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Identifying spawning events of the sea scallop Placopecten magellanicus on Georges Bank.

ABSTRACT Georges Bank is the most productive sea scallop fishing ground in the world, but little is known about the regional spawning patterns. The sea scallop rotational management plan is based on yield projections estimated from shell height/meat weight relationships. Semiannual spawning may influence yield projections, impacting fishery allocations. This study identifies spawning events at two locations on Georges Bank: one in Closed Area I (CAI) and the other in Closed Area II (CAII). We hypothesize that spawning is semiannual, spring spawning is incomplete compared with autumn spawning, and reproductive stage differs temporally between areas. Scallops (n = 1,871) were collected during a monthly dredge survey in these two areas from two sites from March 2011 through June 2013. Tissues from scallops (shell height ~ 130 mm) frozen at sea, were oven-dried and the gonosomatic indices (GSIs) were analyzed to identify spawning events. Oocyte diameter was measured to determine maturity. Bottom temperature was recorded. Semiannual spawning occurred in both closed areas. Spawning rates were similar, but autumn spawning was greater in magnitude than spring spawning. The timing of gametogenesis was similar between sites. Bottom temperature patterns suggest different oceanographic conditions between areas. A semiannual sea scallop reproductive cycle on Georges Bank could influence recruitment and growth assumptions affecting future management decisions.

KEY WORDS: Placopecten magellanicus, sea scallop, semiannual spawning, gonosomatic index, Georges Bank

INTRODUCTION

Georges Bank supports the largest wild scallop fishery globally (Caddy 1989). Three closed areas were established on Georges Bank and closed to all mobile bottom-tendering gears in 1994 (Murawski et al. 2000). The scallop biomass in the closed areas increased 25-fold from 1994 to 2005 (Stokesbury 2002, Stokesbury et al. 2004, Hart & Rago 2006). In 2004, Amendment 10 to the New England Fishery Management Council's Sea Scallop Management Plan established a rotational management strategy that included these closed areas (New England Fisheries Management Council 2003, Hart & Rago 2006). Yield projections are currently derived from shell height/meat weight relationships (Northeast Fisheries Science Center 2010), which could be altered by semiannual spawning.

The sea scallop Placopecten magellanicus (Gmelin 1791) is one of the few gonochoristic broadcast spawners in the pectinid family. The spawning time of sea scallops varies latitudinally across its range, which extends from the Strait of Belle Isle, Newfoundland, to Cape Hatteras, North Carolina (Posgay 1957, Barber & Blake 2006). Annual autumn spawning is typical in Newfoundland (MacDonald & Thompson 1986) whereas semiannual spawning is characteristic of the Mid-Atlantic Bight (DuPaul et al. 1989).

It is generally assumed that spawning occurs in the autumn on Georges Bank, although semiannual spawning has been observed on the Northeast Peak (DiBacco et al. 1995) and has been suggested on the Southern Flank (Almeida et al. 1994, Sarro & Stokesbury 2009). Semiannual spawning may have important implications for fisheries management strategies (DiBacco et al. 1995). Spring-spawned scallops mature more rapidly than autumn-spawned scallops, reaching a harvestable size of 90 mm in 26 mo versus 34 mo, respectively, according to a study in Mahone Bay, Nova Scotia (Dadswell & Parsons 1992). Spring spawning could influence growth estimates and yield projections, and may potentially strengthen the stock-recruitment relationship on Georges Bank.

The objective of this study is to identify sea scallop spawning events at two locations on Georges Bank: one in Closed Area I (CAI) and one in Closed Area II (CAII) (Fig. 1). We hypothesize that spawning is semiannual, spawning intensity is greater in autumn than in the spring, and that reproductive stage differs temporally between areas. Spawning events were determined by analyzing dry gonosomatic indices (GSIs). Rate and magnitude of decreasing GSI was analyzed to quantify spawning intensity. Gonadal tissue was staged reproductively through microscopic examination, and oocyte diameter was analyzed. Bottom temperature was measured directly and compared with modeled estimates. This study will improve our understanding of spawning patterns on Georges Bank, which is important for effective rotational management.

