Lake Trout Mortality and Abundance in Southern Lake Huron

Abstract

Populations of lake trout Salvelinus namaycush in the main basin of Lake Huron collapsed in the 1940s because of predation by sea lampreys Petromyzon marinus and commercial fishing. Efforts to rehabilitate lake trout have emphasized reduction of mortality and the stocking of hatchery‐reared lake trout to reestablish populations. We fit a statistical catch‐at‐age model for lake trout in the southern main basin of Lake Huron using a maximum likelihood approach to estimate mortality rates and abundance during 1984–1993. This represents the first such analysis for lake trout in the Great Lakes, and a flexible application of the approach proved useful for integrating diverse information and assessing population and mortality trends. Sea lamprey‐induced mortality and recruitment of lake trout to age 1 were calculated external to model fitting. Recruitment was based on numbers of lake trout stocked because natural recruitment is negligible. Sea lamprey‐induced mortality rates were based on observed wounding data on lake trout. Other mortality rates were estimated during model fitting. Mortality rates have varied from year to year but do not show trends over time and are currently well below target maximum levels. Lake trout abundance has declined from a peak spawning biomass of 412,000 kg in 1988 to a low of 295,000 kg in 1993. This decline is a result of lower stocking levels since the mid‐1980s. Sea lamprey predation was the dominant source of mortality for lake trout older than age 4. During 1984–1993, we estimated that sea lamprey predation caused 48% and fishing accounted for 16% of deaths of lake trout older than age 2. Although our estimates of sea lamprey‐induced mortality are uncertain, current estimates of lethality of sea lamprey attacks would need to be greatly overestimated to overturn our conclusion that sea lamprey mortality has exceeded fishing mortality in southern Lake Huron.

Population ecology of the sea lamprey (Petromyzon marinus) as an invasive species in the Laurentian Great Lakes and an imperiled species in Europe

“…For example, sea lamprey predation was the predominant source of mortality experienced by the lake trout population in Lake Huron during 1984-1993(Sitar et al 1999). More recently, sea lamprey predation is still believed to be an important source of mortality on lake whitefish populations in northern Lake Huron (M. Ebener, Chippewa Ottawa Resource Authority, personal communication).…”

Sea lamprey wounding in Canadian waters of Lake Huron from 2000 to 2009: Temporal changes differ among regions

“…This approach diverges from other studies that used the number of wounds per fish (Rutter and Sitar et al, 1999). However, the probability of wounding and mean wound number would both be substantially less than one, and therefore very similar in Lake Huron where fish having more than one wound are rare.…”

“…To date, the vast majority of reports examining lamprey wounding rates in the wild have focused on almost exclusively on lake trout (including backcross and splake) without considering alternative prey species (e.g. Madenjian et al, 2008a;Madenjian et al, 2008b;Sitar et al, 1999).…”

“…The objective of our study therefore was to examine changes in lamprey wounding rates during the period of 2000-2009 in Canadian waters of Lake Huron. We focused our analyses on lake trout, as done in similar studies (Rutter and Sitar et al, 1999), and on lake whitefish, a species of commercial importance that also is commonly parasitized by lamprey.…”

Evaluating methods for estimating process and observation error variances in statistical catch-at-age analysis

“…Derivation of error variance values separate from SCAA is the more common approach, with these estimates or their ratio then assumed known during the subsequent SCAA. A plausible estimate of observation error variance for data subsets such as observed annual catch, effort, or abundance indices often can be obtained through analysis of the raw data used to derive these quantities, taking into account the sampling designs (Law and Kelton, 1982;Sitar et al, 1999). Process error variances cannot be estimated in the same way, by analysis of assessment data subsets external to the model, because by themselves these data are not informative about how model parameters such as catchability are varying.…”