WARLEN; ACE AND GROWTH OK LARVAL C.rLF MENHADEN 



were either not in the sampling area or were inac- 

 cessible to the fishing gear used. Although the 

 latter cannot be fully discounted, the former pos- 

 sibility is most likely, since larvae as they g:-ow 

 are known to be transported (Shaw et al. 1985b) 

 toward estuaries. Larvae are about 15-25 mm SL 

 (estimated from Suttkus 1956) when they enter 

 estuaries in Louisiana, and the smallest immi- 

 grating larvae are estimated from the growth 

 model <Fig. 4) to have been at least 30 days old. 

 Larvae then are probably 30-70 days old when 

 they enter Gulf of Mexico estuaries. This range is 

 very similar to that for Atlantic menhaden, B. 

 tyranniis, entering North Carolina estuaries (S. 

 M. Warlen, unpubl. data). The so-called "larval 

 drift" period for gulf menhaden is probably closer 

 to 4-10 weeks than the 3-5 weeks surmised by 

 Reintjes (1970). 



Growth of larval gulf menhaden in the north- 

 ern Gulf of Mexico varied both spatially and tem- 

 porally. For three consecutive years there were 

 significant differences in gi'owth for early season 

 (December) and late season (February) larvae 

 caught off the Mississippi River Delta. The in- 

 crease in length for early season larvae was 

 greater than for larvae hatched in late season. 

 Environmental conditions in this area differed 

 between early season and late season. Mean 

 water temperature measured during the Decem- 

 ber 1979, 1980, and 1981 cruises were 17.4°, 19.4°, 

 and 21.2°C, respectively, while in February 1980, 

 1981, and 1982 the temperatures were 13.8°, 

 15.7°, and 14.7°C, respectively. Although not 

 shown experimentally for gulf menhaden larvae, 

 there is evidence that larvae of some marine 

 fishes grow faster at higher temperatures (Lau- 

 rence 1978; Laurence et al. 1981). Jones (1985) 

 also associated higher water temperatures with 

 higher growth of larvae and found that increase 

 in length of Atlantic herring larvae hatched early 

 in the season was greater than for larvae hatched 

 late in the season. 



Growth rate of larvae caught in the same sea- 

 son but in different years was inversely related to 

 mean water temperature. Larvae caught in De- 

 cember off Louisiana showed a trend of higher 

 growth (1981 < 1980 < 1979) at lower respective 

 mean water temperatures (21.2°, 19.4°, 17.4°C); 

 similarly, the growth rates for larvae caught in 

 February (1981 < 1982 < 1980) was higher at 

 lower respective mean water temperatures ( 15.7°, 

 14.7°, 13.8°C). Other environmental factors in ad- 

 dition to temperature may also affect the growth 

 rate of larval menhaden. Food availability that 



can be an important growth-limiting factor for 

 larval fishes, may be determining the relative 

 growth rates at the lower temperatures in Febru- 

 ary. On the basis of limited data on the zooplank- 

 ton (pelecypod larvae, copepod nauplii, cope- 

 podites, and adult copepods) that could serve as 

 food for gulf menhaden larvae (Govoni et al. 

 1983), food availability (number/100 m'^) is 

 highest in February 1980, lower in 1982, and low- 

 est in 1981. Analogous food abundance data is not 

 available for the December cruises, but levels 

 would probably need to be higher on a per fish 

 basis to support the higher metabolism concomi- 

 tant with the higher December mean water tem- 

 peratures (17.4°-21.2°C). The main and interac- 

 tion effects of growth-limiting (food abundance) 

 and growth-regulating (temperature) factors on 

 larval gulf menhaden growth still must be deter- 

 mined experimentally, preferably using labora- 

 tory-spawned and -reared larvae. 



The apparent growth advantage enjoyed by 

 menhaden larvae spawned early in the season 

 (November) is only typical for a small part of the 

 population. The largest segment of the popula- 

 tion, those spawned in the peak months of Janu- 

 ary and February (Christmas and Waller 1975) 

 and that immigrated to estuaries in February- 

 April, typically had slower growth. Although 

 slower growing, larvae spawned later in the sea- 

 son may be more successful in reaching estuaries. 

 Guillory et al. (1983) found a negative relation- 

 ship between temperature and gulf menhaden re- 

 cruitment into Louisiana estuaries. The larger 

 estuarine recruitment later in the season may be 

 related to winter-early spring (January-April) 

 predominant west-northwest longshore flow of 

 coastal water within and just outside the coastal 

 boundary front producing longshore advective 

 transport and of lesser importance by episodic, 

 short-term cross-shelf advection associated with 

 cold fronts (Shaw et al. 1985b). They hypothesized 

 that longshore currents facilitated the movement 

 of larvae toward shore. Only for December did 

 they note a reverse flow (eastward) that would 

 not allow larvae to be transported toward estuar- 

 ies west of the Mississippi delta. 



The between transect comparisons of growth 

 rate of larvae caught off Texas and Louisiana in 

 February showed a difference in 1981 but not in 

 1982. Again higher growth rates were associated 

 with higher mean water temperatures. Larvae 

 from the LA February 1981 sample (x water tem- 

 perature = 15.7°C) grew faster than larvae from 

 the TX February 1981 sample U water tempera- 



87 



