8 

 30' 



3°W, 



25' 



N. 



-f \o^ 



30' 







'v.-^ao- 



BOUNDARY OF TRAWLING GROUNDS 



Figure 1. — Map of the Tortugas Shelf area showing the sampling grid, regular sampling stations, and 



place names mentioned in the text. 



were slightly displaced from their theoretical grid 

 coordinates because of shoal waters.) 



SAMPLING 



Throughout the study, we sampled about twice 

 each month. A shrimp trawler 20 m. long overall, 

 having an idUng speed of 5.6 km. per hour 

 (3 knots), towed the samphng apparatus. Towing 

 speed varied according to wind velocity and at 

 times was as Iom- as 2.8 km. per hour (1.5 knots). 



Gulf V plankton nets were used throughout the 

 study. This net has been described by Arnold 

 (1959) and consists of a metal frame in which is 

 mounted a monel mesli net fitted with a brass 

 bucket in which the plankters accumulate. The 

 mouth diameter of the Gulf V net is 40 cm., and 

 overall length is 100 cm. The mesh has an aperture 

 size of 0.292 mm. Dobkin (1961) stated that first 

 protozoeae have a maximum width of 0.35 to 

 0.44 mm., and all subsequent larval stages are of 

 greater size. Nauplial stages are considerably 

 smaller (maximum width of the fifth nauplius stage 

 is 0.17 to 0.22 mm.) and were not retained by the 

 mesh. 



Flowmeters were mounted in the mouths of the 

 nets. These meters were cahbrated regularly and 



166 



performed consistently over the entire period of 

 operation. 



Samphng procedures were rigidly controlled, 

 and, in the event of gear failure or malfunction of 

 timing equipment, the sample was discarded, and 

 the tow was repeated. Rephcate samples were 

 taken at each station. 



Step-oblique tows were made with the net 

 towed for 3 minutes at each of 10 levels equally 

 spaced between the bottom and surface. When the 

 net was retrieved, the flowmeter reading was 

 noted, and the bucket of plankton emptied. The 

 net was then hosed down thoroughly, and the 

 additional washings were added to the sample. 

 Washing continued until no more plankton or 

 debris appeared in the bucket. Samples were pre- 

 served with a 3-percent solution of buffered 

 Formahn.^ During the cruise of October 2-6, 1964, 

 when 41 stations were occupied, each level was 

 sampled for 1.5 minutes and tows were reduced 

 to 15 minutes. 



Water temperatures were recorded at each sta- 

 tion. Surface temperatures were measured with a 

 bucket thermometer, and a trace of the vertical 



' Trade names referred to in this publication do not imply endorsement 

 of commercial products. 



U.S. FISH AND WILDLIFE SERVICE 



