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UBC Theses and Dissertations

Reconstructing lingcod biomass in Georgia Strait and the effect of marine reserves on lingcod populations in Howe Sound Martell, Steven James Dean


Commercial lingcod fishing in Georgia Strait was closed in 1990 due to severely depressed catches. Seasonal closures and size restrictions were added to sports fisheries in an attempt to rebuild the stock. There have been no fishery independent surveys to estimate lingcod abundance in Georgia Strait. The rate of recovery is monitored through changes in sports fishery catch rates. In this thesis, I use catch statistics and commercial catch and effort data to reconstruct the lingcod biomass time series for statistical area 4B (Georgia Strait and surrounding areas). This reconstruction was carried out using a technique called Stock Reduction Analysis (SRA), where the dynamics of an agestructured model is driven by annual removals. The SRA model was tested using a fake time series of relative abundance data and known parameter values. The model was able to estimate, within 10%, the correct parameter values used to generate the fake data. Catch and effort data (CPUE) are used as a relative abundance index to estimate unfished biomass and stock productivity. It is assumed that the relationship between CPUE and stock size is hyperstable in commercial fisheries. Results of the SRA indicate that Georgia Strait lingcod stock have been depleted by over 90%, and that their reproductive capacity is much lower than that of other temperate fish stocks. Lingcod in Howe Sound are part of the Georgia Strait Stock. There are 3 small Marine Protected Areas (MPAs) in Howe Sound that are closed to fishing. An in-situ mark-recapture study was carried out to evaluate responses of lingcod populations in closed areas, and to determine whether or not lingcod move across reserve area boundaries. Changes in underwater encounter rates and length frequencies in each of 13 different survey sites provide evidence of small-scale seasonal movement patterns in lingcod. Densities and average lengths of lingcod have higher means in reserve areas compared to fished areas, but the differences are not significantly different. Lingcod in the oldest reserve area are significantly larger than lingcod in recently established reserve areas. However, this result may be due to artificial reefs that are present in the oldest reserve area. The idea of using marine reserves as a fisheries management tool is relatively new in fisheries science. Current ecological questions focus on ideal reserve location and reserve area size. Until contentious issues surrounding the use of marine reserves have been resolved, and proper large-scale field experiments have been conducted to evaluate reserves, computer models can be used to speculate how reserve areas will perform. A spatially explicit computer model (FISHMOD) was used to evaluate the current network of "no-take" areas in Howe Sound. Specifically, the model was used to answer questions pertaining to movement rates in lingcod, spatial distribution of fishing effort, and to compare alternative management policies. Simulation results suggest that a disproportionate amount of fishing effort will be distributed along boundaries of reserve areas, especially at low stock sizes. As exchange rates between reserve area and fished area increase, more fishing effort is distributed along reserve boundaries. In comparison to increasing size limits or reducing fishing season length, a system of MP As in Howe Sound is more effective.

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