MATERIALS AND METHODS

Scallops were collected from a collaborative seasonal bycatch study conducted by Coonamessett Farm Foundation, School for Marine Science and Technology, and Virginia Institute of Marine Science aboard commercial fishing vessels using a 4.57-m-wide New Bedford dredge. Collection sites at CAI and CAII on Georges Bank (Table 1, Fig. 1) were selected because they represent areas of consistently high-concentration scallop aggregations (Stokesbury et al. 2004, Harris 2011). The CAI site is characterized by hard substratum (pebble, cobble) with minimal horizontal current, whereas the CAII site has a sandy bottom and stronger seasonal currents (Naimie et al. 1994, Harris & Stokesbury 2010).

Dry Tissue Weights

Live scallops (n = 16-62) approximately 130 mm in shell height were collected monthly from March 2011 to June 2013 and frozen whole. Scallops were thawed, dissected, and separated into gonad, meat, and viscera. The crystalline style, intestinal contents, and foot were removed from the gonad and included with the viscera (Sarro & Stokesbury 2009).

Tissues were oven-dried at 80[degrees]C for at least 4 days until they reached constant weight, and dry weight was measured. The GSI was calculated as GSI = (Gonad Weight/Total Issue Weight) x 100 (Barber & Blake 2006). Data were tested for normality and equal variance. A Mann-Whitney V test was used to test statistical differences in the GSI between months, because data were not distributed normally. A spawning event was identified as a significant decrease in the GSI between months. Qualitative histological examination verified that a decrease in the GSI was the result of spawning.

Dry meat weights are reported for the purpose of this study because tissues were frozen before processing, and wet weights are influenced by variability in water content as a result of freezing.

Spawning Intensity

Spawning intensity was measured by the rate as well as by the magnitude of spawning. Years were combined for both analyses to increase sample size. Rate was quantified as the slope of the regression line fit to the mean GSI during months of spawning. Differences in slope between spring and autumn spawning events were tested at each station using a variation of Student's t-test (Zar 2010).

Magnitude was quantified as percent decrease in the mean GSI before and after spawning [1 - ([GSI.sub.low]/[GSI.sub.high]) x 100] (Sarro & Stokesbury 2009). Because data were not distributed normally, differences in magnitude between spring and autumn spawning events were tested by station using a Kruskal-Wallis test.

Reproductive Stage

Gonad tissue samples (10 females and 10 males) were collected at each station from June 2011 to November 2012, fixed in 10% formalin, and preserved in 70% ethanol. During processing, tissue was dehydrated and cleared through an ethanol-xylene series, embedded in paraffin wax, and cut into 8-[micro]m-thick sections with a rotary microtome. Sections were mounted on slides and stained with hematoxylin-eosin. Sections were staged reproductively according to the criteria of Naidu (1970).

Oocyte diameter was measured using Image-Pro on a Nikon compound microscope at 100x magnification to determine egg maturity and to verify spawning events. Mature eggs were identified as being 50 pm or larger in diameter, with a vitelline membrane surrounding the nucleus (DiBacco et al. 1995). Diameters of mature and developing oocytes sectioned through the nucleus were measured from a proximal and distal section for each ovary (n = 60 oocytes per individual) from 5-10 females per station for each month. Mean oocyte diameter was graphed by month to identify gametogenic patterns. A Mann-Whitney U test determined whether there was a significant difference in mean oocyte diameter between areas, because data were not distributed normally.

Bottom Temperature

One Vemco Minilog (V3.09) and one Star-Oddi milli-TD deployed in steel sheaths welded to the dredge between the rollers measured bottom temperature at each station from May 2011 to June 2013. Measured data were compared with finite-volume coastal ocean model (FVCOM) estimates, which are continuous and provide a reference regarding interannual variability (Chen et al. 2006).

RESULTS

Dry Tissue Weights

Spawning was semiannual at CAI and CAII in 2011 and 2012, and spring spawning took place in 2013 (Fig. 2). The GSI was greater at CA1 than at CAII, except in September 2012 (Fig. 2). The GSI was significantly different (Table 2; P < 0.05) between months when values were decreasing in both spring and autumn 2011 and 2012, confirming that spawning was semiannual (Fig. 2).

