Food Habits and Dietary Overlap of Newly Settled Red Drum (Sciaenops ocellatus) and Atlantic Croaker (Micropogonias undulatus) from Texas Seagrass Meadows

Abstract

Food habits and dietary overlap of newly settled larval and juvenile red drum and Atlantic croaker were examined during the period when the two species co-occur in seagrass nurseries. A total of 274 red drum (4.00 19.99 mm SL) and 205 Atlantic croaker (8.00 17.99 mm SL) were used for this analysis. Of the red drum stomachs examined, 8.4% were empty while 28.8% of Atlantic croaker stomachs contained no food. Major prey items identified for both species were calanoid copepods, harpacticoid copepods and mysid shrimp across all size classes. Ontogenetic trophic niche shifts were detected for red drum and Atlantic croaker. Type and quantity of food ingested by red drum were similar across all stations (Aransas Bay Station: lH, 2T and 3H) examined. Atlantic croaker ingested the same types of prey at all stations, but contained varying quantities of food throughout the study area. In general, high dietary overlap was observed between red drum and Atlantic croaker with most overlap values (Schoener’s index) exceeding 70%. INTRODUCTION Red drum (Sciuenops ocellutus) spend most of their adult lives offshore and migrate to tidal passes to spawn in late August through mid-November, whereas adult Atlantic croaker (Micropogonius undufutus) occupy gulf coastal waters and congregate offshore to spawn in early October through February (Johnson 1978). Pelagic larvae of both species are transported by currents through tidal inlets and into nursery habitats in bays and estuaries (Rooker et al. 1998). Consequently, larval and juvenile red drum (4 20 mm SL) occupy seagrass beds from late September to early December, while larval and juvenile Atlantic croaker (8 18 mm SL) are found in seagrass beds from early October to February (Holt et al. 1983, Rooker et al. 1998). Both species concurrently occupy seagrass beds in November at similar sizes. Conspecifics and morphologically similar species (i.e., confamilials) occupying similar habitats can potentially compete for food particularly during times when fish densities are high and prey is scarce. Intraspecific and interspecific competition among larval fishes can reduce growth rates, which in turn, may increase early-life stage mortality due to starvation or predation (Houde 1987). Therefore, it is important to understand the trophic relationships of early life stages. Fishes change resource (food) use throughout the course of their lives, especially during larval and juvenile stages. Such ontogenetic niche shifts may divide sizestructured populations into ecologically distinct stages basedon diet (Olson 19%). Duration of stages and transition among stages has the potential to minimize intraspecific competition for food. Although several studies have addressedfood habits of these two species separately (Bass and Avault 1975, Chao and Musick 1977, Oversteet and Heard 1978, Steen and Laroche 1983, Govoni et al. 1983, Cumn et al. 1984, Govoni et al. 1986, and Peters and McMichael 1987), no dietary overlap analysis has been conducted on newly settled red drum and Atlantic croaker. The primary aim of this study was to obtain an understanding of the trophic dynamics of newly settled larval and juvenile red drum and Atlantic croaker occurring in seagrass habitat. Specific objectives were to: 1) quantitatively describe the diets of larval and juvenile red drum and Atlantic croaker; 2) determine ontogenetic changes in diets of the two species; 3) determine ifdiet varies across different sites and habitats for red drum and Atlantic croaker; 4) determine interspecific dietary overlap between red drum and Atlantic croaker; and 5) determine if red drum and Atlantic croaker feed on equal quantities of food at similar sizes during the cooccurring period. MA T E R I A L S AND METHODS Diurnal sampling (0730 1700 h) was conducted weekly from October throughDecember 1994. Fish samples were taken from three stations in Aransas Bay (lH, 2T and 3H) and two stations in Redfish Bay (4H and 5T)(Figure 1). Stations lH, 3H and 4H were in shoal grass (Hulodule wrightii) while stations 2T and 5T were in turtle grass (Thulussiu testudinum) (Figure 1). A 1 m (diameter), 505 pm mesh cone net attached to a 0.75 m (length) x 0.56 m (height)epibenthic sledwas hand-towed for 20 m across the grassbed sites. Three samples f” each site were obtained picked free of grass, and preserved in 5% formalin. Standard