Researchers deploy a new tool to study Chinook salmon fat content on the Fraser River

Whether a Chinook salmon winds up on a backyard barbeque, in the belly of southern resident killer whale, or spawned out in a Rocky Mountain stream, its paramount quality is its fat content. From April to November these salmon, rich with fat filched from far flung foraging grounds, return to the Fraser River, and burn this energy as they wend their way upstream in a dogged drive to reach their spawning grounds. At the Albion test fishery, 50 kilometres from the onset of this freshwater migration, determined UBC scientists are providing new insight into Chinook energy accumulation.

A bit of context is necessary. Chinook are notable for their remarkable diversity. Some Chinook stocks spawn at sea level adjacent to the ocean, while other stocks spawn at over one thousand metres elevation, hundreds of kilometres from the sea. As Chinook stop eating near the onset of their freshwater migration, their different life histories require them to accumulate different amounts of energy to reach their spawning ground, mature, and reproduce. Chinook with longer migrations and longer freshwater holding times tend to have higher energy requirements and consequently need to accumulate more energy.

Humans have long been aware of these differences, and high energy salmon have been valued. Indigenous communities adapted their salmon processing techniques according to variation in fat content among individual stocks. Early Fraser canneries, not known for their thrift, would only use ‘red’ Chinook characteristic of longer migrations and discard ‘white’ Chinook with lower fat content. For species that rely on Chinook, the difference in terms of total calories offered by these different stocks can be extremely important. A Chinook with a short migration to Harrison Lake may offer half the calories to a resident killer whale as a similarly sized Chinook headed to the headwaters of the Fraser River. Despite its significance, quantitative estimates of stock-specific lipid content have been elusive.

The Distell fat meter is a handheld device with a sensor that, when pressed against a salmon, measures microwave reflectance and uses the relationship between water and lipid content to determine lipid content of the muscle below.

To monitor the Chinook salmon return migration, for over 40 years, the Albion test fishery has conducted twice-daily gill net sets where fish have been intercepted, dissected, inspected and, eventually, digested (cleaned salmon are sold just up the bank at Bruce’s Market). For the last two years, however, PhD candidate Jacob Lerner and PI Dr. Brian Hunt have deployed a new tool: a Distell fat meter. Procured through a British Columbia Salmon Restoration and Innovation Fund (BCSRIF) grant, and employed by the supportive three-person test fishery crew, the fat meter is a handheld device with a sensor that, when pressed against a salmon, measures microwave reflectance and uses the relationship between water and lipid content to determine lipid content of the muscle below. (More than one curious researcher has turned the device on themselves and, in their chagrin, been consoled by the fact that the device is only accurate on salmon). For Chinook, the fat meter provides a lipid content measurement only at a discrete location, but Lerner has developed a method to convert these measurements into a more ecologically relevant whole body lipid content estimate.

Calibrating the device for this purpose was no small feat. To do so it was necessary to create regression between the fat meter measurements and the true whole body lipid content of Chinook—a metric that could only be calculated by determining total lipid content of homogenized whole Chinook. This was a problem: a 20kg salmon is not going to fit in a Magic Bullet™️. Lerner was forced to improvise and homogenized samples in an industrial meat grinder nicknamed “Butcher Boy” at the DFO centre in West Vancouver. Initial setbacks aside, the calibration proved to be very effective, converting fat meter measurements to whole body lipid content at a precision of ±1%.

At the test fishery it takes less than 30 seconds to get fat meter measurements from a captured salmon. The test fishery operates throughout the entire salmon migration season and collects Chinook from every major stock on the Fraser. As it encounters thousands of salmon, an unprecedented dataset of Chinook total lipid content has quickly developed. Critically, the DFO genetically identifies each sample to its specific stock.

With daily, stock-specific lipid measurements, this ongoing research project is providing an unprecedented window into the energy levels of individual salmon stocks. Population level differences, long inferred, have come into focus, providing new insight into how distance and elevation drive the energetic requirements of disparate Chinook stocks. This can allow quantification of the value of these individual stocks as prey for endangered resident killer whales.

The multi-year sampling effort also means the effect of ocean conditions on energy accumulation in Chinook is being investigated for the first time. In 2021 vs 2020, for example, Chinook fat levels were lower during the critical summer stocks period. Incoming stable isotope data should provide context on environmental and food web differences between years and between high and low fat individuals. This will provide further insight into environmental determinants of energy accumulation.

The stakes are high: rivers are running warmer, requiring more energy to navigate; obstacles such as the 2019 Big Bar slide are an ever-present threat; and many of the most energy-rich, further migrating stocks are endangered. Jacob Lerner is optimistic about what the results will reveal. Preliminary results have already improved our understanding of stock-specific Chinook energy density. Future insight into determinants of energy accumulation is critical to forecasting the future of these valuable salmon.

Tags: Brian Hunt, British Columbia, fish, fish stocks, fisheries management, IOF students, Pelagic Ecosystems Lab, salmon