Recent numerical studies have shown that mean tidal fields in clusters
of galaxies can cause a significant increase in the planar velocity d
ispersions of stars and gas clouds in the disks of spiral galaxies. Si
nce the perturbations caused by cluster tidal fields are generally wea
k and fairly symmetric, the stellar distributions of the perturbed gal
axies would not necessarily show obvious signs of tidal disturbance. I
n this paper it is shown that the increased noncircular velocities of
stars can result in a symmetric decline (at large radii) of the rotati
on curves of tidally perturbed spiral galaxies in clusters. This decli
ne results from the well-known phenomenon of the ''asymmetric drift''
of a high velocity dispersion stellar population. This suggests that t
he observations of the declining rotation curves of spiral galaxies in
clusters do not necessarily imply that their dark matter halos have b
een tidally truncated as is generally assumed.