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Biology

How much fish does it take to keep a salmon shark warm?

Manishin, K. A., K. J. Goldman, M. Short, C. J. Cunningham, P. A. H. Westley, and A. C. Seitz. 2019. Prey consumption estimates for salmon sharks. Marine and Freshwater Research. https://doi.org/10.1071/MF18345

So what is a salmon shark?

A lamnid, or mackerel shark, the salmon shark looks sort of like a stunted great white; spindle-shaped, dark above and light below – but you won’t find its cousin hunting nearby. These sharks roam the icy waters of the northern Pacific Ocean, from Russia to Alaska and British Columbia. Yet amazingly, they can stand the cold. Salmon sharks are one of the few shark species that can adjust their body temperature, raising it up to 70°F above the surrounding water! Rete mirabile, Latin for “miraculous net,” are special networks of blood vessels around their eyes and muscles that recycle the heat created by their bodies. This keeps their eyes sharp and muscles primed to catch fast-moving prey. Similar adaptations are found in their prey species, like tuna, meaning these sharks have been evolving alongside their food for some time now. These sharks are an evolutionary marvel – but what does it take to keep the heat on?

A salmon shark tagged by NOAA with a spaghetti tag. (Photo credit: Wikimedia Commons, NOAA)

Keeping your body’s thermostat high takes energy, so how do the sharks obtain it? Salmon sharks tend to feed on whatever is available, and often that’s salmon. Since these prey species are economically important, the fishing communities in the areas nearby often view the sharks as pests. Even though they might be bothersome, they’re not heavily targeted; if sharks are caught, it’s usually by accident. Due to their relatively large size, slow growth, and long reproduction cycles, US fisheries agencies consider them vulnerable to overfishing, and have prohibited their harvest in US waters.

Why should we care what a salmon shark eats?

Recently, the salmon shark’s menu favorites have been declining. Some salmon populations in particular have seen drastic drops, and scientists and fisherman are racing to find out why. Salmon sharks can affect populations of prey fish through a phenomenon called ‘top-down control.’ This means that apex predators control the size and number of other species by preying on them. This can be seen all over the world, from sharks on coral reefs to the wolves in Yellowstone, and it’s the reason why scientists say that a healthy ecosystem has a lot of predators. In this instance, our favorite fish and the sharks’ favorite dish overlap, so understanding how salmon sharks use these resources may help fisheries managers conserve both salmon, sharks, and commercial fisheries.

A diagram of top down control. The affect that predators exert on other animal populations ebb and flow, but can be seen throughout the food web. (Diagram: Tyler Plum)

How do you find out how much a shark eats?

Kaitlyn Manishin and her team set out to answer the question of feeding: how much does a single salmon shark eat during the year? We have little information on how often the sharks eat or how much they need to eat to survive. To estimate this, the team used several different models to predict how much energy or fish salmon sharks might require. First, they calculated what percent of the shark’s body weight that it eats, on average, each day (think of when a documentary narrator says something like “the grizzly needs to eat 10% of its body weight each day to prepare for winter” – that number is what the team is trying to find). Since nobody has calculated this number for salmon sharks yet, the team used the numbers for white and mako sharks (which are also endothermic) and applied them to the average body weight of a salmon shark, about 291 lbs. Once the researchers have this number, they can use it to find how much a salmon shark needs to eat per year.

Figures showing comparisons of consumption rates by the different models and the growth curve for the third model. Ages were taken by counting the rings on salmon shark vertebrae! (Manishin et al. 2019)

Next, the team wanted to know how much energy it takes to keep the heat going. Since the bodies of these sharks generate heat, it should require more energy to create that heat than it does for other sharks. Knowing the energy a shark uses based on movement and bodily functions can tell us how much fish a shark needs to eat to meet its energy requirements. Finally, the team looked at how a shark’s energy needs might change as it grows. They used a statistical model based different lengths and ages to find out the amount of energy (or fish) the sharks would need for certain levels of growth.

So, how much?

The different models showed that the sharks may need 577 kg to 20,800 kg of fish per year. The team believes that the bio-energetics model is the most realistic, ranging from 1,610-2,070 kg/yr, because it uses the sharks’ own life history information. The first model borrows measurements and numbers from other species of sharks and the third model relies more on statistics and computer models, making each less accurate. According to these estimates, the salmon shark is on the same fish-eating level as seals and sea lions!

How does this help?

Manishin and her team agree that while the estimates here are too broad to be useful in the direct planning of fishery regulations, it highlights how little we know about salmon predators and emphasizes the need to understand the animals that rely on the species we commercially harvest. The team hypothesized that higher estimates may show that the shark population is able to remove as much salmon from the system as commercial fisher folk do. Knowing the amount of fish that salmon sharks eat also improves the accuracy of our fishery models. Understanding the energy needs of salmon sharks will allow us to design fishery regulations that provide enough fish for both fishers and sharks.

Discussion

2 Responses to “How much fish does it take to keep a salmon shark warm?”

  1. This was an amazing article (and it was very well-written)! Do you think that temporarily halting salmon fishing will yield greater numbers of bigger, healthier salmon and tuna? In this case, I was thinking that the salmon shark will eat the smaller, “weaker” foraging fish, thus allowing the zooplankton population to flourish.

    Posted by Sedona Dorsett | April 27, 2019, 1:37 pm
    • Tyler Plum

      Thank you so much! That’s a great question, and a difficult one to answer! It’s important to realize that the salmon fishery has multiple species, and a variety of factors affecting the size and success of their stocks. However, a temporary closure may not achieve the kind of growth you’re talking about. Since the system is so large and complex, it may very well take years to see the kind of change we’re accustomed to in small coastal fisheries. Salmon sharks are also only one of many predators of salmon, so it’s possible others like orcas and seabirds may fill in the gaps. Today we understand that the the trophic cascades you’re talking about occur more as trophic “trickles”, and are less pronounced in ecosystems with a large number of organisms on the same feeding level. Thanks for reading!

      Posted by Tyler Plum | May 17, 2019, 1:04 pm

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