2007 SOUTHEASTERN NATURALIST 6(4):743–751 Feeding Habits and Mouth Morphology of Young Silver Perch (Bairdiella chrysoura) from the North-Central Gulf of Mexico Gretchen L. Waggy1,2,*, Mark S. Peterson1, and Bruce H. Comyns1 Abstract - We examined predator-prey relationships of young Bairdiella chrysoura (Silver Perch) collected in Mississippi Sound by comparing the diet to fish standard length (2.5–30.0 mm SL) and mouth width (MW). Silver Perch displayed a diel feeding pattern, with the most active feeding occurring from midnight until noon. As Silver Perch SL increased, prey number, frequency, and volume plus prey width increased. Calanoid copepods and mysid shrimp were the dominant prey, with mysids becoming prominent as Silver Perch SL increased. Cluster analysis supported this pattern as Silver Perch ≤5 mm SL consumed a homogenous material and a few copepods, fish 5–10 mm SL preyed upon calanoid copepods, and then fish in larger size classes shifted their diet to mysid shrimp as MW increased and fish became more robust. Silver Perch SL was linearly related to MW (MW = 0.097 [SL] + 0.245; r2 = 0.891). Introduction Bairdiella chrysoura Lacepède, (Silver Perch) is a numerically abundant estuarine resident fish that is ecologically important because it is prey for a great number of economically important predators (Danker 1979, Darnell 1958, Hildebrand and Cable 1930) and also forages on a wide array of species within the estuarine food web (Brooks 1985, Carr and Adams 1973, Chao and Musick 1977, Darnell 1958). They are predators that shift habitat, grow, and mature throughout their life history (Geary et al. 2001, Mok and Gilmore 1983, Peterson and Ross 1991, Rooker et al. 1998), and appear to consume larger, more energetically beneficial prey as they increase in size. However, feeding data are available mainly for fish >30 mm SL, and typically in the studies, diets are presented as coarsely pooled categories across multiple size classes. Finally, studies present no relationships between diet, mouth size, and prey size in small size classes where marked changes in these morphometrics occur. Therefore, this study focused on larvae and young juvenile Silver Perch in size-class increments of 5 mm SL in order to elucidate ontogenetic shifts in feeding habits, mouth morphology, and preysize spectra. The objectives of this study were to 1) examine ontogenetic changes in feeding habits of larvae and young juvenile (≤30 mm SL) Silver Perch in spring and summer, and 2) relate prey species and their body-width distribution to Silver Perch size-specific mouth morphology. 1Department of Coastal Sciences, The University of Southern Mississippi, 703 East Beach Drive, Ocean Springs, MS 39564. 2Current address - Grand Bay National Estuarine Research Reserve, 6005 Bayou Herron Road, Moss Point, MS 39562. *Corresponding author - gretchen.waggy@dmr.ms.us. 744 Southeastern Naturalist Vol. 6, No. 4 Methods and Materials Stomach contents were examined from young juvenile Silver Perch captured in July 2002 along the marsh edge in Mississippi Sound, and Silver Perch larvae (≤5 mm body length [BL]) were collected from the plankton in Mississippi Sound in April 2003. Young larvae were collected with a 330- μm plankton net, whereas older larvae and young juveniles were collected with a 1.83-m Beam Plankton Trawl (0.8-mm outer mesh and a 505-μm cod end). Larvae and juvenile fish were preserved in 95% ethanol, and larvae were measured for BL, whereas juveniles were measured for total (TL) and standard (SL) length (mm), and all were weighed (± 0.001 g). We recognize that the diet of the two stages of Silver Perch may incorporate annual variation in prey availability with our interest in body-size diet variation; however, the potential difference in prey availability between years was probably minimal and likely added no more variation to the diet than collecting fish in different locations given the patchy nature of zooplankton in estuarine systems (Steen 1981). Stomachs were removed from fish ≤30 mm SL, and the contents were separated, identified to the lowest taxon possible, and counted, the % frequency was calculated, and the % volume of each prey item measured with a squash plate of known depth (Snyder and Peterson 1999). The squash plate was calibrated with a glycerin-alcohol solution prior to measurements, and the squash area was calculated with a digital camera attached to a dissecting microscope. The image was transported into MetaVue™ 5.0 (Universal Imaging Corporation), and the squash outline was traced until two areas (mm2) came within 0.1 mm2 of each other. The mean was calculated and multiplied by the depth of the squash plate to calculate the volume (mm3) of the squashed prey. Prey body widths (BW; ± 0.01 mm) of the common prey were measured with an ocular micrometer. The mouth width (MW; ± 0.05 mm) of each fish was measured as the distance between the outer maxillary edges directly beneath the eyes (Lawrence 1958), which is a good estimate of the distance between the cleithral bones, the true limiting factor associated with prey consumption (Peterson and VanderKooy 1996). Ontogenetic changes in diet and prey BW were examined by comparing the primary food item(s) to the individual’s SL and MW. The fish examined ranged from 2.5 to 30 mm SL, and were separated into six 5-mm SL size classes: 1.01–5.0 mm, 5.01–10.0 mm, 10.01–15.0 mm, 15.01–20.0 mm, 20.01–25.0 mm, and 25.01–30.0 mm (n = 15 for each size class except the largest which contained 10). Additionally, time of the most active feeding was estimated by collecting ten or more fish ≤30 mm SL every four hours for 24 h from multiple marsh-edge habitat types, and determining stomach fullness, as a ratio of stomach weight (± 0.001 g) to body weight (± 0.001 g), from each time period (Keast and Welsh 1968). 2007 G.L. Waggy, M.S. Peterson, and B.H. Comyns 745 Statistical analysis Cumulative prey curves were constructed for each of the six size classes to determine if the sample size was sufficient to describe the diet. Cluster analysis based on a hierarchical agglomerative method with the group-average linkage procedure was used to compare percent volume of prey among size classes of Silver Perch with the Bray-Curtis similarity coefficient. The cluster analysis based on Bray-Curtis values was computed using PRIMER (PRIMER-E Ltd, Plymouth, UK); these values range from 0–100%, with 0% being no similarity and 100% being identical (Clark and Warwick 2001). Linear regression was used to compare fish size (mm SL) to prey volume (mm3) or mean prey body width (mm) or fish mouth width (mm), and if the assumptions were not met, we log10 transformed the data prior to analysis. SPSS 11.5 (SPSS, Inc., Chicago, IL) was used to conduct all statistical tests, and all results were considered significant if p < 0.05. Results Cumulative prey curves reached an asymptote in each of the six size classes between 4 and 12 individuals, indicating that the sample size we used was sufficient to describe the diet (Waggy 2004). Stomach-fullness ratios of Silver Perch ranging from 9.4 to 29.6 mm SL were highest at 24:00 and lowest at 20:00. The most-active feeding was at night through the morning (24:00–12:00), tapering off into the afternoon and evening (Fig. 1). Fifteen prey taxa were identified in the diet of Silver Perch <30 mm SL (Table 1), with copepods and mysids being the most frequently consumed. A homogenous material (a dense, granular, translucent substance) was present in all stomachs of all fish in the smallest size class (1.1–5.0 mm SL). Furthermore, 46.7% of the larvae approaching 5 mm SL (upper end of size class) had also consumed copepods. The second size class (5.1–10.0 mm SL) ate mostly copepods, with mysid shrimp becoming a chief prey item in the larger size classes. Generally, as Silver Perch SL increased, prey composition changed (Table 1) and both prey volume and BW increased (Fig. 2). Calanoid copepods and mysid shrimp were the dominant prey, with mysids becoming more prominent as SL increased. Calanoid copepods (r2 = 0.318, p < 0.001, N = 71) and mysid shrimp (r2 = 0.563, p < 0.001, N = 47) both significantly increased in volume (both log10 transformed) as fish SL increased (Fig. 2A). In contrast, BW did not increase significantly with SL for calanoid copepods (r2 = 0.145, p > 0.05, N = 21), but did increase significantly with SL for mysid shrimp (r2 = 0.231, p < 0.001, N = 30) (Fig. 2B, both log10 transformed). Silver Perch SL was linearly related to mouth width (MW = 0.097 [SL] + 0.245; r2 = 0.891, p < 0.001, N = 85), which enabled prey of greater BW to be eaten (Fig. 2). The majority of prey with BWs <0.4 mm were copepods, whereas mysids had BWs >0.6 mm (Fig. 3). The mean MW for size classes 746 Southeastern Naturalist Vol. 6, No. 4 Figure 1. Plot of mean stomachfullness ratio (± 1 SE) over a 24-hr period to determine diel feeding pattern of young Silver Perch. The stomach-fullness ratio is stomach weight (g) divided by eviscerated body weight (g). Table 1. Prey frequency of occurrence (%) for each size class of Silver Perch. Size classes (mm) 1.1–5.0 5.1–10.0 10.1–15.0 15.1–20.0 20.1–25.0 25.1–30.0 Prey Category (N = 15) (N = 15) (N = 15) (N = 15) (N = 15) (N = 10) Nematoda 6.7 Crustacea