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Fishery Bulletin 89(4), 1991 



NEARSHORE 

 LOCATION 



1/ 



GULF OF MEXICO 



Figure 1 



Nearshore and offshore study sites where depth-stratified ichthyoplankton 

 collections were taken in September and October 1984 and 1985. 



samples were taken offshore in the vicinity of a "win- 

 dowshade," subsurface (5m) current drogue as it was 

 tracked from the ship throughout the duration of a 

 cruise in an attempt to repeatedly sample the same 

 patch of fish larvae and zooplankton (Lyczkowski- 

 Shultz et al. 1988). The drogue traveled, on average, 

 13km during a sampling period, typically at varying 

 compass headings over water depths of 18-25 m. Dur- 

 ing each cruise, collections were usually made during 

 early to late afternoon of the first day, then in the 

 middle of the night (usually prior to midnight), and in 

 the morning (after sunrise) of the second day. In all but 

 one instance, dawn and dusk periods were specifically 

 avoided. Times of sunrise and sunset on sampling dates 

 in September and October 1984 and 1985 ranged from 

 0543 to 0600 hours, and from 1733 to 1819 hours cen- 

 tral standard time (CST), respectively. During three 

 of the five cruises, additional samples were taken at 



a fixed, shallow water station (12 m) located 

 15-19km NNW of the offshore sampling 

 area during midday hours of the second day 

 (nearshore, Fig. 1). Times of high and low 

 tide were determined using predictions for 

 tide stations nearest the offshore sampling 

 sites (NOAA-NOS Tide Tables). 



Sampling gear consisted of a lxl.4m, 

 multiple-net Tucker trawl with an effective 

 mouth opening of lm 2 when fished at a 45° 

 angle, and a double-release mechanism oper- 

 ated manually with messengers. Mean tow 

 speeds varied from 1.5 to 2.5 knots (0.8- 

 1.3 m/s). At the offshore location, three 

 Tucker trawl casts were made during each 

 time period. A cast consisted of all three 

 333-/u mesh nets being fished in horizontal 

 5-min hauls at one nominal depth level at a 

 time. These sampling depths were 1,5, and, 

 most typically, 11 or 12 m. At the nearshore 

 location, each of the three nets were fished 

 at 1, 3, and 7 or 9 m, but the nets were of 

 different mesh sizes: 333, 505, and 760^. 

 Offshore catch data from the three net sam- 

 ples at each depth were pooled for analyses 

 for cruises 84-9-1, 84-10-1, and 85-9-1, even 

 though only net 2 was both opened and 

 closed at depth. Contamination in nets 1 and 

 3 was considered to be minimal due to the 

 relatively short time for their deployment 

 and retrieval, less than 30 seconds in total. 

 Comparison of catches from the three nets 

 indicated that contamination had not 

 seriously affected our results, as there was 

 little to no overlap among adjacent sampling 

 depths in the range of observed densities (as 

 measured by each of the three nets) per 

 sampling period (Fig. 2). Catch data from all three nets 

 of different mesh sizes at the nearshore site were com- 

 bined since sampling effort, i.e., number of samples per 

 mesh size at each depth, was the same. Due to time 

 constraints, only net 2 samples (the 333-/J mesh net at 

 the nearshore site) were sorted from cruises 84-9-2 and 

 85-10. 



Fishing depth was monitored throughout each tow 

 via the depth sensor of an electronic conductivity/ 

 temperature/depth probe package (CTD) which was 

 mounted 0.5 m above the trawl frame on the conduct- 

 ing/towing cable. Except for the nearsurface or 1 m 

 depth stratum, actual sampling depths exceeded 

 nominal depths by 0.5m. Target or nominal sampling 

 depths were maintained throughout each tow by ad- 

 justing the amount of wire out. 



Flowmeters in each net measured volume filtered 

 which generally ranged between 200 and 300 m 3 . 



