Ml 



iller et al : Relation between otolith size and larval size at hatching for Gadus morhua 



295 



N 



for faster growing fish. Moreover, after comparing 

 otolith sizes at hatching, Meekan and Fortier ( 1996) 

 suggested that the potential for faster growth ex- 

 pressed by the survivors may have been present at 

 hatching. 



Our ability to detect examples of phenotypic selec- 

 tion evidenced above depends on our ability to quan- 

 tify the distribution of traits in the entire cohort and 

 to hindcast the distribution of traits in the survivors 

 (Miller, 1997). For much of the research discussed 

 above, and for many other individual-based studies, 

 otolith microstructural analysis is used to detect 

 phenotypic selection. However, to apply this ap- 

 proach, three requirements must be satisfied 

 (Francis, 1990): 1) primary increments must be de- 

 posited at a known and consistent rate; 2) there must 

 be a quantifiable relationship between growth and the 

 width of increment rings; and 3) the initial otolith size, 

 defined by the presence of a check mark, must be re- 

 lated to the size of the fish at the formation of the check. 



For cod, the regularity of increment deposition has 

 been verified and validated ( Bergstad, 1984; Radtke, 

 1984; and see Geffen, 1995, for a recent example). 

 Thus, the first condition for back calculation has been 

 satisfied in cod. However, there remains consider- 

 able uncertainty over the status of 

 the remaining two conditions. The 

 relationship between somatic and 

 otolith growth in cod is unclear ( see 

 Geffen, 1995, and Meekan, 1997, for 

 opposing views). The lack of a clear 

 relationship between rates of so- 

 matic and otolith growth may result 

 from the natural variability among 

 and within many cod populations 

 (Brander, 1994; Chambers, 1997). 

 Typcially, cod larvae grow at highly 

 variable rates, and somatic and 

 otolith growth may become disasso- 

 ciated (Suthers et al., 1989; Cam- 

 pana and Hurley, 1989; Geffen, 

 1995). Concerns over the validity of 

 the assumed relationship between 

 fish and otolith size at hatching also 

 arises because of inter- and intrapo- 

 pulation variation. Considerable 

 variation has been reported in lar- 

 val size at hatching both within and 

 among populations (Bolz and Lough, 

 1983; Bergstad, 1984; Radtke, 1984; 

 Knutsen and Tilseth, 1985; Miller 

 et al., 1995). Currently, no clear link 

 between otolith size and larval size 

 at hatching has been established for 

 any population. 



The objective of this paper is to examine the rela- 

 tionship between otolith size at hatching and larval 

 size at hatching for cod. Specifically, we address 

 whether otolith size at hatching is significantly cor- 

 related with larval size at hatching, and whether all 

 otoliths provide an equally accurate and precise es- 

 timate of larval size at hatching. We use data col- 

 lected for cod on the Scotian shelf collected between 

 1991-93 to address these questions. 



Methods 



Sampling was carried out during 29 cruises on the 

 Scotian Shelf between March 1991 and May 1993 

 (Fig. 1). O'Boyle et al. (1984) have given a general 

 description of the Scotian Shelf system. Full details 

 of the sampling method are given by Miller et al. 

 (1995). We summarize the general sampling design 

 here and provide additional details that relate to the 

 specific objectives of this study. 



Twenty six cruises were designed to provide broad- 

 scale information on temporal and spatial distribu- 

 tions and abundance of cod eggs, larvae, and juve- 

 niles (Fig. 1). We sampled a rectangular grid of 45 



44 - 



43 



63 



62 



61 



60W 



Figure 1 



Map of Scotian Shelf showing the area of sampling locations. Shown on the 

 figure are the 50- and 100-m depth contours, the principal sampling loca- 

 tions within the 15-station grid (open circles), and the location of Halifax, 

 NS, Canada, for reference. 



