Alaska Science Center
What is a forage fish?
A forage fish is any fish eaten by large predatory fish, seabirds or marine mammals. They are usually abundant and often swim in large schools. Forage fish are an important link in the marine food web because they transfer energy between primary and secondary producers, such as plankton, to top predators such as seabirds and larger fish. To see and learn more about some of the common Alaskan forage fish, click on their names: Pacific sand lance, juvenile Pacific herring, juvenile walleye pollock, smelts, and juvenile salmonids.
How do we study forage fish?
One of the important things we want to know is what kinds of fish are available to seabirds. In other words, what are the choices on the menu for a seabird dining at its ocean café? We also need to know the fish's depth and location in the water, and how many fish there are. We want to know how old, how nutritious, and how fattening the fish are. To answer these questions we collect a sample of fish each year by a variety of methods. These methods include beach seining, cast netting, bottom trawling, and mid water trawling. We also use hydroacoustic equipment and SCUBA diving to study fish. We even look inside Pacific halibut stomachs to see what kinds of forage fish they are eating.
A beach seine is a long, rectangular net with corks along the top to help it float, and weights on the bottom to make the net hang down and touch the ocean floor. It is used to catch fish that live close to the shore of a sandy or cobbled beach. The beach seine is set parallel to shore, or in the form of an arch, and then pulled in by hand as two to four people walk up the beach holding the ropes that are attached at either end. The seine will hopefully catch all the fish that were between the net and the shore. This is the primary method we use to catch Pacific sand lance because they live close to the shore.
If we see a flock of birds feeding at the surface of the water, one way to sample these fish for identification is with a cast net. A cast net is a special kind of net that only catches fish at or just below the surface of the water. When thrown into the air, the cast net opens wide into the shape of a circle. After it hits the water, weights drag it down and the attached drawstring is pulled to draw the net into a purse with fish (hopefully) caught inside. A cast net is handy because no other nets allow us to sample right at the surface, or seconds after seeing seabirds feeding on fish schools.
We use bottom trawls to study certain kinds of forage fish that live on the ocean floor. While most forage fish live in dense schools and swim in the water column, some species tend to hang out in small groups (or individually) near the ocean bottom. A bottom trawl is a net that is dragged on the ocean floor while it is towed behind a boat. We use a special research trawl that is relatively small (the mouth has a ten foot opening) and we only tow for five minutes. We use it to catch fish that live on the sea floor, such as flatfish or pricklebacks. Everything we catch is identified, counted, weighed, and measured. We want to learn about bottom fish because some seabirds, especially Pigeon Guillemots, eat a lot of bottom fish.
We have used SCUBA diving to study the fish eaten by Pigeon Guillemots and other nearshore predators. SCUBA diving allows us to record information about bottom fish that live in kelp forests and over rocky neashore bottoms. This is the only way to 'sample' some of these nearshore habitats, because the ocean floor may be too rough to use a bottom trawl, and the areas we are interested in may be too far away from shore to sample with beach seines. While underwater, the diver records the names and numbers of all fish seen on standardized line transects in the areas where Pigeon Guillemots live and forage. These nearshore fish communities can then be compared between areas and years.
We use a sophisticated digital fish finder (model: Biosonics DT4000) to determine the location and density of fish schools. The echosounder uses a transducer that is attached to the underside of a metal sled. It is towed underwater alongside a ship and it emits sound waves into the ocean. These sounds bounce off of fish schools, are picked up by the transducer, digitized and sent to a portable computer where they appear on the screen as targets (see figure to right). In the image shown here, the red, undulating line is the ocean bottom, the green line is the bottom as tracked by the computer, and the red-green-blue targets hovering above the ocean floor are walleye pollock. To identify what kinds of fish the echosounder has found, we use our mid-water trawl to catch the school. When we conduct hydroacoustic surveys we simultaneously conduct seabird surveys to see if birds are foraging on these schools of fish.
Most forage fish are "pelagic", meaning they tend to form schools in mid-water rather than on the bottom ("benthic") or near the bottom ("demersal"). To catch bottom-dwelling fish, you simply drag a net along the bottom. Catching pelagic forage fish is trickier, however, because you have to "fly" the net through mid-water, and modify your depth and speed to trap schools concentrated at particular depths. The mid-water trawl we use is a modified herring trawl net with a 30 foot opening at its' mouth. We tow it using a large research vessel (usually the Alaska Department of Fish and Game R/V Pandalus). During these fish surveys, we use a digital echosounder (fish-finder) to detect and pinpoint the location of fish schools in the water column. When a large fish school is detected, we put the trawl in the water and (hopefully) catch the fish. The fish are immediately identified, counted, weighed, and measured.
It is expensive and time-consuming to sample forage fish by using traditional methods such as trawls or seines. One indirect way to sample forage fish is to examine the diets of large predatory fish such as the halibut (small one pictured at right). These natural samplers range widely over the areas we wish to study, and they are a prize catch for sport fisherman. It is relatively easy to aquire their stomachs from helpful fishermen and look inside to see what they have been eating on any given day or area. They typically eat crabs and fish, including some forage fish species such as Pacific sand lance. By knowing what halibut are eating, and where they were caught, we can learn more about what forage fish are present in areas used also by seabirds.
We have found Pacific sand lance to be so important in the diets of seabirds that we’ve devoted extra time and energy to learn more about them. One reason Pacific sand lance are so important is that they are very rich in the energy-yielding fats that seabirds need to maintain their own health and feed their young. We are trying to learn more about Pacific sand lance distribution and abundance- in other words, we want to know where they live in Alaskan waters, how many there are, and what factors influence their availability to predators. We study their reproduction and growth, and also their energetic value. To do this we use beach seines, cast nests, and we dig into sand or gravel beaches at low tide where adult sand lance may be hiding.
To predict how fish communities may change in the future, and how these changes may affect fish-eating birds and mammals, we must examine historical data on fish populations. A lot of historical data on commercial fish are available because long-term records are kept by the fishing industry, and because research has long been conducted on commercial fish species (for obvious reasons) by scientists from government agencies and universities. Unfortunately, the focus on commercial species has limited our knowledge about many species of "forage fish" that are not commercially valuable, but have tremendous ecological importance. One source of historical data on forage species is a long-term database on fish catches in small-meshed trawls conducted for shrimp in the Gulf of Alaska from the 1950's to the present day. Because the mesh used to catch shrimp is very small, lots of small forage fish species get caught during typical trawl surveys. This historical dataset provides strong evidence that major changes in the Gulf of Alaska marine ecosystem occurred over the past 40 years, with large fluctuations in abundance of forage species (including shrimp, capelin, and herring) and commercial species (such as crab, pollock and cod). These changes may be linked to long-term cycles in marine climate and commercial fishing pressure. The bottom line for seabirds is that long-term cycles in forage fish abundance have a profound effect on the ability of seabirds to make an annual living and raise young, which in turn results in long-term cycles in seabird populations.