Identifying spawning events of the sea scallop Placopecten magellanicus on Georges Bank.
KEY WORDS: Placopecten magellanicus, sea scallop, semiannual spawning, gonosomatic index, Georges Bank
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 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.
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.
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).
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 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).
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 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).
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.
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.
Almeida, F., T. Sheehan & R. Smolowitz. 1994. Atlantic sea scallop, Placopecten magellanicus, maturation on Georges Bank during 1993. Northeast Fisheries Science Center reference document 94-13. Woods Hole, MA: NOAA/NMFS/NEFSC.
Barber, B. J. & N. J. Blake. 2006. Reproductive physiology. In: S. E. Shumway, editor. Scallops: biology, ecology and aquaculture. Amsterdam: Elsevier, pp. 377-428.
Barber, B. J., R. Getchell, S. Shumway & D. Schick. 1988. Reduced fecundity in a deep-water population of the giant scallop Placopecten magellanicus in the Gulf of Maine, USA. Mar. Ecol. Prog. Ser. 42:207-212.
Caddy, J. F. 1989. A perspective on the population dynamics and assessment of scallop fisheries, with special reference to the sea scallop, Placopecten magellanicus (Gmelin). In: J. F. Caddy, editor. Marine invertebrate fisheries: their assessment and management. New York: Wiley, pp. 559-589.
Chen, C., R. C. Beardsley & G. Cowles. 2006. An unstructured-grid, finite-volume coastal ocean model (FVCOM) system. Oceanography (Wash. D.C.J 19:78-89.
Chute, A. S., S. C. Wainright & D. R. Hart. 2012. Timing of shell ring formation and patterns of shell growth in the sea scallop Placopecten magellanicus based on stable oxygen isotopes. J. Shellfish Res. 31:649-662.
Cura, J. J. 1987. Phytoplankton. In: R. H. Backus & D. W. Bourne, editors. Georges Bank. Cambridge, MA: MIT Press, pp. 213-218.
Cushing, D. H. 1975. Marine ecology and fisheries. Cambridge: Cambridge University Press. 228 pp.
Dadswell, M. J. & G. J. Parsons. 1992. Exploiting life-history characteristics of the sea scallop, Placopecten magellanicus (Gmelin, 1791), from different geographical locations in the Canadian Maritimes to enhance suspended culture grow-out. J. Shellfish Res. 11:299-305.
DiBacco, C., G. Robert & J. Grant. 1995. Reproductive cycle of the sea scallop Placopecten magellanicus (Gmelin, 1791), on northeastern Georges Bank. J. Shellfish Res. 14:59-69.
Dickie, L. M. 1958. Effects of high temperature on survival of the giant scallop. J. Fish. Res. Board Can. 15:1189-1211.
DuPaul, W., J. Kirkley & A. Schmitzer. 1989. Evidence of a semiannual reproductive cycle for the sea scallop, Placopecten magellanicus (Gmelin, 1791), in the mid-Atlantic region. J. Shellfish Res. 8:173-178.
Gilbert, C. S., W. C. Gentleman, C. L. Johnson, C. DiBacco, J. M. Pringle & C. Chen. 2010. Modeling dispersal of sea scallop (Placopecten magellanicus) larvae on Georges Bank: the influence of depth-distribution, planktonic duration and spawning seasonality. Prog. Oceanogr. 87:37-48.
Gould, E., D. Rusanowsky & D. A. Luedke. 1988. Note on muscle glycogen as an indicator of spawning potential in the sea scallop, Placopecten magellanicus. Fish Bull. 86:597-601.
Harris, B. P. & K. D. E. Stokesbury. 2010. The spatial structure of local surficial sediment characteristics on Georges Bank, USA. Cont. Shelf Res. 30:1840-1853.
Harris, B. P. 2011. Assessment of habitat conditions in persistent high-concentration sea scallop (Placopecten magellanicus) aggregations on Georges Bank, USA. PhD diss., University of Massachusetts. 138 pp.
