The seasonal use of a shallow Georgia creek by flatfishes.
Flatfishes use estuaries as recruitment habitats and nurseries. The purpose of the present study was to assess the seasonal use and abundance (as measured by Catch Per Unit Effort) of flatfishes from September 2004 to February 2006 in Wylly Creek, a shallow estuarine creek in Georgia. Monthly sampling was conducted with a 1-meter beam trawl. The bay whiff Citharichthys spilopterus was the most abundant species collected with a maximum mean CPUE of 34.7 recently settled juveniles in January 2005, while the black-cheek tonguefish Symphurus plagiusa was collected most regularly. The ocellated flounder Ancylopsetta quadrocellata were mostly collected in spring. Variability in the CPUE of newly recruited bay whiff may indicate that Wylly Creek is not a consistent settlement habitat for this species. We documented bay whiffs earlier in the year than previous studies; they may settle in such shallow creeks before moving to deeper water.
Key Words: flatfish, bay whiff, habitat, nursery, settlement
Many flatfishes use temperate and tropical coastal areas such as bays, lagoons, and estuaries (1). Flatfishes may use these areas as nurseries. The seasonal habitat use and abundance of flatfishes varies based on the habitat they occupy and the quality of food in the area. Physical factors such as temperature, salinity, and sediment type could affect the habitat flatfishes occupy (2). Shallow-water creeks may be used as nursery grounds because of the high temperatures frequently found in shallow waters compared with offshore areas (2). The abiotic conditions can affect metamorphosing larvae as well as the distribution and growth of juvenile flatfishes (3). Timing of metamorphosis in young-of-the-year Pseudopleuronectes americanus Walbaum (winter flounder) was closely linked to temperature in four estuaries along the coast of New Jersey. In all of the estuaries, metamorphosis was delayed in colder years (4). In Louisiana, Symphurus plagiusa Linnaeus (blackcheek tonguefish) and Citharichthys spilopterus Gunther (bay whiff) metamorphosed at similar temperatures (20[degrees]C and 19[degrees]C, respectively) (5). Ancylopsetta quadrocellata Gill (ocellated flounder) were collected in a wide temperature range (8-26[degrees]C) in a Georgia estuary (6). Salinity can also affect the distribution and movement of flatfishes in an area. Larval bay whiffs were found in a large salinity gradient (0-31.7%0) in the Cape River estuary, North Carolina (7). Blackcheek tonguefish in Louisiana were found in shallow sites with lower salinity than Symphurus civitatium Ginsburg (offshore tonguefish) (8). In North Carolina, settling sites for Paralichthys dentatus Linnaeus (summer flounder) and Paralichthys lethostigma Jordan and Gilbert (southern flounder) were located in the high-salinity intertidal flats (9). In fact, flatfish growth can serve as a proxy for habitat quality (10). Most flatfishes are associated with sediments rather than hard substrates (2). In Alaska, both age-0 and age-1 Hippoglossoides elassodon Jordan and Gilbert (flathead sole) were most abundant on mixed mud sediments, while both size classes of Lepidopsetta bilineata Ayres (rock sole) were most abundant on sand (11). In laboratory experiments, the highest sediment selectivity occurred in the juveniles of Hippoglossus stenolepis Schmidt (Pacific halibut) and Lepidopsetta polyxystra Orr and Matarese (northern rock sole) (12). Stoner and Ottmar (12) found there was a strong relationship between sediment choice and the ability of flounder to bury. In the Ems-Dollard estuary, no juvenile Pleuronectes platessa Linnaeus (plaice) were collected in areas with more than 10% mud in the sediment (13).
The abundance of flatfishes varies based on the species, location, and time of year. During the period of immigration, larval abundance fluctuates considerably both within and between years (14). In Georgia estuaries during the spring and summer, Reichert and van der Veer (15) collected blackcheek tonguefish, bay whiff, summer flounder, southern flounder, and Etropus cros-sotus Jordan and Gilbert (fringed flounder). Summer and southern flounder were not particularly abundant during their sampling season (15). In Ossabaw Sound estuary in Georgia, southern flounder had high abundances in the spring, while blackcheek tonguefish had high abundances in the winter and summer (16). Ocellated flounder were also present in Georgia estuarine waters in spring (17). Allen and Baltz (5) determined that size and density of flatfishes varied in deeper portions of Georgia estuaries because of abiotic conditions.
