Geology topics
P.J. Rago
Publications and source records attributed to P.J. Rago.
Evaluation of a mark-recapture method for estimating mortality and migration rates of stratified populations
We simulated mark–recapture experiments to evaluate a method for estimating fishing mortality and migration rates of populations stratified at release and recovery. When fish released in two or more strata were recovered from different recapture strata in nearly the same proportions, conditional recapture probabilities were estimated outside the [0, 1] interval. The maximum likelihood estimates tended to be biased and imprecise when the patterns of recaptures produced extremely "flat" likelihood surfaces. Absence of bias was not guaranteed, however, in experiments where recapture rates could be estimated within the [0, 1] interval. Inadequate numbers of tag releases and recoveries also produced biased estimates, although the bias was easily detected by the high sampling variability of the estimates. A stratified tag–recapture experiment with sockeye salmon ( Oncorhynchus nerka ) was used to demonstrate procedures for analyzing data that produce biased estimates of recapture probabilities. An estimator was derived to examine the sensitivity of recapture rate estimates to assumed differences in natural and tagging mortality, tag loss, and incomplete reporting of tag recoveries.
Does pH affect fish species richness when lake area is considered?
Numerous surveys have shown that fish species richness (number of species) is positively correlated with lake pH. However, species richness of fish communities is also correlated with lake size, and low‐pH lakes are often small. Thus, conclusions drawn from examination of fish community structure relative to spatial (among‐lake) variation in pH have been limited by uncertainties regarding the confounded effects of lake area. We used two statistical methods, analysis of covariance and a nonparametric blocked comparison test, to remove effects of lake area and compare fish species richness in low‐pH and high‐pH lakes. Data from six previous surveys of water chemistry and fish communities in lakes of Ontario and northern Wisconsin were examined. Lakes with low pH (≤6.0) contained significantly fewer fish species than lakes with high pH (>6.0) when the effect of lake area was considered. A simple probabilistic model showed that the ability to detect differences in species richness is low when lake areas and the pool of potential colonizing species are small. We recommend the blocked comparison test for separating the effects of lake area and pH on species richness and urge critical examination of inferences based on analyses of fish communities in small lakes.