Temperature dependent otolith growth of larval and early juvenile Atlantic cod (Gadus morhua)

E. Otterlei 0 A. Folkvord 0 G. Nyhammer 0 0 E. Otterlei, A. Folkvord, and G. Nyhammer: University of Bergen, Department of to A. Folkvord: Tel: The otolith (lapillus) size-fish size relationship was examined for offspring of two Atlantic cod stocks, reared at different temperatures. Larvae and early juveniles reared at high temperatures (fast growing), had larger otoliths at a given length than fish reared at low temperatures (slow growing). Within a given temperature group, however, faster growing cod tended to have proportionally smaller otoliths, although the difference was not always significant. Moreover, the otolith radius of Norwegian coastal cod was larger, at given fish lengths, compared to that of the northeast Arctic cod. An ontogenetic shift in the allometric otolith size-fish size relationship occurred at onset of metamorphosis (12 mm). Mean daily otolith growth of the lapillus radius increased with increasing temperature from 4 to 14 C and was size dependent and peaked at a larval length of about 25 mm. The radius of the lapillus at hatching was poorly correlated with larval length at day 56 for both stocks, suggesting that the potential for fast growth may not necessarily be reflected in traits present at hatching. The effects of temperature, stock and ontogeny are discussed with regard to the assumption of constant proportionality between otolith growth and fish growth. 1054-3139/02/040401+10 $35.00/0 - Given the potential use of the otolith as an individual record of size and growth, it is important to examine the factors that might affect the relationship of otolith growth and somatic growth with regard to the proportionality assumption for back-calculation (Hare and Cowen, 1995). The various techniques of backcalculation assume that otolith growth and somatic growth are in constant proportion (e.g. Campana, 1990; Francis, 1990). However, several examples of variable relations between fish growth rate and otolith growth rate have been demonstrated (e.g. Mosegaard et al., 1988; Maillet and Checkley, 1990; Sogard, 1991; Folkvord et al., 1996). Mosegaard et al. (1988) found the response of otolith growth rate to increasing temperature for Arctic char (Salvelinus alpinus) to be totally different from the optimum curve of somatic growth rate, such that otolith growth rate continued to increase at temperatures above those for maximum somatic growth. Faster growing fish also appear to develop smaller otoliths at a given length than slower growing (older) individuals, exposed to the same temperature (e.g. Reznick et al., 1989). A response lag of otolith growth compared with somatic growth, observed for herring (Clupea harengus) (Folkvord et al., 1996), and continuing otolith growth of fish during periods of negligible somatic growth (Maillet and Checkley, 1990; Sogard, 1991), implies that there is not a simple proportional relationship between otolith growth and somatic growth on a daily basis (Hare and Cowen, 1995). The functional relationship between otolith and fish growth may also differ between ontogenetic stages (Campana, 1984; Hare and Cowen, 1995). Throughout the extensive area of distribution in the North Atlantic, the several cod (Gadus morhua) stocks are exposed to a variety of environmental conditions (e.g. Brander, 1995, 1997; Planque and Fredou, 1999), generating systematic differences in ambient temperature between geographic areas. Somatic growth in length and weight of Atlantic cod is significantly influenced by temperature, and stock-specific differences in weight at age are documented (e.g. Brander, 1995; Otterlei et al., 1999). However, the relationship between somatic and otolith growth in cod is more unclear (Geffen, 1995; Miller et al., 1999), and there is little information concerning the relative importance of environmental and genetic factors affecting otolith growth. Here, the main goal was to evaluate the effects of: (i) temperature, (ii) growth rate, (iii) ontogeny and (iv) fish stock on the otolith sizefish size relationship of Atlantic cod larvae and early juveniles and the potential impact on the proportionality of otolith growth and somatic growth. Materials and methods A detailed description of the materials and methods used, somatic growth in length and weight including temperature- and stock-specific survival data is presented in Otterlei et al. (1999). Biological material Northeast Arctic cod (NA) and Norwegian coastal cod (NC) eggs were naturally spawned during two seasons (5 April 1995 and 14 March 1996) at Parisvatnet, ygarden and Austevoll Aquaculture Research Stations in western Norway. Eggs were incubated separately in 70 l aerated black conical tanks at 7.37.9 C and salinity ranging from 32.933.8. In both seasons 50% hatching occurred 12 days after fertilization, referred to as day 0 of larval age. Experimental design Two experiments with similar design were carried out in 1995 and 1996. Initial stocking densities were 1400 larvae, 700 NC and 700 NA, per tank. The two-day-old yolk-sac larvae were individually counted and randomly distributed into replicate green, square, fibreglass tanks holding 500 l. In 1995, the fish were co-reared for eight weeks at two different temperatures ( s.d.); 4.1 0.2 and 8.0 0.1 C and in 1996 at 6.1 0.1, 10.0 0.2, 12.0 0.3 and 14.1 0.2 C. In order to distinguish NC from NA larvae in 1995, we marked the otoliths of the NA stock with alizarin complexone (100 mg l 1 for 24 h) two days before hatching, whereas in 1996 the otoliths of the NC larvae were marked (Tsukamoto et al., 1989; Blom et al., 1994). Feeding and rearing conditions Larvae and juveniles were fed live natural zooplankton in excess (>1000 ind l 1) and cultivated algae, Isochrysis galbana and Rhodomonas baltica, were added to the rearing tanks. A simulated natural light regime (14L:10D increasing to 19L:5D) for the latitude of Bergen (60 25 N) was used. Temperature was measured twice a day throughout the experiments. The water was gently aerated to reduce the patchiness of the prey and larvae. Oxygen concentration (%) was recorded daily and remained above 70% saturation, while salinity ranged from 30.9 to 33.5. Sampling procedure Thirty larvae were routinely sampled weekly from each tank for standard length (SL) measurements. The fish (n=2172) were measured live and transferred individually into marked vials, killed in liquid nitrogen and stored at 80 C for subsequent otolith analysis. The lapilli were extracted under a dissecting microscope equipped with a polarizing filter, and mounted in clear nail varnish on glass slides. Both lapilli were checked for alizarin marks using a fluorescence microscope (Zeiss Axioscope) at 200 magnification and classified as either NC or NA. Lapilli were selected for a number of reasons (Meekan and Fortier, 1996; Miller et al., 1999), but mainly because it was possible to detect the alizarin mark without further polishing. The right lapillus (left when missing) was examined at 40 (...truncated)