Spring spawning occurred in late April into May 2011, reaching a minimum GSI in June (Table 2, Fig. 2A). Gonads recovered in July and were ripe in August at CAI and in September at CAII (Table 2, Fig. 2A). Autumn spawning took place in September and October 2011 at both stations, beginning earlier at CAI (Fig. 2A).

In 2012, spring spawning occurred from May into early June. In autumn, scallops spawned in September and October (Fig. 2B). Scallops at CAI ripened more rapidly and began spawning earlier than scallops at CAII, consistent with 2011 (Fig. 2B). During 2011, the mean GSI was greatest at CAI, whereas in 2012 the mean GSI at CAII was greatest (Fig. 2A, B).

Consistent with the previous 2 y, gametogenesis took place from January through March 2013, the GSI peaked in April, and spawning occurred in May and June at both stations (Fig. 2C). Spring spawning was minor at CAI compared with at CAII in 2013 (Fig. 2C). Autumn spawning was likely in 2013 based on the previous 2 y of data.

Dry meat weight generally reached maximum values in June, except at CAI in 2011, when it peaked in May (Fig. 3).

Spawning Intensity

Spawning rate indicated how quickly gametes were released, whereas spawning magnitude reflected the difference in the GSI from ripe to completely spent. There was no significant difference in spawning rate between spring and autumn spawning periods at CAI (Fig. 4; t = 0.78, df = 536, P > 0.05) or at CAII (Fig. 4; t = 1.27, df = 433, P > 0.05). Autumn spawning was significantly greater in magnitude than spring spawning in both areas in 2011 and 2012 (Table 3; Kruskal-Wallischi square (1) = 104.5, P < 0.05).

Reproductive Stage

Mean oocyte diameter followed the seasonal patterns of the mean GSI in 2011 and 2012 (Fig. 5). Oocytes were mature from August through October 2011, consistent with the timing of ripeness from the GSI (Fig. 5A). Mean oocyte diameter reached maximum values in April and May 2012, confirming that oocytes were mature in the spring (Table 4, Fig. 5B). A decrease in oocyte diameter from May to June 2012 supports the GSI results that spawning took place in the spring (Fig. 5B). Increasing oocyte diameter from June to August 2012 suggests gonadal recovery and maturation of the next cohort of oocytes (Fig. 5B).

Spring spawning was confirmed with qualitative examination of samples collected in June and July 2011. Vacancies in the center of follicles in both ovarian and testicular sections indicate gamete release from spawning.

Bottom Temperature

Bottom temperature measurements were generally warmer at CAI (7-16.6[degrees]C) than at CAII (6.2-11.9[degrees]C) (Table 5, Fig. 6). The bottom temperature was warmer at CAI than at CAII for all months except May 2011 (Table 5, Fig. 6). Difference in bottom temperature measurements between areas was most pronounced from July through October 2011 and 2012, agreeing with modeled data (Fig. 6). Mean daily FVCOM bottom temperature estimates from 2000 to 2009 are warmer at CAI than at CAII for July through October (Fig. 6). June through October measurements at CAI were 2-3[degrees]C warmer in 2012 than in 2011 (Table 5, Fig. 6).

DISCUSSION

Semiannual spawning has important implications for the stock assessment and management of the Georges Bank fishery. There is no clear relationship between sea scallop spawning biomass and recruitment on Georges Bank (Northeast Fisheries Science Center 2010), because recruitment is highly variable between years and is unlikely to be influenced by short-term variability in biomass. Accounting for spring spawning in recruitment models could potentially aid in defining a stock-recruitment relationship. Even if spring spawning does not contribute to recruitment, a biannual spawning pattern affects growth estimates and shell height-meat weight relationships directly, which would alter yield projections and fishery allocations.

There is no evidence of recruitment resulting from two annual spawning events on Georges Bank because of the limited research on this topic in this region. Spat collection experiments on Georges Bank have targeted the autumn spawning event (Larsen & Lee 1978, Tremblay & Sinclair 1990). The most conclusive method of determining the relative contribution of the spring spawning event to recruitment compared with autumn spawning is isotopic analysis of scallop shells. Only two scallop shells from Georges Bank have been analyzed, which verified autumn spawning (Chute et al. 2012). Additional studies that use isotopic analysis to estimate the prevalence of spring-spawned scallops are needed.

