GULF OF MEXICO COMMERCIAL SHRIMP POPULATIONS 



351 



Figure 6. — System for coding origin of shrimp landings and position of commercial shrimping operations in the Gulf of 



Mexico. Shaded circles indicate principal landing ports. 



catch. Fishing positions during concurrent trips 

 of trawlers whose captains ai'e not interviewed are 

 projected and coded accordingly. The quantity 

 of effort expended is calculated by merely dividing 

 their known catches by a projection factor derived 

 from catch-effort ratios of tlie vessels actually 

 sampled for operating data. Effort is recorded 

 to the nearest tenth in terms of days' trawling 

 time or, more precisely, the total number of 

 hours trawled divided by 24. "Day" tlien does 

 not refer to a calendar day but merely represents 

 a coding device. 



Biases affecting the usefulness of effort tlata 

 secured by this technique will be discussed in a 

 later section, but two shortcomings should be 

 pointed out here. The first concerns estimating 

 nonproductive fishing effort. During certain sea- 

 sons considerable amounts of searcliing ("try- 

 netting") and fishing time are expended with 

 negligible results. Under the present system, 

 such activity goes unaccounted for since effort is 

 estimated for and assigned oidy to vessel-trips for 

 which a shrimp sale is recorded. Exclusion of 

 this nonrewarded effort obviouslv leads to under- 



