Hatchery Brook Trout have limited natural reproduction

CONSERVATION AND SCIENCE REPORT

April 2013

By Bill Bakke

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“There was little evidence of successful reproduction by stocked Brook Trout. Similar to the results of other studies, our findings suggest that the stocking of nonlocal Brook Trout strains where a local population already exists results in limited natural reproduction and should be avoided, particularly if the mechanisms governing the ecotype of interest are poorly understood.

Despite the growing recognition of the ecological importance of intraspecific variability the extent of intraspecific diversity has declined in the Great Lakes. Much of the morphological diversity that was once observed among Lake Trout has been lost due to predation by Sea Lampreys Petromyzon marinus and due to overharvest.

Recently, managers seeking to restore the intraspecific diversity represented by coaster Brook Trout to Lake Superior have placed more emphasis on the stocking of Brook Trout strains originating from Great Lakes-region parental populations that display the coaster phenotype.

“Although these data suggest that the experimental stocking program did not attain the desired outcome, the stocking program and the observations derived from it have enhanced the understanding of how interactions between genetics and environment influence intraspecific variation.

Nipigon hatchery strains were observed among the sampled Brook Trout, suggesting that reproduction of hatchery fish did not contribute significantly to populations in the three streams.

“No genetic evidence was found to indicate that the Lake Nipigon hatchery strain has reproduced or was recaptured in the three rivers sampled during our study.

“Previous research suggests that hatchery stocks of Brook Trout have founded new populations when introduced into new areas. However, when Brook Trout are stocked sympatrically with native populations, as was the case in PRNL, introgression rates are consistently low. Although this study is one of the few that have directly looked for hatchery genotypes in Brook Trout from Lake Superior, other genetic analyses have suggested that Brook Trout populations are still maintained by native fish even after those systems have received stocked fish for many years.

“The current NPS management policies (NPS 2006) emphasize the commitment of the NPS to protect natural evolutionary processes and to minimize human interference in those processes. These policies provide clearer guidance regarding native species restoration than was available to park managers in 1997, when the stocking of Tobin Harbor Brook Trout began. Tobin Harbor-strain Brook Trout would not have been stocked under the current policies because the source of the Tobin Harbor strain is geographically remote from the local population and because the strain is derived from a lacustrine coaster population.

“Although native Brook Trout populations at PRNL may have interbred with fish that were stocked before 1997, it remains important to protect local (and possibly naturalized) strains.

“Based on the outcome of the experimental stocking program and on current NPS guidance, Brook Trout stocking in PRNL is unlikely to resume in the near future. Management at PRNL took the conservative view that natural resource actions should be avoided or halted if no perceived positive aspects are evident and if the potential exists for negative impacts. Based on current NPS policies (NPS 2006), if a management need to stock Brook Trout in park waters emerges in the future, the fish would be selected from strains founded from local sources.

“As for any species or form, the historical and contemporary status of coaster Brook Trout should be carefully considered before further management actions, such as stocking, are planned.

“While such historical accounts are important and can be a compelling aspect of any management plan, sufficient historical information may not be available and managers are often asked to make decisions without a complete

historical picture of a system. Regardless of whether historical evidence is available, current scientific data should be acquired in advance of management practices to ensure the best outcomes.

The PRNL Brook Trout stocking program was initiated with little reliable historical data and little recent fisheries data from the streams to provide ecological context for the action, and this lack of information decreased the likelihood that the project’s goals would be reached.

“At present, it is unclear how rehabilitation of the coaster Brook Trout phenotype in PRNL will progress. Current data suggest that coaster Brook Trout should best be viewed as an element of intraspecific diversity that contributes to the overall variability within the species (USFWS 2009).

“Given the excellent Brook Trout habitat available in PRNL, habitat enhancement is unlikely to contribute to rehabilitation.

“A current project evaluating the influence of introduced fish species on the native fish community in two of the three streams we studied will provide information on the ecological effects of nonnative species on native Brook Trout and may offer opportunities for rehabilitation, as may the creative use of harvest regulations.”

Jill B. K. Leonard , Wendylee Stott , Delora M. Loope , Paul C. Kusnierz & Ashwin Sreenivasan (2013):

Biological Consequences of the Coaster Brook Trout Restoration Stocking Program in Lake Superior Tributaries within Pictured Rocks National Lakeshore, North American Journal of Fisheries Management, 33:2, 359-372