Ecosystem Research on the Pacific Hake Acoustic Trawl Survey

During the Integrated Ecosystem and Pacific Hake Acoustic-Trawl Survey, scientists and collaborators collect data on other fish, zooplankton, seabirds and marine mammals, enabling a better understanding of the California Current Large Marine Ecosystem.

West Coast

Enabling a Better Understanding of the Marine Ecosystem

Data on other fish, zooplankton, seabirds, and marine mammals are useful for a number of other studies in the California Current Large Marine Ecosystem, and enables a better understanding of the relationships between Pacific hake (Merluccius productus) and the marine ecosystem they inhabit. The Fisheries Engineering and Acoustic Technologies Team have a number of ongoing ecosystem research projects, which are described below. For more information, contact the FEAT Team lead.

Quantifying Euphausiids Using Acoustic Data

One of the many benefits of the FEAT survey is that fisheries acoustic data from multiple echo sounders are collected and archived. These pre-existing datasets can then be used for new projects to help us gain a better understanding of the California Current Large Marine Ecosystem. One of the first projects to use this archived data involved re-processing acoustic data collected between 2005 - 2019 to identify euphausiids, or krill, in the acoustic backscatter. Our goal was to develop a dataset of biennial estimates, also called a time series, of krill abundance in the California Current.

Krill are large zooplankton that are an important food item for many fish including Pacific hake and Chinook salmon (Oncorhynchus tshawytscha), seabirds like Cassin’s auklets (Ptychoramphus aleuticus), and marine mammals such as blue whales (Balaenoptera musculus). Information about how many krill there are along the U.S. and Canadian West Coast, and where they are concentrated, can improve our understanding about the movement and survival of krill-eating fish, seabirds, and marine mammals, as well as the status of the overall food web in the California Current.

To identify krill in the acoustic backscatter, we use the same basic theory that is used for Pacific hake. However, krill scatter sound differently than hake and other fish because they are much smaller and do not have a swim bladder. Importantly, krill scatter sound more strongly at higher acoustic frequencies than hake, so we use acoustic data from an echo sounder with a higher frequency (120 kHz) than what we use to study hake (38 kHz). To separate krill from the rest of the backscatter generated by other organisms, we use a process called ‘frequency differencing’, where we subtract the amount of backscatter observed at 38 kHz from backscatter at 120 kHz. If the difference between these two frequencies falls within a certain range, we classify the backscatter as krill. Values that fall outside of this specific range are considered ‘noise’ (scattering not associated with krill) and excluded from our analysis. We then use the same echo integration methods described on the Acoustic Sampling Principles page to estimate the total amount of backscatter from krill in a given volume of water. From this, we can estimate the density and biomass of krill present on each survey.

Echogram showing the hake and krill during a hake survey

We are using this time series of krill to look at changes in abundance between years and determine if shifts in ocean conditions, including marine heat waves, affect krill populations. We are also using statistical models to look at how factors such as seafloor depth, proximity to submarine canyons, ocean temperature, and chlorophyll-a relate to krill abundance. This will allow us to make predictions about where and how many krill we might expect in the future.

Examining Co-occurrence of Pacific Hake and Krill in the California Current

Because krill are such an important food item for Pacific hake, we hypothesized that hake will congregate in areas where there are a lot of krill. If hake and krill commonly occur in the same area, we could potentially use krill as an indicator of where we can expect to find hake during our summertime survey. This information will help us make better predictions about how shifts in krill distribution or abundance might affect hake survival, growth, and recruitment in the future.

Using the time series of krill abundance that we developed, we created maps of krill and hake distribution along the U.S. West Coast. Then we calculated how often krill occur in the same area as hake. We also used statistical models to examine how temperature influences hake and krill co-occurrence. Specifically, we looked at temperature measured 100 meters deep in the water column and compared where hake and krill were present and where they were not present to determine if changes in water temperature are an important factor affecting krill and hake occurrence. We found that temperature is an important factor, which allows us to make predictions of how changes in ocean temperature - either cooler or warmer than average - will affect the probability that hake and krill co-occur in the future. From these statistical models, we can predict the impacts of warming oceans due to climate change on krill and hake distributions, and potential impacts to hake growth and recruitment.

How Much Krill Do Pacific Hake Eat?

bolus, or mass of chewed food, found in stomach of Pacific hake, consisting mostly of krill. — Scientists extract a bolus, or mass of chewed food, from the stomach of Pacific hake. The bolus here consists mostly of krill.

Hake are generalist predators, which means they eat krill but also a wide range of other fish and zooplankton. How much krill are eaten by hake may change as the fish get older, and also if the abundance of krill is relatively low compared to other food sources. For example, younger hake (age 1-2) are thought to eat proportionally more krill than older fish, which switch to a diet composed primarily of fish. However, whether this varies with changes in ocean conditions or other factors is currently unknown.

We are using stomach content data from the hake biological samples to examine hake dietary habits. We are especially interested in determining how much krill are being eaten by hake, and if this amount is related to the abundance of krill detected acoustically during our survey. We will use the results of this analysis to update a food web model that is used by researchers working on the California Current Integrated Ecosystem Assessment, based in the Conservation Biology Division. By updating the amount of krill and other prey that hake eat, the effect of this predator on the rest of the food web will be more accurate. We can also use this information to make better predictions of how large and fast hake are growing, which could affect their survival, reproduction, and recruitment to the fishery.

Counting Marine Mammals and Seabirds Along the Survey Transects

We often observe groups of marine mammals, such as humpback whales, during our acoustic-trawl survey for Pacific hake. We also see a lot of seabirds flying along the coast, and sometimes encounter large feeding flocks of species including sooty shearwaters. Beginning in 2021, we will begin to systematically record the locations of marine mammals and seabirds seen during our summertime survey. Our goal is to explore relationships between marine mammal and seabird species, the abundance of krill, hake, and other pelagic species detected on our survey, and changing ocean conditions that may influence management of fisheries and protected resources.