Proceedings of the Third World Fisheries Congress: Feeding the World with Fish in the Next Millenium—The Balance between Production and Environment
Impacts of Egg Size and Hatching Temperature on Yolk Size at Hatching, Larval Hatching Time, and Larval Size of Atlantic Cod Gadus morhua
Yingming Zhao, Yong Chen, Joseph A. Brown
Fish year-class strength can be established at very early life stages, such as the egg and larval stages (Bailey and Houde 1989). The survival and growth of fish at these life stages can greatly influence the number or biomass of fish recruited to fisheries, and thus the sustainability of the fisheries stocks. Small variations in these parameters between years can result in large fluctuations in recruitment. It is therefore important to identify and understand the factors that may influence the growth and survival of larval fish.
Egg quality is one of many factors that can influence the survival and development of fish eggs. However, because of limited knowledge of the factors affecting egg quality and large variation among fish species in egg quality, there is little agreement regarding a general index that can be used to assess egg quality (Brooks et al. 1997). In some studies, buoyancy has been used as an indicator (McEvoy 1984; Kjorsvik et al. 1990). Features such as appearance and development of larvae also have been used (Bromage et al. 1994; Kjorsvik 1994). Although little evidence indicates that egg size can be an indicator of egg quality for many fish species (Springate and Bromage 1985; Bromage et al. 1992), egg size has been found to be positively related to larval size at hatching and future larval survival and growth (Blaxter and Hempel 1963; Knutsen and Tilseth 1985; Marteinsdottir and Able 1992; Miller et al. 1995). Moodie et al. (1989) reported that egg size was positively correlated with yolk and oil volume in walleye Stizostedion vitream; the larvae from large eggs tended to be significantly larger, and their survival, growth, and feeding were better than those from small eggs. Rijnsdorp and Jaworski (1990) used survey data and mathematical models to demonstrate that egg survivorship was size-selective in Pleuronectes platessa plaice and Gadus morhua cod, and they concluded that egg mortality rate decreased as egg size increased.
For fish larvae, growth and survival are size dependent (Beyer 1989; Pepin 1993; Pope et al. 1994; Houde 1997; Otterlei et al. 1999). Initial larval size is an important factor in determining subsequent survival, growth, and feeding behavior (Miller et al. 1988; Moodie et al. 1989). In addition to the impacts of egg size on larval size at hatching described previously, Pepin et al. (1997) reported that hatching temperature significantly influenced hatching time and larval size at hatching. Miller et al. (1995) reported that temperature played a significant role in determining both egg diameter and larval size and thus could explain seasonal variation in egg size and larval size.
Atlantic cod Gadus morhua fisheries in the northwest Atlantic Ocean have provided food and income for eastern Canada for more than 400 years (Trippel 1995). For the past 15 years, however, all cod stocks have experienced dramatic declines (Baird et al. 1992; Sinclair 1993). Many hypotheses have been developed to explain this phenomenon. It has been attributed to recruitment failure, which may have be caused by overfishing of spawning stocks, unfavorable environmental conditions causing increased mortality of fish early in life, or both.