Enclosed Channel-Section Studies 
After the first spraying and prior to the second one an additional 
means of determining blue crab mortality was devised. It involved the use 
of sets of screen barriers placed across certain creek or ditch channels. 
Each barrier consisted of a board fence with removable screens of }-inch 
hardware cloth in the central section (see Fig. 2). Measured Leneths of 
creek or ditch channels were thus enclosed by barriers in each of the four 
study areas. 
Since the screen-covered portion of the barrier was constructed 
like a sluice gate, it could be opened at low water, closed during the 
following high tide, and re-opened during low tide after dead crabs had 
been counted and removed. This arrangement, enabling "outside" crabs to 
come into the enclosure during high tide and deterring their escape with 
ebb tide, was employed in the units that received heavy dosages. In the 
other two units (0.4 and 0.2 areas), screen barriers at both ends of the 
channels were shut the day before spraying and were not re-opened until 
the end of the experiment. Observations on mortalities within the channel 
enclosures in the study areas are summarized below. 
The channel enclcsed in the 1.6 area was part of a large ditch 
about feet deep and varying from 2 to l, feet in width. Since the ditch 
was a dead-end one, only a single barrier was required. This barrier shut 
off about 500 linear feet of the ditch. After the spraying, no live crabs 
were seen during three visits to the enclosure. Dead crabs were found as 
follows: 
Days After Spraying: 1 5 10 
Dead Crabs Found and Removed: 
In the 0.3 area about 800 linear feet of ditches approximately 
3 feet wide and averaging 3 feet in depth were enclosed by three screen 
barriers, one across each of two upper branches and another across the main, 
lower part of the ditch. Only the lower screen barrier was opened and 
closed with the tides. Again no live crabs were found in three visits 
following the spraying. Dead crabs found were as follows: 
Days Following Spraying: 2 5 10 
Dead Crabs Found and Removed: 96 20 16 
Another enclosure in this study area included approximately ], 300 
linear feet of winding creek and tributary ditches. The creek was about 
14 feet wide and 4 feet deep. Following the spraying, only one live crab 
was seen-~10 days after the treatment. Dead crabs were noted as follows: 
Days Following Spraying: 2 5 10 
Dead Crabs Found and Removed: 269 122 3 
The enclosure in the 0.4 area embraced 290 linear feet of a creek 
averaging about , feet wide and 3 feet deep. Observations were made daily 
or every other day until there was no further evidence of mortality-~18 days 
15 
