Lake Roosevelt is one of 11 lakes and reservoirs within the Columbia River drainage in Washington State that supports a Burbot population. Standardized stock assessment data is needed to facilitate management of the Lake Roosevelt Burbot population. In the absence of standardized stock assessment data, Burbot populations in Washington State, including Lake Roosevelt, have been managed conservatively. A suspected increase in abundance and high survival suggests that the population in Lake Roosevelt could potentially support greater harvest, although this needs confirmation.

For the 2011 Resident Fish Categorical Review, the Colville Confederated Tribes (CCT) submitted a proposal to develop a standardized stock assessment program for Burbot in Lake Roosevelt. The objective was to facilitate management of the Burbot population to provide for sustainable harvest through the development of a status and trends monitoring program. Because burbot are commonly observed in the bycatch of annual Lake Roosevelt Fall Walleye Index Netting (FWIN) surveys, the Independent Scientific Review Panel (ISRP) and Northwest Power and Conservation Council (NPCC) recommended that the CCT first evaluate the potential for using FWIN as the basis for Burbot stock assessment in Lake Roosevelt. Accordingly, in 2013 the CCT hired a subcontractor to analyze ten years of FWIN data (2003-2012) and assess its adequacy for Burbot stock assessment. The overarching conclusion of the analysis was that the FWIN program is adequate for monitoring year-to-year differences in Lake Roosevelt Burbot population indices.

Having determined that FWIN is generally adequate for Burbot stock assessment in Lake Roosevelt, we are working toward our goal of achieving a Burbot population that has stable abundance and is managed to provide some harvest. The primary objective of this project is to provide technical advice to the Lake Roosevelt Co-Managers regarding management alternatives so that they can develop realistic fishery targets and appropriate implementation strategies. Our approach will be to estimate the harvest potential of Lake Roosevelt Burbot under current management and population characteristics using Fishery Analysis and Model Simulation (FAMS) software. Model inputs are recruitment, mortality (natural and fishing), and growth, which will be derived from the FWIN data with some supplemental field sampling to evaluate biases. Thus, we (with a subcontractor) will analyze the FWIN data set with the 2016 data included.

Despite the demonstrated utility of FWIN as a tool for Lake Roosevelt Burbot stock assessment, there is a need to evaluate the size selectivity of the gill net gear. Gill net size selectivity can be estimated indirectly by fishing multiple mesh gill nets with equal effort and comparing catch frequencies within each of the mesh sizes. We will use a maximum-likelihood approach to estimate parameters of log-linear selectivity models (Normal location, Normal scale, Gamma, Lognormal, and Bi-normal), also known as the SELECT method (Share Each Length’s Catch Total), and correct the length data prior to the calculation of length-based stock assessment indices.

Age bias resulting from gill net selectivity can be corrected by applying the proportion of fish of each age within each length bin to the bias-corrected length data. To increase sample sizes of small (young) and large (old) Burbot, supplemental sampling with alternative gears will be conducted. We will conduct supplemental sampling concurrent with FWIN using towed trawls (to capture small fish), and modified cod traps (large fish) – both of which have been used to sample Burbot populations.

