American Fisheries Society Symposium 5:1-6, 1988 Copyright by the American Fisheries Society 1988 Two Worlds for Fish Recruitment: Lakes and Oceans JOHN J. MAGNUSON Center for Limnology and Department of Zoology, University of Wisconsin-Madison Madison, Wisconsin 53706, USA Abstract.-Lakes and oceans have many similarities for the ecology of larval and adult fishes, but they differ in at least one important respect. Lakes have characteristics of islands, poorly connected wjth one another and subject to locally extreme environmental conditions. These circumstances inhibit immigration of fishes and increase their chances of extinction. Adjacent ocean patch~s. in contrast, are broadly connected and environmentally similar, conducive to immigration but not to extinction. At similar spatiai scales, then, recruitment of species and year classes may be influenced principally by extinction processes in lakes and by colonization processes in oceans. Interests in larval fish ecology span two of the world's major waterscapes-lakes and oceans. Most scientists who study recruitment work in Jakes or in oceans, but not in both. Does this distinction simply indicate a pragmatic difference in the choice of field sites, or are there fundamental differences in the ecology of fish recruitment in lakes and oceans? Just how similar or how different are lakes and oceans in respect to characteristics fundamental to larval fish biology and recruitment? When the Society of Limnology evolved to become the Society of Limnology and Oceanography, Alfred C. Redfield, an oceanographer interested in physiology and chemistry, wrote in the announcement of the journal Limnology and Oceanography (1956, volume 1, number 1) that "Just as it had been accepted from the start that students of the biological, chemical and physical aspects. of limnology had much to gain by association, so it was soon realized that the differences between fresh and salt water systems were trivial, when compared to the common principles with which limnologists and oceanographers alike are concerned.'' Some of us, when we compare things, see everything as different, and we stress dissimilarity. Others of us tend to see everything as the same, and we stress similarity. Certainly there is an infinite number of differences as well as an infinite number of similarities between lakes and oceans. To me, the similarities are the fundamental process-oriented things. For example, oxygen is relatively rare, and important sets of adaptations are apparent during the egg and larval stages for providing sufficient oxygen. Pelagic eggs and larvae are advected by currents. Buoyancy and 1 vertical mixing are important both to the larvae and to their food supply. Fishes are ectotherms, and temperature and seasonality are major environmental factors. Fish are size-selective feeders. Yolk-sac absorption is often a critical process. Predation and cannibalism are important mortality factors. Mortality rates are high. The differences seem to be the small, fascinating details of specific cases. The density of water is a little different: 1.000 g/ml for fresh water versus 1.025 g/ml for seawater. Fresh water can hold a little more dissolved oxygen than seawater. Different species of fish and their prey abound in the two environments. Many of the differences we could call to our attention are no more than those found between different lakes or different parts of the sea. Some of the assumed differences do not hold up in any absolute sense. For example, lakes are fresh and oceans are salty but many inland waters are brackish or even hypersaline, like Great Salt Lake in Utah, and many parts of the ocean are brackish or almost fresh, like Bothnian Bay in the Baltic Sea. The similarities provide the matrix in which larval ecology operates; the differences provide the interesting specific cases scientists study. Members of the Early Life History Section of the American Fisheries Society are drawn together by the similarities of larval fish biology and ecology among all aquatic environments. As individuals study specific problems of specific taxa in specific environments, they bring the interesting variability to each other to test against the underlying questions of larval fish biology and recruitment. Ocean versus lake would not appear to be a fundamental dichotomy for their science and understanding.