Leffler and Shaw: Age. growth, and mortality of larval Chloroscombrus chrysurus 



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Temperature (Laurence 1978, Laurence et al. 1981, 

 Houde 1989) and food availability (Methot and Kramer 

 1979, Laurence et al. 1981, Lyczkowski-Shultz et al. 

 1988, Warlen 1988) play important roles in larval 

 growth and survival. Atlantic bumper growth rates 

 were highest in August 1986, when mean surface-water 

 temperatures and zooplankton biomass estimates were 

 greatest. 



The Atlantic bumper growth rate calculated over the 

 two cruises in September 1987 may have been higher 

 than the September 1986 growth rate because of the 

 increase in zooplankton availability (Tables 1 and 2). 

 Zooplankton displacement volume values calculated 

 from the samples taken in 1986 declined from August 

 to September. Relative zooplankton biomass values 

 have peaked, however, as late as October off the 

 Chandeleur Is. within Chandeleur Sound (102,000 

 animals/100 m^; Gillespie 1971; Fig. 1). Our zooplank- 

 ton standing-stock estimates were high compared with 

 values obtained from Mississippi River plume fronts 

 during July 1987 (0.04-0.43 mL/m^; R.F. Shaw, un- 

 publ. data). 



Atlantic bumper larvae had a dry weight-length ex- 

 ponent value of 3.25 which is similar to that of 3.32 

 determined for larval northern anchovy Engraulis 

 mordax (Lasker et al. 1970). This power term, however, 

 is lower than values determined for seven laboratory- 

 reared, cold-water marine larval species (3.76-4.77; 

 Laurence 1979), or the hypothesized standard value for 

 developing larval fish (4.0; Power 1989). 



The highest Atlantic bumper instantaneous daily 

 mortality estimate (M 0.62), observed during August, 

 was similar to that reported for estuarine larval spotted 

 seatrout Cynoscion nehulosus (0.64; Peebles and Tolley 

 1988) and, to some extent, another carangid, jack 

 mackerel Trachurus symmetricus (0.80; Hewitt et al. 

 1985). Mortality estimates, which declined throughout 

 September 1986 (0.35-0.18) and 1987 (0.30-0.17), were 

 similar and were within the reported range for several 

 larval marine species (Essig and Cole 1986, Houde 1987 

 and 1989, Pepin 1991). The highest daOy mortality rate 

 was associated with highest temperatures, highest 

 macrozooplankton displacement volumes, and highest 

 larval Atlantic bumper densities (Tables 1-3). In late 

 September 1986, however, there was a low mortality 

 rate during a time of relatively high Atlantic bumper 

 densities, lower zooplankton biomass estimates, and 

 lower temperatures. Two factors— larval size and lower 

 water temperatures— may have influenced this lower 

 mortality rate (Weinstein and Walters 1981). Mean 

 larval Atlantic bumper standard lengths (1.2mm) were 

 similar for all the cruises. Lower surface-water tem- 

 peratures, therefore, may have enhanced survival. 



reducing the Atlantic bumper mortality estimate. 

 Larval growth (i.e., daily development) and mortality 

 rates have been reported to increase wath temperature 

 (Houde 1989, Pepin 1991). The high growth-rate and 

 mortality estimate observed in August 1986 is consis- 

 tent with these findings. 



The high natural mortality observed in August is 

 probably related to predation, based on two existing 

 theories. Larval Atlantic bumper are usually aggre- 

 gated in patches (Leffler 1989) and, therefore, may 

 offer exceptional feeding opportunities to any pred- 

 ator that encounters them (McGurk 1987). Pepin (1991) 

 suggested that increased mortality rates were asso- 

 ciated vdth increasing growth rates, resulting from in- 

 creased encounters with predators. These higher 

 growth rates require a higher intake of food, causing 

 increased activity which leads to increased predator 

 encounters. 



Another possible cause for the high August mor- 

 talities may be associated with competition for limited 

 food resources, i.e., density-dependent mortality (Gush- 

 ing 1974). Food availability as indexed by the zoo- 

 plankton biomass estimate was highest during August, 

 but the high total larval fish density may have rapidly 

 depleted the food source, causing elevated mortalities. 

 Larval Atlantic bumper density was high during the 

 August cruise (608.9 larvae/100 m^) as was the total 

 larval fish density (1838.1 larvae/100 m^; Leffler 

 1989). 



This study provides preliminary information on the 

 early life history of larval Atlantic bumper. Further 

 studies need to be conducted on larval Atlantic bumper 

 to determine the relationship between these early-life- 

 history parameters and fluctuating temperatures and 

 food availability. 



Acknowledgments 



The authors would like to thank J. Ditty, D. Drullinger, 

 R. Raynie, and K. Edds for assistance in the field and 

 laboratory. Also we would like to acknowledge 

 L. Rouse, E. Turner, G. Wilson, R. McMichael, and two 

 anonymous reviewers for critical reviews of the manu- 

 script, and M. Mitchell and B. McLaughlin for their 

 assistance in preparing the graphics. Special thanks to 

 the captains and staff at Gulf Goast Research Lab- 

 oratory, Ocean Springs, MS, for use of their boats and 

 aquarium facilities. 



Financial support was provided through the Loui- 

 siana Sea Grant Gollege Program, a part of National 

 Sea Grant Gollege Program maintained by NOAA U.S. 

 Department of Gommerce. 



