GULF OF MEXICO 



509 



coast. Onl_v a few narrow passes connect these 

 inner baj^s with the open Gulf. Because of the 

 low range of tide levels this results in weak circu- 

 lation of water between the bays and the Gulf. 

 As a result, these bays exhibit wide ranges in 

 temperature and salinity. The rise in the rivers 

 following the winter rains causes a great drop in 

 salinity; many of the baj^s are almost fresh for 

 periods of a few months. Many of them normally 

 have a salinity around 15 to 20 parts per thousand 

 contrasted with nearl}^ 35 parts in open ocean 

 water. During the winter a strong, cold, north 

 wind occasionally drops the temperature very sud- 

 denlj' and many of the cold-sensitive fish are 

 killed before they can reach deep water. 



The importance of the passes connecting the 

 bays to the open Gulf is shown by the life histories 

 of many of the species. Thus, the redfish or red 

 drum, Sciaenops oceUata, the croaker, Micropogon 

 undulatus, the black drum, Pogonias cromis, the 

 spot, Leiostomus xanthurus, and the striped mullet, 

 Mugil cephalus, all hnportant sport and commer- 

 cial species, crowd tlu-ough these narrow passes 

 during the fall and early winter to reach the open 

 Gulf. Here they spawn, chiefly in the vicinity of 

 the passes. The post-larval and young of these 

 species are later observed in vast schools entering 

 the passes from which they spread throughout the 

 shallow, inner bays. A few species, such as the 

 spotted sea trout, speckled trout, or squeteague, 

 Cynoscion nebulosus, spawn within the inner bays. 



One of the most interesting areas biologically 

 is the Laguna Madrc, a narrow bay 115 miles long, 

 paralleling the Texas coast. No rivers enter the 

 Laguna, and its onlj' present connection with the 

 Gulf is through Corpus Christi Bay at the northern 

 end. In depth it ranges from a few inches to 4 

 feet, with occasional deep holes. As a result of 

 the shallow depths, the lack of permanent stream 

 drainage, the high evaporation rate, and the poor 

 connection with the Gulf, the monthlj^ average 

 salinity of the upper Laguna is above 50°/oo, and 

 in some j^ears salinities well over 100°/oo are found. 



Despite these conditions it produces a large 

 quantity of fish. When the salinity rises above 

 a critical point (about 72°/oo, Gunter 1945b) fish 

 start dying by the thousands. This happens every 

 few years. Because of the life histories of the 

 species involved and the absolute necessity that 

 the young find suitable conditions in the inside 



bays, perhaps the chief fishery problem of tiie 

 region is the maintenance of proper conditions in 

 these bays. 



Because of the lack of any major streams the 

 important feature in the Laguna Madre is the high 

 salinity. The shallowness of the water makes it 

 impossible to obtain any significant circulation 

 through a narrow pass, no matter what its depth, 

 so that the only major changes in salinity occur 

 when there is a rise in the water level across the 

 wide, northern entrance to Corpus Christi Bay. 

 As the tide ranges are slight the extra high levels 

 occasionallj^ attained through the piling up of 

 water by strong, inshore winds are of major im- 

 portance to the circulation in the Laguna. 



Excluding the Laguna Madre, most of the bays 

 behind the barrier islands and reefs are entered by 

 large rivers. The problem in these bays is, in 

 part, similar to that of the Laguna. They differ, 

 however, in that while high salinitj^ is the problem 

 in the Laguna, these other bays are troubled 

 chiefly by low salinity. The diflSculty has height- 

 ened with the passing years as soil erosion and 

 sparse vegetative ground cover caused by over- 

 grazing has intensified the magnitude of the floods. 

 The problem in these other bays may be solved 

 eventually through better agricultural practices 

 and through flood control and power dams that 

 will assure a steadier flow of fresh water. A certain 

 amount of fresh water is needed to prevent con- 

 ditions similar to those in the Laguna, but too 

 much fresh water in a short period drops the 

 salinity to almost zero. Because of the poor circu- 

 lation these flood waters take many weeks to 

 become mixed with water from the Gulf. 



In addition to those species that depend on the 

 ecological conditions in the inner baj's, there are 

 many species on the perimeter of the Gulf whose 

 life histories, so far as known at present, render 

 them more or less independent of conditions in the 

 inner bays. These probably include the men- 

 haden, Brevoortia, the pompano, Trachinotus caro- 

 linus, the butterfish, Poronotus triacanthus, and 

 the Spanish mackerel, Scomberomorus maculatus. 

 The actual degree to which any one species is 

 dependent on the inside waters is at present 

 largely a matter of speculation. The answer lies 

 in continued research. 



There are also many species of estuarine and 

 anadromous fishes in the Gulf. Gunter (1945a) 



