Ontogeny and systematics of fishes : based on an international symposium dedicated to the memory of Elbert Halvor Ahlstrom

Abstract

mortalities occurred at first feeding. Measurements of mortality rates of eggs and larvae at sea tend to show a high but continuing mortality of perhaps 5-20% per day. The results of sea surveys are, however, often difficult to interpret because of the need to sample within a discrete larval population over a long time.May (1974), in his review of this subject, concluded that starvation at the end of the yolk-sac stage may often have a major influence on brood strength but that mortality from fertilization to the O-group stage is the ultimate determinant.The results of modelling and the tests of the patchiness hypothesis which have already been discussed support the idea that first feeding is a critical time, although not having, necessarily, the dominant effect claimed by Hjort. Experimenters and modellers have also derived further concepts for testing. The major sources of mortality are identified as starvation and predation. Starvation, of course, only operates from the end of the yolk-sac stage. Blaxter and Hempel (1963) used the expression "point-of-no-retum" to express the point at which larvae, as a result of starvation, are too weak to feed even if food becomes available. Sometimes called "ecological death" or "irreversible starvation" this is a useful concept for assessing the chances of larval survival under different conditions. For larvae in a good nutritional state the time to the point-of-no-retum may be only 1-2 days in a small newly feeding larva like the anchovy, but Ontogeny, Systematics, and Phylogeny D. M. Cohen THE work of Ahlie and his students and colleagues has brought to the fore great amounts of descriptive information about the early life history (ELH) stages of fishes gathered over many years. These data are of broad provenance, many being the results of original research by the Ahlstrom school, others being taken from the literature. Only a scientist with Ahlie's capabilities-an extensive knowledge of fishes and their ontogeny, a fine sense of order in nature, and a critical intellect-could perceive pattern in the bewildering diversity represented by the early life history stages of fishes. As would any good scientist, Ahlie questioned the meaning of these patterns, and it is chiefly to further this inquiry that this symposium was convened.Most students of comparative fish ontogeny know more about adult fishes than ichthyologists who study adults know about larval fishes; they have to. Ahlie stated in his lectures. "Larval taxonomy is just an adjunct to adult taxonomy and you have to start with the adults to know the larvae." Early on he discovered that data from early life history studies did not always confirm classifications based on adults alone. We all want to know which data sets most closely approximate phylogenetic relationships; how apparent conflicts best can be resolved; how the data of ontogeny can be integrated into the overall field of fish systematics? Answering these questions is not easy, especially within the framework dictated by the widespread adoption of new meth...

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