Seasonal Abundance, Age Structure, and Spawning Period of Gizzard Shad in the Hydrologically Altered Upper Barataria Estuary, Louisiana

Gizzard shad (Dorosoma cepedianum) are an important component of many water bodies in the southeastern United States that contributes energy and nutrients to piscivores and impacts nutrient cycles. Spawning movements from the lower Barataria Estuary, Louisiana, into the upper reaches of the Barataria Estuary results in seasonal congregations of gizzard shad in the upper estuary. Historically, these spawning movements may have been initiated by the predictable annual Mississippi River floodpulse; however, the Barataria Estuary is currently cut off from the Mississippi River and no longer receives a predictable annual floodpulse. Gizzard shad were sampled biweekly from 22 November 2005 to 6 September 2006 using monofilament gill nets to assess gizzard shad spawning behavior, the timing of spawning, and growth rates in the altered floodplain of the upper Barataria Estuary. Catch per unit effort (CPUE; number per hour) and female gonadosomatic index peaked in March and subsequently declined. Sampling indicated that gizzard shad migrated from the lower Barataria Estuary to the upper Barataria Estuary to spawn, and spawned from April to June. The first post-spawn (spent) female was collected on 7 April 2006 and the last pre-spawn (non-spent) female was collected on 7 July 2006. Gizzard shad relative abundance was not related to changes in water level or dissolved oxygen, but appears to be related to spawning activity. Maximum age was four years for males and five years for females. Male and female gizzard shad were similar in size for ages 1 and 2, but females were larger than males for ages 3 and 4. Size at age for gizzard shad from the upper Barataria Estuary was larger than size at age reported in other studies. The lack of a predictable floodpulse in the upper Barataria Estuary did not negate gizzard shad spawning behavior and did not negatively affect growth.

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