Evaluation of sea lamprey-associated mortality sources on a generalized lake sturgeon population in the Great Lakes

Lake sturgeon populations in the Laurentian Great Lakes experience two age-specific mortality sources influenced by the sea lamprey Petromyzon marinus control program: lampricide (TFM) exposure-induced mortality on age-0 fish and sea lamprey predation on sub-adults (ages 7–24). We used a generic age-structured population model to show that although lampricide-induced mortality on age-0 lake sturgeon can limit attainable population abundance, sea lamprey predation on sub-adult lake sturgeon may have a greater influence. Under base conditions, adult lake sturgeon populations increased by 5.7% in the absence of TFM toxicity if there was no change in predation; whereas, a 13% increase in predation removed this effect, and a doubling of sea lamprey predation led to a 32% decrease in adult lake sturgeon. Our estimates of lake sturgeon abundance were highly dependent on the values of life history and mortality parameters, but the relative impacts of ceasing TFM treatment and increasing predation were robust given a status quo level of predation. The status quo predation was based on sea lamprey wounding on lake sturgeon, and improvements in this information would help better define tradeoffs between the mortality sources for specific systems. Reduction or elimination of TFM toxicity on larval lake sturgeon, while maintaining TFM toxicity on larval sea lamprey, can promote lake sturgeon restoration and minimize negative impacts on other fish community members.     

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

Lake Huron has undergone a number of substantial changes in recent years, including changes to management of the parasitic sea lamprey, Petromyzon marinus. While control strategies of lamprey involving lampricides have had some success, lamprey spawning in St. Marys River has been a major and persistent problem and has led to intensified treatment beginning in 1998. The objective of our study was to broadly examine lamprey spatial wounding dynamics of lake whitefish (Coregonus clupeaformis) and lake trout (Salvelinus namaycush) within the Canadian waters of Lake Huron from 2000 to 2009. Temporal trends were evident and these differed among regions (North Channel, northern Main Basin, southern Main Basin, northern Georgian Bay, and southern Georgian Bay). There was a monotonic annual increase in probability of wounding for both lake trout and lake whitefish in three of the five regions, with high increases seen in both northern and southern Georgian Bay. The increases in three of the five regions are unexpected given the ongoing treatment of St. Marys River.     

Variation in larval sea lamprey demographics among Great Lakes tributaries: A mixed-effects model analysis of historical survey data

Understanding variation in fish populations is valuable from both a management and an ecological perspective. Great Lakes sea lampreys are controlled primarily by treating tributaries with lampricides that target the larval stage. Great Lakes streams were divided into four categories based on their regularity of parasitic lamprey production inferred from the historic regularity of chemical treatments. This categorization was intended to direct future assessment efforts, but may also reflect differences in early demographics. We analyzed assessment data collected from 1959 to 2005 using mixed-effects models and variance components analyses to test for differences in recruitment and growth to age 1 among stream categories. Recruitment was twice as large in regularly treated streams as in irregularly treated streams, indicating that age-1 year-class strength is correlated with consistent chemical treatments. We found no differences in length at age 1 among stream categories; however, Lake Superior streams with irregular treatment histories exhibit more variation in length at age 1 than streams that are treated regularly. The majority of variation in length at age 1 was due to within-year variation, which was fairly consistent across stream types within each lake. Our results indicate that early life history differs among subsets of the Great Lakes sea lamprey population, and management practices should be modified to account for these differences. Mixed-effects models and variance components analyses are useful tools for analyzing large historical datasets for patterns of demographic variation within and among populations, whether the ultimate goal is pest control, harvesting, or conservation.     

Control rule performance for intermixing lake whitefish populations in the 1836 Treaty waters of the Great Lakes: A simulation-based evaluation

We conducted a simulation-based evaluation exploring the effects of population intermixing on the current 65% total annual mortality control rule used to manage lake whitefish (Coregonus clupeaformis) fisheries in the 1836 Treaty waters of the Laurentian Great Lakes. The simulations incorporated intermixing among four populations with characteristics similar to those of lake whitefish in northern lakes Huron and Michigan. Dynamics of each population were simulated for 100 years with each stock exploited by a single fishery. An age-structured assessment of each stock was conducted every third year, with the abundance, mortality, and recruitment estimates used with the current control rule to set future harvest limits. Overall aggregate yield of the modeled system was generally not affected by intermixing, but did depend on assumed productivity levels. Mean annual yields for individual fisheries were sensitive to mixing levels, with yields from fisheries coinciding with low productivity spawning populations often similar to those coinciding with high productivity populations when intermixing occurred. Variability in yield tended to decrease slightly as mixing rates increased. Intermixing did not have a large influence on measures of spawning population sustainability, but detection of declining population sizes in low productivity populations may be difficult when intermixing occurs because of fishery yields remaining high due to immigration of fish from other spawning areas. This raises concern about the current control rule given that low productivity populations had spawning population sizes less than 20% of the unfished level in most years.