Volume and Occurrance of Food Items in Salmonid Stomachs: Moosehead Lake
FISHERIES INTERIM SUMMARY REPORT SERIES NO. 02-9
By Scott A. Roy
Maine Department of Inland Fisheries and Wildlife
Fisheries and Hatcheries Division
As part of the Department of Inland Fisheries and Wildlife long-term creel survey of Moosehead Lake, fisheries biologists collected and examined a sample of stomachs from angler-caught lake trout, landlocked salmon, and brook trout. Between 1967 and 2002, a total of 9,402 stomachs were examined to identify, count, and measure food items comprising the diet of Moosehead Lake salmonids. The results are summarized by the species of game fish and the season in which they were caught.
Most apparent among the observations was the high reliance on smelts for forage by landlocked salmon and lake trout. Landlocked salmon growth rates, condition (robustness), and survival decreased as the volume of smelts in salmon stomachs decreased. Lake trout growth rates appeared less affected by changes in smelt availability, but body condition followed trends in smelt volumes. Brook trout utilized smelts when they were available, but brook trout growth, condition, and survival were less reliant on smelts.
Of the 24 species of fish present in Moosehead Lake, only 13 were identified in the stomach contents of salmonids. No single fish species other than smelts made a major contribution to the diet of Moosehead Lake lake trout, landlocked salmon, or brook trout. Smelts were the most abundant food item in the winter stomachs. Insects occurred in the greatest proportion of summer stomachs and comprised the highest volume of food observed in game species.
Decreased smelt availability (the relative abundance of smelts vs the number of predators) has been linked to increased abundance of wild lake trout. To reduce the number of predators and maintain acceptable growth rates, lake trout regulations have been liberalized and landlocked salmon stocking rates have been reduced.
Attempts were made to augment Moosehead Lake’s smelt population by transferring smelt eggs and adult smelts. Stomach examinations failed to show any increase in smelt availability as a result.
The detailed examination of thousands of salmonid stomachs has been instrumental in understanding and documenting the importance of smelts as forage. Changes observed in the occurrence and abundance of smelts in lake trout and landlocked salmon stomachs helped to explain changes measured in growth rates, condition, and survival, and resulted in management and regulation recommendations. The thorough analysis of stomach contents provided the means to determine that forage augmentation in Moosehead Lake was not successful and pointed to the need to control predator-forage relationships. The fact that only half of the species of fish found in Moosehead Lake were identified in game fish stomachs suggests that not all small fish are either equally or even aggressively sought after for food.
The Maine Department of Inland Fisheries and Wildlife initiated annual clerk creel surveys of the Moosehead Lake fishery in 1967. As part of the survey, stomachs were removed from angler-caught fish to assess the occurrence and volume of food items utilized by lake trout, brook trout, and landlocked salmon. It was assumed that by measuring forage observed in fish stomachs, it would be possible to evaluate the relative availability of various food items and that fluctuations in the average annual volume of stomach contents would be a predictor of changes in growth, condition factor, and possibly survival. If the assumptions proved true, the study of stomach contents would be a valuable asset in determining stocking rates and size and bag limits. During ice fishing seasons, fisheries clerks collected 7,787 stomachs from unfrozen fish observed while conducting on-the-ice surveys. During open water seasons, 1,615 fish stomachs were collected. Regional fishery biologists of the Greenville office of the Maine Department of Inland and Wildlife examined all 9,402 stomachs. Food items were identified, counted, measured volumetrically, and lengths were obtained on identifiable whole fish.
Most apparent among the observations is the high reliance on smelts for forage by landlocked salmon and lake trout. When smelt availability decreased, salmon experienced a decrease in growth rates, condition factors, and survival. Lake trout growth was less affected by decreases in smelt availability; however, condition factors followed trends in smelt availability. The stomach examinations also documented a high degree of utilization of adult terrestrial insects for forage by all species in the summer samples
This report presents an overview of a large volume of food habit observations and documents the availability of the data for other investigators. The appendices show the results by season and year. The in-text tables present summaries and comparisons of annual stomach investigations. Results of the stomach examinations have been used to formulate stocking recommendations, regulation changes, and management strategies. The investigations have also provided additional basic life history data on Moosehead Lake salmonids and reveal how critically important smelts are to the diets of salmon and lake trout.
For more information or a compete report, please contact:
Maine Fish and Wildlife
Greenville, Maine 04441
Telephone: (207) 695-3756