Comyns et a\: Spatial and temporal variability in growth and mortality of fish larvae in the Gulf of Mexico 



17 



i 



0) 100 - o 

 CO 50  > 



B 



Sep, 14-16 1991 



24/36 stations 



938 larvae Z=0 25 



r^=097 SE=0.03 



71 



5 



55 



180 

 160 

 140 

 120 

 100 

 80 

 60 

 40 

 20 

 



all 



II 



Sep 21-23 1991 

 29/35 stations 

 975 larvae Z=0.29 

 71 r-=0.95 SE=0.05 



L 



II 



200 . 

 180 1 ^ 

 160 H ^ 



120^ ^ 

 100 ^ y 

 80 '^"' 

 60 > 

 40 i ^ 

 20 J K 

 



25 



lis 



^j / / 



D 



Sep. 27-29 1992 50 



29/37 stations 45 



458 larvae Z=0-18 "o ^ pj 

 35 . < 

 r=0.87 SE=0,03 



30- :j > 



25 /■ K 



20 

 15 

 10 



1 1/1 , g^ 



Sep. 19-21 1993 



11/32 stations 



210 larvae 



Z=0.25 





3 5 4 4.5 5 55 25 3 



Size class (mm) 



Figure 7 



Size-frequency distributions of vermilion snapper larvae collected during four 

 cruises conducted in the northcentral Gulf of Mexico during September 1991, 

 1992. and 1993. Estimates of Z, SE, and r- refer to mortality curves produced 

 from duration-corrected age-frequency distributions. The fraction listed for 

 each cruise refers to positive (larvae were collected) stations/total stations 

 sampled. Abundance of each size class was pooled from estimates of station 

 abundances. 



stations that we observed. However, predation pressure 

 seems unlikely to have been the primary cause of this 

 variability. If among-station variability in size-selective 

 mortality was largely responsible for the differences in 

 larval growth rates, one would expect the variability in 

 size-at-age at each station to be quite variable and this 

 was not the case. Stations where the effects of size-selec- 

 tive mortality were minimal (or less) should have had 

 both fast and slow growing larvae present; yet coefficients 

 of determination (r^) were >0.90 for age versus length 

 regressions at all stations. Furthermore, there was no 

 correlation between observed growth rates and r- values 

 which would be expected if size selective predation was 

 largely responsible for the variability in growth rates that 

 we observed. 



Many studies have shown that food availability has a 

 large influence on growth rates of larvae (e.g. Houde and 

 Schekter, 1981; Buckley et al., 1987; Pepin, 1991i and it 

 is likely that station differences in food availability influ- 

 enced our observed differences in larval growth rates. We 

 did not collect the small size-fraction of prey eaten by fish 

 larvae, but our data did reveal extensive spatial and tem- 

 poral variability in the abundance of macrozooplankton. 

 Macrozooplankton biomass at station 42, where relatively 

 slow growth of Atlantic bumper occurred, was 2.6 mg/100 

 m-' whereas at station 41, where larvae were growing 



faster, macrozooplankton dry weight (3.9 g/100 m') was 

 bWc higher. When all stations were considered, there 

 was no correlation between macrozooplankton dry weight 

 and growth coefficients of lan'ae, but macrozooplankton 

 biomass was certainly very patchily distributed. For most 

 stations there was at least a 509^ difference in macrozoo- 

 plankton dry weight between one of the adjacent sta- 

 tions. It is equally likely that the smaller size fraction of 

 zooplankton that fish lai-vae eat were also very patchily 

 distributed. In addition, several other studies have shown 

 that primary production in the northern GOM is dynamic 

 and spatially heterogenous (Lohrenz et al., 1990, 1994; Re- 

 dalje et al., 1994), although these studies have focused on 

 regions influenced by discharge from the Mississippi and 

 Atchafalaya rivers. 



In many studies the significant spatial variability in 

 growth rates of field-caught lai-\'ae cannot be explained by 

 changes in water temperature. These reported differences 

 in growth rates have often been associated with factors 

 such as storm events (Lasker, 1975; Maillet and Checkley, 

 1991). different geographical locations (Mokness, 1992; 

 Nixon and Jones, 1997; Allman and Grimes, 1998;), or 

 distinct hydrographic features such as tidal fronts (Munk, 

 1993) and riverine discharge plumes (Govoni et al., 1985; 

 DeVries et al., 1990; Lang et al., 1994). All studies in the 

 GOM that have reported spatial differences in larval 



