Spawning of king scallops, Pecten maximus (L.) in Mulroy Bay and the relationship with spatfall intensity.

ABSTRACT The major restriction to expansion of scallop culture and scallop fishery enhancement in Europe is a reliable source of scallop spat. In Ireland, production of spat from the natural settlement in Mulroy Bay has exhibited considerable variation over the last 25 years. In this study data available on king scallop spawnings have been reviewed with the objective of understanding why in some years spat are abundant and in others scarce. Similar spawning periods of king scallops, Pecten maximus in two locations 10 km apart, supported the use of scallops from the Broad Water for gonad monitoring rather than from the North Water spat production site where scallop dredging is legally prohibited. Using weekly determination of gonad index, gonad weight and relative gonad height, five partial spawnings were recorded between April to August in 1 year, with scallops capable of spawning 1 week and re-maturing to a pre-spawning level the following week. Sampling frequency significantly influenced the number and size of spawnings recorded, date of the spawning and duration of the spawning event. These results have major implications for data collected using sampling intervals greater than 1 week and purporting to represent the reproductive cycle of this scallop. Based on weekly monitoring of scallop gonads during summer months between 1993 to 2004 there was no significant correlation between intensity of the scallop spatfall and size of the gonad prior to spawning, magnitude of spawning event, date of spawning period within June and July and duration of the spawning event.

KEY WORDS: Pecten maximus, scallop spawning, spatfall intensity

INTRODUCTION

Attempts to develop a king scallop, Pecten maximus (L.) culture industry in Ireland based on natural spat collection in Mulroy Bay commenced in 1979 after accidental collection of millions of scallop spat on mussel ropes. Despite early promise proffered by the site and the many scallop fishery projects planned for around the coast, commercial development has been limited. The major restriction to expansion of scallop culture and scallop fishery enhancement has been seed availability. Yields of natural spat collected in the site since its initial discovery have varied widely, there being years of plenty and years of complete absence. To assist in understanding why in some years spat are abundant and in others scarce this review of some of the available data on scallop spawnings at this location has been undertaken. Data reviewed have been confined to adult scallops and does not include any consideration of scallop larvae or any association that may exist with environmental parameters at the site.

Considerable research on reproductive cycles of the scallop P. maximus in waters around the Isle of Man (Fullarton 1890, Dakin 1909, Tang 1941, Mason 1958, Brand et al. 1980); Scotland (Comely 1974, Mackie 1986, Mackie & Ansell 1993); Ireland (Gibson 1956, Wilson 1987, McDonough 1998); the Channel Islands (Jory 2000); France (Lubet et al. 1987, Paulet et al. 1988, Paulet & Boucher 1991, Lubet et al. 1995); Spain (Roman & Acosta 1995, Pazos et al. 1996, Roman et al. 1999, Roman et al. 2001) and Norway (Strand & Nylund 1991, Duinker & Nylund 2002) has been undertaken. Various numbers of spawnings of different magnitude using a range of techniques have been reported.

Barber and Blake (1991) in a review of scallop reproductive physiology outlined a wide range of measures that have been used to monitor scallop gametogenic cycles. Anatomy of the scallop P. maximus is such that the gonad is largely separate from the rest of the visceral mass allowing for easy removal and analysis (Beninger & LePennec 1991). As the gonad matures, its weight and size increase, it becomes rounder in cross-section as gametes become larger and more numerous, and the creamy-white color of testis and orangey-red color of ovary become brighter. Several authors have used gross visual observation of size, shape and color of the gonad to assess the reproductive cycle of this species. Subdivision of the maturation cycle into a series of stages and application of numerical values to these stages has allowed essentially qualitative descriptions to be converted into quantitative measures. Complications exist however because different authors have used different numbers of developmental stages; for example, Tang (1941) and Gibson (1956) used 5 stages, Sastry (1963) and Duinker et al. (2002) used 6 stages, Mason (1958) and Campodonico et al. (2001) used 8 stages and Naidu (1970) used 10 stages.

