Wexler et al.: Temporal variation m larval growth of Thunnus albacores in the Panama Bight 



15 



time. Interactive effects of food availability and tem- 

 perature may have a profound effect on growth, and we 

 would expect that energetic demands become greater 

 at the higher water temperatures, in which case the 

 potential for faster growth would be attained through 

 increased food consumption (Houde, 1989), providing 

 food resources are not limited. Thus, it may be more 

 reasonable to assume that food availability has a more 

 significant impact on growth than SSTs during the 

 reduced upwelling season when temperatures are con- 

 sistently greater than 27°C. 



Although water temperature has been shown to regu- 

 late the formation and short-term growth of otoliths in 

 some marine fish species (Barber and Jenkins, 2001), 

 the causal factor affecting otolith growth could also 

 be associated with food availability (Govoni et al., 

 1985; Johnson et al., 2002) and composition (Woodbury, 

 1999). Otolith growth appears to be a conservative 

 measure of somatic growth (i.e., otoliths continue to 

 grow even with decreases in somatic growth; Campana 

 and Neilson, 1985); therefore a significant change in 

 otolith growth could signal a dramatic change in the 

 larval feeding environment. We observed significant 

 interannual differences between relationships of otolith 

 size and fish size. Otoliths were disproportionately 

 larger in slower growing groups of larvae, but addi- 

 tionally, during the period of 1997 when SSTs were 

 abnormally high, otoliths were growing at a greater 

 rate in relation to fish size (Fig. 6). Elevated water 

 temperatures have been shown to increase short-term 

 otolith growth (Hoff and Fuiman, 1993; Barber and 

 Jenkins, 2001). Althoughtemperature may have af- 

 fected otolith growth in 1997, lower food levels may 

 have been the causal factor for a significantly slower 

 otolith growth rate and the smaller mean otolith di- 

 ameter of the 1991 group. 



Recruitment implications 



The probability of survival from early stages of devel- 

 opment to recruitment in marine fishes is thought to 

 be influenced by prerecruit starvation and predation 

 mortality associated with slower growing and nutrition- 

 ally weakened individuals (Gushing, 1975; Houde, 1987; 

 Margulies, 2001) and predator-prey interactions and 

 densities (Cowan and Houde, 1992). However, Peterman 

 et al. (1988) have demonstrated that the survival rate 

 of prerecruits older than 19 days of age is more vari- 

 able than earlier life stages, and it is this stage that 

 determines recruitment strength in northern anchovy 

 (EngrauUs mordax). Yellowfin tuna in the Pacific Ocean 

 exhibit a pattern of reproduction that has strong poten- 

 tial for regulation of recruitment during prejuvenile 

 stages, when initial numbers in a cohort are quite 

 large and vital rates (e.g., growth, mortality) are high 

 (Houde, 1987; Margulies, 2001). However, the poten- 

 tial for recruitment fluctuations is also high for the 

 relatively long juvenile stage of yellowfin tuna as well 

 (Houde, 1987; Margulies, 2001). In the EPO, yellowfin 

 tuna are recruited to the fishery at a fork length of about 



30 cm and at an age of approximately 6 months (Wild, 

 1994; Maunder and Harley^°). Recruitment estimates 

 calculated at quarterly intervals for yellowfin tuna in 

 the EPO are variable and may be influenced by envi- 

 ronmental fluctuations (Maunder and Harley^"). It is not 

 clear what effect slower or faster growing cohorts may 

 have on recruitment, but the growth- and stage-specific 

 mortality rates (Houde, 1987; Pepin, 1991; Comyns et 

 al., 2003) of a cohort may determine whether it survives 

 to recruitment or when it enters the fishery. We were 

 unable to estimate mortality rates for the larvae col- 

 lected in each of the three years because during some 

 years they were collected on single sampling dates 

 (Essig and Cole, 1986). Nonetheless, the recruitment 

 estimate (ca. 1.44x10^ individuals) following the period 

 in 1991, when the smallest size-at-age and the slowest- 

 growing larvae were present was approximately half the 

 amount estimated (ca. 3.11x0' individuals) following 

 the period in 1990 (Maunder and Harley^"; Maunder"), 

 when larvae were larger and growing more rapidly. The 

 recruitment estimate following the 1997 period (ca. 

 3.06x10' individuals) was slightly less than that fol- 

 lowing the 1990 period. Larvae of the 1997 group were 

 growing at a rate similar to that of the 1991 group, but 

 the mean size-at-age was significantly greater, which 

 may indicate a size advantage favorable for prerecruit 

 survival (Miller et al., 1988). The growth rates and 

 conditions estimated for yellowfin tuna larvae within 

 the small scale area of our study may apply to local- 

 ized recruitment estimates within the Panama Bight 

 and not those of the entire EPO, given that restricted 

 movements of yellowfin tuna occur in the EPO (Schaefer, 

 1991; Wild, 1994). Although these inferences may be 

 applicable only to recruitment within the Panama Bight, 

 they may still indicate that growth rates and the mean 

 size-at-age during the larval and early-juvenile stages 

 are a contributing factor to recruitment variability of 

 yellowfin tuna in the EPO. 



Our study provides the first examination of factors 

 affecting larval growth and possibly prerecruit sur- 

 vival of yellowfin tuna in the Pacific Ocean. Future 

 research is necessary to better understand small-scale 

 variability in growth and mortality rates of yellowfin 

 tuna larvae within their feeding environment. To that 

 end, we are conducting further studies at the Achotines 

 Laboratory in Panama to examine vital rates of this 

 species and the interactions of these vital rates with 

 biological and physical processes to complement our 

 field measurements and to gain more insight into pre- 

 recruit survival of yellowfin tuna. 



1" Maunder, M. N., and S. J. Harley. 2004. Status of yellowfin 

 tuna in the eastern Pacific Ocean in 2002 and outlook for 

 2003, p. 5-119. Inter-American Tropical Tuna Commission, 

 stock assessment report 4. lATTC, 8604 La Jolla Shores 

 Drive, La Jolla, CA 92037. 



1' Maunder, M. 2004. Personal commun. Inter-American 

 Tropical Tuna Commission, 8604 La Jolla Shores Drive, La 

 Jolla, CA 92037. 



