Sonar Reveals How Herring Respond to Predators

By Ron Heintz¹, Kevin Boswell² and John Moran³

Throughout much of coastal Alaska Pacific herring are important prey to whales, seals, sea lions, birds and other predators, but little is known about how these predators influence herring populations. Last winter, NOAA’s Undersea Research Program teamed up with NOAA Fisheries’ Auke Bay Laboratories, Louisiana State University and the University of Alaska Southeast to study how predators affect herring abundance. Scientists integrated imaging sonar (DIDSON) with a traditional echosounder (Simrad, 38 and 120 kHz) capturing what are believed to be the first images showing sea lions feeding on herring at depth.

Why are herring important for fisheries management?

The Pacific herring is commercially and ecologically important to coastal communities throughout Alaska. Herring are typically harvested near shore, so fishermen in small boats are able to participate in the fishery. In Alaska, this harvest has an average annual value of around $10 million. Perhaps more importantly, herring are the linchpin in marine ecosystems. Herring have high nutritional value and are preferentially consumed by whales, seals, sea lions, salmon, groundfish and sea birds. Despite the value of herring to coastal communities and ecosystems, relatively little is known about the factors regulating their abundance.

(Left) Sampling in winter offers Alaskan scientists unique opportunities to get out of the office and enjoy the great outdoors while gaining valuable data. (Right) Whales are attracted to winter herring aggregates and may represent a significant source of mortality to herring.

Scientists studied herring in the winter months to understand the factors that regulate their abundance. During winter months, adult herring often form dense aggregates in predictable locations. These schools can be enormous consisting of a layer of fish 150 feet thick stretching over several miles, permitting predators such as whales and seals to easily find them. Estimates of winter mortality for juvenile herring can be as high as 95%, making this work critical to understanding the relative contribution of predation to this mortality and for developing ecosystem-based management approaches for herring populations.

ecosystem approach to management: management approach that is adaptive, specified geographically, takes into account ecosystem knowledge and uncertainties, considers multiple external influences, and strives to balance diverse social objectives.

Why combine imaging sonar and echosounders?

Traditional echosounders are used widely by fisheries scientists to find fish and estimate their abundance. However, they display fish schools only as bands of color but do not image individuals. To verify that the fish on the display are the species of interest and to determine the size distribution and number of fish, it is necessary to trawl through the school. This can be very time consuming and expensive due to the associated cost of large trawl vessels, sophisticated instrumentation for directly observing, filming and collecting samples, and the need for a dedicated fishing crew.

In contrast, DIDSON sonar produces video-like images that resolve individual fish and can be used to accurately identify the species and size distributions without having to trawl (for examples of video imagery view www.cfi.lsu.edu/hydroacoustics). Smaller, less expensive boats outfitted with DIDSON can be used to estimate population sizes with better accuracy without impacting the fish schools. These cost savings enable more frequent surveying that allows scientists to directly measure survival in discrete schools over relatively short time periods. In addition, scientists can study individual behavior, including how individual herring respond to attacks and identify who attacks them.

Comparison of DIDSON (a) and traditional echosounder images (b and c). In (a) you can clearly see a school of herring and the spacing between the fish. The size distribution of the fish in (a) is shown in panel (d). These length measurements were made directly from the image, without trawling. In (b) the red and green bands depict a herring school. The top panel shows the display for the 38KHz transducer and the bottom panel the shows a 120 kHz transducer. Fish are visible only as color bands and the spacing between them is inferred from the color. Fish must be trawled to determine their size. (Larger image)

A humpback whale feeds on a school herring as seen through a traditional echosounder. Images such as this are difficult to obtain and leave most of the fine scale behaviors of both predator and prey open to interpretation.

Pacific herring at night under attack by a Steller sea lion (2.9 m long) in Fritz Cove, AK. The sea lion (white arrow) is moving up through the school at approximately 3.3 m s-1. Mean fish size is 22.2 cm measured from DIDSON data. The DIDSON was deployed in a downward orientation at 50 m water depth. (Larger image)

For fishery scientists, the combined application of DIDSON sonar and traditional echosounding represents an important advance towards an ecosystem approach to fisheries management. DIDSON sonar and traditional echosounding together provide critical information to better understand foraging success of predators and the contribution of particular predator species to the overall mortality of prey species. This new technique will help scientists understand how changes in abundance of a given species affect the foraging success of its predators and/or the abundance of its prey.

What is learned from DIDSON and traditional echosounding?

An immediate result of the study was to demonstrate the potential of DIDSON as a powerful tool to observe and document interactions between large predators and their prey in situ without influencing their behavior. Scientists observed the responses of individual herring to attacks and estimated the relative capture success of sea lions during each attack. This enables them to calculate the energetic cost of predator avoidance in Pacific herring, an important factor in winter survival, as herring do not feed in winter.

In addition, scientists were provided with added value by not only mapping the extent of school with traditional echosounding, but the ability of DIDSON adds value to traditional echosounder mapping of fish schools by determining the spacing of the fish in the school, their size distribution and biomass at a relatively inexpensive cost. This approach can now be applied to other forage fish species that form large aggregates and benefit scientists and fisheries managers with greater insights into the interactions between forage fish and their predators.

What is the future direction for fisheries management?

The Magnuson-Stevens Act requires fishery scientists to develop an ecosystem based approach to fishery management. Fishery scientists currently lack the tools to help manage multiple species. This study provided two important advances that are likely to have far reaching implications for fishery scientists. First, the combination of imaging sonar with traditional echosounder’s provided an inexpensive method for monitoring survival in a discrete herring school. Second, the ability to combine estimates of fish school size with direct observation of predation events provided valuable insights into the contributions of predator-prey relationships and how natural mortality contributes to ecosystem health. The ability to understand how herring survival is regulated by different predator-prey relationships is an important advance that will assist scientist and policymakers with the development of ecosystem-based approach to fisheries management.

¹Ron Heintz - NOAA Fisheries' Auke Bay Laboratories
²Kevin Boswell - Louisiana State University
³John Moran - NOAA Fisheries' Auke Bay Laboratories