Chinook Salmon Studies

Chinook salmon studies: Two studies of Lake Huron chinook salmon are nearing completion. One looks into chinook life history, and stocking methodologies and stocking sites and their effects on post-stocking survival. The other investigates the incidence of reproduction of chinook salmon in Lake Huron. Both of these studies were funded by Federal Aid to Fish Restoration (link to FWS DJ page) and Michigan DNR license fees.

Chinook life history, stocking methods, and post-stocking survival - Chinook salmon spend relatively little time in their natal streams before migrating to Lake Huron. Egg deposition is in October and November. The eggs hatch during late winter and fry emerge from their gravel nests, known as redds, in late March and April. By mid-June the juvenile chinooks have grown to about 3 inches in length and begin "smolting", that is, their colors change to bright silver and they begin migrating to the lake. Chinook salmon only spend about 9 months (including the egg stage) in streams and are not present in Lake Huron's tributaries during the warmer summer months. Thus, it is likely chinook salmon are able to reproduce in streams that might be too warm for other trout and salmon. Steelhead trout and coho salmon, for example, have a one- to two-year stream residency and thus must live in streams through the warmest summer months.

For a few weeks after entering the lake, chinook smolts inhabit the beach zone. We found high numbers of wild and recently stocked chinooks by seining after dark along sandy beaches of north and central Lake Huron. By June 15 most of the smolts appeared to have migrated off shore beyond the reach of beach seines.

During July through September we found young of year chinooks, ranging in size from 6 inches to 13 inches, near the surface in water depths of less than 50 feet. Stomach contents indicated the mainstay of the diet of these juvenile salmon was airborn insects, principally ants, bees, and flies. These young chinooks were most commonly found off points such as North Point of Thunder Bay and Tawas Point. We found both wild and hatchery chinooks were mingled at this point.

By late fall, young-of-year alewives and juvenile smelt appeared in chinook diets and chinooks remained dependent on fish for their diet thereafter. Yearling and older chinooks were found to be suspended between the surface and depths of well over 100 feet. The diet of these fish was principally alewives.

Our stream sampling and beach seining data showed that chinook smolts were exposed to predation by birds and nearshore predator fish such as walleyes and smallmouth bass. We hypothesized that by holding hatchery-raised juveniles in acclimation raceways the hatchery smolts would be protected from predation, and would be larger and better acclimated at time of release than if stocked directly from the truck into tributary streams. To test this hypothesis, we used coded-wire tags to mark two test groups of chinooks: those stocked directly from the truck into the AuSable River and those acclimated in off-stream raceways before release. The raceways were located on the lower AuSable River and were in fact abandoned harvest-weir raceways supplied with pumped AuSable River water. The fish were fed in these pens for an average of 3 weeks prior to release. We then monitored catches of the chinooks to see whether raceway acclimation indeed produced better survival, and thus harvest, of stocked chinooks. The results showed that the AuSable River pen-reared fish produced nearly three times the harvest in the open water, summer fishery than the groups stocked the traditional way by truck.

Furthermore, the raceway-acclimated chinooks appeared to be better imprinted and produced nearly 6 times the returns of conventionally stocked fish to the AuSable River as mature (spawning condition) fish during fall. Further coded-wire tagging has shown that other acclimation rearing sites, Harrisville and Harbor Beach, are experiencing enhanced survival rates. The pens on the AuSable River are maintained by Lake Huron Sportfishing Inc. of Oscoda. The Harrisville acclimation raceways are operated by local volunteers and members of the Lions Club in Harrisville. The Thumb Steelheaders organization operated rearing pens in Harbor Beach. Thanks in large part to the hard work of volunteers from these groups, stocking success remains relatively high in central Lake Huron. Reported harvest of various study groups of coded-wire tagged chinooks stocked in Lake Huron is given in Table 1.

Figure 1. Measuring growth of chinook in the AuSable River acclimation raceways.
Figure 1. Measuring growth of chinook in the AuSable River acclimation raceways.

