Table 2. — Fishing effort and recoveries of stain-marked 

 shrimp, Tortugas grounds, December 14, 1963 to February 

 7, 1953— Continued 



Assumption 2: 



No losses due to predation during release. 



Justification: 



The e.xperimental groups, as noted earlier, were 

 retiu-ned to the bottom in a release box designed to 

 avoid predation. The effectiveness of this release 

 device had been demonstrated previously by 

 underwater observations (Rounsefell, 1963). 

 Assumption 3: 



NegUgible loss of recaptured marked shrimp 

 because of failure to detect them in the commercial 

 catch or failure to report them. 



Justification : 



Assurance that a high percentage of recaptures 

 were recognized and returned (recovered) was 

 given by the foUowdng evidence: 



a. Just before these experiments, most fishermen 

 and processing-plant personnel in the area were 

 shown samples of stain-marked shrimp. 



b. Eight experiments \vith stain-marking had 

 been performed recently in these areas, and most 

 fishermen and processing-plant personnel in the 

 Sanibel and Tortugas areas were famihar with 

 stain-marked shrimp. 



c. A reward was paid for each recovery when 

 vessels arrived in port or as soon as marked shrimp 

 were found in a processing plant. 



d. During both experiments, marked shrimp had 

 many chances to be recognized and returned be- 

 cause aU shrimp were "headed" by hand. All 

 Sanibel shrimp were headed at sea, but some 

 Tortugas catches were headed ashore. Ordinarily, 

 a single fisherman may remove heads from 8,000 

 or more shrimp in a single night. During the San- 

 ibel experiment, fishermen, two to a boat, removed 

 the heads from an average of only about 4,000 

 shrimp each night (table 3). When they handled 

 less than their capacity, fishermen had time to 

 examine each shrimp and recognize the marked 

 animals. Considerably more shrimp were handled 

 per night by Tortugas fishermen than by Sanibel 

 fishermen. Crew members from many boats told us, 

 however, that they spread catches of shrimp on 

 the deck before heading so that marked shrimp 

 might be easily noticed. Most of the total re- 

 coveries (93 percent) were recognized by fishermen 

 at sea and removed from the catch before the 

 return to port (Allen and Costello, 1966). Ashore, 

 Bureau personnel daily reminded workers in proc- 

 essing plants to watch for marked shrimp that 

 passed unnoticed by fishermen at sea. 



e. "Planting" experiments indicated a high 

 ratio of recoveries to recaptures. During the 

 time marked shrimp occurred in commercial 

 catches from the Tortugas grounds, small num- 

 bers were placed secretly in catches of whole 

 shrimp being unloaded at shore processing plants. 

 These shrimp were of identical size and were 



Table 3. — Fishing effort and pink shrimp catch (indi- 

 viduals), south Sanibel grounds, March to May 1962 ' 



' Based on 171 interviews of boats landing shrimp at Fort Myers and Fort 

 Myers Beach. Compiled by Bureau of Commercial Fisheries Branch of 

 Statistics. 



: Rounded to the nearest thousand. 



496 



U.S. FISH AND WILDLIFE SERVICE 



