Annual Production of Corydalus cornutus (Megaloptera) in the Guadalupe River, Texas

Annual production of the predaceous stream insect Corydalus cornutus (Megaloptera) was determined at five sites along the longitudinal profile of the Guadalupe River in S-central Texas using the size-frequency method. Production values progressively decreased from 22.9 g ash-free dry mass/m2/year to 4.6 g ash-free dry mass/m2/year from the upstream to the downstream site. These values are among the highest reported for a single species of aquatic insect. INTRODUCTION During the past 10 years there has been a resurgence of interest in the determination of secondary production in stream ecosystems, largely due to improved methods for estimating secondary production (e.g, Waters and Crawford, 1973; Benke, 1984). These methods have promoted studies on the trophic dynamics of individual species and their role in the ecosystem (Benke and Wallace, 1980; Benke et al., 1984). The production of carnivorous species is of particular interest since its magnitude is often a reflection of prey populations and the factors which influence prey populations. This study deals with the production of the predator Corydalus cornutus L. (Megaloptera: Corydalidae) at five sites along the longitudinal profile of a Texas river. Corydalus cornutus is widely distributed throughout central and eastern North America (Merritt and Cummins, 1978) where it normally inhabits riffle regions of welloxygenated streams. The larvae, which are often referred to as hellgrammites, have been reported as being predators feeding on a variety of aquatic insects (Stewart et al., 1973). Pupae are terrestrial and the aerial adults deposit egg masses on objects which overhang the stream. After hatching, the larvae drop down into the water to begin the aquatic phase of their existence. STUDY AREA The Guadalupe River arises in the Edwards Plateau region of S-central Texas and flows in a southeasterly direction for 712 km before entering the Gulf of Mexico at San Antonio Bay. Throughout its length the river is highly buffered and exhibits high conductivity (Table 1). The water temperatures normally range from near 10 C during winter to over 30 C at times during the summer (Table 1). Five sites located along the longitudinal profile of the river from near the headwaters to within 112 km of the coast, all riffle areas with rubble and gravel substrata, were sampled. The major difference TABLE 1.-Physical and physicochemical characteristics of the Guadalupe River at each sampling site. Physicochemical characteristics are shown as annual means and ranges