Heather Dawson PhD Candidate Recruitment variation in Great Lakes sea lamprey populations

I received my Bachelor's degree in biology at University of Michigan and my Master's degree in ecosystems biology at Eastern Michigan University before beginning this project. My research interests focus on researching and applying new techniques in the management of exotic fish species. Being a lifelong Michigander I have concentrated on fish who pose a threat to the Great Lakes ecosystem. For my Ph.D. research at MSU,I have studied the Great Lakes sea lamprey, which have parasitized large fish species in the upper Great Lakes since their accidental introduction in the 1920s.

There were three components to this study. The first involved gaining information on sea lamprey recruitment dynamics in contrasting streams in the Great Lakes system. The second was the incorporation of recruitment variation and other sea lamprey demographic information into management models for sea lamprey control. The third was the creation of a standard protocol for larval sea lamprey age assessment, using both length-frequency and statolith age information to allow for the quantification of recruitment with less error. This study allowed me to pursue my interest in research on management techniques for the control of exotic species.

Sea lamprey gained access to the Great Lakes in the 1920s and were instrumental in the collapse of lake trout fisheries in Lakes Superior, Huron, and Michigan in the 1940s and 1950s. The Great Lakes Fishery Commission has been managing this species since 1954, mostly through the application of a chemical called TFM (3-trifluoromethyl-4-nitrophenol) in streams where sea lamprey larvae have been found. Because of a wish to reduce reliance on chemicals, alternative methods of control such as the use of pheromones, adult trapping, barriers to block spawning migrations, and the release of sterile males are also being applied and evaluated. TFM removes lamprey recruits, while alternative methods of control are aimed at reducing the number of adult spawners before recruitment occurs. Alternative control strategies depend on the reduction of spawning stock resulting in concomitant reductions in recruitment. Having a better understanding of the sea lamprey stock-recruitment relationship is necessary to determine the efficacy of sea lamprey control through alternative control methods, and incorporating the natural variation found in recruitment into management models will make them more realistic than they are at present.

To gain a better understanding of sea lamprey population dynamics, we measured recruitment at age 1 in contrasting streams by introducing a known number of spawners in barricaded-off sections of streams. Each fall we conducted standardized surveys to estimate larval abundance and recruitment.  Sea lamprey stock-recruitment data combined from streams across the Great Lakes basin indicated both compensation (density-dependent survival) and a large amount of density-independent recruitment variation (Figure 1).

Figure 1.  Observed sea lamprey stock and recruitment data for 90 stream-years.

We tested factors that may significantly affect recruitment, such that different streams might require different alternative control spawner target abundances to ensure low levels of recruitment. Streams described by sea lamprey program staff as having a regular and predictable cycle of lampricide treatment experienced significantly higher recruitment than less predictable (irregular) streams. Lakes Superior and Michigan tributaries experienced significantly higher recruitment than streams from Lakes Huron or Ontario. Recruitment was also significantly higher in streams where the number of lamprey competitors (sea lamprey and native lamprey) to age-1 sea lamprey recruits was greater. We interpret this counter-intuitive result as being the consequence of habitat differences among streams. Streams with more and better habitat can support larger populations of competitors and higher sea lamprey recruitment.

We incorporated density-independent recruitment variation found from this study, larval assessment uncertainty, and other sea lamprey demographic information into a management model to facilitate a realistic comparison of the effectiveness of different sea lamprey control strategies. The model was a stochastic age-structured population model (MUSTR) that simulated the existing control program for Lake Michigan, to allow comparison of the effect of using only lampricide control to combinations of both lampricide and adult control methods. Assuming our best estimates of adult control costs and efficacy, results suggest that increasing adult control efforts at the expense of lampricide use will result in an increased abundance of sea lamprey in the Great Lakes(Figure 2a).  By simultaneously increasing the efficacy of alternative control and decreasing the initial unit cost of alternative control we found a few hybrid strategies (using both control methods) that perform better than using only lampricide control (Figures 2b and c).

Figure 2.  The median number of spawners produced versus the cumulative cost of alternative control when using alternative control costs and spawner reductions achieved by alternative control of a) $39500 + $0.06 ∙ larval habitat area and 88% (our best estimates of alternative control costs and efficacy); b) $39500 + $0.03 ∙ larval habitat area and 94%; and c) $39500 + $0.04 ∙ larval habitat area and 96%.  The horizontal line in each graph represents the median number of spawners resulting from the application of only lampricide control.  Points below the line represent strategies that result in better performance than lampricide control alone. The number above each point represents the number of streams treated by alternative control; one to ten of the largest streams received alternative control, added in order of decreasing size.

Because estimating the recruitment of sea lamprey involves assessing the larval population in streams and separating the population into age classes, the accuracy of these estimates can be limited by errors in larval age determination. Developing a standard method of age-assessment using both statolith and length-frequency data requires the validation and improvement of both methods of age interpretation. Therefore, we established known-age populations in two contrasting streams by introducing a single cohort of sea lamprey and then compared the age determined by statolith interpretation to the known age using two different methods for statolith preparation and evaluation.  Multiple independent age readings of sea lamprey statoliths indicated that the overall average percent error indicated a higher bias in age estimates using the Crystal Bond method as opposed to the Immersion Oil method. The statolith data were bias-corrected and combined with length-frequency data in the statistical model to determine proportion-at-age in our “known-age” sea lamprey populations, which resulted in a substantial increase in the precision of this estimate (Table 1).

This project, funded by the Great Lakes Fishery Commission, and in collaboration with U.S. Fish and Wildlife Service and the Canadian Department of Fisheries and Oceans has produced insight into the population dynamics of sea lamprey. To better understand recruitment and growth dynamics of sea lamprey gets us further ahead in the struggle to manage this species and thus, protect many of our Great Lakes fish and fisheries.