The study area was located in Wylly Creek (31[degrees]59'52"N, 81[degrees]03'18"W), which is a small tributary of the Herb River in Georgia, USA (Fig. 1). Wylly Creek has a semidiurnal tide and has a sand content of 54.3-72.4% as determined by a sieving (Rhodes and Curran, unpubl. data). The channel is approximately 15 m wide and is bordered by Spartina alterniflora Loisel (smooth cord grass) and small oyster beds on both sides. At low tide, large sections of the creek become dry, but in one small area the maximum depth was 2.59 m (Rhodes and Curran, unpubl. data). Wylly Creek was chosen for our study because we have preliminary data indicating that flatfishes were present there in higher numbers than other creeks sampled such as the Herb River. Our goal was to determine how flatfishes used such shallow creeks that are inaccessible to boats at low tide. The purpose of the present study was to assess the seasonal use and abundance of flatfishes in a shallow-water creek. We hypothesize that there would be seasonal changes in abundance as measured by Catch Per Unit Effort (CPUE) and size of flatfishes.
[FIGURE 1 OMITTED]
MATERIALS & METHODS
Wylly Creek was sampled monthly from September 2004 to February 2006 during ebbing tide (3 hours before low tide). Three replicate two minutes hauls were made with a 1-m beam trawl with a 3-mm mesh net. Tows were conducted consecutively from the mouth to the end of this small creek. Therefore, the same areas were sampled each time. The towing speed was 2-2.5 knots. All flatfishes were identified, counted, and measured to the nearest mm TL. Surface water temperature and salinity were measured using a stem thermometer and refractometer, respectively. Length frequency distributions were generated to follow changes in size classes over time. Results are reported as CPUE, which is a mean value of the abundance of each species for the three tows. The normality of the data was determined by the Shapiro-Wilk test using the Univariate procedure in SAS (18). Statistical significance of the data was determined by a nonparametric analysis of variance procedure using SAS software because data were not normal.
During the study, water temperature ranged from 11-32[degrees]C and the salinity ranged from 13-25 psu (Table I). Temperature levels fluctuated from year to year, particularly during September and October. For example, in September 2004 the temperature was 23[degrees]C, but was much higher (27[degrees]C) in 2005. In October 2004, the temperature was 24[degrees]C but was only 16[degrees]C in 2005. The largest difference in salinity between years was in September when the salinity was 25 psu in 2004 and 19 psu in 2005. The species of flatfish present in the estuary and the mean length of flatfishes varied by sampling month (p<0.0001). There was a significant effect of temperature on the CPUE and length of flatfishes with a p-value for both F tests of <0.0001.
At Wylly Creek, a total of 333 flatfishes were collected including 189 bay whiffs, 84 blackcheek tonguefish, and 32 ocellated flounder. Bay whiff (CPUE = 3.5) was the most abundant species (Table I). There was a maximum CPUE of 34.7 recently settled individuals in January 2005 with fish ranging from 5-25 mm TL. Fewer individuals were collected in February (CPUE = 8.0) and March 2005 (CPUE = 11.3), but they were larger, ranging from 12-40 mm TL in February and 25-89 mm TL in March. No bay whiffs were collected from April-September 2005. The largest bay whiffs were collected in the fall (52-108 mm TL). In January 2006, there was no settlement pulse with a CPUE of only 2.3 (17-24 mm TL). There was a CPUE of 2.7 in February 2006 (19-46 mm TL). Modal size for bay whiffs was smaller in January and February than during any other months (Fig. 2).