Gonad and meat weights are related closely physiologically; however, past studies have reported variable seasonal patterns in the relationship between gonadal and somatic weights on Georges Bank. Energy reserves in the form of glycogen and lipids are reallocated from the adductor muscle to the gonad during gametogenesis, thereby decreasing meat weight (Robinson et al. 1981, MacDonald & Thompson 1986, Gould et al. 1988). It follows that meat weight would be greatest in June, between spawning events, when the GSI is lowest (Figs. 2 and 3). Sarro and Stokesbury (2009) observed maximum wet meat weight in June on the majority of Georges Bank, whereas summer and winter peaks on the Southern Flank were attributed to semiannual spawning. Hennen and Hart (2012) observed maxima in wet meat weight in June and December on Georges Bank. Although the summer peak in meat weight is consistent, there have been variable reports on the winter peak.

The unimodal pattern in meat weight observed in this study may be attributed to dry weight analysis. This study was designed to examine the reproductive cycle of sea scallops rather than seasonal meat weight patterns; hence, why dry tissue weights were analyzed as opposed to wet weights. Wet meat weight could not be measured accurately from frozen samples because water is lost during the process of freezing. It is possible that the winter peak in wet meat weight observed in past studies is the result of water content and was therefore not detected in this analysis. Additional research is needed to explain more completely the seasonal relationship between gonad and meat weights by region on Georges Bank.

This study confirmed that semiannual spawning occurs on Georges Bank in two very different regions. Temperature measurements indicated differing bottom temperature patterns at CAI and CAII, possibly representing different physical oceanographic conditions. A well-mixed water column at CAI may have resulted in the wide range of seasonal water temperatures observed, whereas seasonal stratification at CAII explains less extreme seasonal fluctuation in bottom temperature. That semiannual spawning was observed in such different environments suggests that it may be more widespread on Georges Bank. Additional reproductive data collection is needed to confirm this and to determine whether semiannual spawning is a regular occurrence.

Spawning was more complete at CAI than at CAII, possibly as a result of spatial differences in oceanographic conditions. Spawning magnitude may be associated with the degree of water temperature fluctuation, because thermal shock acts as a cue for spawning induction. Seasonal bottom temperature variability was greater at CAI (Fig. 6), which may have resulted in stronger scallop spawning cues.

Spawning magnitude was generally greater in autumn than in spring. This could be a result of a faster rate of change in water temperature in autumn than in spring. Scallops acclimate to decreasing temperatures much more slowly than to increasing water temperature (Dickie 1958). Therefore, rapidly decreasing temperatures in autumn may produce a stronger spawning cue than gradually increasing temperatures in spring.

Differences in spawning magnitude could also be explained by variable food availability. The spring bloom was more pronounced in 2011 at CAI, perhaps resulting in a greater spawning magnitude in spring 2011 than in spring 2012. Chlorophyll a levels were low at both areas in spring 2012 and 2013 (Fig. 7), possibly explaining less pronounced peaks in the GSI in those seasons (Fig. 2).

The GSI and oocyte diameter differed between areas when scallops were spent or developing. This could be explained by differences in vertical mixing between areas, because low food availability can result in lower gamete production and small oocytes in scallops compared with scallops exposed to greater food abundance (Barber et al. 1988). A more stratified water column at CAII could result in low food availability for benthic organisms, because plankton is isolated in the surface layer. It is possible that scallops in areas of low food availability allocate energy reserves to somatic tissue growth rather than to reproduction.

Spring-spawned larvae may have a distinct resource advantage over autumn-spawned larvae. According to the match-mismatch hypothesis, larval survival is influenced by the degree of overlap between food abundance and spawning time (Cushing 1975). Therefore, larvae spawned in April and May, during the spring phytoplankton bloom, would have greater food availability than autumn-spawned scallops. The spring bloom coincided with peak scallop ripeness in 2011 and 2012 (Fig. 7), indicating favorable larval feeding conditions.

Although feeding conditions may be ideal for planktonic scallop larvae in the spring, predation on scallop larvae is likely to be greater in spring than in autumn. Predators such as zooplankton and pelagic fish larvae are most abundant after the spring bloom, when phytoplankton are highly concentrated in the upper layers of a stratified water column (Cura 1987). Phytoplankton concentrations decrease and vertical mixing disperses potential predators in autumn (Cura 1987).

