Technical Paper 28: Bureau of Sport Fisheries and Wildlife 



Figure It— R. V. Dolphin , offshore research vessel of the Sandy Hook Marine Laboratory. 



SURVEY DESIGN 



We wanted to sample ichthyoplankton of 

 the continental shelf from Nantucket Shoals to 

 Cape Lookout as frequently as possible during 

 the year . We planned to complete each cruise 

 in 2 weeks, with cruises about 6 weeks apart. 

 This is the minimum interval consistent with 

 ship and equipment upkeep, logistics, processing 

 of collections, and sharing the ship with other 

 laboratory research projects . Although gear 

 breakdowns, adverse weather, and the usual 

 problems of ship use delayed or prolonged some 

 cruises, we completed eight plankton surveys 

 with the Dolphin in the period December 1965 to 

 December 1966. 



We selected the Gulf V plankton net for 

 sampling fish eggs and larvae because of the 

 following favorable characteristics: (1) The 

 Gulf V can be towed at speeds over 5 knots and 

 thus should have higher capability for capture of 

 larvae during daytime than stramin nets which 

 must be towed at 2 knots or less and thus allow 

 many larvae to escape because of visual warning; 

 (2) the Gulf V has a larger mouth opening than 

 most other high-speed plankton nets, yielding 



higher catches of eggs and larvae per tow and 

 providing sample sets of higher reliability for 

 comparison; (3) flow -through characteristics of 

 the net are good enough to prevent extensive 

 damage to larvae which could make identification" 

 difficult; (4) the Gulf V is simple in design and 

 rugged in construction, thus guaranteeing a 

 minimum of trouble in constant use aboard ship. 



The northern sector of the Atlantic 

 continental shelf is characterized by a seasonal 

 thermocline that develops in the spring and re- 

 mains through early fall (Walford and Wicklund, 

 1968). The thermocline is typically shallower 

 nearest shore, starting at 8 to 10 meters (4 .4 to 

 5.5 fm.) and is deeper offshore, starting at 15 to 

 30 meters (8.2 to 16.4 fm.). Therefore, to in- 

 sure sampling of the whole water layer above the 

 thermocline we decided to collect to a depth of 

 33 meters (18 fm.). We used two Gulf V nets 

 simultaneously, at all stations where depth per- 

 mitted, in order to sample separately the upper 

 and lower sectors of our sampling depth range. 

 The tows were step-oblique, the upper net sam- 

 pling from to 15 meters, the lower from 18 to 

 33 meters. 



