[ XLi/l/VU-Ti UO 



Suspension of the Habitat Traps 



The strong surf and current conditions 

 prevailing around the pier pilings required the 

 development of a special system for suspending 

 the habitat traps, as shown in Figure 3. This sys- 

 tem allowed the trap to rise and fall during the 

 tidal cycle, which has a vertical range of 

 approximately 2.5 m, and in response to wave mo- 

 tion, thereby always remaining at the surface. The 

 counterweight maintained a taut line and 

 counteracted drag on the habitat trap caused by 

 currents. No bottom anchor was necessary, thus 

 eliminating problems of retrieval and fouling on 

 kelp, and allowing the traps to be raised and 

 lowered easily for examination. The traps also 

 were held away from the pier pilings by the pole 

 extension in order to prevent damage and en- 

 tanglement. 



Paired Neuston Nets 



The results of night-lighting operations, dis- 

 cussed in a later section, suggested that the 

 puerulus stage of P. interruptus swims within a 

 few centimeters of the ocean surface. Thus, 

 plankton or other nets towed below or near the 

 surface probably would fail to catch this stage. 

 Even surface tows made with a net breaking the 

 surface might not be successful if it were trailed 

 directly behind a vessel, because the "snow plow" 

 action of the vessel might effectively push surface 

 organisms away from the path of the net. Thus, 

 paired neuston nets which could be mounted 

 laterally on a small boat were developed to allow 

 unobstructed surface water sampling, as shown in 

 Figure 4. The presumed rarity of the puerulus 

 stage required that these nets be of relatively 

 large mesh size (5.0 mm) to allow reasonably fast 

 towing speeds with a small boat (approximately 3 

 knots) and the filtration of large volumes of water. 

 The short length and lateral position of the nets 

 allowed their cod ends to be removed and emptied 

 periodically without hauling the entire net from 

 the water. The nets were towed with approximate- 

 ly 30% of the frontal area above water to ensure 

 continual filtration of the top few centimeters of 

 surface water, even during periods of low surface 

 waves. The average frontal net area maintained 

 submerged was approximately 0.7 square meter 

 per net. Only rough estimates were made of the 

 volumes and surface areas of water filtered during 

 initial trials conducted during this study, as these 



Figure 3.-The method employed in suspending habitat traps 

 from the Scripps Institution pier. 



were primarily concerned with evaluation of the 

 sampler and with qualitative rather than quanti- 

 tative results. The estimates of the water volume 

 and surface area sampled were based on average 

 boat speed and duration of the tow. 



RESULTS 



Night-Light Observations 



Night-light observations were first conducted 

 briefly in May 1968, and then resumed for a 2-mo 

 period in September 1968. Night-light observa- 

 tions were also conducted again in the spring and 

 summer of 1969, primarily to supplement infor- 

 mation from the habitat traps also maintained 

 during this period. The results of these observa- 

 tions, together with pertinent environmental da- 

 ta, are summarized in Table 1. During calm eve- 

 nings in September 1968, pueruli were collected 

 from the Scripps Institution pier at an average 

 rate of approximately 4 per hour, with the highest 

 rate reaching 12 pueruli collected within 90 min (8 

 per hour). No pueruli appeared during night- 

 lighting conducted in October 1968, and further 

 attempts were curtailed due to the onset of rough 

 winter surf conditions which made the operation 

 difficult. Night-lighting activities also were con- 

 ducted at other San Diego localities during the 

 period September-October 1968. These were: 

 Quivira Basin in outer Mission Bay, Shelter Island 

 in outer San Diego Bay, and the Imperial Beach 

 pier. No pueruli were observed at any of these 



363 