[FIGURE 2 OMITTED]
Blackcheek tonguefish (CPUE = 1.6) were caught more consistently than any other species, although they did not exceed the number of bay whiffs collected (Table I). The CPUE varied from 1.0-1.7 for blackcheek tonguefish that were collected from September 2004 to March 2005. They ranged in size from 14-76 mm TL. In April 2005, the greatest number of blackcheek tonguefish was collected at one time (CPUE = 8.3) and the sizes ranged from 22-83 mm TL, with a modal size of 50 mm TL (Fig. 2). Only a few blackcheek tonguefish were collected for the remainder of the study and none was captured in July 2005. In general, the smallest blackcheek tonguefish were caught in the winter, while larger individuals were captured from April-June 2005.
Ocellated flounder (Ancylopsetta quadrocellata) were collected starting in March 2005 with an overall CPUE of 0.6 for the study (Table I). Smaller ocellated flounder (40-42 mm TL) were captured from April-May 2005. Larger individuals were caught in April 2005 (94 mm TL) and June 2005 (92 mm TL). The CPUE varied from 0.3-6.7, with the peak in April 2005. Modal size in April 2005 was 60 mm TL (Fig. 2).
Very few individuals of other species were collected. Southern flounder (Paralichthys lethostigma) (total n = 15) were first caught in April 2005. Smaller southern flounder were captured in the spring (45-49 mm TL), while the largest individual was caught in the fall (171 mm TL). The only time that summer flounder (Paralichthys dentatus) (n = 5) were collected was in October 2004 and they ranged from 76-163 mm TL. Fringed flounder (Etropus crossotus) (total n = 8) were captured in June and September 2005 with 4 individuals being collected in each month. The smallest individual (49 mm TL) was collected in June 2005, and large individuals were collected in both June and September 2005 (80-95 mm TL).
The major finding of this study was that flatfishes used Wylly Creek at different times of the year possibly because of differences in larval transport mechanisms, environmental parameters, or resource availability. The variability in the CPUE of flatfishes over the course of the study could be due to life-history differences such as seasonal habitat use, timing of spawning, or migration. The seasonal habitat use in Wylly Creek was most pronounced for the bay whiff. A maximum CPUE (34.7) of recently settled bay whiffs was obtained in January 2005 and larger individuals were caught in February and March 2005 after which they were not collected again until October 2005. In Louisiana, the smallest size class ([less than or equal to] 15 mm) of bay whiffs were collected at higher salinities, while larger bay whiffs (30-100 mm) were found in lower salinity (5). In Georgia from late March to August 1990, bay whiff and blackcheek tonguefish had a mean abundance of 3.5 ind/100 [m.sup.2] and 10.3 ind/100 [m.sup.2], respectively (15). Blackcheek tonguefish were collected consistently throughout the present study, but most were captured in April 2005 when bay whiffs were no longer in Wylly Creek. Therefore, there could be partitioning of resources. Dahlberg and Odum (19) determined that fringed flounder, summer flounder, southern flounder, and blackcheek tonguefish were found in trawls nearly year round, while we only observed such a broad annual occurrence by blackcheek tonguefish. Dahlberg and Odum (19) sampled a larger deeper area, while we sampled a small creek that is not navigable at low tide. In the Ogeechee River-Ossabaw Sound estuary in Georgia, the blackcheek tonguefish had a maximum density of 8 ind/100 [m.sup.2] in both the upper estuary in December and in the lower estuary in June (15). Switzer et al. (8) observed that the abundance of blackcheek tonguefish changed with the season, with the highest abundances occurring in fall and winter followed by a decrease in the spring. However, we observed peak CPUE of blackcheek tonguefish in April 2005 when recently settled bay whiffs were no longer present in Wylly Creek. As seen with blackcheek tonguefish, most of the ocellated flounder were collected in April 2005 when bay whiffs were no longer present. Our findings support those of Stickney et al. (17), who found that ocellated flounder were most abundant in the spring.
