Differing demographic structures had a direct effect on the total availa- 
ble biomass at the two beaches. Absence of larger size classes at the nour- 
ished beach therefore resulted in a drastic reduction in available biomass. 
In any event, the adult population of overwintering FH. talpotda never returned 
to the beach at Fort Macon following nourishment. They could have been (1) 
forced to move out of the nourished area due to the nourishment activities, 
(2) killed in their offshore overwintering grounds, or (3) killed on the beach 
as they attempted to recolonize in early spring while nourishment was underway. 
Hayden and Dolan (1974) suggested that #. talpotda responds to beach nour- 
ishment by relocating in adjacent but unimpacted areas until nourishment is 
over and then returns to the nourished area. This seems an unlikely expla- 
nation for the failure of #. talpotda to repopulate the Fort Macon beach 
because no increased densities were observed in areas immediately adjacent to 
the nourishment area (Fig. 18); few adult #. talpotda ever returned to the 
beach. It is also possible that adult #. talpotda were killed on the beach 
during the nourishment process. The diffuser head shown in Figure 6 gives an 
idea of the volume and speed of the material being laid down. However, at any 
given time only a small fraction of the total beach front was being nourished, 
and it seems likely that at least some of the adult EF. talpotda would have 
survived. 
The most likely explanation for failure of adult crabs to return to the 
Fort Macon beach is that they were killed in their nearshore-offshore over- 
wintering grounds. The probable cause of this mortality was increased turbid- 
ity. The mole crab is a fairly unselective filter feeder (Efford, 1966), and 
its stomach could have easily been clogged with undigestable inorganic matter 
from the turbid waters. Mortality of offshore macrofauna by such a mechanism 
has been attributed to turbidity and siltation by Courtenay, et al. (1974). 
b. Donax parvula and Donax vartabilis. Donax parvula were slightly more 
abundant than D. vartabtlts at Fort Macon, while the reverse was true at 
Emerald Isle. These differences, although not statistically significant, do 
suggest the possibility that D. parvula may prefer the slightly less rigorous 
environment provided by the partially protected beach at Fort Macon (C. Lytle, 
Department of Zoology, North Carolina State University, Raleigh, North 
Carolina, personal communication, 1978) or beaches near inlets (H. Porter, 
personal communication, 1977). 
During the study period D. parvula and D. variabilis reached their highest 
densities at Fort Macon in late June and early August 1977 (Figs. 22 and 23); 
however, Leber (1977) reported that population peaks for Donax spp. occurred 
much earlier in the year. The data from the Emerald Isle beach for the spring 
of 1978 (Figs. 22 and 23) support Leber's observations. Therefore, it is 
concluded that the peak populations of Donax spp. in 1977 for Fort Macon were 
probably missed by starting the sampling in June. 
After the peaks were reached, populations of Donax spp. decreased somewhat 
as summer progressed and began to decline steadily during fall. Adult Donax 
spp. were rarely encountered on the beaches between December and March. 
Recruitment began in March at Emerald Isle and, in contrast to the pattern 
exhibited by mole crabs, the first recruits to the beach were young of the 
year from pelagic larval stocks (Figs. 24 and 25). 
4| 
