Lake Pend Oreille Bull Trout population appears stable

October 26, 2016

A moratorium on angling and lake trout eradication efforts to address the threats of overfishing and a voracious non-native species appear to have stemmed the decline of bull trout in Lake Pend Oreille, according to a recent study. The study compares the mean age, growth, maturity and abundance from surveys done in 2006 and 2008 with a 10-year old study of bull trout. It found that management had addressed current threats to the fish – overfishing and an invasive species—and in doing so, had increased the likelihood of their long-term persistence in the lake. Bull trout have inhabited the Idaho lake since the ice age and it once supported a “world-renowned” bull trout fishery, according to the study. The bull trout population in the 1998 survey was 12,134 and today that population is holding steady 12,513 fish. “We found that Bull Trout size and age structure, mortality, growth, maturity, and abundance were consistent with that of a population that was rebuilding,” said lead researcher Michael Hansen. “Collectively, these findings suggest that management effectively addressed current threats to increase the likelihood of long-term persistence of the Bull Trout population in Lake Pend Oreille.” The adfluvial bull trout population – those that live in lakes and spawn in streams – in Lake Pend Oreille began to decline with the construction of hydro-electric dams both upstream in the Clark Fork River (Cabinet Gorge Dam) and downstream in the Pend Oreille River (Albeni Falls Dam). Lake trout were introduced in 1925 and the addition of opossum shrimp to the food supply resulted in a 356 percent increase in the number of lake trout between 1998 and 2005. In 1966, the Idaho Department of Fish and Game placed a moratorium on fishing for bull trout in tributaries (with the exception of the Clark Fork River), and a predator removal program for lake trout using trap and gillnets was initiated in 2006. “Demographic Characteristics of an Adfluvial Bull Trout Population in Lake Pend Oreille, Idaho,” was published online October 7 in the North American Journal of Fisheries Management, http://www.tandfonline.com/doi/full/10.1080/02755947.2016.1209602. Hansen and co-author Jonathan McCubbins were with the College of Natural Resources, University of Wisconsin-Stevens Point, when conducting the study. Hansen is currently with the US Geological Survey as a Supervisory Research Fishery Biologist and McCubbins is a fishery biologist in Glacier National Park for the National Park Service. Other co-authors are Joseph DosSantos, retired, aquatic program manager with Avista Utilities, and Andrew Dux, regional fishery manager with the Idaho Department of Fish and Game. Although the abundance of bull trout is similar today as it was in 1998, other attributes – size, age structure and growth, among others – have changed. Bull trout were larger in 1998 at 546 millimeters (21.5 inches) in length than in 2006 – 2008 at 494 mm (19.44 inches), while mean age was similar at 7.6 years for the earlier survey and 7.3 years for the later survey. In the current study, bull trout grew faster to age four, but slower afterwards, and were generally longer up to age four and shorter after age six than in 1998. Male and female bull trout matured at a similar age, but females grew faster than males, thereby maturing at a larger size. The rise in the abundance of lake trout between 1998 and 2005 coincided with a period when age-4 and older bull trout declined and the consumption of kokanee, a primary food source for both lake and bull trout, increased, resulting in competition between the species for a diminishing number of kokanee, the study says. That may have led to the slower growth of ages four – nine bull trout. “If Lake Trout suppression allows abundance of kokanee to increase,” the study says, “interspecific competition would decline further, thereby allowing Bull Trout growth to increase in the future.” The later survey found bull trout to be younger at age of maturity and that, the study says, is consistent with early growth, which would cause fish to mature at an earlier age, “thereby reducing the size and age structure.” The shorter fish and declining age structure suggests that bull trout “may be compensating for increasing density of competitor species by reducing age at maturity through increased early growth,” the study says. The shift to smaller fish, while maintaining abundance, could be due to increased recruitment, it added, although the number of redds (nests) declined from 726 in 1998 to 584 in 2008 “as would be expected if age and size structure shifted downward.” Still, size and age structure, mortality, growth, maturity and abundance “are consistent with that of a population that is rebuilding,” the study says. That could be good news for anglers. “Given the stable population status since the time of harvest closure, the Bull Trout population may be able to support limited angler harvest,” Hansen said. “Providing Bull Trout harvest opportunity for anglers is a management goal for the Lake Pend Oreille fishery, and reinstituting some level of harvest may be justified if tributary habitat conditions and population status remains stable in the future.