tonic stag-es have been published for the pink 

 shrimp by Dobkin (1961) ; for the seabob, 

 Xiphopeneiis kroyeri (Heller) by Renfro and 

 Cook (1963) ; and for the rock shrimp, Sicyonia 

 brevii'ostris Stlmpson by Cook and Murphy 

 (1965). All three species occur in the northern 

 Gulf. 



Because of the limited amount of taxonomic 

 material available, as well as the occurrence of 

 about 35 penaeids (Burkenrcad, 1936; Springer 

 and Bullis, 1956; Bullis and Thompson, 1965) 

 in the Gulf of Mexico, one of the major prob- 

 lems in this study was the identification of the 

 planktonic stages. Although H. L. Cook (per- 

 sonal communication) has been able to rear 

 the planktonic stages of brown, white, and pink 

 shrimp, differentiation of the species among 

 these forms is not yet possible. Consequently, 

 penaeids encountered in plankton samples were 

 identified only to genus by using the generic 

 key developed by Cook (1966a). Data on only 

 Penaeus spp. are presented in this report. 



METHODS AND MATERIALS 



STUDY AREA 



During 1961, sampling was conducted at 11 

 stations (fig. 1) over an area of about 20,725 

 km.- (8,000 square miles). During cruises at 

 3-week intervals, plankton hauls were made at 

 stations where water depth was about 14, 27, 

 46, and 82 m. 



SAMPLING GEAR AND CALIBRATION 



Plankton samples were obtained with the 

 Gulf-V plankton net described by Arnold 

 (1959) . This gear consists of a metal frame, to 

 which a conical monel net with a mesh size of 

 31.5 strands per centimeter is attached. The 

 diameter of the net mouth is about 40.5 cm. 

 Plankton was collected in a cup attached to the 

 end of the net. After each tow the net was 

 thoroughly washed down and the plankton re- 

 moved and preserved in 5 percent Formalin. - 



Estimates of water volume filtered during 

 each tow were calculated from a flowmeter po- 



sitioned in the center of the net mouth. Both 

 TSK and Atlas flowmeters, calibrated by the 

 technique outlined by Ahlstrom (1948), were 

 used. Each tow lasted 20 minutes, and towing 

 speeds averaged 4.6 km. per hour (2.5 knots). 

 Flowmeter readings indicated that during each 

 tow the net filtered about 100 m.' of water. 

 Catches are reported in numbers of organisms 

 per 100 m.' of water strained. 



DEPTHS FISHED AND TOWING CABLE PROFILE 



Each of four depths was fished for 5 minutes 

 during each tow: 3 m. above the bottom, two 

 intermediate depths, and 3 m. below the sur- 

 face. The two intermediate depths fished were 

 equally spaced vertically within the water col- 

 umn and depended on the total water depth. 

 Sampling depths were determined by the trig- 

 onometric function of the wire angle and length 

 of towing cable. Realizing that this technique 

 assumed the profile of the towing cable to be 

 a straight line, we attached a bathykymograph ■ 

 (Model T-la, Marine Advisors, Inc.) to a Gulf-V 

 net and made tests to determine the reliability 

 of this technique. Results are given in table 1. 



A plot of mean actual depths vs, calculated 

 depths provides an estimate of the error be- 

 tween the actual and assumed towing cable 

 profiles (fig. 2). Agreement was close to a 



100 



ASSUMED WIRE PROFILE \ 



- Trade names referred to in this publication do not imply 

 endorsement of commercial products. 



"Calibration accurac;- ^ 1 percent full scale. Sensitivitj- 0,.', per- 

 cent of full scale 



20 40 60 80 100 



CALCULATED DEPTH (METERS) 



Figure 2. — The calculated and actual profiles of the 

 towing cable attached to a Gulf-V plankton net. 



U.S. FISH AND WILDLIFE SERVICE 



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