Bioproduction /The Living World
Stay or Go: Life-history Plasticity in Salmon
Munetaka Shimizu , Lecturer
Faculty of Fisheries Sciences / Graduate School of Fisheries Sciences
High school : Toyota Minami Senior High School in Aichi Prefecture
Academic background : Graduate School of Fisheries, Hokkaido University
- Research areas
- Fish physiology, comparative endocrinology
- Research keywords
- Salmonid fish, life history, hormone, biomarker, biodiversity
What are your goals?
Figure 1. Masu salmon captured in Hokkaido river in the spring
I’m trying to elucidate the mechanisms determining whether salmonid fish head out to sea or stay in rivers, in the hope that such knowledge will be useful both for aquaculture and conservation of salmonid species. Salmonids hatch in freshwater, and generally migrate to the sea after spending a certain period of time in the river. Once they have grown sufficiently at sea, they return to the river of their birth to spawn. Salmonids that follow such a lifecycle (life history) include chum salmon and pink salmon. There are, however, some salmon species such as masu salmon that, depending on conditions, do not migrate to the sea. Masu salmon are often referred to as yamame in Japanese when they are in the river, but they are the same species. Interestingly, masu is Japanese for “trout”, but the masu salmon is a genuine Pacific salmonid species.
In Hokkaido, masu salmon normally descend rivers to enter the sea in the spring of their second year. However, not all of them head out to sea. Whether they do so or not is determined by their growth up to the summer of their first year in the river (Figure 1). Males that have grown the most by this stage remain in the river and mature a year earlier, while those that have grown only moderately develop the ability to tolerate seawater (seawater adaptation) in the spring of their second year and migrate to the sea. Those showing the poorest growth remain in the river for another year without becoming fully mature or developing seawater adaptability.
My research is focused on the way in which growth in salmon determines whether they migrate to the sea or not and how they grow once out at sea. I am attempting to elucidate the mechanisms behind growth and the relationship between growth and seawater adaptation. Ultimately I hope to understand the mechanisms by which differences in growth determine the branching we see in the life histories of salmon. I also hope to be able to contribute in various ways to the fisheries industry through this research. For example, a better understanding of the growth mechanisms of salmon could be applied to aquaculture to the benefit of future food security. Also, elucidating the mechanisms behind seawater adaptation could lead to the development of methods for rearing healthy salmon juveniles for stocking the rivers.
What kind of experiments are you carrying out?
Figure 2. Dissecting a small chum salmon juvenile
I’m collecting blood, liver, gill, muscle, and other tissue samples from salmon juveniles in various physiological states, and analyzing the hormones, proteins and genes they contain. For example, I’m measuring the hormones involved in growth by collecting blood from the juveniles, and collecting the gills of fish on their way to the sea to measure the activity of enzymes that are instrumental in seawater adaptation. Fish have growth hormone and other protein hormones similar to human hormones, but while they show similar amino acid sequences, amino acids in many locations are substituted as a result of molecular evolution. This means that we need to develop dedicated immunoassays for measuring fish hormones. So far, we have developed a method for measuring insulin-like growth factor, a hormone that promotes growth, and have found that the blood level of this hormone shows good correspondence with fish growth. We are currently using this hormone as an index (biomarker) of growth to evaluate the state of growth of chum salmon juveniles migrating along the Abashiri coast (Figure 2). We are also using E. coli to produce hormones and proteins involved in growth and seawater adaptation, and then investigating their physiological effects using a fish cell and organ culture system.
What do you plan to do next?
Figure 3. Masu salmon strains with different life histories
I am eager to investigate the extent to which the tendency of masu salmon to migrate to the sea is genetically determined, and the extent to which it can be affected by environmental factors. Masu salmon are widely distributed in Japan from Hokkaido to southern Kyushu, with life histories adapted to the local environments (Figure 3). In Hokkaido, many female masu salmon and some males migrate to the sea, but the further south one goes, the smaller is the percentage of individuals that migrate to the sea. Also, masu salmon in the Pacific Ocean side of the southwest Japan are called amago, and migrate to the sea in the fall of their first year rather than the spring of their second year. This is thought to be a strategy for avoiding high summer water temperatures. Biwa salmon, a subspecies of masu salmon so named because they are found only in Lake Biwa in Shiga Prefecture, spend their whole lives in freshwater. It is thought that they have been landlocked for so long that they are losing their seawater adaptability, but we don’t know the mechanism involved. We have determined that one of the reasons that the Biwa salmon’s seawater adaptability does not develop is the failure of the hormone involved in seawater adaptation to be secreted when the fish migrated to the lake. We intend to elucidate the mechanism behind the Biwa salmon’s inability to adapt to seawater by looking at mutations in the genes related to seawater adaptation. The fact that amago salmon migrate to the sea in fall has also suggested to us that ancestors of the masu salmon may have migrated to the sea in both fall and spring, and we want to see if there is any evidence to corroborate this theory. Hopefully our research findings will be useful to fostering understanding of masu salmon biodiversity and the utilization of genetic resources.