Regional jet Captain reports low altitude alert from CHS tower during visual approach to Runway 15 during day VMC with the First Officer flying.

Narrative

We were on a heading to intercept the LOC for Runway 15 at CHS. We were given clearance to descend from 11;000 FT to 1;600 FT. My First Officer did not hear the descent clearance and asked me what altitude we were cleared to descend to. I told him 1;600 FT. He still acted doubtful; so exercising good CRM; I asked Approach to confirm our descent altitude. They told us again 1;600 FT. By tht time we had covered some distance and needed to get the descent going. I told the First Officer that he needed to get the descent going and he selected Vertical Speed and at one point had as much as 3;000 FPM rate. As we got lower; I told him he needed to slow the descent rate. He acknowledged; but did nothing. I told him a few seconds later again that he needed to slow the descent rate. He responded 'I got it' and had his finger on the Vertical Speed dial; so I thought he was going to slow it. I was about to take the controls when he finally began to arrest it. About the time we were leveling off; the Tower called us with a low altitude advisory. I had flown with this First Officer for several weeks and he is a competent pilot. For some reason; unknown to me; he fixated on something and did not do a good job of leveling off to continue a stable visual approach to the runway outside of the OM. When I brought this to his attention twice; he indicated that he was correcting when in fact he was either not correcting or was doing it too slowly. He did finally get the aircraft slowed and configured for a stabilized approach the rest of the way in.