Science: Lake trout rehab effort in the Northern Refuge failing to produce a natural population

The lake trout Salvelinus namaycush was once the top predator in Lake Michigan, but overfishing and parasitism by sea lamprey Petromyzon marinus led to a collapse in the population and by 1960 the population was wiped out.  A rehabilitation effort began in 1965, driven by stocking.  In 1985, as part of the rehabilitation effort, refuges, areas where fishing for lake trout is prohibited, were established.  In 2008, biological recommendations for the rehabilitation of lake trout populations were given to the Great Lakes Fishery Commission.  Part of these recommendations gave stocking priority to lake trout refuges, including the Northern Refuge.  Madenjian and Desorcie (2010) performed a study to assess the progress towards rehabilitation in the Northern Refuge.

            Progress was assess by: reviewing the lake trout stocking history, tracking return rates of adult lake trout to the Northern Refuge, determining if spawner abundance changed between the 1990’s and 2000’s, determining if annual mortality rates, sea lamprey wound rates or growth rates changed between 1990’s and 2000’s, tracking the abundance of wild age-0 lake trout and determining the proportion of wild lake trout in the adult population.

            Lake trout were captured with gill nets within the Northern Refuge from 1991 to 2008.  They were measured (mm total length) and weighed (g).  The presence of fin clips and sea lamprey wounds were recorded.  Sex and maturity of each individual was determined by visual examination of the gonads.  Heads were collected from fish with the adipose fin clipped so coded wire tags could be collected and used to identify the year-class of the fish and gather any stocking information on the fish.  Age-0 fish were collected with beam trawl.

            The number of lake trout stocked annually from 1986 to 2008 was found in the Great Lakes Fish Stocking Database.  Relative return rates (RRR) were calculated using a ratio of catch per unit effort (CPUE) to the number of fish stocked.  Abundance of spawner lake trout was determined by calculating the CPUE.  The proportion of wild fish in the population was calculated as the proportion of gill net-captured lake trout with unclipped fins.  The annual mortality rate was determined by catch curve analysis.  Differences in growth rates were determined by comparing lengths at ages-5 and 6.   

            Since 1985, lake trout have been stocked within the Northern Refuge at Boulder Reef, Gull Island Reef, Richards Reef and nearshore waters of High Island, with some exceptions.  Through the early 1990’s, the three reefs received approximately 250,000 yearling lake trout a year.  In the mid to late 1990’s, they received 150,000 yearlings a year.  In the 2000’s, stocking decreased and became more sporadic.

            Lake trout growth did not change significantly between the 1991-99 period and the 2000-08 period.  The authors, therefore, concluded that the supply of prey has been adequate to support the growth of lake trout.  Annual mortality rate decreased between the two periods, but not significantly.  Sea lamprey wounding rates approximately doubled between the two periods.  A lake trout was 2.27 times more likely to have a sea lamprey wound during the later period, a significant increase.

Between the 1991-99 period and 2000-08 period, lake trout spawner abundance decreased by 93% on Boulder Reef and 86% on Gull Island Reef.  RRR of adult lake trout to the Northern Refuge decreased by an order of magnitude from the 1990-93 year-classes to the 1995-98 year-classes.  Only 2.1% of lake trout caught during the study were unclipped.  The percentage of unclipped fish annually ranged from 0.0% to 5.1% (excluding 2008, which was 28%) and the annual mean percentage was 2.6%.  No wild age-0 lake trout were caught during the entire study.

The absence of wild age-0 fish and the low percentage of unclipped fish (except in 2008) is a sign that there has been little recruitment of naturally produced lake trout to the Northern Refuge.  The authors offer a couple potential explanations: First, alewives Alosa pseudoharengus interfere with natural reproduction because they feed on lake trout larvae and feeding on alewives can reduce egg survival.  Second, the density of spawners was too low to support a self-sustaining population. 