Table 1. Returns by open-water troll fishery per 100,000 coded-wire tagged chinook salmon released, comparing acclimated and conventionally stocked lots of fish, AuSable River at Oscoda.

Year Acclimated Conventional
1993 293.1 154.7
1994 332.6 116.8

Chinook salmon reproduction in Lake Huron - From 1989-1992 the DNR marked all hatchery-origin chinooks with oxytetracycline (OTC) to determine incidence of reproduction. The estimate from that study was that 15% of chinook salmon in Lake Huron originated from the wild. Subsequent to that study catch rates of chinook salmon rose and growth rates declined. Concerned about declining growth rates and condition of chinooks, in 1998 Michigan DNR and Ontario Ministry of Natural Resources reduced stocking of chinooks by 20% to balance predator numbers with available prey fish. In spite of the stocking reduction, catch rates reached a new record high in 2002 and growth of chinook salmon remained slow. Ontario and Michigan therefore decided to repeat the reproduction study and mark all stocked chinooks from 2000-2004. Michigan again marked its stocked chinooks by feeding them the antibiotic oxytetracycline just prior to stocking. This produces a florescent zone on the vertebrae that can be detected by viewing the vertebrae under a microscope with black-light illumination (a marked chinook vertebra is illustrated in Figure 2). Ontario used fin clips to mark their hatchery chinooks. To date, we have examined catches of chinooks that originated with the 2000, 2001, and 2002 year classes. Chinook salmon collected during the spawning run at two Michigan stocking sites, the Swan Harvest Weir and the lower AuSable River, were almost all of hatchery origin. However, nearly 85% of chinook salmon caught from the open water of Lake Huron during spring and summer, prior to spawning migrations, lacked the OTC mark and were presumably wild. The mark composition of mid-summer recreational catch Figure 3.

Figure 2. Chinook salmon vertebra with OTC mark near center.
Figure 2. Chinook salmon vertebra with OTC mark near center.

Figure 3. Chinook salmon marked with OTC
Figure 3. Percent chinook salmon marked with OTC, from open water troll fishery at Alpena and Rogers City, by year class.

The United States Geological Survey Great Lakes Science Center (USGS) assesses prey populations in the Great Lakes. In Lake Huron, the USGS estimates of adult alewife abundance in fall 2003 were the lowest on record. By spring of 2004, alewives had become a minor component of the prey assessment catch. Rainbow smelt, the other leading prey fish in Lake Huron, also appeared to decline. A positive note was that smelt and bloater chubs produced strong year classes in 2003 that persisted into 2004.

What's it mean? It appears several circumstances conspired to produce increasing trout and salmon catch rates, an unanticipated abundance of chinook salmon, and declines in alewife and smelt abundance in Lake Huron. What set off this chain of events?

  • Exotic species and the food web. The introduction of zebra and quagga mussels (from ballast water discharges of ocean-going vessels) to the Great Lakes appears to have somehow caused the depletion of the native crustaceans (shrimp like animals) Mysis and Diporeia. These native shrimp fed on organic matter and plankton that settled to deeper strata of Lake Huron and in turn became prey for such fish as deep water chubs, whitefish, rainbow smelt, and to some degree alewives. Zebra and quagga mussels are far less nutritious and less available to fish as food than the native shrimp. In addition, the invasion of Lake Huron by spiny water fleas (Bythotrephes cederstroemi) displaced native plankton. Spiny water fleas, by virtue of their long, calcareous spine, are less suitable as prey than native plankton. These invaders, together, have apparently reduced Lake Huron's productive capacity for alewives, smelt, and other planktivorous small fish chinook salmon depend on for prey.
  • Winter severity: Alewives are near the northern extremity of their range in Lake Huron; Lake Superior, for example, is too cold for alewives. The winter of 2002-2003 was one of the most severe in the last 20 years and the winter of 2003-2004 was slightly colder than average. Alewife survival during these two winters was much lower than during the previous several years.
  • Reproduction of chinook and enhanced survival of pen-cultured chinook salmon: Our marking studies showed that pen culture significantly increased survival of stocked chinook salmon while at about the same time natural reproduction rates for salmon rose sharply. Natural reproduction now accounts for approximately 7 of every 8 chinook salmon caught during the summer fishing season.
  • Sea lamprey control: Finally, Lake Huron's sea lampreys have come under nearly they same level of control as the other Great Lakes. Advances in technology and increased funding ($3,000,000 from the State of Michigan) led to control of sea lampreys in the St. Marys River. The St. Marys River was the largest producer of sea lampreys in the Great Lakes. Mostly as a result of the St. Marys River treatment, sea lamprey wounding rates and lamprey-induced mortality of salmon and trout have declined approximately 50% since 1999.

