SHERIDAN ET AL.: RECAPTURE PATTERNS OF TAGGED PENAEID SHRIMP 



TX 

 239,006 HR 



TA 

 57,511 HR 



0-9 10-18 19-27 28-37 38-46 47-55 56-64 65-73 74-82 

 FISHING DEPTH (M) 



Figure 9.— Comparison of fishing effort (h) by depth strata between Texas (TX) and Tamaulipas (TA) 



interviews. 



all recaptures were made within 40 days after re- 

 lease. Thus, in contrast to the present study, the 

 mark-recapture analyses reported by Sheridan et al. 

 (1987) were of limited use since they described 

 primarily a small nimiber of long-distance, long- 

 duration recaptures. The difference in results is 

 most likely due to the higher quality, finer scale data 

 collected during the present experiments and to the 

 use of all recaptures, not just those for shrimp mov- 

 ing long distances. 



The factors "species" and "state" may actually 

 describe different habitat requirements and differ- 

 ent levels of fishing effort in the western Gulf of 

 Mexico. Trawl catches indicate brown shrimp utilize 

 a greater depth range (0-160 m) and are found over 

 a wider variety of substrates (sand, silt, clay) than 

 pink shrimp (0-65 m depth; coarse sand and shell) 

 (Hildebrand 1954, 1955; Williams 1958; Cook and 

 Lindner 1970; Costello and Allen 1970; Grady 1971; 

 Renfro and Brusher 1982). Offshore substrate pref- 

 erences of brown shrimp and pink shrimp have not 

 been directly tested, however. Coarse substrates 

 tend to lie in pockets or bands paralleling the Texas 

 coast (McGowen and Morton 1979); thus pink shrimp 

 recaptures could reflect longshore movement seek- 



ing these substrates. The major influence on recap- 

 ture patterns was the difference in fishing activity 

 between the two states: fishing effort off Tamau- 

 lipas was only 13% of the effort expended off Texas 

 and occurred in deeper waters. Consequently, fish- 

 ing mortality off Tamaulipas was lower and tagged 

 shrimp generally exhibited a lesser recapture rate, 

 greater times at large, greater distances travelled, 

 and greater depths at recapture. Collection and 

 utilization of fishing effort data thus seems impera- 

 tive for interpretation of tag recapture patterns. 

 A potential source of error common to all mark- 

 recapture experiments involves the precision of 

 reported recapture locations. Trawl tows are of 

 variable durations and distances, and there is no way 

 of knowing at what point along a towing track that 

 a marked shrimp is captured. In addition, the exact 

 locating of recoveries depends on the precision of 

 navigational equipment, which is usually not re- 

 corded and which could vary from dead reckoning 

 to satellite navigation. These factors would affect 

 estimates of distance travelled, speed, and direction 

 but not recapture depth (shrimpers tow along, 

 rather than across, depth contours to avoid gear ad- 

 justments) or days at large. Effects on R/f values 



309 



