WARLEN and CHESTER: LARVAL SPOT OFF NORTH CAROLINA 



Estimated Age and 

 Back-Calculated Length 



We counted the number of growth increments on 

 otoHths (eg., Pannella 1971; Brothers et al. 1976) to 

 estimate age (in days) of each larva. Laboratory- 

 reared larval spot have been shown to deposit an 

 average of 1 ring/d on their otoliths (Peters et al. 

 1978; Warlen 1984^), but do not begin to do so until 

 5 d after hatching, a time coincident with yolk-sac 

 absorption and first feeding activity at 20°C (Peters 

 et al. 1978). Therefore, we added 5 to the number 

 of counted increments to estimate age. The spawn- 

 ing date of each larva was estimated by subtracting 

 age in days from date of capture 



After we measured each larva in alcohol to the 

 nearest 0.1 mm standard length (SL), we teased the 

 largest pair of otoliths (sagittae) from the surround- 

 ing tissue, cleaned them in distilled water, and 

 mounted them on a glass microslide under a thin 

 layer of Flo-Texx^ mounting medium. They were ex- 

 amined with a compound microscope fitted with a 

 television camera. Growth increments were counted 

 from images of otoliths on a video monitor at 

 magnifications of at least 400 x. For selected larvae, 

 otolith radius and the growth increments along it 

 were measured to the nearest 0.1 yim with a filar 

 ocular micrometer. We then used Lee's (1920) 

 modification of the direct proportion formula to 

 back-calculate lengths and reconstruct the growth 

 of each fish. In addition to the assumption that 

 growth increments be daily, the reliability of back- 

 calculated lengths requires that grov^h of the otolith 

 must be linearly related to growth of the fish. We 

 found, for larvae 2.2-12.4 mm SL, that the relation 

 between body length and otolith radius was linear: 



body length (mm) = 2.202 + 0.045 * otolith 



radius (/.<m) 

 n = 32, r2 = 0.95. (1) 



Weight-Length Relationships 



Because larval fish are not weighed in many 

 ichthyoplankton field studies, a weight-length rela- 

 tionship is required to describe the growth of popula- 

 tions, assess production in terms of dry weight, and 



^Warlen, S. M. 1984. Rates of increment formation in otoliths 

 of larval gulf menhaden, Brevoortia patronus and spot, Leiostomus 

 xanthurus. Unpubl. manuscr. Southeast Fisheries Center 

 Beaufort Laboratory, National Marine Fisheries Service, NOAA, 

 Beaufort, NC 28516-9722. 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



estimate weight where only length is known. We 

 determined a dry weight-length relationship from 

 125 laboratory-reared larvae and early juveniles 

 (2.7-29.6 mm SL). Live fish were anesthetized in a 

 solution of MS-222 (tricane methanesulfonate), 

 removed from the solution, and measured to the 

 nearest 0.1 mm SL. Fish were then rinsed in distill- 

 ed water, placed on preweighed Nuclepore^ mem- 

 brane filters, freeze-dried, and weighed to the 

 nearest 1 ^g. 



RESULTS 



Spawning and Larval Movement 



The temporal pattern of spawning found here, 

 though perhaps influenced by the particular dates 

 and stations sampled (Ikble 1), indicated that spot 

 is a late October-early March spawner (Fig. 2). The 

 majority (67%) of fish collected during the fall and 

 winter of 1979-80 were spawned during December 

 or January (Fig. 2). 



The offshore larval distribution by estimated age 

 and length suggests that spawning occurred over the 

 outer continental shelf. Both mean age and length 

 varied inversely with distance from shore (Figs. 3, 

 4). Youngest (<25 d) and smallest (<4 mm) larvae 

 were found most often near or in the Gulf Stream, 

 80-100 km off Beaufort Inlet, paralleling the 183 m 

 depth contour (stations 5, 16, 17, 18). However, com- 

 parable ages and sizes also were found in the mid- 

 shelf area (stations 1, 14, 15) early in the spawning 

 season (December 1978; November and December 

 1979). Older (40-50 d) and larger (>8 mm) larvae 

 generally occurred closer to shore within about 40 

 km of Beaufort Inlet in <25 m of water (stations 

 10-13, 19), except during February and March 1980 

 when some larvae were caught 50 km offshore at 

 station 14. Spawning apparently is continuous 

 between late October and late February, since young 

 larvae occurred every month at the three offshore 

 stations (16-18). Most spawning off North Carolina 

 probably occurs between 75 and 95 km offshore, ex- 

 cept for some activity in the mid-shelf area early in 

 the spawning season. Our age-length data provided 

 no evidence that spot spawn near shore 



Estuarine Immigration 



Larvae entered the Newport River estuary over 

 a 4-mo period from about mid-December to mid- 

 April (Figs. 2, 5). None were caught prior to 

 December 1979 nor after 17 April 1980. Relative 

 abundance of larvae collected at Pivers Island dur- 



589 



