Table 1. — Hesiilts </ tests to determine retiiibiliti/ of calcuhiled 

 sampling depths attaiited by using wire angle and length 

 of lowing cable 



depth of 50 m., but became progressively poorer 

 beyond this depth. Consequently, the deepest 

 samples from stations in 82 m. of water proba- 

 bly were taken at a distance above the bottom 

 that averaged considerably greater than the in- 

 tended 3 m. 



DAY vs. NIGHT CATCHES OF PLANKTONIC-STAGE 

 PENAEUS SPP. 



The oblique-step tow was used throughout 

 this study in an attempt to eliminate possible 

 differences in day and night catches caused by 

 diurnal migrations of larval shrimp. Russell 

 (1925, 1928) has shown that, in general, deca- 

 pod larvae undergo diurnal vertical migrations. 

 More recently Temple and Fischer (1965) ob- 

 served similar migrations in planktonic stages 

 of penaeid shrimp in the northwestern Gulf 

 of Mexico when temperature profiles (bathy- 

 thermograph traces) indicated a stratified 

 water column. 



Hydrographic conditions over the Continen- 

 tal Shelf in the northwestern Gulf of Mexico 

 appear to be seasonal, as there are definite 

 times when temperatures of the waters are 

 either stratified or isothermal (Harrington, 

 1965). The time and extent of these seasons 

 vary, however, depending on total water depth 

 and to some degree on distance offshore. In 

 general, water is stratified at a total depth of 

 14 and 27 m. during May to July, at 46 m. dur- 

 ing May to September, and at 82 m. during 

 April to October. In other months, tempera- 

 tures are essentially isothermal. 



Average day and night catches were calcu- 

 lated for stations at which water was stratified 

 and for those at which the water column was 



isothermal (table 2). Hauls with no shrimp 

 were excluded. Catches made at the 82-m. sta- 

 tions also were excluded because no larvae 

 appeared to be present during either the day 

 or night at certain times of the year, and water 

 deeper than 50 m. was not sampled adequately. 

 After a test of homogeneity indicated a need 

 for logarithmic transformation of the data, 

 statistical treatment revealed that day and 

 night catches did not differ significantly during 

 either stratified or isothermal conditions. As 

 used in this study, the oblique-step tow appar- 

 ently did prevent possible differences in day 

 and night catches caused by diurnal migrations 

 of larval shrimp. 



'I'ahi.k 2. — T-lest of average eatrhes of planktontc-stage Penaeus 

 sp/i. in dai/ and night sam/iles during different temperature 

 conditions in the n'lrtlneeslern (lulf of Mej:ico, UHil 



INiniiliir |iir lou m." of water strninedl 



Stage 



Water stratified 



Water isothermal 



Day I Night "t" ' Day 1 Night "t" 



N'auplius 12.5 



l*rotozoea 67.2 



Mysis 22.2 



I*osllarva .0 



Allstages 04.5 



I "t" value at .05 level. 



ASSOCIATED PHYSICAL DATA 



In addition to the plankton sampling at each 

 station, temperature and salinity measurements 

 were taken with a Foxboro Dynalog at selected 

 depths. Temperatures were recorded to 0.1° C. 

 and salinities to 0.1 p.p.t. (parts per thousand). 



LABORATORY PROCEDURES 



In the laboratory, plankton samples were 

 transferred from 32-fluid-ounce (9.6 dl.) jars 

 to 8-fluid-ounce (2.4 dl.) jars, and the 5 percent 

 Formalin solution was replaced by a new 5 per- 

 cent solution with glycerin and borax added. 

 Each sample was examined under a microscope 

 at magnifications that ranged from 0.7X to 

 6.0X. All planktonic .stages of penaeids were 

 removed, sorted to developmental stage, iden- 

 tified to genus, and counted. Po.stlarvae of the 

 genus PenaeitH were measured and identified to 

 species by using characteristics described by 

 Baxter and Renfro (1967). 



326 



DISTRIBUTION, SHRIMP IN GULF OF MEXICO 



