GULF OF MEXICO PLANKTON INVESTIGATIONS: 

 1951-53 



The Gulf of Mexico, despite its 

 economic importance, is one of the world's 

 least known major bodies of water, in regard 

 both to mass water movements and to the 

 abundance, distribution, ranges, and life 

 histories of many species of fishes found 

 within its boundaries. With the exception 

 of several cruises of the Blake and Mabel 

 Taylor in the late 1880's, the relatively 

 small accumulation of scientific knowledge 

 was confined largely to inshore areas. 

 Although supporting a number of fisheries 

 for many years, notably those for shrimp, 

 snappers, oysters, and menhaden, the total 

 fishery production of the Gulf has been 

 considerably less than would normally be 

 expected from an area of approximately 

 700,000 square miles. Recognizing the need 

 for a more comprehensive general knowledge, 

 it was decided to make a general oceano- 

 graphic survey when the Gulf Fishery Inves- 

 tigations was established in the summer of 

 1950 at Galveston, Texas, by the Fish and 

 Wildlife Service. The primary objective 

 was to determine the spawning areas of 

 various fishes. Studying the distribution 

 of their eggs and larvae by means of plank- 

 ton tows offered the best method for attain- 

 ing this objective. 



Data are presented in this report on 

 the offshore plankton collections made in 

 the Gulf from the MA Alaska , research 

 vessel of the Gulf Fishery Investigations. 

 To aid in the comparison of data, the Gulf 

 was divided arbitrarily into the eight sub- 

 areas shown in figure 1 (see page 2). Ten 

 cruises were made from March 1951 to July 

 1953. Originally it was planned to cover 

 the entire Gulf annually in a series of 

 three cruises, repeated each year. Unfor- 

 tunately, the requisitioning of the Alaska 

 for 24 weeks of sea-time disrupted the 

 planned continuity. 



As the distance between stations in 

 the basic pattern (approximately 40 miles 

 north and south, 100 miles east and west) 

 seemed too great for obtaining representa- 

 tive plankton collections, I designed a 

 sampler that could be towed at cruising 

 speed between stations. It consisted essen- 

 tially of a removable "Monel" wire-cloth 



mesh net housed within a "Monel" tube. 

 Details of construction are given in Fish 

 and Wildlife Service Special Scientific 

 Report — Fisheries No. 88. Designated as 

 model G-IA, the sampler was fabricated and 

 ready for use prior to the start of the 

 second coverage of the Gulf (cruise 4-2A) 

 in January 1952, and was operated success- 

 fully until inactivation of the Alaska in 

 June 1953. 



Continuing with the wire-cloth mesh 

 principle, an all-metal net was designed 

 and fabricated for making tows on station. 

 Fundamentally a modified larger version of 

 the G-IA model, this net was designated as 

 model G-III, and is described in detail in 

 the Special Scientific Report already cited. 



It seemed logical to assume that the 

 G-III sampler, towed for the same period of 

 time at 4 to 5 knots, and with the same net 

 opening, would catch substantially more 

 plankton than a half-meter silk net, towed 

 at one-fourth the speed. Moreover, we 

 hoped that the increased speed, plus an 

 opening unobstructed by a towing bridle, 

 would result in the capture of the more 

 agile planktonic forms. 



Collecting operations in January 1952, 

 during cruise 4-2A, afforded the first 

 opportunity to compare the catching abili- 

 ties of the G-III sampler and a conventional 

 half-meter silk net. I used numbers and 

 sizes of fish larvae — as a basis for com- 

 parison. 



Ten tows at 4 to 5 knots were made 

 with the G-III sampler, 23 tows at approxi- 

 mately 1 knot were made with the half-meter 

 net. All tows were of 30-minute duration. 

 Both nets were fished horizontally, either 

 at or within 10 meters of the surface. One 

 G-III tow that yielded 959 larvae was con- 

 sidered atypical and was not used in the 

 comparison. In the 9 remaining tows, the 

 G-III net captured 1,334 fish larvae as 



1/ The term "larvae", used in this paper, 

 includes all immature forms. 