During our fall (2014) pilot burbot sampling we were having difficulty assessing the stage of maturity of captured fish by visually inspecting the gonads. Our FWIN analysis subcontractor (Golder Associates) also reported widely varying proportions of mature burbot in the FWIN data sets, suggesting an inability to accurately assess burbot maturity in the fall, several months before spawning. This is consistent with other maturity studies conducted on marine stocks of cod. In 2015, the CCT contracted a pilot study to evaluate the use of endocrine markers (estradiol and vitellogenin – females; 11-ketotestosterone – males) to assess burbot maturity. The contractor analyzed plasma estradiol in N=28 females, follicle stimulating hormone (FSH) in N=33 females and 11-ketotestosterone in N=17 males. The amount of estradiol quantified in the plasma represents the major source of estrogen secreted by the premenopausal ovary. The 11-ketotestosterone is a key secondary androgenic steroid secreted in fish plasma that functions as a reproductive hormone in males. FSH has been shown to represent the regulatory reproductive processes of the body (i.e., development, growth, pubertal maturation). Based on the results of the pilot study, the tested hormonal markers can be measured in burbot plasma.  In addition, for our dataset, the models for hormonal markers measured in blood appear to have a positive relationship with Gonadosomatic Index (GSI) and maturity state during the spawning period. However, it was uncertain if the endocrine markers would be useful for determining maturity prior to the spawning period (i.e., during FWIN sampling in the fall), thus assessments of maturity from burbot sampled in the fall will be confirmed with histological examination of gonad tissue. Contractor will analyze archived blood plasma and gonad tissue from up to 200 (100 male and 100 female) burbot captured from Lake Roosevelt during supplemental sampling. Blood plasma will be analyzed for enodcrine markers indicative of maturity using immunoassays for estradiol and vitellogenin (females) and 11-ketotestosterone (males). Gonad tissue will be analyzed using histology to confirm state of maturation.

Spawning frequency of sexually mature burbot is also currently unknown, and is assumed that these fish can skip spawning cycles in a given year (Pulliainen and Korhonen, 1994). In order to determine whether a burbot will successfully spawn, or skip spawning, a comprehensive endocrine profile is needed that examines not just plasma levels of the main reproductive hormones (estradiol and testosterone), but prior steroid precursors and indicators of oocyte maturation, yolk production, and spermatogenesis. Fish that skip a spawning year may arrest spawning progression at several stages, and a recent study has shown that burbot raised in a hatchery can proceed to the vitellogenic age stage prior to absorption of eggs (Foltz et al, 2012). Rideout and Tomkiewicz (2011) examined the phenomenon of skipped spawning in cod, and found that endocrine profiles showed distinct annual expression levels that were indicative of spawners compared to non-spawners. This characterization of annual endocrine profiling can be used to determine spawning frequency for use in subsequent stock assessments and biomass contribution for a managed fishery. In 2016, we initiated a two-year study to characterize the annual variation in reproductive hormones in burbot. The CCT captured 75 burbot from Lake Roosevelt in November 2016 and delivered them to the US Fish and Wildlife Bozeman Fish Technology Center (BFTC) aquatic research lab, where they will be held and endocrine profiles will be determined on a monthly basis and related to maturity (i.e., spawning activity). In 2017, CCT will collect and transport approximately 175 burbot to (n=100 spring; n=75 fall) to the BFTC for endocrine profile sampling.

Recruitment variability can complicate the management of a fishery. If we can identify density-independent factors that are affecting recruitment, then we may be able to address them in a manner that would provide for more stable recruitment at a higher level. We will investigate the factors that are responsible for the apparent recruitment variability in the Lake Roosevelt Burbot population. Multivariate statistical techniques will be utilized to explore relationships between mean age 2 catch per unit effort in FWIN gill nets and the abiotic and biotic conditions in the year of birth for each year class.

The purpose of this project is to improve understanding of the factors that influence population abundance of burbot (Lota lota) in Lake Roosevelt. The Lake Roosevelt population, which is considered healthy, is an important fishery resource for local stakeholders, but interest in expanding fishing opportunities for the species has raised concern about the potential effects of increased harvest rates on population productivity. Although catch rate and distribution data for burbot in Lake Roosevelt have provided baseline information regarding population status, important knowledge gaps regarding the age, spawning frequency and spawning habitat of the adult population remain. These knowledge gaps complicate the efforts of the resource managers to identify the appropriate age structure and biomass for annual harvests that will help ensure a sustainable fishery. Moreover, Lake Roosevelt is subject to wide variation in flow and water level elevation that may affect spawning success and recruitment. The development of reliable techniques to determine age-specific contributions to recruitment and identify critical spawning habitat will provide managers with valuable tools to implement policies and regulations that aid in effective management of burbot in Lake Roosevelt.