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
(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.
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
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
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
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
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
2Kevin Boswell - Louisiana State University
3John Moran - NOAA Fisheries' Auke Bay Laboratories