140 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 1. — Submarine iUuininalion data (irrnrdivy to the Woods Hole Oeeanoyraphic Institution {WHOI) and Dana reports 



(D) — Continued 



I Photronic photometer derived extinction coefficients. 



was reported as tiie depth in which the disc dis- 

 appeared. These are converted here into ex- 

 tinction coefficients These values are entered in 

 table 2. More detailed iaformation for 51 values 

 in this area is tabulated in the same paper by 

 Taylor. 



Table 2. — Extinction coefficients of the Dry Tortugas area 

 based on locations and data of Taylor (1928) 



Secchi disc 

 "K" 



Lagoon.- -_ o 227-(i H4 



Outside of reefs - ,207-. 121 



Outside island group . .o.«*9-,(k>l 



Gulf Stream (16 to 20 miles olTshore) .067- ,047 



East of Loggerhead Key ' .189-, 150 



West end of Garden Key Cliannel 227- . 174 



Clarke (1938) calculated the extinction coeffi- 

 cients for a series of stations east of the Mississippi 

 Delta. These values are indicated in table 1 as 

 series 439 through 442. 



For security reasons, Hulburt's paper (1940) on 

 transparency and visibility of submerged objects 

 in the Key West sector has been classified. 



Bumpus and Clarke (1947) have connected 

 equal points of known "K" values on a chart 

 which includes the Gulf of Mexico. It must be 

 remembered that these lines are based on very 



few observations, and considerable interpolation 

 was necessary. 



The Special Scientific Reports, Fisheries No. 8, 

 by Butler (1949), and \o. 14, by Butler and 

 Engle (1950), of the United States Department of 

 the Interior contain turbidity indices for selected 

 points of the Mississippi Sound and Lake Pont- 

 chartrain. Turbidity is expressed as the per- 

 centage-transmission of light through the sample. 

 No correlations with extinction coefficient values 

 or <lirect Secchi disc measurements are presented. 



It is obvious that the transparency data for the 

 Gulf of Mexico are quite inadequate to meet the 

 increasing demands of researchers. 



It is admitted that the Photronic photoipeter 

 is more accurate than the Secchi disc methods of 

 determining the extinction coefficients, but it is 

 also more time consuming. It is suggested, there- 

 fore, that the Secchi disc be used. Not more 

 than 2 or 3 minutes are required to lower the 

 white disc (usually 20 or 25 cm. in diameter) until 

 it disappears, and to record the depth. The whole 

 operation is simple and can readily be carried out 

 by unskilled persons. 



In view of the limited transparency observa- 

 tions in the Gulf of Mexico it would be difficult 

 to draw accurate comparisons with other areas. 