The authors propose that the lower abundance of spawners during the 2000-08 period was due primarily to reduced stocking rates from 1995 to 2005.  But they also suggest other factors could be involved.  Burbot Lota lota feed on newly stocked lake trout yearlings with the Northern Refuge.  The Isle Royale strain of lake trout stocked in the Northern Refuge during the late 1990’s exhibited lower survival that other strains.  And there was relatively higher fishing mortality during the late 1990’s. 

The estimate of annual mortality was close to the target level established by the Lake Michigan Lake Trout Technical Committee, so it is unlikely that the mortality rate caused the decline in spawner abundance.  The authors speculate that the most likely explanation for the decrease in mortality between the 1991-99 and 2000-08 periods is that a decrease in fishing mortality compensated for the increase in mortality due to sea lampreys.  For example, the 2000 Consent Decree decreased commercial large-mesh fishing effort from 4,716 km/year during the 1990’s to 1,073 km/year during 2000-08 in statistical district MM-3( and area which contains the Northern Refuge).  Also, it is doubtful that double-breasted cormorants Phalacrocorax auritus feeding on newly stocked lake trout played a part in the decline of spawner abundance because they have not been seen near the reefs when lake trout were being stocked.

Because seven of the 25 lake trout caught in 2008 were unclipped but no wild age-0 lake trout were caught, the authors postulate that they may not have come from the Northern Refuge.  A study in Lake Huron reported that after the collapse of the alewife population in 2003 there was a resurgence of the lake trout population.  Perhaps these fish traveled to the Northern Refuge.

The authors suggest that spawner abundance and the proportion of wild fish on a particular reef can be increased by stocking 250,000 yearlings per reef annually.  The lake trout rehabilitation guide suggests stocking 480,000 yearlings annually in the Northern Refuge.  The authors, therefore, recommend that stocking should occur on two reefs, Boulder and Gull Island, and stocking should only be stopped if spawner abundance and the proportion of wild fish continue to increase into the future.  The authors also recommend that annual mortality should be reduced to below the target level (0.4) by maintaining the low levels of fishing mortality produced by the 2000 Consent Decree and reducing sea lamprey abundance.

Implications for anglers

            Targeting or possessing lake trout while in the refuges is prohibited.  To avoid costly tickets, trout and salmon anglers should steer clear of the refuges since targeting Chinook salmon or brown trout might be interpreted as targeting lake trout.  The refuges should also be avoided because any taking of lake trout in these areas could hamper rehabilitation effort.  If fishing for lake trout outside of the refuges, anglers can dampen their impact on the rehabilitation effort by keeping smaller fish and releasing larger fish.  Older, larger fish not only demonstrated they are evolutionarily fit for life in Lake Michigan but  possess greater fecundity, meaning they contribute more to subsequent generations than smaller fish. 

            Proponents of lake trout rehabilitation argue that success will bring ecological stability to Lake Michigan.  Success would also mean that funds used to rehabilitate lake trout could be diverted to other species.  However, this study demonstrates that decades of effort and millions of dollars have failed to produce positive results.  With lower interest for lake trout among trout and salmon anglers, and a number of factors working against them, it may be time for Lake Michigan fisheries managers, to cut their losses.  The money and effort may be better spent managing more valuable fisheries resources.  Anglers, scientists and managers should discuss the possibility of a Lake Michigan without lake trout.

Selected definitions

2000 Consent Decree- an agreement between native American tribes and the state of Michigan that governs allocation, management, and regulation of state and tribal fisheries in the 1836 Treaty Waters of the Great Lakes

Adipose fin- a soft, fleshy, rayless fin posterior to the dorsal fin

Catch per unit effort- measure of density or population size calculated as total catch divided by the total amount of effort used to harvest the catch

Fecundity- measure of the ability to reproduce

Fishing mortality- rate at which individuals are removed from a population by fishing

Spawner- a fish capable of spawning

Year-class- individuals of a given species that were all spawned in the same year

Yearling- an individual that is one year old or in the second year of its age

Madenjian, C.P. and T.J. Desorcie. 2010. Lake trout population dynamics in the Northern Refuge of Lake Michigan: implications for future rehabilitation. North American journal of fisheries management. 30:629-641.