370 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Cursory observations suggest, however, that 

 whereas adult popuhition segments distribute 

 themselves over the same depth range as that 

 occupied by brown shrimp, maximum density 

 occurs at a somewhat lesser depth. This is 

 particularly apparent in instances where both 

 species inhabit the same general area. But, as 

 speculated upon earlier when discussing the dis- 

 tribution of brown shrimp, the underlying factor 

 may be substrate composition. On the basis of 

 extensive field observations, Hildebrand (1955) 

 concluded that over the same bathymetric range, 

 browai shrimp prefer a very soft mud substrate 

 and pink shrimp a somewhat harder and coarser 

 bottom. A similar conclusion was reached by 

 Williams (1958), who conducted substrate selec- 

 tivity experiments under laboratory conditions 

 with juveniles of both species. As additional 

 factors controlling the distribution of littoral 

 Penaeidae, food preferences or habits as they 

 relate to substrate type should not be discounted. 

 Pink shrimp, like brown and white shrimp, also 

 display a marked tendency to move to deeper 

 water with advancing age (Iversen, Jones, and 

 Idyll, 1960). This movement presumably occurs, 

 for the most part, over substrate characterizing 

 the species habitat. 



The Sanibel-Tortugas Population 



Undergoing intensive exploitation for the first 

 time about 1950, the pink shrimp population 

 occurring off southwest Florida has since sup- 

 ported a valuable commercial fishery. Arbi- 

 trarily delimited, its habitat extends coastwise 

 from the vicinity of Tarpon Springs on the nortli 

 to just beyond the Keys on the south (fig. 6). 

 The seaward limit at all points has been set at the 

 45-fathom contour although population fragments 

 may be found in deeper water. Within these 

 boundaries the population disperses itself over a 

 projected bottom area of some 23,000 square 

 nautical miles with Florida Bay and adjacent 

 estuaries serving as "nursery" grounds for its 

 immature phases. Studies are currently being 

 undertaken to determine whether the population 

 is actually' continuous as presumed, or whether it 

 is separated into two discrete units at about the 

 26th parallel. 



Commercial fishing on the Sanibel-Tortugas 

 grounds now continues with varying intensity the 

 year round (fig. 18). Spatial distribution of 



100 ro ipoo 

 >iooo 



MMJ5NJMMJSNJMMJSN 

 1957 1958 1959 



Figure 18. — Distribution (schematic) of fishing effort in 

 the Sanibel-Tortugas area, 1956-59. 



trawling effort follows a somewhat regular pattern. 

 Greatest concentrations occur between 5 and 25 

 fathoms just north of the Dry Tortugas (statistical 

 subarea 2) and in 5-15 fatboms off Sanibel Island 

 (subarea 4). Operations are gradually extending 

 to intermediate and outlying areas though un- 

 trawlable bottom precludes intensive fishing in 

 many of these. 



Condensed data of commercial effort and 

 landings for the Sanibel-Tortugas area are given 

 in appendix tables 1 and 3, respectively. 



Commercial yield. — Over the period 1956-59, 

 annual pink shrimp production in the Sanibel- 

 Tortugas area fluctuated about an average of 19.2 

 million pounds, this representing about 11 percent 

 of the total Gulf production of all species. Land- 

 ings ranged from a high of 24.7 million pounds in 

 1958 to a low of 13.9 million pounds in 1959. 

 Despite a practically constant expenditure of 

 effort, the 4-year production trend was down 

 appreciably, its slope reflecting an average decline 

 of about 1 .5 million pounds per year. Particvdarly 

 interesting is the fact that the catch of 1958 was 

 the largest ever recorded for the so-called Tortugas 

 fishery, and that of 1959 the second lowest (cf. 

 Idyll, 1957, table 4). This dramatic drop to- 

 gether with a long-term waning trend in produc- 

 tion have stimulated concern for the fishery's 

 future welfare. 



The jneld curve derived from monthly landings 

 reveals that peak production in the Sanibel-Tortu- 

 gas fishery is attained during winter and early 

 spring (November-May), 80-85 percent of each 

 calendar year's catch being made in that period 



