Heather A. Dawson

The Ecology of Larval and Metamorphosing Lampreys

The life cycle of lampreys typically begins in streams where fertilized eggs hatch into small, wormlike larvae (ammocoetes) which burrow into soft stream bottoms where they filter feed on organic matter until the onset of metamorphosis. The relative importance of habitat variables can change with ammocoete size (and depending on the spatial scale measured), but habitat must provide adequate substrate for burrowing and a regular supply of the suspended organic matter upon which larval lampreys feed. Larval movement occurs significantly more often at higher densities and in warmer temperatures, and typically occurs in a downstream direction at night. Sex ratio of some lamprey species is often related to differences in larval density, with the proportion of males increasing with relative density. Larval mortality is thought to be high in the egg phase, immediately following hatching, and at metamorphosis. The duration of the larval period in the life cycle of lampreys has been found to vary among and within species, but generally ranges from 3 to 7 years. However, analyses of larval growth and duration of larval life have been hampered by the unreliability of age assessment methods for larval lampreys. Metamorphosis begins during the summer months, when water temperatures are the most favorable, and is completed by winter or early spring.

Factors Affecting Recruitment Dynamics of Great Lakes Sea Lamprey (Petromyzon marinus) Populations

ABSTRACT Knowledge of stock—recruitment dynamics is as important for control of pest species such as the sea lamprey (Petromyzon marinus) as it is for sustainable harvest management of exploited fish species. A better understanding of spatial and temporal variation in recruitment of pest populations may inform managers on where and when to effectively apply different control methods. Sea lamprey stock—recruitment data combined from streams across the Great Lakes basin into a Ricker stock—recruitment model indicated both compensation (density-dependent survival) and a large amount of density-independent recruitment variation. We evaluated the use of a mixed-effects model to look at common year effects and stream-level variables that could affect productivity and growing season length, with the objective of identifying factors that may explain this density-independent variation in recruitment. After selecting the “best model”, we tested factors that might affect recruitment variation, using a Great Lakes dataset comprising 97 stream— years. Lake Superior tributaries, streams with larger numbers of lamprey competitors, and streams regularly requiring lampricide treatment showed significantly higher recruitment. Alkalinity and thermal regulation did not affect the observed recruitment pattern among streams. In four long-term study streams we observed significant variation among streams, tested as a fixed effect, but no evidence suggested a common pattern of variation among years. Differences in recruitment among streams were consistent with evidence of quality of spawning and larval habitat among streams. Our findings suggest that management models should account for differences in recruitment dynamics among sea lamprey-producing streams, but not common year effects.

An Operating Model for the Integrated Pest Management of Great Lakes Sea Lampreys

Models of entire managed systems, known as operating models or management strategy evaluation (MSE) models, have been developed in recent years to more fully account for uncertainty in multiple steps of fishery manage- ment. Here we describe an operating model of sea lamprey management in the Great Lakes and use the model to compare alternative management strategies for sea lamprey control in Lake Michigan. Control of sea lampreys is mainly achieved through the application of chemical lampricides that target stream-dwelling larvae before they become parasites. The op- erating model simulated uncertainty due to process variation in larval population dynamics, the accuracy of population as- sessments used to direct selection of areas to be chemically treated, and the effectiveness of these treatments. We used the operating model to compare the performance of stream selection strategies that either rely on assessments to direct chemi- cal treatments or eliminate the assessment process altogether by relying on prior but uncertain knowledge of stream-level sea lamprey growth rates to specify a fixed schedule for chemical treatments. The fixed schedule strategy led to a modest improvement in expected suppression of parasitic sea lamprey abundance over the assessment-based strategy so long as assessment cost savings were allocated to chemical treatment when assessment was not used to select streams for treat- ment. We also evaluated the sensitivity of the assessment-based strategy to differing but plausible levels of assessment uncertainty. A moderate reduction in assessment uncertainty led to a large increase in suppression of parasitic sea lamprey abundance for the assessment-based selection strategy, emphasizing the importance of both accurately measuring and re- ducing assessment uncertainty.