As a result of variable seasonal circulation on Georges Bank, spring-spawned larvae could follow different dispersal patterns than autumn-spawned larvae. Residual stream patterns result in relatively low recirculation on Georges Bank in the winter and spring months, with the strongest recirculation occurring in September/October (Naimie et al. 1994). Seasonal variability in recirculation may lead to lower larval retention (~ 20%) and greater loss to downstream locations (~ 30%) in spring versus autumn on Georges Bank (Gilbert et al. 2010). Therefore, autumn spawning may contribute significantly to local recruitment, whereas spring spawning could supply larvae to southern regions such as the Mid-Atlantic Bight.

Semiannual spawning has important applications in management of the scallop fishery. Seasonal spawning closures in areas of low recruitment could help to protect spawning stocks and to enhance recruitment. Furthermore, knowledge of the timing of spawning and associated shifts in meat weight can provide insight to times of optimum meat yield (Sarro & Stokesbury 2009). More accurate representation of scallop spawning behavior in management will bring us closer to promoting a healthy sea scallop stock and a more efficient fishery.

ACKNOWLEDGMENTS

We thank our collaborators Coonamessett Farm Foundation and Virginia Institute of Marine Science, Dr. Aswani Volety for his assistance in reproductive staging, and committee members for their guidance. Thank you to all who assisted with sample collection, dissections, and measuring, and thank you to Dan Ward and Karen Thompson for their editorial assistance. This research was supported by NOAA (grants NA10NMF4720288 and NA12NMF4540034). This work uses the Gulf of Maine FVCOM model-predicted database created by Dr. Chen's research team at the Marine Ecosystem Dynamics Modeling Laboratory, University of Massachusetts Dartmouth. This database was built with support from NSF, NOAA (NERACOOS, MFI, and MIT Sea Grant) and private foundations, and is being maintained with support from NERACOOS.

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KATHERINE J. THOMPSON, * ([dagger]) SUSAN D. INGLIS AND KEVIN D.E. STOKESBURY

Department of Fisheries Oceanography, School for Marine Science and Technology, University of Massachusetts Dartmouth, 200 Mill Rd, Suite 325, Fairhaven, MA 02719

* Corresponding author. E-mail: kthompsonl@umassd.edu

([dagger]) Current address: Coonamessett Farm Foundation, 277 Hatchville Road, East Falmouth, MA 02536.

DOI: 10.2983/035.033.0110

TABLE 1. Location and depth of primary stations (in bold type)
and backup stations (with month sampled) for Closed Area I (CAI)
and CAII.

Station   Date            Latitude              Longitude

CAI-26                    41.06747[degrees]N   -68.70043[degrees]W
CAI-18    February 2012   41.13206[degrees]N   -68.70043[degrees]W
CAI-27    May 2012        41.06747[degrees]N   -68.63585[degrees]W
CAI-35    April 2011      41.00289[degrees]N   -68.57126[degrees]W
CAII-22                   41.28097[degrees]N   -66.65744[degrees]W
CAII-23   March 2011      41.28097[degrees]N   -66.55522[degrees]W

TABLE 2. Mean gonosomatic index (measured as a percentage; [bar.X]),
SD, and sample size at Closed Area I (CAI) and CAII March 2011 to
June 2013.

                          CAI

Month          [bar.X]    SD         n

March 2011     15.3        6.8       22
April          22.7        7.1       24
May            13.8 *      4.0       40
June            6.6 *      3.0       52
July           11.9 *      7.5       23
August         26.6 *      7.8       50
September      16.0 *     10.0       49
October         6.6 *      7.2       42
November        3.5        2.1       38
January 2012    7.7 *      5.2       37
February       14.8 *      5.5       40
March          15.6        6.7       40
April          18.3        7.0       40
May            19.3        6.4       35
June           12.0 *      7.1       40
August         18.0 *      7.5       40
September      15.1       11.8       40
November        3.3 *      1.3       38
December        4.4 *      2.3       24
January 2013   11.4 *      6.6       20
March          15.8 *      6.2       26
April          16.9        7.1       30
June           11.4 *      8.7       26
Total                                816

