Salmonid Spawning Habitat in Rivers: Physical Controls, Biological Responses, and Approaches to Remediation
Salmonid Embryo Development and Pathology
G. Russell Danner
The development of each salmonid egg follows a predetermined course. In order for a salmonid to emerge from the redd gravel, all developmental steps from oogenesis, fertilization, embryonic development, hatching, yolk sac absorption, and emergence must be completed in the proper sequence. Knowing the developmental sequence between oocyte formation and fry emergence aids in live identification of species and determines the etiology of morbidity and mortality in these young fishes. This chapter provides an overview of fish gametogenesis, fertilization physiology and behavior, and documented embryo development stages. With this knowledge, it may be possible to collect developing or dead embryos from redds, identify developmental abnormalities, trace their origins, determine causes of death, predict survival rates, and/or back-calculate local spawning dates. Detailed comprehension of salmonid ontogeny helps fisheries biologists evaluate causes of mortality; provides pertinent information to project managers, engineers, and geomorphologists useful in assessing river salmon spawning habitats before and after construction projects; and improves quantitative assessment of spawning habitat remediation projects in disturbed habitats.
Proper collection techniques and thorough examination of fish embryos provides a great deal of information regarding their development. Historically, salmonid eggs have not been examined much prior to the eyed egg stage; unfortunately, this means many causes of infertility and early fry mortality get lumped together. Salmonid eggs are generally hardy enough to survive transport back to a laboratory for examination with adequate care to prevent desiccation and mechanical or thermal shock. Eggs can be removed from nests, or redds, with gentle suction and placed into a container of ambient water for live transport or can be placed into fixative for preservation. Oxygen consumption and waste elimination by the embryo is minimal, and live eggs kept in a few milliliters of cold water will survive several days without significant deterioration. Developing embryos change rapidly during early development, but changes are progressively slower as more detailed structures develop and differentiate. The most crucial consideration during the collection of live eggs is maintenance of incubation temperature. Eggs are sensitive to rapid changes in temperature, so they should be kept as close to their local temperature as practical. Changes in temperature also alter the developmental rate of the embryo. Increasing water temperature will cause embryos to develop more rapidly, while decreasing temperature will arrest development. If arresting the developmental stage is preferred, then eggs should be chilled to 4°C or killed by preservation in 10% pH neutral buffered formalin before examination. Freezing salmonid eggs generally ruptures the chorion and distorts the developing embryo; therefore, it is not a recommended method of preservation.