In situ determination of an acoustic target strength to total length relationship for age-0 gizzard shad

Acoustic estimates of biomass and density require an understanding of the relationship between acoustic target strength (TS) and size of fish targets.  However, no such relationship is available for gizzard shad Dorosoma cepedianum, which can dominate reservoirs in terms of both biomass and density.  To explore this relationship, we generated paired estimates of dorsal aspect target strength and total length (TL) of age-0 gizzard shad at five sites in Dillon Reservoir, Ohio, biweekly during May through August 2007.  Mean TS from acoustic surveys was weakest on 10 May and increased progressively through August.  Similarly, mean TL of age-0 gizzard shad was smallest on 10 May and also increased during this period, resulting in a relationship between mean TS and TL that explained 83% of observed variation in TS.  Estimates of gizzard shad target strength from this equation were greater than predicted by general equations of Love (1971) and Foote (1987) and an equation for the related alewife Alosa pseudoharengus (Warner et al. 2002).  Underestimation of gizzard shad TS by these equations would result in overestimation of gizzard shad density.  Thus, use of the equation developed herein would improve accuracy of density estimates calculated from acoustic surveys in reservoirs dominated by age-0 gizzard shad.