Interactions between Larval White Crappie and Gizzard Shad: Quantifying Mechanisms in Small Ponds

—To test potential competitive interactions between larvae of white crappie Pomoxis annularis and of gizzard shad Dorosoma cepedianum. we added adult gizzard shad to eight of twelve 0.1-ha ponds that had been stocked with adult white crappies. Larval white crappies and larval gizzard shad appeared within 1 week of one another and larval white crappie density did not differ between treatments. Zooplankton density declined similarly between treatments, and zooplankton species composition and size did not differ. Although larval white crappie and larval gizzard shad had moderate diet overlap, prey selection by white crappie did not differ between treatments. During April, more than 70% of larval gizzard shad had empty stomachs, compared with less than 20% of larval white crappies. Age-0 gizzard shad recovery was extremely low (< 10 kg/ha) in five of eight ponds. Daily ring counts from otoliths indicated that late-spawned gizzard shad had higher growth rates than early-spawned gizzard shad, whereas no such relationship existed for white crappie. At a given size, age-0 white crappies weighed more in ponds with gizzard shad than in ponds without them. Contrary to our predictions, gizzard shad did not negatively affect age-0 white crappies. The white crappie Pomoxis annularis is a valuable sport fish. In the USA, 11.7 million anglers spend an estimated 229.8 million days annually targeting crappies Pomoxis spp. (Hooe 1991). Numerous management techniques are used to enhance white crappie populations, including the imposition of size and bag limits (Colvin 1991), management of water levels (Mitzner 1981; Beam 1983), and control of excessive aquatic macrophytes (Maceina et al. 1991). In small waters, most crappie management effort has been directed at reducing white crappie density, increasing prey density, or both (Gabelhouse 1984). Predation by a dense population of small (200-300 mm total length, TL) largemouth bass Micropterus salmoides can reduce crappie density in 0.7-11.1-ha impoundments (GaDelhouse 1984) and similar results have been obtained in larger impoundments up to 27 ha in Texas (Cichra et al. 1984) and up to 106 ha in Oklahoma (Boxrucker 1987). Prey enhancement, including manipulation of populations of shad Dorosoma spp., although sound from a predator-prey perspective, has not always produced the desired results (DeVries and Stein 1990). Inconsistent results may be due to competition between age-0 crappies and their prey. 1 Present address: Department of Wildlife and Fisheries Sciences, South Dakota State University. Brookings, South Dakota 57007, USA. Although large white crappies (larger than 160 or 170 mm TL) are piscivorous, small white crappies are paniculate-feeding zooplanktivores (Siefert 1969; O'Brien et al. 1984). Similarly, age-0 (about 25-30 mm TL or smaller) gizzard shad D. cepedianum are paniculate-feeding zooplanktivores (Kutkuhn 1958; Cramer and Marzolf 1970; Van Den Avyle and Wilson 1980), although larger gizzard shad are filter-feeding omnivores that feed on detritus, phytoplankton, zooplankton, and occasionally insects (Miller 1960; Bodola 1966; Baker and Schmitz 1971; Jester and Jensen 1972; Drenner et al. 1978). Thus, young gizzard shad and young white crappies may compete for food. Both white crappie and gizzard shad spawn in the spring, the former when water temperatures are 14-23°C(Tin 1982a), and the latter at 10-24°C (Tin 1982b). Thus, the spawning times of white crappie and gizzard shad, relative to one another, may influence the outcome of interactions between their age-0 progeny. For example, if the water warms slowly, gizzard shad will spawn first and their larvae may gain an advantage over laterspawned white crappie larvae. In 0.23-0.35-ha ponds, total number and biomass of age-0 white crappie decreased when either gizzard shad or threadfin shad D. petenense were present (Guest et al. 1990). Mechanisms suggested as explanations for this decline were (1) predation by shad on white crappie larvae and eggs, and (2) competition for food between white crappie and shad (Guest et al. 1990). Detection of predation