Gravimetric measures used to assess the reproductive cycle include mean gonad weight (GWt), or weight relative to some other parameter such as weight of soft parts as in gonad index (GI) and gonadosomatic index (GSI) or to size of the scallop shell as in relative gonad height (RGH). In relating GWt to weight of the visceral mass, some researchers have used total soft tissue weight including the gonad (Slater 2003), some soft parts remaining after removal of the gonad (Latrouite & Claude 1979) and some weight of the shell (Paulet & Boucher 1991). Further difficulties arise because some methods have used wet weight and others dry weight making comparison of results impossible in many cases. Barber and Blake (1991) expressed the opinion that dry weights were preferable because of variability in water content, which can be encountered seasonally and among the different tissues.

Histological methods provide the means to definitively examine and assess gamete development (Barber & Blake 1991). Techniques for the quantification of data derived from histological studies include, assigning numerical values to developmental stages to produce a "histological index" (Naidu 1970, Burnell 1983, Wilson 1987); measurement of the mean oocyte diameter ([D.sub.m]), because oocytes increase in size as they develop, achieving a maximum size prior to spawning (Barber & Blake 1981 and 1983, Pazos et al. 1996, Roman et al. 2002); and measurement of the oocyte volume fraction (OVF) that takes advantage of the fact that as maturation progresses more and more of the gonad tissue comprises gametes (Beuinger, 1987; Lubet et al. 1987; Roman et al. 2002). While histological techniques provide an opportunity for examination of gonad development at the microscopic level, sample preparation, sectioning and staining result in such techniques being expensive and time-consuming and thus impractical in an aquaculture context.

Despite numerous studies, most research studies performed to elucidate annual reproductive cycles have been of short-term duration, most commonly covering a period of 2 to 3 years. Sampling frequency during these investigations has varied from weekly (Mason 1958, Pereira & Molina 1997); fortnightly (Mason 1958, Racotta et al. 2003, Roman et al. 1999); every 3 weeks (Roman et al. 2002); monthly (Mason 1958, Sastry 1963, Naidu 1970, Campodonico et al. 2001, Strohmeier et al. 2000, Saout et al. 1999, Penchaszadeh et al. 2000, Luna-Gonzalez et al. 2000); to every 4-6 weeks (Roman et al. 2001). Each sample in these investigations varied between 10 to 50 scallops.

The objective of this study is to review available data from 1980 onwards on scallop spawnings in Mulroy Bay. The date and duration of spawning period, size of the gonad immediately prior to spawning and magnitude of the spawning event have been compared with spat collection intensities since 1993 as part of a review of the characteristics that result in a good scallop spatfall.

MATERIALS AND METHODS

Gonad Monitoring

In 1980 adult king scallops for gonad assessment were dredged from around the Stookans in Massmount Bay, North Water of Mulroy Bay (Fig. 1). Samples comprising 22-27 individuals of mean shell height 113.4 [+ or -] 1.8 mm were collected at intervals ranging from 4-13 days (mean = 8.4 days) between June 5 to August 30, 1980. Scallops were also dredged from Mc Swynes Bay in the south of the Broad Water of Mulroy Bay, some 10 km distance from the North Water site (Fig. 1). Samples comprising 22 to 26 individuals of mean shell height 120.7 [+ or -] 1.6 mm scallops were collected at intervals ranging from 7-13 days (mean 9.3 days) between July 5 to August 30, 1980. Soft tissues were dissected from each scallop, pooled and weighed. GI and RGH were calculated according to the formulas below:

GI (%) = Total wet wt. of gonad/Total wet wt. of soft tissue x 100/1

RGH ([gmm.sup.-3] = Mean wt. of gonad in g/(Mean shell ht. in mm) x [10.sup.6]

[FIGURE 1 OMITTED]

In 1981 and 1982 spawning periods were determined using scallops dredged from around the Stookans, North Water of Mulroy Bay. Scallop dredging in this area was prohibited from 1982 onwards following intense scallop spat settlements in 1979, 1980 and 1981 in the North Water and spat collection intensities exceeding 3000 spat per collector. The aim of the Mulroy Bay (North Water and Moross Channel) (Prohibition on Dredging for Escallops) Bye Law No. 625, 1982 was protection of broodstock scallops from exploitation and hence protection of the area as a national center for scallop spat collection. This legislation necessitated the use of king scallops from the Broad Water as a source of stock for gonad monitoring.

