of freshwater mollusca in Britain. J. Anita. Ecol., 5, 116-86. Brander, T. (1957). Aktuelles fiber die Flussperlmuschel, Margaritana margaritifera (L.) in Finnland. Acta Soc. Fauna Flora Jenn., 74, 1-29.
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Comfort, A. (1957). The duration of life in molluscs. Proc. Malac. Soc. Lond., 32, 219-41. Earl of Cranbrook. (1976). The commercial exploitation of the freshwater pearl mussel, Margaritifera margaritifera L. (Bivalvia: Margaritiferidae) in Great Britain. J. Conch., Lond,, 29, 87-91. Eagar, R. M. C. (1977). Shape of shell in relation to weight of Margaritijera margaritifera (L.) J. Conch., Lond., 29, 207 18. Hendelberg, J. (1960). The freshwater pearl mussel Margaritijera margaritijera. On the localisation, age and growth of the individual and on the composition of the population according to an investigation in Parlalven in Arctic Sweden. Rep. Inst. Freshwat. Res. Drottningholm, 41, 149-71. Jackson, J. W. (1925). The distribution of Margaritana margaritijera in the British Isles. J. Conch., Lond., 17, 195-211,270, 278. Kerney, M. P. (1975). European distribution maps of Pomatias elegans, Discus ruderatus, Eobania vermiculata, and Margaritifera margaritifera. Arch. Molluskenk., 106, 243 9. Kerney, M. P. (ed.) (1976). Atlas of the non-marine Mollusca of the British Isles. Cambridge, Institute of Terrestrial Ecology. Okada, Y. & Koba, K. (1933). Notes on the distribution of the freshwater pearl mussel Margaritifera margaritifera in Japan. Proc. imp. Acad. Japan, 9, 337-9. Okland, Jan. (1975). Utbredelsen av noen ferskvannsmuslinger i Norge, og litt om European Invertebrate Survey. Fauna, Oslo, 28, 61-70. Pliny, Naturalis historia, 9, 35. Suetonius. De vita Caesarum. Divus Julius, 47. Wellman, G. (1938). Untersuchungen fiber die Flussperlmuschel (Margaritana margaritifera (L.)) und ihren Lebensraum in Bachen der Luneburger Heide. Z. Fisch., 36, 489-603.