Practical equivalence of laboratory and field measurements of gut passage time in two penaeid shrimp species

Gut passage times (GPTs) of 2 species of juvenile penaeid shrimp Farfantepenaeus aztecus (Perez Farfante & Kensley, 1997) and Litopenaeus vannamei (Perez Farfante & Kensley, 1997) were measured in field and laboratory feeding trials using soy-based feeds varying in percent dry weight of either fiber (2.3 to 11.3 %), protein (20.1 to 45 %), or lipid (3.5 to 13.5 %). To trace feed movement visually through the gut, feeds were thoroughly mixed with inert fluorescent latex beads. Because it was not possible to continuously view feed passage through the shrimp guts in the field trials (as is possible in the laboratory), we developed indirect methods that allowed us to obtain periodic 'snapshots' of feed movement through shrimp guts at 10 min intervals, which were then compared with direct observations of feed movement used in the laboratory trials and used to calculate GPT and gut passage rate. Indirect methods underestimated GPTs in field feeding trials, necessitating their adjustment before comparison with the directly measured laboratory GPTs. Overall, mean GPTs for L. vannamei fed 13 different fiber, protein, and lipid feeds were not significantly different between laboratory and field feeding trials. In general, mean laboratory GPTs for L. vannamei were slightly shorter than mean GPTs from field feeding trials (1 to 20 min). Utilizing only 1 feed (base feed: 30.1 % protein, 5.3 % fiber, 7.5 % lipid), mean GPTs of F. aztecus in laboratory feeding trials were again slightly shorter (12 min) than mean field GPTs. We suggest that a temporal change in mean GPT of 1 to 20 min for shrimp fed the same feed in laboratory and field settings may be trivial enough to justify use of simpler laboratory measurements to extrapolate the GPTs of field populations for design of more efficient feeding regimes.