FISHERY BULLETIN: VOL. 75, NO. 4 



was reported as 20%-35% brown shrimp (presum- 

 ably P. subtilis) and 65%-80% pink shrimp (pre- 

 sumably mostly P. brasiliensis) (Far Seas 

 Fisheries Research Laboratory see footnote 3). 



SIZE COMPOSITION 



The data on temporal and spatial distribution of 

 sizes of shrimp provide information necessary for 

 management of fishery stocks (Rounsefell and 

 Everhart 1953). In particular, data on size reveal 

 information on progressive changes as an indi- 

 cator of rates of growth, population structure, 

 maturity stages, and potential use of habitat by 

 shrimp of different sizes, the latter being related to 

 spawning, recruitment, and migration. 



In our study the more precise data came from 

 landing records, which we used to measure tem- 

 poral changes in size composition. Size data from 

 logbooks (less precise) were used to measure areal 

 differences in size composition. 



The majority of shrimp in U.S. landings for the 

 entire period of 1972-74 were in the 26-30, 21-25, 

 16-20, and 11-15 tails-per-pound size categories 

 (Figure 5). The sizes of shrimp caught may repre- 

 sent a true picture of size availability, but in many 

 instances are also governed by factors which cause 

 fishermen to select certain sizes (e.g., market price 

 of shrimp, fuel price, feasibility of operation, and 

 physical condition of the boat). 



In studying the temporal and, to a lesser extent, 

 the areal distribution of shrimp, we plotted from 



o 



IT 



s 



n — 



FISHING LOG REPORTS 



, LANDING RfPORTS 



ti-4- 



36/40 31/35 35/30 



SIZE CATEGORY 



31/ 35 10/10 11/10 



FIGURE 5. — Size composition of the shrimp catches of U.S. ves- 

 sels as reported in landing records and fishing log reports for the 

 period July 1972 to December 1974. The size categories given are 

 the number of heads-off shrimp per pound. 



landing data the average size of shrimp for all 

 ports combined and for Cayenne, French Guiana, 

 only (Figure 6). The landings for all ports include 

 catches from the entire fishery, while Cayenne 

 landings reflect catches primarily off French 

 Guiana and northern Brazil. The shrimp landed at 

 Cayenne generally averaged slightly smaller than 

 the shrimp from all ports combined. The smallest 

 average size of shrimp, for both total landings and 

 Cayenne landings, were in April and October in 

 1972 and 1973. In 1974, when fishing success was 

 markedly lower than in 1973, small shrimp were 

 present in October, but the expected April peak of 

 small shrimp was less evident. Shrimp were larger 

 in December-January and in June-August of 

 each year. 



Trends in average size of shrimp calculated from 

 the logbook data were similar to those of average 

 size calculated from the landing data (Figure 7). 

 This similarity suggested that the size data from 

 logbooks, although less precise, could also be used 

 to compare areas; the smallest shrimp occurred in 

 zones 69-70 and 77-81, and the largest in zones 

 71-76 (Figure 7). The magnitude of fluctuations in 

 average size calculated from the log data were less 

 than one size category unit, because the original 

 data reported by fishermen are averages. 



At present we have no satisfactory explanation 

 for the greater prevalence of smaller shrimp off 

 French Guiana and Brazil than in other areas of 

 this fishery. We can offer some plausible hypoth- 

 eses. Cayenne landings consist primarily of small 

 brown shrimp caught off French Guiana and 

 northern Brazil. These shrimp are probably re- 

 cently recruited to the fishable population. Small 



° "^ JASON 0|J FMAMJ JASON OjJ FMAMJ J A S O N D| 



1972 1973 1974 



MONTH OF LANDING 



FIGURE 6. — Distribution by month of the mean size index of 

 shrimp calculated from landing records of U.S. vessels at all 

 ports of the Guianas shrimp fishery (marked as total on the 

 graph) and at the Port of Cayenne, French Guiana. The size 

 index was calculated as described in the text. 



708 



