The influence of temperature on the stage at hatch of laboratory reared Gadus morhua and implications for comparisons of length and morphology A. J ORDAAN*, S. E. H AYHURST AND L. J. K LING School of Marine Sciences, University of Maine, 214 Libby Hall, Orono, Maine, 04469, U.S.A. (Received 9 September 2004, Accepted 23 May 2005) A staging table for yolk-sac larvae of Atlantic cod Gadus morhua was developed and applied to determine the development stage at hatch. Larvae (n ¼ 20) were sampled during the period from 50% hatch to first-feeding from four temperature treatments (c. 2, 4, 8 and 12  C). Data recorded included standard length, development stage, yolk-sac area and evidence of feeding by larvae. The results indicated that the stage of development at hatch was positively correlated to larval size. Furthermore, developmentally based changes in morphology shortly after hatch reduced much of the initial difference in size, although some difference still existed. No relationship between size at hatch and the incidence of first-feeding larvae was found. Based on these observations, it is recommended that the development stage of hatching larvae be reported in studies investigating size and morphological differences when observing individual larval fish that originate from different populations, batches or species. # 2006 The Fisheries Society of the British Isles Key words: Atlantic cod; batch comparisons; length; morphology; stage; temperature. INTRODUCTION Laboratory studies have shown that higher egg incubation temperatures produce smaller larvae at the time of hatch (Laurence & Rogers, 1976; Blaxter, 1992; Canino, 1994; Pryor & Brown, 1998). Pepin et al. (1997), however, showed that Atlantic cod Gadus morhua L. eggs incubated at 1  C had smaller mean size (standard length, L s ) at hatch (c.29 mm) compared to eggs incubated at 1–3  C (c.36–37 mm) and 5–7  C(c.40–41 mm). The combined observations of recent studies suggest a dome-shaped relationship between size at hatch and incubation temperature (Chambers, 1997; Galloway et al., 1998), much in the same way that some meristic characteristics, such as vertebral and fin ray counts, are related to incubation temperature (Blaxter, 1992). Pepin et al. (1997) surmise that an increased metabolic load incurred by Atlantic cod embryos incubated at temperatures near the lower end of their thermal tolerance range results in lost growth potential and a smaller length at hatch. These observations suggest that, *Author to whom correspondence should be addressed. Tel.: þ1 207 581 4432; fax: þ1 207 581 4388; email: adrian.jordaan@umit.maine.edu Journal of Fish Biology (2006) 68, 7–24 doi:10.1111/j.1095-8649.2005.00857.x, available online at http://www.blackwell-synergy.com 7 # 2006 The Fisheries Society of the British Isles