The Adult Salmon Diet Program is a Pacific Salmon Foundation (PSF) funded collaboration between University of Victoria salmon researchers and recreational anglers to study the diets of adult Chinook and coho in British Columbia waters.
Understanding salmon diets is important; it provides a salmon’s perspective on the health of our coastal ecosystems. Since salmon are not picky eaters, their diets also can be used to understand the status of prey species, including Pacific herring. Analysis of thousands of stomachs collected in the Salish Sea from 2017 to 2021 shows that Pacific herring are the most important prey of adult Chinook. Moving forward, we are continuing to investigate salmon diets, searching for new insights into the biology of both salmon and their prey.
Salmon and their Prey
We are looking at Chinook stomachs to learn about herring migrations and other aspects of herring biology that are poorly understood. We found that Chinook commonly eat adult herring (2+ years old) in the Strait of Georgia in the summer. While this is likely no surprise to experienced Strait of Georgia anglers, it will be a surprise to many fisheries scientists. The conventional understanding is that most Strait of Georgia herring spend the summer feeding on the west coast of Vancouver Island and only return to the Strait of Georgia in fall and winter, prior to spawning in early spring. The importance of adult herring in summer diets of Chinook suggests that herring migrations are not as simple as has been thought. This has been the goal of the Adult Salmon Diet Program from the start: to transform the observations and knowledge of recreational fishers into rigorous scientific data that can inform understanding and management of the ecosystems supporting our fisheries.
We decided to investigate this further – what is the contribution of different ages of herring to Chinook diets in the Strait of Georgia? Answering this question was not as easy as just counting large herring in stomachs. Most stomach contents are quite digested, which means we often can’t use the length of consumed herring to estimate their age. Instead, we used otoliths—fish ear structures—to determine the ages of herring found in salmon stomachs.
Otoliths are a fish biologist’s best friend.
Otoliths are hard structures inside fish ears made of calcium carbonate, which is the same material of which clam shells are made. Like tree rings, otoliths grow in a circular pattern that provides a record of a fish’s age. Additionally, the size of the otolith is proportional to
a fish’s length. This means we can reconstruct the age and length of digested herring in salmon stomachs using their otoliths. To do this, we counted the number of otolith annual rings to determine the age of minimally digested herring whose length could be accurately measured. From these otoliths, we made a set of rules that assigned an age class (age 0, 1, or 2+) to each herring based on its length. However, we did not know every herring’s length due to digestion. To deal with this, we determined the statistical relationship between herring otolith width and herring length. Now, we could estimate a herring’s length from the width of its otoliths, and therefore predict its age.
With this otolith-based approach, we found that adult herring are Chinook’s most important food source in the Strait of Georgia during the summer. We (as well as many recreational anglers) knew that some adult herring were present in the Strait of Georgia in the summer, but it was exciting to find that adult herring made up almost half of the summer Chinook diet in the Strait of Georgia. This suggests that many herring stay in the Strait of Georgia to feed during the summer. Yet, we still lacked an explanation of why certain herring stay in the Strait of Georgia in the summer while other herring spend the summer on the west coast. Do individuals in these two groups differ, say in body size or early growth? Understanding the abundance of resident herring and the different traits of residents and migrants is important for management of the herring fishery and understanding fluctuations in herring abundance.
To answer key questions about these adult herring found in the summer in the Strait of Georgia, we need a way to differentiate “residents” and migrants. UVic master’s student Jess Qualley began studying the migration patterns of Strait of Georgia herring. Jess’ research aims to use the chemical composition of herring otoliths to track their migrations—using lasers, believe it or not!
Different regions of the Pacific Ocean have different levels of chemical elements, which are reflected in the chemical composition of otoliths as a fish grows and moves from one region
to another. As a result, otolith chemistry can tell us where a fish has been during its life, and we can ask the following question: Did a herring born in the Strait of Georgia seasonally migrate to the west coast of Vancouver Island to feed and grow, or did it stay in the Strait of Georgia for its entire life? Once we know the migration history of an individual, we can begin to ask questions about why a herring opted to migrate or stay.
The Adult Salmon Diet Program transforms the observations and knowledge of recreational fishers into rigorous scientific data.
So far, we know that adult Chinook mostly eat herring in the Strait of Georgia. Many of these herring are adults spending the summer in inside waters. (Sounds like a nice staycation until you get eaten?!) Jess’ research will reveal some of the complexities of herring migrations in the Strait of Georgia. There is still much to be learned about salmon and the forage fish prey that they rely on.
You Can Participate in the Adult Salmon Diet Program
By participating in the Adult Salmon Diet Program, you can support our research on salmon and forage fish in BC. Collecting and submitting adult salmon stomachs to the Adult Salmon Diet Program is easy! When you catch and keep a Chinook or coho, instead of throwing away the guts or using them as crab bait, put the esophagus, stomach, and intestinal tract in a ziplock bag. Fill out an Adult Salmon Diet Program data card (available at participating tackle shops or by email request) with the information about your catch. Drop off the stomach and data card in the same bag at a participating tackle shop (see the list at the end of this article). We will then dissect the stomachs in the lab to see what your salmon ate before you caught it.
In addition to helping further our understanding of salmon and their prey in BC, there are many great benefits for participating anglers. At the end of each year, we send each angler a personalized report describing the contents of the stomachs they submitted. Additionally, every stomach you submit enters you into our annual raffle for amazing fishing gear. Previous years’ raffles have included generous donations from Islander Reels and AP Tackleworks – by collecting stomachs, you can both help salmon science and have a chance to win awesome gear like an Islander MR2 reel and an AP Tackleworks Salish Sea Spoon Pack! Additionally, anglers who submit 10 or more stomachs during 2022 will receive an awesome Adult Salmon Diet Program t-shirt!
We thank the over 100 recreational anglers who have submitted stomachs to the Adult Salmon Diet Program.
We hope that, if you are not already participating in the program, you will begin to submit stomachs and help us better understand the biology of adult salmon and forage fish in BC! If you have any questions about the Adult Salmon Diet Program (or are curious about
Jess’ research), please email us at [email protected].
You can also check out our previous Island Fisherman articles about the Adult Salmon Diet Program: