34 



Fishery Bulletin 92(1). 1994 



Materials and methods 



Tripletail larvae were obtained from museum collec- 

 tions throughout the Gulf of Mexico to determine 

 their spatial and temporal distribution. These in- 

 clude collections from the Southeast Area Monitor- 

 ing and Assessment Program's (SEAMAP) ichthy- 

 oplankton surveys of the Gulf from 1982 through 

 1986 (SEAMAP 1983-1987 1 ); National Marine Fish- 

 eries Service (NMFS, Panama City, Florida) and 

 Louisiana State University (LSU) collections from 

 within riverine and oceanic frontal zones off the 

 Mississippi River delta; and collections made by the 

 Gulf Coast Research Lab (GCRL), Ocean Springs, 

 Mississippi, and by Freeport-McMoRan Inc., New 

 Orleans (Appendix Tables 1 and 2). 



SEAMAP collections from 1982 to 1986 represent 

 the first time-interval for which a complete set of 

 data were available. Standard ichthyoplankton 

 survey techniques as outlined by Smith and 

 Richardson (1977) were employed in data collection. 

 SEAMAP stations sampled by NMFS vessels were 

 arranged in a systematic grid of about 55-km inter- 

 vals. NMFS vessels primarily sampled waters >10 

 m deep. Each cooperating state had its own sam- 

 pling grid and primarily sampled their coastal wa- 

 ters. Latitude 26°00'N was the southern boundary 

 of the survey area. Hauls were continuous and made 

 with a 60-cm bongo net (0.333-mm mesh) towed 

 obliquely from within 5 m of the bottom or from a 

 maximum depth of 200 m. A flowmeter was mounted 

 in the mouth of each net to estimate volume of wa- 

 ter filtered. Ship speed was about 0.75 m/sec; net 

 retrieval was 20 m/min. At stations <95 m deep, tow 

 retrieval was modified to extend a minimum of 10 

 minutes in clear water or 5 minutes in turbid wa- 

 ter. Tows were made during both day and night 

 depending on when the ship occupied the station. 

 Overall, 1,823 bongo-net tows were collected and 

 processed during these years. The SEAMAP effort 

 from 1982 to 1984 also involved the collection and 

 processing of 814 neuston samples taken with an 

 unmetered 1x2 m net (0.947-mm mesh) towed at 

 the surface for 10 minutes at each station. SEAMAP 

 sampling during April and May was primarily be- 

 yond the continental shelf, whereas that during 

 March and from June through December was over 

 or immediately adjacent to the shelf at stations <180 

 m deep. No samples were taken during January and 

 February Additional information on the temporal 

 and spatial coverage of SEAMAP plankton surveys 



1 SEAMAP. 1983-1987. (plankton). ASCII characters. Data for 

 1982-1986. Fisheries-independent survey data. National Ma- 

 rine Fisheries Service, Southeast Fisheries Center: Gulf States 

 Marine Fish. Comm., Ocean Springs, unpubl. data. 



is found in Stuntz et al. ( 1985), Thompson and Bane 

 (1986, a and b), Thompson et al. (1988), and Sand- 

 ers et al. (1990). 



Collections from frontal zones off the Mississippi 

 River delta include 311 surface-towed 1x2 m neus- 

 ton net samples (0.333-mm mesh) made by NMFS. 

 NMFS samples were collected during May, August, 

 September, and December (1986 to 1989), although 

 not all four months were sampled each year (Appen- 

 dix Table 1). We also examined 63 surface-towed 

 1-m 2 Tucker trawl samples (0.363-mm mesh) taken 

 at seven stations during July 1987, and 45 surface- 

 towed multiple opening/closing net and environmen- 

 tal sensing system (MOCNESS) (Wiebe et al., 1976) 

 samples (0.363-mm mesh) collected at five stations 

 during April 1988. These samples were from LSU 

 collections. In addition, we examined 17 samples 

 from stations taken by LSU inside the 100-0m 

 isobath during October 1990. The sampling area 

 during October 1990 extended 140 km west from 

 Southwest Pass of the Mississippi River delta along 

 the inner-to mid-shelf. Samples were collected with 

 a 60— cm bongo net (0.333-mm mesh) towed ob- 

 liquely to the surface from 5 m of the bottom or from 

 a maximum depth of 50 m (Appendix Tables 1 and 2). 



Museum collections from GCRL and Freeport- 

 McMoRan, Inc. were primarily taken off Mississippi 

 Sound and within the Barataria Bay system of Loui- 

 siana, respectively. Gear type and most environmen- 

 tal data were not available from these two institu- 

 tions (Appendix Table 2). 



Temperature and salinity data were from the sea 

 surface. Hydrographic data from stations where lar- 

 vae were taken were multiplied by the total num- 

 ber of larvae collected at each station to derive 

 median and mean hydrographic values. This method 

 gives weight to distribution of larvae rather than to 

 distribution of stations. We used percent cumulative 

 frequency for defining the relationship between dis- 

 tribution of larval tripletail and water temperature, 

 salinity, and station depth. Percent frequency indi- 

 cates the range of hydrographic conditions most of- 

 ten associated with occurrences of tripletail larvae. 

 Median, mean, and percent cumulative frequency 

 statistics were calculated (SAS Institute, 1985). 



An examination of tripletail larvae was made to 

 describe developmental morphology. Body measure- 

 ments were made on 21 tripletail between 2.2 and 

 23.0 mm SL (Table 1) according to the methods of 

 Hubbs and Lagler (1958) and Richardson and 

 Laroche (1979). Measurements were made to the 

 nearest 0.1 mm with an ocular micrometer in a dis- 

 secting microscope. We follow Leis and Trnski's 

 (1989) criteria for defining length of preopercular 

 spines, body depth, head length, eye diameter, and 



