10 



Abstract— Extensive plankton collec- 

 tions were taken during seven Septem- 

 ber cruises (1990-93) along the inner 

 continental shelf of the northcentral 

 Gulf of Mexico ( GOM ). Despite the high 

 productivity and availability of food 

 during these cruises, significant small- 

 scale spatial variability was found in 

 larval growth rates for both Atlantic 

 bumper (Chloroscombrus chrysurus, 

 Carangidae) and vermilion snapper 

 {Rhomboplites aurorubens, Lutjani- 

 dael. The observed variability in larval 

 growth rates was not correlated with 

 changes in water temperature or asso- 

 ciated with conspicuous hydrographic 

 features and suggested the existence of 

 less-recognizable regions where condi- 

 tions for growth vary. Cruise estimates 

 of mortality coefficients (Z) for larval 

 Atlantic bumper (n=32,241 larvae from 

 six cruises) and vermilion snapper (n = 

 2581 larvae from four cruises) ranged 

 from 0.20 to 0.37 and 0.19 to 0.29, re- 

 spectively. Even in a subtropical cli- 

 mate like the GOM, where larval-stage 

 durations may be as short as two 

 weeks, observed variability in growth 

 rates, particularly when combined with 

 small changes in mortality rates, can 

 cause order-of-magnitude differences 

 in cumulative larval survival. To what 

 extent the observed differences in 

 growth rates at small spatial scales 

 are fine-scale "noise" that ultimately 

 is smoothed by larger-scale processes is 

 not known. Future research is needed 

 to further characterize the small-scale 

 variability in growth rates of larvae, 

 particularly with regard to microzoo- 

 plankton patchiness and the temporal 

 and spatial pattern of potential preda- 

 tors. Small-scale spatial variability in 

 larval growth rates may in fact be the 

 norm, and understanding the implica- 

 tions of this subtle mosaic may help 

 us to better evaluate our ability to 

 partition the causes of recruitment 

 variability. 



Small-scale spatial and temporal variability 

 in growth and mortality of fish larvae 

 in the subtropical northcentral Gulf of Mexico: 

 implications for assessing recruitment success 



Bruce H. Comyns 



Department of Coastal Sciences 



College of Marine Sciences 



The University of Southern fVlississippi 



703 East Beach Drive 



Ocean Springs, Mississippi 39566 



E mail address bruce.comyns(g)usm.edu 



Richard F. Shaw 



Department of Oceanography and Coastal Sciences 

 School of The Coast and Environment 

 Louisiana State University 

 Baton Rouge, Louisiana 70803 



Joanne Lyczkowski-Shultz 



Southeast Fisheries Science Center 

 National Manne Fishenes Service 

 P,0- Drawer 1207 

 Pascagoula, Mississippi 39568 



Manuscript accepted 1 1 July 2002. 

 Fish. Bull. 101(2):10-21 (2003). 



For many marine fishes year-class 

 strength undergoes large fluctuations 

 because of the inherent variability in 

 larval, postlarval, and juvenile survi- 

 vorship (Hjort, 1914; Gushing, 1975; 

 Lasker, 1975; Hunter, 1982; Houde, 

 1987; Goshorn and Epifanio, 1991; 

 Pepin and Myers, 1991). Understanding 

 and quantifying recruitment variability 

 remains one of the greatest challenges 

 in fisheries science today (Fritz et al., 

 1990; Gushing and Horwood, 1994; 

 Leggett and Deblois, 1994; Mertz and 

 Myers, 1995). Early survival rates are 

 influenced not only by predation pres- 

 sure but also by gi-owth rate which can 

 alter the duration of the larval stage 

 when larvae are exposed to accumula- 

 tive high mortality rates (Houde, 1987; 

 Ghambers and Leggett, 1987; Ander- 

 son, 1988; Bailey and Houde, 1989). 

 Pepin (1991) formalized this concept by 

 depicting the cumulative mortality (C) 

 of a population from stage a to older 

 stage b as the direct function of the 

 instantaneous growth ig{x\) and mor- 

 tality (AfUl) rates such that 



■mx] 



g\x] 



dx, 



where .v are factors that influence the 

 vital rates (M andg) such as food avail- 

 ability, temperature, and abundance of 

 potential predators. 



Many questions remain concerning 

 the causes of recruitment variabil- 

 ity. Reasons for variability include the 

 following: the inherent variability in 

 growth and mortality rates and result- 

 ing survivorship; difficulties in estimat- 

 ing mortality rates with sufficient ac- 

 curacy and precision; and the complex 

 interrelationships among factors that 

 affect survivorship of larvae (Parrish, 

 1973; Laurence, 1979; Houde, 1987; 

 Beyer, 1989; Pepin, 1991 ). Houde ( 1989) 

 hypothesized that cohort survivorship 

 is more sensitive to small changes in 

 vital rates in high latitude systems 

 than in tropical or subtropical systems 

 because the colder temperatures cause 

 slower growth rates and longer larval 

 stage durations, i.e. up to 100 days. 