Since the timing of settlement is closely related to the spawning period, a major reason for seasonal difference in habitat use could be that some of the species settle earlier in the year. For example, bay whiffs hatch from February-March in North Carolina (20). Reichert and van der Veer (15) conducted plankton tows as early as February, but no bay whiffs were collected, though they caught juveniles in their earliest beam trawl (March). These findings seem to support our observation of an earlier settlement period (January) for this species. One reason that we found bay whiffs earlier in the year may be because they may settle in shallow creeks in January before moving into deeper water as the year progresses. Dahlberg and Odum (19) found larger bay whiffs in Georgia estuaries from May to November. The blackcheek tonguefish have a spawning period from summer to fall (21). Reichert and van der Veer (15) observed that blackcheek tonguefish settled from June to September in Georgia. In Louisiana, the recruitment of blackcheek tonguefish began in the fall and abundance peaked in the winter (22). We observed a peak CPUE in the spring. Ocellated flounder recruitment occurs in the spring when more were collected in Georgia estuaries (17). We also observed more ocellated flounder in the spring. Stickney et al. (17) collected no bay whiff smaller than 50 mm in otter trawls because they sampled larger rivers, while we collected smaller ocellated flounder with a beam trawl in shallower creeks.
The mechanisms that cause larvae to be transported into nursery areas are not completely known (23). Variability in water circulation may affect larval transport and retention (24). Winter flounder were found to be transported into shallow estuarine coves by a small-scale eddy (25). The bay whiffs could have been carried by small eddies and currents into Wylly Creek. The migration of bay whiffs to deeper water during the summer might be due to the rising water temperature (15). In Louisiana, juvenile blackcheek tonguefish moved to coarser substrate and higher salinity as size increased (5). Larval and juvenile migration have been observed from the coastal shelf to estuarine areas for blackcheek tonguefish (21). Ocellated flounder migrated into the lower and middle areas of a Georgia estuary during the winter and spring (6).
Physical factors such as temperature and salinity could affect the distribution and abundance of flatfishes in estuaries (9, 2, 26). Favorable environmental factors for blackcheek tonguefish in Barataria Bay, Louisiana included a water temperature of 10.2-32.4[degrees]C and a salinity of 0-29.2 psu (8). In Wylly Creek, the water temperature ranged from 11-32[degrees]C and the salinity ranged from 13-25 psu, which are near the range described by Switzer et al. (8). This could be why blackcheek tonguefish were collected more consistently than any other species. Wylly Creek was sampled because initial studies indicated that more flatfishes were present in Wylly Creek than elsewhere in Wassaw Sound even though we were restricted to sampling during ebbing tide because the creek was inaccessible at low tide, which is when flatfish are usually most concentrated. Since we only sampled one summer, the data collected during the spawning season for some species were limited.
The fluctuations between yearly recruitment of flatfishes could be caused by their natural mortality due to food limitations or predation (27). Plaice move into shallow water after they settle (28). The concentration of the juvenile stages of many flatfish species in shallow water indicates that water depth plays a role in their distribution (2). Wylly Creek does not appear to be a consistent nursery for flatfishes. We collected a pulse of settling bay whiffs in only one year that was probably controlled by physical transport mechanisms. Low CPUE numbers in some years indicate that Wylly Creek is not a consistent nursery ground. Six different flatfish species occupied Wylly Creek and their numbers varied by season. The bay whiffs had the highest CPUE in the winter, while the blackcheek tonguefish, ocellated flounder, and southern flounder had their highest CPUEs in the spring. The fringed flounder were observed in the summer, while summer flounder were only collected in the fall. Cain and Dean (29) observed a seasonal trend in the number of species collected in a small intertidal creek in South Carolina. The subtropical species (bay whiff, blackcheek tonguefish, ocellated flounder, and fringed flounder) are not fished commercially and less information is known about their life history. In conclusion, we documented an earlier seasonal use of shallow creeks by bay whiffs than previously reported. We have provided evidence of the seasonal use by flatfishes of a very shallow tidal creek that is not navigable at low tide. We determined that although flatfishes utilized this shallow area, only bay whiffs used it as a potential settlement site albeit inconsistently.
G.D. would like to thank Dionne Hoskins and Matthew Gilligan for their guidance, knowledge, and assistance while pursuing his Master's degree. We would also like to thank Heidi Schaffner, Robert Froncko, Karen Harris, Carl Poe, Shanell Moss, Ajmal Gordon, April Pressler, Joseph Butts, and Tracey Modeste for their time and assistance in the field. We would like to thank Emily Sekula for the analysis of the sediment samples and Elizabeth King Rhodes for her assistance with the sediment samples and the creek depth analysis. Finally, we thank the NOAA Living Marine Resources Cooperative Science Center for funding this project. This is Contribution Number 1466 of the Belle W. Baruch Institute for Marine and Coastal Sciences at the University of South Carolina.