In 1983 and 1984 farmed-scallops from a single year class, (and) greater than 100 mm shell height and suspended in lantern nets at 5-m depth from longlines at Deegagh Point in the Broad Water were used for determination of the spawning period in the North Water. The GI and RGH of scallop samples comprising 24 to 26 individuals of mean shell height 115.8 [+ or -] 3.6 mm were assessed at 3-7-day intervals. After reduced intensity of the scallop spatfall in 1982 and complete absence of a spatfall in 1983, 1984 and 1985, caused by the effects of tributyl tin (TBT) antifoulants (Minchin et al. 1987), gonad monitoring ceased in 1985 because of financial constraints. Although a spatfall at a much-reduced intensity by comparison with the early 80s returned from 1986 (Slater 1995), gonad analysis was not resumed until 1993.

An intensive study was undertaken in 1993 to determine if alternative spawnings to those previously identified in the June to July period existed that might support an improvement in spat collection intensity. GI, GWt and RGH of samples of 24 scallops of mean shell height 102.0 [+ or -] 8.6 mm were determined at weekly intervals from April 3 to August 21, 1993. Tissues dissected from each scallop were pooled and weighed and GI and RGH calculated. Between 1994 to 2004 the determination of GI and RGH was undertaken at weekly intervals in the summer of each year using samples comprising 17-33 individuals of similar-sized farmed scallop.

Scallop Spat Collection

The pioneering scallop farming company, Deegagh Point Shellfish Ltd., established in 1980 to commercially exploit king scallop spat collection in Mulroy Bay provided spat collection results presented here. Data used in the comparative study with GI between 1993 to 2004 were obtained using consistent techniques in each of the years. Scallop spat were collected in Japanese spat collector bags of 0.8 m x 0.3 m dimensions and 5 mm x 3 mm mesh size containing 2 m x 1 m of lightweight garden netting (Netlon[R]) of 6 mm x 6 mm mesh size. All collector bags were suspended from 300 m longlines attached to the shore at one end in a site at Lurgacloghan in the North Water (Fig. 1). Branch lines holding 12 collector bags were attached at 1-m intervals on a main line at the optimum depth for spat collection. Total carrying capacity of each longline was 3,600 collector bags and up to five such longlines for spat collection were used in any one-year period.

Spat collectors were installed over a 3 to 4-day period in each year at peak settlement time. Gonad analysis and scallop larval monitoring were used to predict spat settlement periods and hence the dates for installation of collector bags to maximize spat collection each year (Slater 2005a, Slater 2005b).

RESULTS

Comparison of scallop GI from Massmount Bay in the North Water and Mc Swynes Bed in the Broad Water over the period July to August 1980 demonstrated that GI from the Broad Water site was higher than from the North Water site (Fig. 2). There was a highly significant correlation between GI at the sites ([F.sub.1,5] = 29.471, P = 0.003) and the spawnings, indicated by a decrease in GI in late June to early July and from late July onwards occurred in the same period at both locations.

[FIGURE 2 OMITTED]

An intensive investigation performed at weekly intervals between April to August 1993 demonstrated that the 3 measures of reproductive condition; GI, GWt and RGH exhibited similar trends (Fig. 3). GI ranged between 12.3% to 26.9%, GWt between 3.8-11.3 g per gonad and RGH between 4.1-12.7 [gmm.sup.-3]. Five partial spawnings were recorded between April to August 1993 and were clearly exhibited by all three measures of reproductive condition.

[FIGURE 3 OMITTED]

The importance of a weekly sampling frequency was demonstrated by the spawning recorded between May 8 to May 15 as evidenced by the decline in GI, GWt and RGH. Only one week later, the GI, GWt and RGH had all returned to a pre-spawning level. The possibility exists that this spawning would not have been recorded had weekly monitoring not been performed. The largest spawning release during 1993, estimated by GI decrease, was 7.3% of total wet weight between June 12 to June 18.

Significance of sampling frequency on the number of recorded spawnings using GI was demonstrated using 1993 data. The trend in GI based on a weekly sampling frequency was compared with those based on fortnightly data---odd weeks only (weeks 1, 3, 5 etc), on fortnightly data--even weeks only (weeks 2, 4, 6 etc) and on monthly data only (Fig. 4.) Compared with five partial spawnings exhibited by weekly data, three partial spawnings were exhibited by fortnightly data--odd weeks, two partial spawnings by fortnightly data--even weeks and one partial spawning by monthly GI data. In summary the number of spawnings recorded, spawning dates, spawning duration and magnitude of spawning varied depending on sampling frequency. These results have significant implications for data collected using sampling intervals greater than 1 week and purporting to represent reproductive cycles of Pecten maximus.