It now appears the combined effects of food web changes caused by exotic invaders, relatively harsh winters, improved survival of stocked chinooks through acclimation rearing, prominence of natural reproduction, and control of sea lampreys (which led to much better survival of large predator fish) was just too much for the alewife prey base. By 2003 alewife numbers were in serious decline and alewife numbers appeared to decline further in 2004. Evidently, salmon and trout numbers exceeded the prey supply during this period.

What does the future hold? Nowhere is there a reproducing, freshwater chinook salmon fishery of the magnitude of Lake Huron's. Lake Superior is probably the only freshwater lake with conditions somewhat resembling Lake Huron. However, lake trout, not chinook salmon, are the keystone predator of Lake Superior; chinooks play a relatively minor role there. Lake Superior tells us that, with a reproducing lake trout population and almost no alewives, rainbow smelt and lake herring can combine to produce a sustainable prey base. Chinook salmon reproduce in Lake Superior, although apparently in much lower numbers than Lake Huron. Thus, based on the Lake Superior example, it appears that Lake Huron's prey base is likely be become predominantly composed of smelt and perhaps lake herring. Chinook salmon should persist in Lake Huron, although in perhaps smaller numbers and/or smaller average sizes. Should alewife numbers remain low, prospects for reproduction of native species, including walleyes, yellow perch, and lake herring are good. In fact, early indications are that Saginaw Bay's walleye and perch populations are doing exactly that: 2003 produced record year classes for both species (link to Saginaw Bay). Alternatively, alewives may take advantage of occasional mild winters to stage recoveries and could in certain years continue to be a staple for Lake Huron's chinook fishery. Only time will tell us the outcome.

In the mid 1980's Lake Michigan apparently experienced a prey shortage. There, salmon contracted bacterial kidney disease, which killed a large proportion of Lake Michigan's salmon population annually until the early 1990s. Of immediate concern on Lake Huron is whether Huron's chinook salmon will also become victims of bacterial kidney disease. Fish tend to fend off most common pathogens, such as bacterial kidney disease, when in good health, but Lake Huron's chinooks may be experiencing stress from shortage of prey. This stress could render the fish more vulnerable than usual to diseases such as bacterial kidney disease. The DNR will be monitoring chinook salmon this summer more closely than usual for incidence of disease. We will also be collecting vertebra for OTC detection and heads for coded-wire tag extraction from the angler catch. The cooperation of recreational anglers is vital to the collection of these samples with these data collections is appreciated.

Figure 4. Collecting vertebra samples from chinook salmon during Alpena's Brown Trout Festival.
Figure 4. Collecting vertebra samples from chinook salmon during Alpena's Brown Trout Festival.

In summary, Lake Huron is going through a period of adjustment. Alewife numbers will eventually recover or other prey will fill the void left by the decline in alewife numbers. Until that happens, though, predator fish such as chinook salmon will find prey to be scarce and will be smaller and lower condition (thinner) than usual. Some may show signs of stress or disease. Eventually, perhaps in just a year or two, the ecosystem will adjust to a more appropriate balance of predators and prey.