Chance Favors Only the Prepared Mind How does a scientist go about the task of pushing back the curtains of the unknown? Certainly the romance of tackling the mysteries of nature provides the motivation, for who would not be inspired by the remarkable life history of this romantic beast, the salmon. After living in the Pacific Ocean for several years, salmon swim thousands of kilometers back to the stream of their birth to spawn. I have always been fascinated by the homing migration of salmon. Noone who has seen a 20-kilogram salmon fling itself into the air repeatedly until it is exhausted in a vain effort to surmount a waterfall can fail to marvel at the strength of the instinct that draws the salmon upriver to the stream where it was born. But how does it find its way back? I was puzzling over this problem during a family vacation in 1946. Inspired by the work of the great German Nobel Laureates, Karl von Frisch and Konrad Lorenz, I had been conducting research with my graduate student Theodore Walker, since 1945, on the ability of fishes to discriminate odors emanating from aquatic plants. Von Frisch had studied schooling minnows and discovered that, if broken, their skin emitted a con specific chemical substance, termed Schreckstoff, which caused other members of its school to disperse and hide.
Salmon are one of the most popular and commonly eaten fish and are among the most important fishery resources in the world. They are born and die in fresh water but can live in both fresh water and seawater where they migrate between rivers and oceans, showing amazing abilities to home to their natal stream precisely. However, their dynamic life cycles and mysterious abilities of natal stream imprinting and homing migration are not well understood. Physiological Aspects of Imprinting and Homing Migration in Salmon: Emerging Researches and Opportunities is a pivotal reference source that introduces the dynamic and complicated life cycle of salmon connected with fish migration and climate changes and presents physiological mechanisms of natal stream imprinting and homing in salmon with special references to hormone, olfaction, memory, and behavior. Additionally, salmon resources concerning salmon commercial fisheries, aquaculture, and global propagation systems are discussed. This book is ideally designed for ichthyologists, environmentalists, pisciculture professionals, fisheries, marine biologists, scientists, researchers, academicians, and students seeking coverage on one of the most integral species of fish in the world.
In the following study, I tried to link hormonal background conditions to successful olfactory imprinting in sockeye salmon by employing behavioural, endocrinological and electrophysio logical experiments. In the initial experiments, sockeye salmon were exposed to potential imprinting odorants, with or without additional treatment with thyroid hormones, during several juvenile stages between fertilization and beyond the PST. After two years of rearing, these fish were tested for behavioural responses to test odorants in two behavioural arenas. Neither immature nor mature fish reacted behaviourally to the odorants that they had been exposed to previously. Therefore, exposure of juveniles to odorants did not lead to imprinting to those odorants under hatchery rearing conditions. In contrast, juvenile fish that were exposed to test odorants and treated with a combination of T3 and T4 (in all cases) or T3 (in one case) the two most common forms of thyroid hormones, did exhibit an odorant recognition response two years later. However, the response differed between immature and mature fish. Mature fish were attracted to the imprinting odorant, whereas immature fish were repelled by the it. When immature fish were injected with GnRH before testing, their behavioural response was reversed. No behavioural response could be detected in fish that had been challenged with either T3 or T4 alone, in contrast to a combined treatment with both forms. Thus, I found evidence that a combination of T3 and T4 initiated imprinting and that GnRH motivated odorant recognition. To examine the underlying hormonal processes, I first determined plasma thyroid hormone concentrations in sockeye salmon before and after hormonal challenges with thyroid hormones or GnRH. In addition, the activity of the deiodinase enzyme that converts T4 into the other possible forms of thyroid hormones was investigated in sensory and non-sensory tissues. The results suggested that only a combined T3T4 treatment increased the availability of both thyroid hormone forms in blood plasma, while a separate challenge with T4 suppressed T3 availability and vice versa. Moreover, the results provided evidence for deiodinase activity in the olfactory epithelium and the retina and demonstrated that GnRH can modulate the T4 conversion process. This inform ation was helpful for planning and interpretation of the remaining experiments. Results obtained from a classical conditioning paradigm (heart-rate-conditioning), provided support for the hypothesis that GnRH lowers the threshold to an imprinting odorant and that the influence of GnRH was not restricted to an enhancement of motivation. To investigate whether hormonal action could also modulate the sensitivity of the peripheral olfactory system, electrophysiological responses from the olfactory epithelium (electro-olfacto-grams or EOGs) were recorded. The EOG results established that thyroid hormones and GnRH increased the EOG response of adult naïve (never imprinted to an odorant) fish, as did maturity. In the last experiments, I conducted EOG recordings on fish that had been imprinted at a juvenile stage. In summary, EOG recordings revealed that the imprinting process increased sensitivity to the imprinting odorant at maturity, while sensitivity in immature fish was decreased in comparison to non-imprinted fish. In combination with my behavioural results, this could explain why salmon do not enter their natal stream before they reach maturity. At maturity however, I also encountered desensitization to non-imprinting odorants, which might increase the ability to focus the olfactory system to the task of homing.
