Abundance of mysid shrimp (Tenagomysis chiltoni) in shallow lakes in the Waikato region and implications for fish diet

Abstract

Tenagomysis chiltoni, a species of mysid shrimp, is widely distributed amongst the riverine lakes of the lower Waikato basin. They appear to thrive in turbid waters, with the greatest abundances found in lakes such as Waahi and Waikare, which have low Secchi transparencies and sparse aquatic macrophyte communities representing remnants of formerly dense beds (Kirk, 1983; Chapman el al., 1991). Maximum mysid abundances of 2,868 and 857 individuals m⁻² in Lake Waahi and Waikare respectively were recorded by Chapman et al. (1991) in March-April 1987. Anecdotal evidence suggests that mysid abundance in Lake Waikare is markedly reduced since the late 1980s (Gary Watson, Te Kauwhata, pers. comm.) with the arrival and proliferation of koi carp (Cyprinus carpio) presumed to be the cause. Koi carp arrived in Lake Waikare after 1987 and by 2007 it was estimated that over 80% of the fish biomass present in Lake Waikare was comprised of koi carp (Hicks, 2007). Sable isotope studies on carp (Matsuzaki et al., 2007) have shown that mysid shrimp can form a significant component of their diet. This suggests that mysid shrimp may be predated on by koi carp in the Waikato which has implications on mysid shrimp abundance as well as the abundance of native fish species which rely on mysid shrimp as a food source (Champman et al., 1991). The objective of this study was to measure mysid abundance in three shallow, turbid lakes in the lower Waikato basin (Lake Waikare, Whangape and Waahi) to compare with previous abundance estimates made in the late 1980s. A second objective was to determine whether mysid shrimp form a significant component of the diet of koi carp in the study sites by examining their stable isotope signatures.