How many fish are there and how many can we kill? Improving catch per effort indices of abundance and evaluating harvest control rules for lake whitefish in the Great Lakes.

摘要

My dissertation has two main objectives: (1) to explore alternative ways to use commercial lake whitefish fishery catch per effort (CPE) data as an index of abundance in 1836 Treaty-ceded waters of the Great Lakes, and (2) to evaluate alternative harvest control rules for lake whitefish. Chapter 1 was directed at exploring alternative ways to use commercial lake whitefish fishery CPE data, while Chapters 2 and 3 covered topics related to harvest control rules.;Fishery CPE data is often used to assess relative fish abundance, and assessments used in 1836 Treaty-ceded waters of the Great Lakes assume that commercial CPE (i.e., ratio of aggregate catch to aggregate effort in each year) from gill-net and trap-net fisheries is proportional to abundance. However, CPE may change due to factors other than abundance. In Chapter 1, I developed general linear mixed models (GLMMs) to account for sources of variation in CPE unrelated to abundance, and used the least-squares means (LSMs) for each year as an alternative to the current index of abundance. Effects such as license holder, boat size, and month accounted for much of the variation in CPE. LSMs and the current CPE index displayed different temporal trends among years in some areas, suggesting the importance of adjusting fishery CPE for effects like boat size, season, and license holder.;Harvest policies use control rules to dictate how fishing mortality or catch and yield levels are determined. Common control rules include constant catch, constant fishing mortality rate, and constant escapement. The "best" control rules for meeting common fishery objectives (e.g., maximizing yield) is a source of controversy in the literature, and results are seemingly contradictory. In Chapter 2, I conducted a detailed review of the relevant harvest control rule literature to compare control rules for their ability to meet widely used fishery objectives and identify potential causes for contradictory results. The relative performance of control rules at meeting common fishery objectives was affected by: fishery objectives, whether uncertainty in estimated stock sizes was included in analyses, whether the maximum recruitment level was varied in an autocorrelated fashion over time, how policy parameters were chosen, and the amount of compensation in the stock-recruit relationship. More research is needed to compare control rules while considering these and related factors.;In Chapter 3, I used an age-structured simulation model that incorporated stochasticity in life history traits and multiple uncertainties to compare the current harvest control rule for lake whitefish (constant fishing rate; CF) with a range of alternative control rules, including conditional constant catch (CCC), biomass-based (BB), and CF and BB rules with a 15% limit on the interannual change in the target catch. The CF and BB rules simultaneously attained higher average yield and spawning stock biomass than other control rules, while the CCC rule and limiting the target catch changes by 15% had the lowest yearly variability in yield. The low yearly variability in yield provided by limiting target catch changes to 15% comes at the cost of frequently reducing biomass to low levels, so that in many situations other control rules would be preferred.