Ontario Ministry of Natural Resources - Lake Simcoe Fisheries Assessment Unit
Update No. 2001-5

Lake Simcoe Lake Trout:
Evidence of Natural Reproduction

This summer, two naturally reproduced lake trout were captured during bottom trawling being conducted by the Lake Simcoe Fisheries Assessment Unit (LSFAU). This finding is significant because it is the first indisputable evidence of lake trout natural reproduction since the 1980's.

The primary purpose of this year's trawling program was to monitor the abundance of adult lake trout prey fish, particularly rainbow smelt. Other LSFAU monitoring programs (creel surveys, index netting) have shown a dramatic decrease in rainbow smelt and lake herring abundance in recent years, raising concerns over the availability of food for stocked lake trout. The trawls caught only 26 rainbow smelt in 2001, as compared to 1,685 in 1991 and 356 in 1992, confirming the decline in the abundance of this species.

The lake trout were captured on July 17, 2001 at two different 30 metre sampling stations. Surprisingly, the bottom trawling captured not only these two naturally reproduced lake trout but also 1,368 spoonhead sculpin. This species was first documented in Lake Simcoe during the 1991 bottom trawling program when only one specimen was caught. Lake Simcoe is near or at the southern limit of its geographic distribution.

The two young lake trout were approximately 40 mm in length, and were approximately 3-4 months of age. Due to the difficulty in identifying very young fish, these specimens were sent to experts at the Royal Ontario Museum for positive identification.

Where is no doubt that these fish were naturally produced, since stocked lake trout are put into Lake Simcoe at one year of age and at a length of about 130 mm. Also, these two fish had all their fins, while stocked lake trout have a fin removed at the hatchery so that they can be identified as stocked fish.

Does this mean that lake trout have started to naturally reproduce again in Lake Simcoe?

The capture of these small lake trout shows that there is some natural reproduction of lake trout in Lake Simcoe, but whether or not this is new, is unknown. Keeping in mind that only two naturally reproduced lake trout were captured, there are essentially two possible scenarios:

A)    naturally reproduced lake trout less than one year old have continued to be present in Lake Simcoe but have not been seen, nor have they contributed to the adult population;
or
B)    the presence of these two fish represents renewed, or at least increased, natural reproduction of lake trout.

We know that 99.9% of lake trout caught by anglers and by the LSFAU fish sampling programs have fins missing, indicating that they were stocked. The very few seen without fin clips could more easily be attributed to fin regeneration or fin-clipping error than natural reproduction. However, these programs rarely sample lake trout that are less than 3 years old, so it is possible that very young lake trout have been present in the lake all along.

It is currently unknown at which life stage natural recruitment to the fishery fails (egg, alevin or first summer?). We know that egg incubation experiments conducted in Lake Simcoe in 1990 found that survival of lake trout eggs to the hatched alevin stage varied from 0 to 94% between 15 field incubators (Sly and Evans 1996). This indicates that naturally spawned lake trout eggs are capable of hatching. The recent capture of young naturally produced lake trout indicates that there is at least some extended survival after the hatch.

On the other hand, it may be possible that finding these two fish represents renewed, or increased, natural reproduction of Lake Simcoe lake trout. The more extensive trawling programs conducted in 1990 and 1991 failed to find any naturally produced lake trout, indicating that extended survival to at least 4 months of age may be a more recent phenomenon. Should this be the case, LSFAU sampling programs are in place to detect and monitor such a change.

There have been many changes to the Lake Simcoe ecosystem in recent years including the introduction of zebra mussels with a subsequent increase in water clarity, invasion of the spiny water flea, dramatic decrease in rainbow smelt abundance, near extinction of lake herring, and a build up of the stocked lake trout population, to name a few. The impacts of these changes are not fully understood, however, it is possible that some of these changes could favour the survival of young lake trout.

Why is there a lake trout natural reproduction problem?
Natural recruitment failure affects a number of coldwater fish species in Lake Simcoe. Lake trout, lake whitefish and lake herring populations have all suffered drastically such that lake herring are now virtually gone from the lake while lake trout and lake whitefish populations are being maintained only through fish stocking.

The problem stems from excessive nutrient loading into Lake Simcoe, which in turn, is adversely affecting the habitat of the coldwater fish species. Efforts to combat this problem are coordinated through the Lake Simcoe Environmental Management Strategy (LSEMS). LSEMS is a multi-agency partnership whose goal is to protect, maintain and improve the health of the Lake Simcoe ecosystem by reducing nutrient loading and restoring a naturally reproducing coldwater fishery.

What next?
The LSFAU will continue long-term monitoring and assessment of Lake Simcoe fish populations and the factors affecting them (water quality, habitat, fishing, species introductions). In addition, plans are being developed to conduct further sampling of the deepwater, summer fish community. In the summer of 2002, this will include more extensive bottom trawling and an index netting program. This will help us to determine the extent of natural lake trout reproduction in Lake Simcoe as well as population changes in other members of the fish community. Perhaps the most important message from finding these two fish is that restoring natural reproduction of lake trout would indeed seem possible. Efforts to control nutrient loading and protect fish habitat in Lake Simcoe are as important as ever.

This update was prepared by Cam Willox, LSFAU Manager.
Acknowledgements
We wish to thank Erling Holm and Marty Rouse of the Centre for Biodiversity and Conservation Biology, Royal Ontario Museum, for providing positive species identification.

References
Auer, N. A. 1982. Identification of larval fishes of the Great Lakes basin with emphasis on the Lake Michigan drainage. Great LakesFishery Commission, Spec. Publ. 82-3: 744 pp. Sly, P.G. and D.O. Evans. 1996. Suitability of habitat for spawning lake trout. Journal of Aquatic Ecosystem Health 5:153-175