Fertilization and Predator Control to Increase Growth Rate and Yield of Trout in a Natural Lake

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Abstract
Enrichment in 1946 of Crecy Lake (20 ha; mean depth, 2.4 m), New Brunswick, with commercial fertilizers to provide 210 ppb of nitrogen (N), 390 ppb of phosphorus (P), and 270 ppb of potassium (K) had increased the growth rates of native and planted brook trout. In further studies, control of predation by fish-eating birds and mammals and reduction in number of eels, along with another application of fertilizers in 1951, improved the percentages of planted brook trout taken by anglers. Maximum percentages before and after predators were controlled were 17 and 88 for planted yearlings and 4 and 42 for planted underyearlings. Yields of planted trout to anglers and catches per unit effort improved commensurately. Autumn-stocked yearlings were largely (96%) angled in April before appreciable growth was made in the lake. Increases in yields of trout flesh produced in the lake resulted largely from the growth and higher survival of planted underyearlings. Harvested weights were greater than weights of plantings only when 16% or more of planted underyearlings were angled.When underyearling and yearling rainbow trout were planted instead of brook trout, with a third fertilization in 1959 and continued predator control, they grew faster than brook trout of comparable size. Rainbow trout produced more fish flesh in the lake than brook trout when the percentage returns to anglers for the two species were similar. However, the percentage return to the anglers from most plantings of underyearling rainbow trout was low, with the net result that the rainbow trout did not provide as consistently good angling as the brook trout nor did they utilize the productive capacity of the lake more effectively.Brook and rainbow trout spawned in the littoral areas of the lake. However, yields of naturally reared trout were poor, particularly when the survival of planted trout was high.Killifish increased in numbers with fertilization, with or without predator control. Cyprinids were much less abundant than killifish, and sticklebacks disappeared from the lake.

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