                          CAII

Month          [bar.X]    SD         n

March 2011      8.2       4.6        16
April          15.7 *     6.9        31
May            11.4 *     4.5        21
June            5.3 *     3.0        24
July            6.4       2.7        40
August         11.2 *     5.4        61
September      15.7 *     5.6        50
October         7.5 *     5.7        41
November        1.6 *     0.6        40
January 2012    3.1 *     2.7        36
February        6.4 *     3.9        38
March          11.0 *     5.2        62
April          13.8 *     4.6        41
May            18.1 *     6.0        40
June            4.9 *     2.4        40
August         16.6 *     5.4        40
September      26.0 *     5.8        40
November        2.2 *     1.1        40
December        2.6       1.7        25
January 2013    8.0 *     4.1        20
March           9.5       2.8        30
April          18.3 *     4.3        30
June            5.0 *     3.4        30
Total                                836

* Statistically significant difference (P < 0.05) between preceding
and referenced months using the Mann-Whitney U test.

TABLE 3.
Percent decrease in gonosomatic index during months of
spawning, from spring 2011 to spring 2013.

Station   Year   Spring (%)   Autumn (%)

CAI       2011     70.93        86.84
          2012     37.83        81.67
          2013     32.55
CAII      2011     66.24        89.81
          2012     72.93        91.54
          2013     72.68

TABLE 4.
Mean oocyte diameter (in micrometers; [bar.X]), SD, and
sample size at Closed Area I (CAI) and CAII June 2011
to November 2012.

                            CAI                       CAII

Month             [bar.X]   SD      n       [bar.X]   SD      n

June 2011         46.9      13.3    5       53.5      8.3     5
July              48.7       6.0    5       38.3      9.2     5
August            55.1       7.8    10      52.3      8.5     10
September         50.4      19.0    10      53.4      6.9     10
October           52.1       8.1    10      49.9      8.3     10
November          34.3 *    11.7    10       *         *      10
April 2012        61.6       8.9    10      59.0      8.1     10
May               61.1       8.3    9       60.3      7.5     10
June              46.2      10.2    10      37.8      9.7     10
August            58.2       7.5    10      58.5      7.9     10
September         53.7       8.7    10      55.3      6.7     10
November          35.0 *    17.8    10      49.1 *    6.9     10
Total                               109                       110

* Egg absence.

TABLE 5. Mean bottom temperature (in degrees Celsius;
[bar.X]) measurements from 30-min tows recorded at 30-sec
intervals at stations in Closed Area I (CAI) and CAII from
May 2011 to June 2013.

                          CAI                        CAII

Month          [bar.X]    SD      n       [bar.X]    SD      n

May 2011        8.7       0.0     52       8.7       0.2     70
June            9.7       0.1     40       6.9       0.1     65
July           11.7       0.1     60       7.5       0.1     62
August         14.6       0.4     62       9.7       0.1     60
September      16.0       0.1     59      10.5       0.2     62
October        16.7       0.1     67      10.6       0.4     68
November       13.6       0.1     66      10.4       0.1     67
January 2012   10.1       0.0     54       9.0       0.1     64
February        7.4       0.1     62       6.7       0.0     62
March           7.0       0.1     62       6.2       0.0     62
April           7.4       0.0     59       6.5       0.1     62
May             8.6       0.1     62       8.0       0.0     61
June           12.2       0.1     62       9.4       0.1     62
August         15.4       0.1     62       9.8       0.3     59
September      17.6       0.1     62      11.9       0.2     62
November       15.3       0.0     61      12.0       0.2     67
December       12.9       0.3     44      10.4       0.1     62
January 2013    7.1       0.0     61       7.2       0.0     61
March           5.9       0.0     66       8.1       0.1     69
April           7.1       0.0     62       6.9       0.1     62
June            9.4       0.2     61       8.8       0.1     65
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Author:Thompson, Katherine J.; Inglis, Susan D.; Stokesbury, Kevin D.E.
Publication:Journal of Shellfish Research
Article Type:Report
Geographic Code:1USA
Date:Apr 1, 2014
Words:5131
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