Canada's Greatest Fisheries Biologist: Inventor of the Ricker Curve for describing fish population dynamics
"Try and arrange that you’re doing something that you’re interested in. There’s quite a bit of routine in research work but I’ve never worked on a project that I wasn’t very interested in."
Fisheries biology is the study of fish habitat and population. Knowing the number of spawners in a given year is crucial for predicting how many fish will be available for future harvest. Ricker knew salmon runs like some baseball fans know World Series statistics. He had kept track of the Canadian Salmon Commission’s estimates of the Fraser River sockeye ever since 1938.
Each year different numbers of salmon return to spawn, depending on the species. The five salmon species in British Columbia go through different cycles. For example, the big Fraser sockeye run of 2001 was the same “line” or “cycle” as the record run of 1913, which was around 100 million fish. For sockeye, such huge numbers occur only every four years. Other years are one-tenth as numerous or even less.
Ricker was the first scientist to suggest several possible reasons for the cyclic variation in returning salmon stocks. Biologists are still collecting evidence to determine the correct explanation. The Fraser sockeye are on a four-year cycle possibly because most of the fish mature at four years of age. Farther north, age five is also common. Pink salmon have a two-year life cycle. For coho it’s usually three years, while chinook or “spring” salmon return at any age from two to seven years. Chinook are the largest and most powerful of all salmon. In fact, the largest salmon ever caught in the world was a chinook weighing in at over 57 kilograms.
Ricker is famous for his mathematical model of fish population dynamics, now called the Ricker Curve, which he first described in a book he wrote in the 1950s on the computation of fish population statistics. Today that book is known throughout the world as the “Green Book.”
Ricker retired just before the age of personal computers, and he never used one. According to his son, Eric, his father was a man who had little interest in things that changed established, well-functioning habits. He never had a dishwasher, for example. Ricker used to tell a story about the time he uncovered some serious computer calculation errors with a few quick moves on his trusty old slide rule.
1. The Ricker Curve is still used all over the world to determine average maximum catches for regional fisheries. Each curve represents a different type of fish population. This is what governments use to decide how many days commercial fishers can be allowed to fish for salmon or cod, so that there are enough fish left to reproduce more than their current numbers the next year.
Line of natural replacement: Along this line, spawners (adult fish that lay eggs or fertilize them) are replaced by an equal number of progeny (fish that grow up to be adults).
2. Line of natural replacement: Along this line, spawners (adult fish that lay eggs or fertilize them) are replaced by an equal number of progeny (fish that grow up to be adults).
3. Salmon are interesting because their Ricker Curve looks more like this. At this point, at the top of the curve you have reduced the population to 40 percent of the natural equilibrium by fishing, but some fish, such as salmon, produce many more mature progeny when their spawning grounds are less crowded.
4. Natural equilibrium: At this point, spawners equal progeny. If there were no commercial fishery, this is where the fish population would tend to stay. In nature, fish don’t go much beyond this level because they get too crowded. Spawning beds get messed up and many eggs die.
5. For a fish species that followed Ricker Curve A, if commercial fishers were allowed to catch 20 percent of the mature spawners the fish population would be at this point. There would be fewer progeny, but enough to sustain the catch — a 20 percent surplus.
6. The point of maximum sustainable catch on any Ricker Curve is shown by the curving dotted line (“Sm” shows the maximum point for salmon). Note that it’s actually a bit to the left of the peak of a curve. This is because the distance between the curve and the natural replacement line is the greatest at this point. Anywhere to the left of this line, you are overfishing and will reduce the next generation’s harvest.
So You Want to Be a Fisheries Biologist
Ricker felt that anyone planning a career in science should be sure to choose a subject that is of great personal interest. Some aspects of scientific work can be boring, consisting of very repetitive experiments or endless data collection. Only a keen interest in the subject will make the tedium of the day-to-day work tolerable.
David R. Montgomery, King of Fish: The Thousand-Year Run of Salmon, Westview Press, 2003.
William Ricker, Computation and Interpretation of Biological Statistics of Fish Populations, Bulletin of the Fisheries Research Board of Canada no. 191, 1975. (This is “the Green Book.”)
Website of the Pacific Fishery Management Council.
Cyber Salmon website by Alaska Region of the U.S. Fish & Wildlife Service and the Fairbanks Fish & Wildlife Field Office.
Salmon lifecycle on the GoldSeal website.
ACTIVITYhas 2 Activities for you to try in the Activities section.
- Apply your knowledge of the physical characteristics of Pacific salmon and identify them.
- Learn about different kinds of fish in your local grocery market or lake/river.