Copepoda Calanoida 46.7 100.0 73.3 100.0 100.0 90.0 Cyclopoida 13.3 20.0 20.0 20.0 Parasitic copepod 10.0 Isopoda Bopyridae 13.3 6.7 6.7 10.0 Amphipoda Gammaridae 6.7 20.0 Caprellidae 6.7 Unidentified amphipod 13.3 Decapoda 6.7 Mysida 20.0 73.3 80.0 93.3 70.0 Unidentified Shrimp 13.3 Crab megalopae 13.3 Grapsidae megalopae 6.7 6.7 6.7 Caridean shrimp zoea 6.7 10 Mollusca Bivalvia 6.7 6.7 Mollusc siphon 6.7 Homogenous material 100.0 Amorphous debris 13.3 26.7 46.7 26.7 40.0 2007 G.L. Waggy, M.S. Peterson, and B.H. Comyns 747 1 (1.1–5.0 mm), 2 (5.0–10.1 mm), and 3 (10.1–15.0 mm) were 0.45, 1.10, and 1.58 mm, respectively (Fig. 3). These MW differences coincided with Figure 2. Plot of volume (A) and mean prey body width (B) of calanoid copepods and mysid shrimp (± 1 S.E.) compared to Silver Perch size class. Data and least squares regression lines are presented in nontransformed format for clarity. 748 Southeastern Naturalist Vol. 6, No. 4 the shift in diet from a homogenous material and a few copepods in size class 1 to solely calanoid copepods to mysid shrimp as a major portion of the diet (Table 1, Fig. 3). A cluster analysis on prey volume also indicated ontogenetic feeding shifts at about 5 mm and 10 mm SL (Fig. 4). It also indicated size classes 3 and 4 were most similar in relation to percent volume of prey (73.74%). Size class 5 ranked next in similarity (65.08%), followed by 6 (62.34%) and 2 (46.89%). Size class 1 (16.23%) was the least similar to the other size classes. Discussion Silver Perch in the Mississippi Sound were most actively feeding at night and into the morning hours and exhibited ontogenetic diet shifts. These occurred in Silver Perch first at ≤5 mm SL, and then around 10 mm SL, with a shift from larval feeding to a stage of juvenile feeding. Figure 3. Prey body width (mm) in relation to predator mouth width (mm) with 1:1 line that has been adjusted to axis scale. Prey items: 􀂄 = Calanoida, = Mysida, 􀂋 = Cyclopoida, 􀂕 = other prey; also displayed is the mean mouth width for size class 1 (0.45 mm), 2 (1.10 mm), and 3 (1.58 mm) Silver Perch. 2007 G.L. Waggy, M.S. Peterson, and B.H. Comyns 749 At this size, mysids became more prominent within the diet, and prey BW increased along with the volume of a prey consumed. The first shift illustrated the rather abrupt change in diet in fish approaching 4–5 mm SL from homogenous material with a few copepods. This shift was followed by another ontogenetic shift coinciding with an increase in the mean MW between size classes 5.1–10.0 and 10.1–15.0 mm SL, and was evidenced by mysids appearing in the diet. Several other studies that used wider size-class intervals reported a dietary shift in Silver Perch from copepods and few mysids to a diet of mysids, caridean shrimp, and penaeid shrimp around 50 mm SL (Brooks 1985, Carr and Adams1973, Darnell 1958). Another dietary shift was reported to occur around 70 mm SL, with fish becoming an important component (Carr and Adams 1973, Chao and Musick 1977, Dietz 1976). Because we only examined individuals up to 30 mm SL, these ontogenetic dietary shifts had not yet occurred. However, the increasing importance of mysids in the diet of Silver Perch >15.0 mm SL precedes these ontogenetic shifts seen in the larger size classes. According to optimal foraging theory, a predator utilizes available prey in the most energy efficient manner possible (Pyke et al. 1977). Silver Perch between 5.1 and 10.0 mm SL primarily preyed upon calanoid copepods and then shifted toward mysid shrimp in larger size classes (i.e., MW increased and fish became more robust). As Silver Perch become larger, more energy is needed for general survival activities (Brooks 1985). Mysids likely become increasingly more important in the diet because an average-sized mysid shrimp provides 26 times more energy than an average-sized calanoid copepod (Thayer et al. 1974). Unless calanoid copepods are extremely abundant, Figure 4. Percent similarity (Bray-Curtis) dendrogram based on a hierarchical agglomerative method with the group-average linkage cluster analysis comparing prey volume (mm3) to Silver Perch size class (mm SL). 750 Southeastern Naturalist Vol. 6, No. 4 the same volume of copepods equal to one mysid would still be less energy efficient due to prey-handling time. In conclusion, we found that young juvenile Silver Perch fed primarily on copepods with an ontogenetic shift toward mysid shrimp in fish larger than 10 mm SL. These small Silver Perch are preyed upon by larger predators and become one of the pathways in the estuarine energy flow. 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