"Atlantic salmon imprint on streams as juveniles and undergo long distance homing migrations back to their natal streams years later to reproduce. Dissolved free amino acids (DFAAs) in streams have been identified as a likely olfactory imprinting cue used by salmon. To test if DFAAs act as imprinting odorants for Atlantic salmon, we exposed juveniles to five DFAAs known to naturally occur in Atlantic salmon streams. Exposure to the "imprinting DFAA mixture" occurred during the parr-smolt transformation (PST), a critical imprinting period that occurs when fish migrate from their natal stream to a lake or ocean. We explored four DFAA treatments to better characterize the timing and duration of imprinting during the PST: continuous (March 30-May 25), early (March 30-April 13), peak (April 27-May 11), and control (no DFAAs added). Behavioral responses to the imprinting DFAA mixture versus five novel DFAAs were tested in a two-choice maze during the fall homing migration period two-and-a-half years later, when the fish were three years old. Of 321 fish tested in the maze, 117 swan into a maze arm indicating a preference for a DFAA mixture. Fish from DFAA exposed treatments preferred the imprinting DFAA mixture versus novel DFAA mixture (X21=3.25, p=0.04). Fish from the control treatment showed no preference for either DFAA mixture (X21=0.04, p=0.42). Small sample size limited inference on timing of imprinting during the PST, but it appeared DFAA exposed fish from the continuous (X21=1.96, p=0.08) and early (X21=1.86, p=0.09) treatments preferred the imprinting of DFAA mixture while peak treatment (X21=0.04, p=0.26) fish showed no preference for either DFAA mixture. These results provide behavioral evidence that mixtures of DFAAs are viable olfactory imprinting odorants for Atlantic salmon to use in homing to their natal stream."
Fish comprise more than 50% of all living vertebrates and are found in a wide range of highly diverse habitats like the deep sea, the shoreline, tide pools, tropical streams and sweetwater ponds. During evolution, the senses of fish have adapted to the physical conditions of the environment in which different species live. As a result, the senses of fish exhibit a remarkable diversity that allows different species to deal with the physical constraints imposed by their habitat. In addition, fish have evolved several `new' sensory systems that are unique to the aquatic environment. In this book, examples of adaptation and refinement are given for six sensory systems: The visual system, The auditory system, The olfactory system, The mechanosensory lateral line system, The taste system, The electrosensory system. In each case, the environmental conditions under which a particular group of fish lives are analyzed. This is followed by a description of morphology and physiology of the sensory system and by an evaluation of its perceptional capabilities. Finally, the sensory adaptations to the particular conditions that prevail in the habitat of a species are highlighted. The various examples from different groups of fish presented in this book demonstrate the impressive capability of fish sensory systems to effectively overcome physical problems imposed by the environment.
This book describes in general how the chemosensory systems of fish function at various levels. In many ways, fish are typical vertebrates differing only slightly from other vertebrates including humans. In other ways, their aquatic environment imposes strict requirements or offers unique opportunities which have resulted in some unusual functions having no counterpart in higher vertebrates. This new volume is necessitated by advances in many vital areas as the field of chemical senses continues to grow at a rapid pace. Most significant is the application of the contemporary electrophysiological technique of patch-clamping, recognition of a second messenger system in chemosensory transduction processes and the identification of hormonal pheromones in fish reproductive behaviour. The last major synthesis of our knowledge about fish chemoreception, Chemoreception in Fishes, was published ten years ago (Elsevier, Amsterdam, 1982). In that volume four aspects of fish chemoreception, Le. morphology of the peripheral chemoreceptors. primary sensory processes, roles in behaviour, and its interactions with environment, were discussed. This book is intended to be helpful to students, scientists and aquacul turists not only as a source book but also as a textbook on chemical senses.