Use of Electric or Bubble Barriers to Limit the Movement of Eurasian Ruffe (Gymnocephalus cernuus)

ABSTRACT Eurasian ruffe (Gymnocephalus cernuus) is an aquatic invasive species accidentally introduced via ballast water to the Great Lakes in the mid-1980s. Fish barrier technology is being studied to stop the spread of invasive fish species such as ruffe. Electrical barriers have been constructed, most notably in the Chicago Sanitary and Ship Canal, to prevent non-indigenous species such as ruffe from spreading into areas where they are currently absent. Information on the response of an invasive fish to barriers can help managers determine strategies to prevent the spread of these species via artificial waterways. In this laboratory study electrical barriers were set up to determine effectiveness of four electrical settings for repelling Eurasian ruffe measuring 10 cm or more in length. In separate tests, air-bubble curtains with two bubble sizes and densities were created to test this type of barrier in blocking movement of ruffe less than 10 cm in length. The most effective electrical settings found (5 ms, 6 Hz) repelled only about half of the attempted passes. When ruffe were offered food or shelter on the opposite side of the electrical barrier, neither food-starved nor shelter-deprived ruffe made significantly more attempts to cross the barrier. Ruffe were significantly repelled by all air-bubble curtains, but a large proportion of passes (4.5 passes per fish on average in the treatments) were still observed. Electrical barrier settings and air-bubble curtains used in this study were found ineffective at completely blocking the movement, but somewhat effective at inhibiting the passage of ruffe.

An Assessment of Age Determination Methods for Great Lakes Larval Sea Lampreys

Abstract Estimating the age composition and recruitment of populations of invasive sea lampreys Petromyzon marinus necessitates the validation and improvement of age assessment methods that rely on statoliths and length-frequency data. Determining age based on length-frequency distributions is subjective because of heterogeneity in the growth rates of larval sea lampreys (ammocoetes) within and across streams and the resulting overlap in lengths between age-classes. Statolith-based age assessment methods have never been validated for more than 1 year. We established “known-age” ammocoete populations in two streams by introducing a single cohort of spawners above barriers and compared estimates of ammocoete age based on statoliths with the true ages. In five additional streams, we used microsatellite data from adults and ammocoetes to assign parents to ammocoetes produced the same year and compared the age determined by statolith-based interpretation with the age based on parentage assignment. We combined ...

Validation of a relationship between statolith size and age of larval Great Lakes sea lamprey (Petromyzon marinus)

Sea lampreys (Petromyzon marinus) are parasitic pests in the Great Lakes that have negatively affected game fish populations. Accurate aging of pest species such as sea lampreys can provide estimates of growth and mortality rates, which can direct control efforts. Because growth of larval sea lampreys is both slow and variable, determining age based solely on visual assessments of length-frequency distributions is subject to considerable uncertainty. Otoliths have been used to estimate age in teleosts through annuli counts and otolith size metrics. Lampreys do not have otoliths, having instead an analogous structure called a statolith. Determining age based on statolith annuli counts has been found to be imprecise and inaccurate. Therefore, we evaluated whether statolith size was correlated with ammocoete age using known-age populations of ammocoetes from two Great Lakes streams with contrasting physical conditions that affect larval sea lamprey growth. A morphometric system was used to measure length, width, and height of statoliths from these known-age populations. Statolith width was found to be the measurement that best distinguished the age-classes within the populations. A likelihood-based statistical model was used to assess ammocoete population age composition. Even though statolith width was clearly associated with age, combining statolith width data and length-frequency did not improve estimates of proportions at age relative to those estimated using only length-frequency information.

Effects of coded-wire-tagging on stream-dwelling sea lamprey larvae

AbstractThe effects of coded wire tagging Sea Lamprey Petromyzon marinus larvae from a known-aged stream-dwelling population were assessed. Tagged larvae were significantly shorter on average than untagged larvae from 3 to 18 months after tagging. However, 30 months after tagging, the length distribution of tagged and untagged larvae did not differ and tagged Sea Lampreys were in better condition (i.e., higher condition factor) and more likely to have undergone metamorphosis than the untagged population. The reason why tagged larvae were more likely to metamorphose is not clear, but the increased likelihood of metamorphosis could have been a compensatory response to the period of slower growth after tagging. Slower growth after tagging was consistent across larval size-classes, so handling and displacement from quality habitat during the early part of the growing season was likely the cause rather than the tag burden. The tag effects observed in this study, if caused by displacement and handling, may be m...

Feasibility of Passive Integrated Transponder Technology to Study In Situ Movements of Larval Sea Lamprey

Abstract In the Laurentian Great Lakes, sea lamprey Petromyzon marinus is an invasive species controlled primarily through application of selective toxicants (lampricides) to tributaries expected to contain the most large larvae (>100 mm). Current assessment techniques make the assumption that larvae occupy all stream habitats in the same proportion irrespective of size or life history stage. Testing this assumption relies on the availability of a marking method to determine individual animal movement between habitats over time. To evaluate the feasibility of using passive integrated transponder (PIT) technology to detect sea lamprey larvae in situ, we implanted two sizes of PIT tags (8 and 9 mm) in larvae of less than 120-mm average length to assess survival, tag loss, behavior, and detectability in situ. Larval mortality and tag loss were lower when smaller tags were used, but mortalities were still high (60%) due to the small body cavity severely restricting internal tag implantation. Burrowing perform...