Bull shark, continued 



Table 5.14. Relative abundance of bull shark in 31 

 Gulf of Mexico estuaries (from Volume I). 



Life stage 



Relative abundance: 



O Highly abundant 



® Abundant 



O Common 



V Rare 



blank Not present 



na No data available 



Life stage: 



A - Adults 

 M - Mating 

 J - Juveniles 

 P - Parturition 



Ecuador and possibly to northern Peru (Lee et al. 

 1980). 



Within Study Area : This species is common in inshore 

 waters and estuaries from Texas to Florida, and is fairly 

 abundant in Louisiana and Florida estuaries (Table 

 5.14). It is generally the most common shark species 

 in brackish water areas of the Gulf of Mexico, and is 

 known to enter fresh water (Shipp 1986). 



Life Mode 



Bull sharks are demersal predators. They are euryha- 

 line and occur from the nearshore marine zone to 

 freshwater rivers (Fischer 1 978, Lee et al. 1 980, Shipp 

 1986). 



Habitat 



Ty pe: This species is predominantly a coastal species 

 that is frequently found in shallow waters, especially in 

 bays and river estuaries (Fischer 1 978, Lee et al. 1 980, 

 NMFS1993). 



Substrate : No particular substrate preference by this 

 species has been noted, but it is considered a bottom 

 dweller (Fischer 1978). 



Physical/Chemical Characteristics 

 Temperature: Thomerson and Thorson (1977) sug- 

 gested water temperatures to be the limiting factor for 

 the advancement of bull shark up the Mississippi River. 

 Only when temperatures are above 24°C, particularly 

 during the summer and fall, do the sharks ascend the 

 Mississippi River. Snelson and Williams (1981) col- 

 lected juvenile bull shark in temperatures from 20 to 

 32°C, and reported that two individuals had succumbed 

 to hypothermal stress around a temperature of 8°C, 

 during January. Branstetter (pers. comm.) suggests 

 that 18°C is the minimum temperature necessary 

 before bull sharks advance into estuaries. 



Salinity: The bull shark occurs in brackish or freshwa- 

 ter, mainly as pups and juveniles but also as adult 

 females. This occurrence may be related to inshore 

 migrations of the females for parturition (Garrick 1 982, 

 Snelson et al. 1 984). As a result, juveniles often spend 

 considerable time in these brackish waters (Garrick 

 1982). Branstetter (1986) noted that the fishery for 

 these is located primarily near freshwater inflows. One 

 study reported the collection of juveniles from a salinity 

 range of 1.6 to 2.3%o (Kelley 1965). Thomerson and 

 Thorson (1977) report that the bull shark is the only 

 shark known to withstand the osmotic demands of 

 either fresh water or sea water for periods of at least 

 months and probably years. Other sharks may be 

 capable of withstanding these osmotic conditions, but 

 do not typically enter freshwater (Branstetter pers. 

 comm.). 



119 