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Guy' Donald DuBeck and Mary Carla Curran*
Marine Science Program
Department of Natural Sciences and Mathematics
P.O. Box 20600
Savannah State University
Savannah, GA 31404
Mary Carla Curran
Table I. Mean CPUE (number per tow) [+ or -] 1 SD of the three most common flatfishes collected in Wylly Creek from September 2004-February 2006 (Temp = temperature; Sal = salinity). The size range and average size in mm TL of each species is included in parenthesis. Temp Sal Month/Year ([degrees]C) (psu) Bay whiff Sept 2004 23 25 1.7 [+ or -] 0.6 (52-76; 60.6) Oct 2004 24 15 0 Nov 2004 18 24 1.0 [+ or -] 1.0 (65-93; 81.3) Dec 2004 14 23 0 Jan 2005 11 20 34.7 [+ or -] 14.0 (5-25; 14.8) Feb 2005 12 21 8.0 [+ or -] 4.3 (12-40; 19.0) Mar 2005 12 14 11.3 [+ or -] 3.5 (25-89; 38.9) Apr 2005 24 14 0 May 2005 18 18 0 June 2005 30 13 0 July 2005 32 14 0 Aug 2005 30 13 0 Sept 2005 27 19 0 Oct 2005 16 18 1.0 [+ or -] 1.0 (97-108; 102.7) Nov 2005 17 20 0.3 [+ or -] 0.6 (106.0) Dec 2005 11 19 0 Jan 2006 13 20 2.3 [+ or -] 2.1 (17-24; 21.6) Feb 2006 12 20 2.7 [+ or -] 3.8 (19-46; 27.0) Mean CPUE 3.5 [+ or -] 1.7 [+ or -] SD Month/Year Blackcheek tonguefish Ocellated flounder Sept 2004 1.3 [+ or -] 1.5 (30-66; 44.0) 0 Oct 2004 1.0 [+ or -] 1.0 (20-76; 48.7) 0 Nov 2004 1.0 [+ or -] 0 (34-62; 51.3) 0 Dec 2004 1.7 [+ or -] 1.5 (40-64; 51.6) 0 Jan 2005 0 0 Feb 2005 1.3 [+ or -] 0.6 (14-34; 23.5) 0 Mar 2005 1.3 [+ or -] 1.2 (43-65; 49.3) 0.7 [+ or -] 1.2 (75-80; 77.5) Apr 2005 8.3 [+ or -] 2.5 (22-83; 53.8) 6.7 [+ or -] 4.2 (40-94; 55.9) May 2005 1.7 [+ or -] 1.5 (68-89; 77.6) 2.3 [+ or -] 0.6 (48-68; 60.7) June 2005 1.7 [+ or -] 1.5 (65-87; 76.6) 0.7 [+ or -] 1.2 (81-92; 86.5) July 2005 0 0 Aug 2005 2.3 [+ or -] 2.1 (29-65; 45.3) 0 Sept 2005 1.0 [+ or -] 1.0 (38-68; 55.3) 0 Oct 2005 0.7 [+ or -] 1.2 (52-64; 58.0) 0 Nov 2005 0.7 [+ or -] 0.6 (38-56; 47.0) 0 Dec 2005 1.0 [+ or -] 1.0 (29-35; 32.7) 0.3 [+ or -] 0.6 (55.0) Jan 2006 0.7 [+ or -] 0.6 (29-37; 33.0) 0 Feb 2006 2.3 [+ or -] 2.1 (35-70; 46.7) 0 Mean CPUE 1.6 [+ or -] 1.1 0.6 [+ or -] 0.4 [+ or -] SD
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|Title Annotation:||Wylly Creek|
|Author:||DuBeck, Donald; Curran, Mary Carla|
|Publication:||Georgia Journal of Science|
|Date:||Dec 22, 2007|
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