Hart, D. R. & P. J. Rago. 2006. Long-term dynamics of U. S. Atlantic sea scallop Placopecten magellanicus populations. North Am. J. Fish. Manage. 26:490-501.
Hennen, D. R. & D. R. Hart. 2012. Shell height-to-weight relationships for Atlantic sea scallops (Placopecten magellanicus) in offshore U.S. waters. J. Shellfish Res. 31:1133-1144.
Larsen, P. F. & R. M. Lee. 1978. Observations on the abundance, distribution and growth of post-larval sea scallops, Placopecten magellanicus, on Georges Bank. Nautilus 92:112-116.
MacDonald, B. A. & R. J. Thompson. 1985. Influence of temperature and food availability on the ecological energetics of the giant scallop Placopecten magellanicus'. II. Reproductive output and total production. Mar. Ecol. Prog. Ser. 25:295-303.
MacDonald, B. A. & R. J. Thompson. 1986. Influence of temperature and food availability on the ecological energetics of the giant scallop Placopecten magellanicus: III. physiological ecology, the gametogenic cycle and scope for growth. Mar. Biol. 93:37-48.
Murawski, S. A., R. Brown, H. L. Lai, P. J. Rago & L. Hendrickson. 2000. Large-scale closed areas as a fishery-management tool in temperate marine ecosystems: the Georges Bank experience. Bull. Mar. Sci. 66:775-798.
Naidu, K. S. 1970. Reproduction and breeding cycle of the giant scallop Placopecten magellanicus (Gmelin) in Port au Port Bay, Newfoundland. Can. J. Zool. 48:1003-1012.
Naimie, C. E., J. W. Loder & D. R. Lynch. 1994. Seasonal variation of the three-dimensional residual circulation on Georges Bank. J. Geophys. Res. 99:15967-15989.
New England Fisheries Management Council. 2003. Final amendment 10 to the Atlantic sea scallop fishery management plan with a supplemental environmental impact statement, regulatory impact review and regulatory flexibility analysis. Newburyport, MA: New England Fishery Management Council.
Northeast Fisheries Science Center. 2010. 50th Northeast Regional Stock Assessment Workshop (50th SAW) assessment report. U.S. Department of Commerce, Northeast Fisheries Science Center reference document 10-17. Woods Hole, MA: Northeast Fisheries Science Center. 884 pp.
Posgay, J. A. 1957. The range of the sea scallop. Nautilus 71:55-57.
Robinson, W. E., W. E. Wehling, M. P. Morse & G. C. McLeod. 1981. Seasonal changes in soft-body component indices and energy reserves in the Atlantic deep-sea scallop Placopecten magellanicus. Fish Bull. 79:449-458.
Sarro, C. L. & K. D. E. Stokesbury. 2009. Spatial and temporal variation in the shell height/meat weight relationship of the sea scallop Placopecten magellanicus in the Georges Bank fishery. J. Shellfish Res. 28:497-503.
Stokesbury, K. D. E. 2002. Estimation of sea scallop abundance in closed areas of Georges Bank, USA. Trans. Am. Fish. Soc. 131: 1081-1092.
Stokesbury, K. D. E., B. P. Harris, M. C. Marino II & J. I. Nogueira. 2004. Estimation of sea scallop abundance using a video survey in off-shore U. S. waters. J. Shellfish Res. 23:33-40.
Tremblay, M. J. & M. Sinclair. 1990. Sea scallop larvae Placopecten magellanicus on Georges Bank: vertical distribution in relation to water column stratification and food. Mar. Ecol. Prog. Ser. 61:1-15.
Zar, J. H. 2010. Biostatistical analysis, 5th edition. Upper Saddle River, NJ: Prentice Hall. 944 pp.
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: email@example.com
([dagger]) Current address: Coonamessett Farm Foundation, 277 Hatchville Road, East Falmouth, MA 02536.
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|
|Date:||Apr 1, 2014|
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