We hypothesized that shallow-bodied crucian carp should have a competitive advantage over deep-bodied fish as they avoid costs associated with the induced defense.
Crucian carp is common in ponds in this area, and the populations either consist of few deep-bodied fish coexisting with piscivores or, in the absence of predators, a large number of small, shallow-bodied fish (Bronmark and Miner 1992, Bronmark et al.
The experiment started on 18 June 1993 when treatments were randomly assigned to the enclosures, giving five replicates to each of the following treatment combinations: deep-bodied fish in low density, shallow-bodied fish in low density, deep-bodied fish in high density, and shallow-bodied fish in high population density.
As we used deep-bodied and shallow-bodied focal fish of similar body mass, they differed in total length (due to their morphology).
Deep-bodied and shallow-bodied crucian carp increased on average 25.9 [+ or -] 2.4 and 24.6 [+ or -] 0.6 mm, respectively (means [+ or -] 1 SE), at low density, and 13.7 [+ or -] 4.0 and 16.9 [+ or -] 1.4 mm at high density.
In the low-density treatment, deep-bodied and shallow-bodied crucian carp practically doubled their body masses [ILLUSTRATION FOR FIGURE 2 OMITTED], gaining 25.8 [+ or -] 2.7 and 28.0 [+ or -] 0.9 g, respectively (means [+ or -] 1 SE).
The shallow-bodied and deep-bodied crucian carp used in this experiment came from different populations but were affected in a similar way by the experiment, and differences in growth rates during the experiment were not due to interpopulation differences in responsiveness to changing resource levels (cf.
The liver index was rather variable [ILLUSTRATION FOR FIGURE 4 OMITTED] and did not differ between deep-bodied and shallow-bodied crucian carp (morphology: [F.sub.1,12] = 0.004, P = 0.951).
shallow-bodied focal fish had no effect on cladoceran biomass (MANOVA, [F.sub.1,12] = 0.388, P = 0.545).
Throughout the experiment, shallow-bodied crucian carp remained more fusiform than the deep-bodied cruclan carp (MANOVA, [F.sub.1,12] = 243.540, P [less than] 0.001).