Gizzard shad population dynamics in eutrophic Missouri reservoirs with emphasis on environmental influences on their growth

Abstract

Gizzard shad (Dorosoma cepedianum) is an ecologically important species in many waters of the USA. Their versatile feeding and nutrient-recycling capabilities and importance as prey, combined with their commonly high biomasses, result in widespread ecosystem effects. Despite their importance, few studies have focused on variables that influence their population dynamics, especially in mid-latitude waters. The objectives of this study were to: (1) describe population dynamics including size and age structure, growth, and mortality and (2) determine potential environmental influences on annual variations in growth increments of gizzard shad. Gizzard shad populations were sampled annually in three eutrophic reservoirs in Missouri, over a successive 5-year period. Size and age structure were temporally variable owing to variable year-class strength and differing growth rates. Strong year-classes arose from age-0 cohorts that experienced relatively high overwinter survival. Annual variations in growth increments across all cohorts and reservoirs appeared to be affected by the initial length of fish at the beginning of the growing season and by food availability. Annual mortality estimates ranged from 0.38 to 0.70 for ages 3-7 among the reservoirs. Due to their dynamic nature, effects of these populations on their ecosystems also must be temporally variable and complex. Future studies should focus on the importance of these temporal effects on ecosystem dynamics.

Increased light availability and nutrient cycling by fish provide resilience against reversing eutrophication in an agriculturally impacted reservoir

“…Our results suggest gizzard shad provide a source of resilience to impacted lakes. Given that gizzard shad are widely distributed across the eastern half of the U.S.A., and is perhaps the most abundant fish (by biomass) in productive lakes across the Midwest and Southeastern U.S. (Vanni et al ; Michaletz ), their effects are broadly important, and the consideration of the role of gizzard shad nutrient excretion in lake management efforts should not be ignored (Persson et al ; Vanni et al ; Domine et al ; Godwin et al ). Indeed, efforts in Florida have shown that deliberate reduction in gizzard shad biomass can reduce nutrient loading to the water column through a reduction in excretion as well as the P pool contained in shad biomass (Schaus et al ); however, it is likely that biomanipulation efforts need to be large in scale to be effective because of the high reproductive capacity of gizzard shad (Jeppesen and Sammalkorpi ; Catalano et al ).…”

Responses of gizzard shad and white crappie to introductions of palmetto bass

“…Long Branch Lake is a shallow (mean depth D 4.4 m), frequently turbid, eutrophic reservoir that contains a stunted white crappie population, a largemouth bass population with sparse recruitment, and a gizzard shad population with fast-growing age-0 individuals (Michaletz 2012(Michaletz , 2013Michaletz et al 2012; Missouri Department of Conservation, unpublished data). Most crappies (both black crappie and white crappie) never reach sizes desirable to anglers.…”

“…Gizzard shad were collected during six electrofishing transects, usually 20 minutes in duration (Michaletz 2012). All gizzard shad were counted and measured for each transect and sagittal otoliths were removed for aging from a subsample (10 fish per 10-mm TL class) of these fish.…”

“…Ages of palmetto bass were estimated from scales and whole otoliths and ages of gizzard shad and white crappie were estimated from sectioned otoliths (Michaletz 2012(Michaletz , 2013. Annuli were counted on projected images of these bony structures.…”

“…Water temperature and dissolved oxygen profiles, total phosphorus, and total nitrogen were measured at one fixed site per area on the same dates. Water quality and zooplankton biomass data were collected, processed, and analyzed following procedures in Michaletz (2012Michaletz ( , 2014. Estimates of all variables were averaged across all sites and sampling dates for each year to provide a single annual estimate.…”