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Angling quality, effort response, and exploitation in recreational fisheries : field and modeling studies on British Columbia Rainbow Trout (Oncorhynchus mykiss) lakes

University of British Columbia

Creel surveys and mark-recapture experiments were used to assess the dynamics of catch per unit effort (CPE), fishing effort, and exploitation on eight rainbow trout lakes in British Columbia's southern interior. CPE was high 3.8 fish per angler day (ad), and fishing effort and gross exploitation (15 ad ha⁻¹ and 0.15, respectively) were both low on two limited-access lakes. CPE was generally low (1.7 fish ad⁻¹) on lakes that were easily accessible (open-access) to anglers and fishing effort and gross exploitation were both high (50 - 100 ad ha⁻¹ and 0.50 - 0.60, respectively). Seasonal patterns of CPE and effort were similar among-lakes with peak values being observed during the late-spring/early summer, followed by more or less rapid declines as the season progressed. Maximum exploitation rates in the range 0.50 to 0.60 were observed over a wide range of total seasonal fishing effort density (50 ad ha⁻¹ to 100 ad ha⁻¹), which suggested that fish vulnerability may have been limited. Fish population characteristics of growth, age- and size-at-maturity, vulnerability to harvest, and natural mortality were assessed using fall gillnet survey and mark-recapture data. Estimates of the von Bertalanffy growth parameter K were between 0.19 yr⁻¹ to 0.36 yr⁻¹, but most single-species (rainbow trout only) lake estimates were between 0.25 yr⁻¹ and 0.36 yr⁻¹. Asymptotic body length (Zoo) estimates varied among-lakes from 416 mm to 887 mm. Size-at-50% maturity for female rainbow trout varied among-lakes from 290 mm to 387 mm, but age-at-50% maturity fell within the narrow range 2.95 yr to 3.08 yr. Size-at-50% maturity of female trout was approximately equal to 50% of L[sub ∞]. Male rainbow trout typically matured during their second year at body lengths between 150 mm and 250 mm. Patterns of size-selective exploitation showed rapid increases with body length, and occasional decreases at body lengths greater than 450 mm. Increasing natural mortality, or behaviours associated with spawning, may explain these apparent decreases in exploitation at large size. Exploitation rates on fully vulnerable (large body size) rainbow trout ranged from 0.21 to 0.36 in low effort lakes and from 0.60 to 0.80 in high effort lakes. Relative vulnerability of smaller fish followed a smooth power function with lengths-at-50% vulnerability between 204 mm and 345 mm. Age-at-50% vulnerability ranged from 1.70 yr to 2.79 yr suggesting that most fish become vulnerable to harvest during their second year in the lakes. Estimates of natural mortality for adult fish in two lakes were 0.41 yr⁻¹ and 0.46 yr⁻¹ (annual survival rates of 0.64 and 0.63, respectively. Egg to age 2+ survival was 0.0014 for one lake where virtual population estimates of total eggs laid and age 2+ recruitment could be obtained. Maximum egg to age-2+ survival estimated from a life history model was 0.0028 age 2+ fish egg⁻¹. A model of angling quality, effort response, and exploitation was developed for recreational fisheries (limited vulnerability/effort response model), where anglers remove fish from behaviourally reactive pools. Angling quality (CPE) is predicted to decline with increasing angler effort for given fish abundance, and the rate of decline depends on hypotheses about reactive/unreactive exchange dynamics of fish. Effort responses to fish abundance were modeled under the assumption that anglers attempt to equalize CPE among-fisheries within a region. Solutions to the model equations taken over the fishing season predict: (/.) total seasonal effort that is linearly proportional to initial fish abundance with slope inversely proportional to the expected catch rate c[sub 0]; (ii.) a lower limit to fish abundance (N[sub ∞]) below which angler effort is no longer attracted (y-axis intercept <0); (iii.) asymptotic exploitation rates < 1. Numerical analysis results show that analytical solutions based on equilibrium assumptions are generally robust to moderate deviations from the equilibrium conditions. Fitting the exploitation component of the model to observed fully vulnerable exploitation rates on B.C. rainbow trout lakes gave an asymptotic exploitation rate estimate of 0.79. The limited vulnerability/effort response model was fitted to observed effort and stocking rate data for rainbow trout lakes in three management regions (Regions 3,5, and 8) in British Columbia's southern interior. The observed effort response appeared linear in all regions, and all regional estimates of effort response slope were significantly greater than zero. Statistically significant differences could not be detected between the regional effort response slopes. Intercept values for Regions 5 and 8 were less than zero, while Region 3 showed a positive intercept. Multiple comparisons among intercept values revealed that only Regions 3 and 5 were significantly different from one another. Effort response parameters implied by the linear model coefficients were 1.35 fish ad⁻¹ for the pooled catch rate (c[sub 0]) and -99, 180, and 43 fish ha⁻¹ for the lower abundance limit (N[sub ∞]) in Regions 3,5, and 8, respectively. I developed an approach for combining equilibrium calculations implied by the limited vulnerability/effort response model with an age structured population model based on life history and fishery characteristics. Results from this method show that classic recruitment over-fishing is possible on B.C. rainbow trout lakes. In particular, where access to lakes is open, and (c[sub 0]) values are low (0.10 - 0.85 fish ad⁻¹), effort would be much higher and recruitment much lower than levels necessary to give MSY. The limited vulnerability/effort model was also fitted to 105 lake-specific time series of fishing effort. Annual lake-specific stocking rates were used to drive the model after accounting for density-dependent fish survival and harvest. This analysis provided (z.) a test of among-year stationarity in (c[sub 0]) values, (ii.) a broad test of the assumption that anglers equalize catch rates among-lakes within-regions, and (iii.) a method by which (c[sub 0]) values may be predicted as a function of access. Model results showed that catch rates tend to be stationary among-years within-lakes, (c[sub 0]) values are predictable from access factors, and catch rates do tend to be equalized after accounting for access differences. Therefore, the effects of access control on angling quality and sustainability of fisheries on wild-stocks can be directly assessed.

Thesis/Dissertation

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2009-07-20

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http://hdl.handle.net/2429/11055

Resource Management and Environmental Studies

University of British Columbia

UBCV

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