[FIGURE 4 OMITTED]

The dates, duration and magnitude of 13 spawnings occurring in June and July during 1993 to 2004 inclusive and from which spat were collected are provided in Table 1. A further seven spawnings were recorded but have been omitted from Table 1 on the basis that spat collection from these spawnings was not performed because of poor larval numbers and hence predicted poor spatfalls. Good spatfall years, defined as years providing more than 500 spat per collector, 50% of the level used to distinguish between a good spatfall and a poor spatfall in Japan, were recorded from spawnings in 1995, 1997, 2001 and 2002 (Taguchi & Walford 1976).

Based on data in Table 1 there was no significant relationship between gonad size prior to spawning and spatfall intensity ([F.sub.1,11] = 0.314, P = 0.586). GI prior to spawning ranged from a maximum of 24.3% in 1995 to a minimum of 7.9% in 2004. In 1993, 1994, 1995 and 1999; gonad size prior to spawning exceeded 20% of soft tissue weight, yet of these years only 1995 yielded a good spatfall. In summary development of a large gonad prior to spawning was not an indicator of a good spatfall.

The magnitude of the spawning event, in terms of GI decrease, exhibited no significant correlation with the spatfall intensity ([F.sub.1,11] = 0.622, P = 0.447). The highest GI decrease was recorded in 1998 at 11.3% of the soft tissue weight, but failed to result in a significant spatfall. A large spawning signified by high GI decrease was not an indicator of a good spatfall.

Relationships between early and late spawnings and short and prolonged spawnings and spatfall intensity were investigated. Spatfalls resulted from spawnings occurring from early June to late July. There was no significant correlation between date of the start of spawning referenced to June 1 and spatfall intensity ([F.sub.1,11] = 0.029, P = 0.867). In some years, spawning duration, based on the period over which GI decreased, was short, extending for only 1 week and in some years a more prolonged spawning period occurred, extending over 2 or 3 weeks. There was no significant correlation between spawning duration and spatfall intensity ([F.sub.1,11] = 1.496, P = 0.247).

DISCUSSION

The objective of gonad monitoring in Mulroy Bay was identification of spawning periods in scallops in the North Water, which might result in larvae and hence spat collection in this location. Commercial spat collection is not practiced in the Broad Water, trials having demonstrated that it occurs at a much lower intensity compared with the North Water. A comparative study performed in 1980 with scallops from the North Water and Broad Water demonstrated that it was feasible to use scallops from the Broad Water as an indicator of scallop spawning in the North Water. Experimental data since 1980 on scallop larval occurrence in the North Water have confirmed the use of spawning of cultured scallops in the Broad Water as an indicator of spawning in the natural population in the North Water. Similar findings were reported by Acosta and Roman (1994) in a comparison of spawning periods of king scallop, Pecten maximus from a natural population in the Ria de Arosa with that of cultured scallops from the same embayment, similar spawning periods being recorded despite different sampling locations. By contrast populations of sea scallop, Placopecten magellanicus, separated by short distances, have been observed to spawn at different periods (Bonardelli 1985). While spawning periods in the North Water and Broad Water were similar, GI in scallops from the Broad Water site in 1980 were higher than those from the North Water site, possibly reflecting reduced water depth in the Broad Water sampling site (Minchin & Ni Donnachada 1995) or slightly larger size of scallops from this location.

A wide range of methods has been reported for assessment of scallop reproductive condition and identification of spawning periods. Whereas each method has its advantages and disadvantages, and different techniques purport to provide that little extra information, several factors were taken into consideration in method selection including time for analysis, simplicity, equipment and cost. Methods used required no more than a standard kitchen balance and a plastic ruler. Results from 1993 demonstrated that of the 3 parameters, GI, GWt and RGH, each parameter could be used for determination of spawning periods. Of more importance than which parameter was measured was the fact that use of these simple techniques allowed identification of spawning, hence the period in which scallop larvae might be expected in plankton samples.

