GULF OF MEXICO (XIMMERCIAL SHRIMP POPULATIONS 



353 



Species Composition 



Along: iiuioh of the Gulf coast, processors dis- 

 tinguish between commercial varieties commonly 

 occurring together by assigning slightly different 

 ex-vessel prices to each. A breakdown by species 

 for each landing is thus obtained automatically 

 when transcribing landing data from dealers' 

 records. 



In some areas, however, closely related species 

 are not differentiated by price. Here mi.xcd land- 

 ings of two species may be described as entirely 

 composed of one or the other, resulting in dis- 

 torted catch figures for both. Examples of areas 

 in which this situation periodically exists are 

 southwest Florida and Texas. In the former 

 area, Trachypeneus spp., which have little com- 

 mercial value because of their small size, frequently 

 enter pink shrimp landings in small amounts. In 

 contrast, pink shrimp often dominate spring land- 

 ings at Freeport, Aransas Pass, and perhaps other 

 Texas ports, being purchased and entering dealers' 

 records as brown shrimp. The degree to which 

 past landing data from these and other areas are 

 so biased has not been determined. An attempt 

 is being made to rectify this problem by establish- 

 ing a Gulf-wide catch-sampling program. 



No evidence of preference on the part of the 

 commercial fleet for a particuhir species (in situa- 

 tions where more than one were equally available) 

 was detected in the present study. 



Size Composition 



Ex-vessel sales are prorated on the basis of 

 each landing's size composition as well as its 

 species composition, with larger shrimp bringing 

 higher prices. Landings are thus recorded ac- 

 cording to the sizes purchased from the fisherman, 

 the breakdowTi being carried through to final 

 tabulation. 



Although such a practice might appear to 

 obviate the need for sampling shrimp landings to 

 secure a picture of population size or age struc- 

 ture, closer scrutiny raises some doubt as to the 

 commercial data's usefulness for this purpose. 

 Comparability of size composition data from 

 different Gulf areas may be suspect due to the 

 following biases of unknown degree: (I) varying 

 minimum-size laws; (2) differential dealer and 

 gear selectivity; (3) changing prices; and (4) 

 different grading methods. Further discussion 

 of these factors will be deferred to a later section. 



CONVERSION FACTORS 



As a convenience to commercial interests, 

 shrimp landing statistics are compiled in terms 

 of "tail" or headless weight. In keeping with 

 the ecological convention of maintaining unit 

 correspondence between yield and biomass, all 

 landings reported herein have been converted to 

 whole or "heads-on" weight. This was accom- 

 plished by applying the factor 1.68 to catch data 

 published for eacii common species. 



Unfortunately, the statistical reliabilitv of this 

 factor has not been established. Moreover, 

 current studies indicate that among commercial 

 Penaeidae, the factors relating headless to whole 

 weight vary widely between species and to a 

 lesser extent between sexes and from season to 

 season within species, and are measurabh' less 

 than formerly believed. Ratios between total 

 and tail weight for the brown, pink, and white 

 shrimp and seabob (se.xes combined over all 

 seasons) have been found to deviate only slightly 

 (coeficients of variation are 3 percent or less) 

 from 1.61, 1.60, 1.54, and 1.53, respectively. 

 These represent significant departures from the 

 traditional 1.68. 



Conversion from headless to whole weight 

 would not constitute a problem if all shrimp were 

 landed and weighed heads-on. Published data 

 could be restored to their original and desired 

 state by simply applying the reciprocal of what- 

 ever factor was used to convert them to heads-oflf 

 units. But commercial shrimp are not handled 

 in uniform fashion around the Gulf. Many are 

 landed heads-on, many heads-off, the former 

 being converted to heads-off units immediately 

 upon being landed. The degree to which either 

 practice is followed in each area is unknown and, 

 consequently, so is the relative accuracy of ad- 

 justed landings data. If landings heads-on pre- 

 dominate, the inaccuracy of data converted using 

 a generalized factor will be minimal. But if 

 heads-off landings are the rule, data converted 

 using the same factor (1.68) will not reflect true 

 catch (heads-on) weiglits for all species. In either 

 case, landing data will not be comparable from 

 area to area and, in some instances, from port to 

 port within an area. 



Further complications arise if conversion of 

 catch-by-sizc data is desired. .Vll such data are 

 recorded in terms of number of headless shrimp 



