BIOLOGY OF THE COMMERCIAL FISHES OF THE GULF OF MEXICO 



By George A. Rounsefell, Fish and Wildlife Service, United States Department oj the Interior 



The ecology of the fishes of the Gulf of Mexico 

 differs in many respects from that of the Atlantic 

 coast. The offshore waters of the Gulf (as indi- 

 cated by current investigations by the Fish and 

 Wildlife Service) are low in nutrients; the largest 

 fish populations are thus found in the littoral 

 zones where the nutrients necessary to grow the 

 organisms forming the base of the food chain are 

 washed from the land by rains and floods and 

 carried in by rivers (Riley 1937). 



At 1 ,200 fathoms the water masses in the Carib- 

 bean Sea are warmer and less dense than those 

 outside the perimeter of the Caribbean. Those 

 latter cold-water masses are derived from water 

 that has sunk in high latitudes (Parr 1937, 1938). 

 It therefore intermittently runs over the sills be- 

 tween the islands of the Antilles and flows down 

 into the Caribbean and Cayman Basins. An in- 

 termediate water mass above 1,000 fathoms moves 

 westward through the Caribbean between depths 

 of 245 and 500 fathoms. This water, of Ant- 

 arctic origin, is rich in nutrients. Between 100 

 and 250 fathoms lihe entering water is chiefly of 

 South and North Atlantic central water origin. 



There is little surface upwelling in the Carib- 

 bean, but on the Venezuelan coast the tilt of the 

 water layers brings nutrient-rich waters up to the 

 euphotic zone. The Gulf of Mexico derives its 

 deep waters from water flowing from the Carib- 

 bean Sea over the sill in the Yucatan Channel 

 which is not so deep as the main entrances to the 

 Caribbean Sea. Proximity to this inflowing cur- 

 rent may account for the productivity of the 

 fisheries of the Campeche Banks. 



Most of the new water entering the Gulf ap- 

 parently flows out again through the Straits of 

 Florida so that the main part of the Gulf is more 

 or less of a cul-de-sac. This may influence the 

 low nutrient content of the offshore waters of the 

 Gulf. 



As in most subtropical waters the high tem- 

 peratures cause rapid growth. The same or re- 



lated species in the Gulf tend to grow faster than 

 on the northern Atlantic coast; they attain ma- 

 turity at younger ages and are usually smaller in 

 size. The life histories of many of the fishes of 

 the Gulf are practically unknown. Some of those 

 that occur both in the Gulf and along the Atlantic 

 coast have been studied on the Atlantic coast, 

 and presumably their life histories in Gulf waters 

 are similar. Within the Gulf proper, mention 

 should be made of the studies by Pearson, Gunter, 

 and Gowanloch. However, the area is so vast, the 

 species so numerous, and the conditions so diverse 

 that the total knowledge is meager when com- 

 pared to that of the Pacific or Atlantic coasts. 



It is known that certain species can be caught 

 in certain localities, but no detailed study is 

 available on many of the most abundant species 

 such as the menhaden, the anchovy, the Spanish 

 mackerel, the groupers, and the snappers. Gins- 

 burg (1930), in describing the biology of the com- 

 mon red snapper, Lutianus aya, says, "the red 

 snapper is one of the important food fishes of this 

 country. . . . Among the commercial food fish 

 of the Gulf coast . . . the red snapper is second 

 in point of quantity obtained, being exceeded 

 only by the mullet ... it is significant that 

 practically nothing is known regarding the life 

 history of the red snapper." 



The relative abundance of the diflferent species 

 of fish is not accurately known, especially for 

 those not landed by fishermen or only taken inci- 

 dentally while in pursuit of other species. In esti- 

 mating relative abundance, Gunter (1945a) uses 

 the term "total species mass." He states that, 

 "The estimates of relative species mass of the 

 fishes given here are based on general impressions 

 and observations, bolstered to some extent by 

 data, and are admittedly more subjective than is 

 desirable. It is quite certain, however, that 

 irrespective of their rank in species mass, the 

 species discussed are the most numerous fishes 

 in Texas coastal waters." For the inshore fishes 



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