Consideration of data from the 1993 sampling program on a weekly, fortnightly or monthly basis clearly demonstrated the importance of sampling frequency in determination of the spawning cycle of king scallops. Weekly data (A) demonstrated that scallops with a GI of 25.8% on May 8 spawned between May 8 to May 15, GI declining to 20.6% by May 15 and had recovered to a GI of 26.9% by May 22. While a fortnightly sampling regime based on odd weeks (B) showed some evidence of this spawning, a fortnightly regime based on even weeks (C) failed to record this large spawning event. The monthly sampling regime (D) exhibited evidence of a final partial spawning only and provided only a general trend over the sampling period. The fact that this scallop is capable of maturing, spawning partially and recovering within such a short period raises questions as to the validity of data obtained from studies with a sampling frequency greater than 1 week, particularly when such data is extrapolated to present evidence of the scallop spawning cycle. For example Mason (1958) using a combination of weekly and fortnightly sampling reported that Pecten maximus in the Isle of Man exhibited 2 main spawnings, one in the spring and one in the autumn with only a minor spawning in the summer. Gibson (1956) reported a similar bimodal spawning pattern for Pecten maximus in Irish waters. McDonough (1998) reported that Pecten maximus in Strangford Lough, Northern Ireland exhibited interannual variability in reproduction, a single protracted spawning being recorded in 1995 and a bimodal pattern in 1996. Pazos et al. (1996) using a monthly sampling frequency reported two main spawnings every year for Pecten maximus in Galicia, Spain, the first in April to May and a second in August to September. In support of the proposal mentioned earlier that trends based on a monthly sampling frequency reflect only a general pattern, Ramonell et al. (1990) in Galicia, Spain reported settlement of three batches of larvae between May to June, which originated from three separate spawning events between April to June.

With regard to gonad size prior to spawning and its relationship to spatfall success, only a limited number of long-term datasets exist in the literature. In a 15-year review of data from natural spat collection trials with the Chilean scallop, Argopecten purpuratus, in Tongoy Bay, Pereira et al. (1997) concluded that high values of GI and temperatures exceeding 15[degrees]C resulted in good spatfalls and low values of GI and temperatures below 15[degrees]C resulted in poor spatfalls. Robinson and Martin (2003) in a decadal study of spawning patterns and settlement of sea scallops, P. magellanicus in the Bay of Fundy reported that mean temperatures in Passamaquoddy Bay had increased 1[degrees]C in the last 12 years and concurrent with this observation, maximal gonadosomatic index (GSI) and mean size of spat in collector bags had also increased over the last decade. It was suggested that these environmental changes might have implications on the early life history processes of the scallop. By contrast in the Broad Water of Mulroy Bay, GI of king scallops prior to spawning exhibited an annual decline since 1999. It is suggested that in this more enclosed site, this decline may be linked to an increased biomass of cultured mussels over the last 5 years possibly limiting the food supply for gonad growth in the scallop. Martinez and Perez (2003) used measurement of the mean GI of three individual scallops to assess the effect of water temperature on scallop conditioning in the hatchery. Increased G1 was taken to reflect greater success of one temperature regime compared with another. While significant differences exist between natural spat production and spat production in hatcheries, results obtained in this study demonstrated no significant correlation between gonad size prior to spawning and scallop spatfall intensity.

With regard to the magnitude of spawning, Mason (1958) in a 2-year study from October 1950 to October 1952 recorded a large spring spawning (April) and an even larger autumnal spawning (September) with a small partial spawning (July) in the summer. Mason suggested that spat collection resulting from the autumnal spawning might be expected to be higher than that from the spring spawning because of the much greater amount of spawn released in autumn by comparison with spring. Results in this study demonstrate no significant correlation between the magnitude of spawning and intensity of scallop spatfall. For example in 2002, spawning resulting in a GI decrease of only 1.6% produced an excellent spatfall, whereas in 1998, spawning with a GI decrease of 11.3% resulted in a poor spatfall. While the autumn spawnings of Mason (1958) and the spawning example in 1998 may be significant in terms of numbers of gametes released, this factor alone does not correlate with settlement success. For a spat settlement to occur, some spawning release must inevitably occur, however spawning represents only the beginning of a life cycle, many other factors such as fertilization success, growth and survival through the various stages of larval development, metamorphosis; environmental factors such as water temperature and food availability; biological factors such as the presence or absence of predator species and their abundance, competitors for the larval food supply etc., will all have an influence on spatfall intensity.

The time of spawning within the months June and July varied from year to year in this study. There was no significant correlation between early or late spawning events and spatfall intensity. Annual variation in the spawning dates of P. maximus in Irish waters were reported by Gibson (1956) and Wilson (1987) however their investigations were performed in sites with spatfalls of limited intensity, and no results on spatfall intensity were reported.

Over the 12 years of study of gonad development and spawning some spawnings were of short duration, extending for only a week, others were more protracted, continuing for over 3 weeks. Duration of the spawning period, recorded using GI, exhibited no significant correlation with spat collection intensity. By contrast Mason (1958) in a 2-year study in the Isle of Man used increased sampling frequency of P. maximus as gonad maturation progressed to precisely identify the actual spawning event. Spring spawnings of 1951 and 1952 lasted for 7 days and 3 days respectively; larger autumnal spawnings lasted for 5 and 6 days respectively. No evidence of protracted spawning events was reported in the 2-year duration of the study. Similarly Bonardelli et al. (1996) reviewed spawning data of giant scallops, P. magellanicus over the period 1984 to 1991. A total of 33 spawnings between July to mid September were recorded in this 8-year study, however all spawnings were reported to be abrupt.

In summary based on this dataset collected over a 12-year period of investigation and comprising 20 spawnings, 13 of which resulted in a spatfall, there was no significant correlation between spatfall intensity and gonad size prior to spawning; magnitude of the spawning event; date of spawning nor duration of the spawning event. For a successful spatfall to occur in a site, spawning of adult broodstock is inevitable, however reasons for spatfall success are complex and spawning alone is not the sole factor in determining success or failure of spat collection or recruitment in a fishery.

ACKNOWLEDGMENTS

The author thanks postgraduate students, Ms. Yvonne Bogan and Ms. Anne-Louise Harkin for assistance in analysis of scallop samples in 2003 and 2004. Deegagh Point Shellfish Ltd. provided funding for all gonad analysis data used in this review. Mr. John Boyce and the staff of Deegagh Point Shellfish Ltd. assisted in collection of adult samples and scallop spat collection. Financial support provided in 2002 under PDOC/01/004 by the Marine Institute, Ireland to Dr. Gavin Burnell, University College Cork enabled registration of John Slater for a Ph.D. and provided the impetus for preparation of this manuscript.

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JOHN SLATER

Letterkenny Institute of Technology, Department of Science, Port Road, Letterkenny, County Donegal, Ireland

E-mail: [e-mail]

TABLE 1.
The time, duration and magnitude of spawnings during June to July
1993-2004 producing king scallop spatfalls.

                     Spawning                  Spawning
                    start date                 end date    Spawning
        Spawning    referenced    Spawning    referenced   duration
Year   start date    to Jun 1     end date     to Jun 1     (days)

1993   02/07/1993       32       16/07/1993       46          14
1994   29/06/1994       29       11/07/1994       41          12
1995   10/06/1995       10       23/06/1995       23          13
1995   13/07/1995       43       29/07/1995       59          16
1996   13/07/1996       43       20/07/1996       50           7
1997   12/07/1997       42       04/08/1997       65          23
1998   17/07/1998       47       31/07/1998       61          14
1999   24/06/1999       24       08/07/1999       38          14
2000   21/06/2000       21       07/07/2000       37          16
2001   06/07/2001       36       11/07/2001       41           5
2002   04/07/2002       34       11/07/2002       41           7
2003   08/07/2003       38       14/07/2003       44           6
2004   20/07/2004       50       27/07/2004       57           7

          G.I.         G.I.         G.I.         Spatfall
         start         end        decrease       intensity
Year      (%)          (%)          (%)       (spat/collector)

1993     22.10        18.70         3.40            150
1994     20.60        16.10         4.50            150
1995     24.30        13.10        11.20           1000
1995     19.00        16.30         2.70            300
1996     14.00        11.30         2.70             10
1997     14.10        11.60         2.50           4400
1998     19.70         8.40        11.30             20
1999     23.30        20.60         2.70             20
2000     18.00        11.20         6.80             20
2001     17.20        12.30         4.90           1100
2002     15.60        14.00         1.60           3000
2003     12.20         7.10         5.10             10
2004      7.90         6.70         1.20             10