Publications: 1970–1979

Abstract: The assumption of equal catchability of marked and unmarked animals in mark-recapture sampling is examined. Violation of this assumption may come from two basic sources, heterogeneity or contagion, and both produce a bias in the estimate of population size. Two approaches to this problem are discussed. First, it is noted that bias can be avoided by statistically independent sampling procedures for marking and recapturing. Second, a new regression method is presented for multiple captures on a closed population, which combines mark-recapture with “simulated” removal and tests whether the assumption of equal catchability holds, regardless of the source of bias. Furthermore, if the ratio of catchabilities between unmarked and marked animals is constant over at least two successive samples, the method provides an estimate of population size even if the assumption of equal catchability does not hold. Two suggestions are made to supplement the regression method. First, noncapture sampling to obtain the ratio of unmarked to marked animals has several advantages over trapping. Second, a subclass of the marked animals may satisfy the constant ratio assumption even when all marked animals do not. A worked example is provided to illustrate the method.

Abstract: A new approach based on mark-and-sample tracking is presented that may substantially reduce errors in censusing small mammals. A test is included to check the underlying assumption of equal tracking from marked and unmarked mice. A procedure for applying the tracking approach to estimating births and deaths between samples is also presented. The tracking approach was demonstrated on a population of Peromyscus maniculatus. The number of tracks left nightly by males was significantly greater than by females. However, the tracking rate was about the same each night for all individuals of the same sex, even though tracking fluctuated from night to night. The test revealed no strong difference in the tracking rates of marked and unmarked mice. It is concluded that population estimates based on tracking can be highly precise and are far superior to estimates based on trapping alone.

Overview: This paper explores electronic remote sensing as a non-capture method for censusing small-mammal populations. Using deer mice as a test case, it develops several sampling approaches and field-tests electronic cables designed to detect animals as they cross them. The paper evaluates the performance and limitations of the detection devices, examines how patterns of mouse activity affect population estimates, and considers the further development needed to make electronic remote sensing a practical ecological censusing technique.

Abstract: The activity of Peromyscus maniculatus in an Ontario forest and Peromyscus truei and Peromyscus californicus in California chaparral was observed directly and monitored electronically. The activity of all three species outside the burrow was pulsed through the night, with different individuals not in tight synchrony and a depression in overall activity in the middle of the night. Activity of P. californicus was similar to that of P. truei, but P. californicus had in addition a pronounced burst of activity before dawn. As summer progressed, the amount of activity of P. maniculatus diminished and was compressed into the early part of the night. Wind and moonlight had no strong effect upon activity within a night.

From night to night, P. truei had conspicuous fluctuations in total nightly activity (at a period of about 4 days), the entire population usually being in synchrony. Night-to-night fluctuations in P. californicus activity were in phase with those of P. truei. Night-to-night fluctuations in vole tracks in Quebec (unpublished data from R. Bider) had low correlations with P. maniculatus tracks in the same area.

Night-to-night fluctuations in P. truei activity had a positive correlation with barometric pressure and a negative correlation with low pressure weather variables (humidity, temperature). There was no correlation with wind or cloud cover. None of the weather variables had a strong periodicity at 4 days, but P. truei activity was in phase with the 4-day-component of barometric pressure, out of phase with temperature, and leading humidity. The evidence suggested that sundown temperature was the key variable.

Fluctuations in tracks of P. maniculatus (Quebec) had similar properties, except they were in-phase with low pressure weather variables. Vole tracks (Microtus pennsylvanicus and Clethrionomys gapperi) showed the same periodicities as weather and were tightly correlated with weather.

Abstract: A general equation is derived for the quantity of different foods a predator consumes in response to food preference and food abundance. It is supposed the predator apportions its efforts among different search modes in order to minimize the discrepancy between actual and desired quantities of each food consumed. Included are the conflict between quantity and composition of the diet and the competition between feeding and nonfeeding activities. The properties of the equation are examined and discussed in light of the literature.

Overview: This report analyzes fish catch records from the Kenyan portion of Lake Victoria, examining geographic differences, long-term trends, and recurring fluctuations among major fish species. It documents contrasting catch patterns inside and outside Nyanza Gulf, changes in the abundance of several important species, and seasonal and other periodic patterns in catches.

Abstract: A computer program is available for fitting any linear or nonlinear ordinary differential equations to data, and for estimating transfer rates and/or compartment sizes from time-series tracer data in a compartmental system. The program is easy to use because compartment problems are specified simply in terms of the geometry of the components and transfers between them. The program generates confidence limits for the estimates, a feature that can be used to examine in advance the effectiveness of proposed tracer experiments.

Overview: This study uses time-series analysis of Lake Victoria fish catches, prices, rainfall, lake level, and wind to examine recurring fluctuations and relationships among them. It compares patterns at Masese and in Nyanza Gulf, identifying seasonal and longer-term cycles in catches and prices and assessing the extent to which these patterns are associated with climatic conditions.

Overview: This paper develops a method for evaluating overfishing from fish length-frequency data without requiring detailed prior estimates of growth and natural mortality. Using Lake Victoria fisheries as its application, it relates average fish length to mortality and fishing intensity and shows how comparisons among populations exposed to different fishing pressures can be used to assess existing fishing levels against an estimated optimum.

Overview: This report presents initial results from a survey of the Lake Victoria fishery in Kenya undertaken to assess breeding stocks and evaluate existing fisheries regulations. It compares commercial catches with fish populations in adjacent waters, examines fishing effort, maturity and gillnet selectivity, documents trends among major fish species, and evaluates the effects of different mesh sizes and seines on immature fish.

Overview: This report presents recommendations for the management and further study of the Lake Victoria fishery based on research conducted from 1973 to 1975. It addresses regulation of the inshore artisanal fishery, development and limits of offshore trawling, possibilities for aquaculture, and priorities for fisheries research, with particular attention to protecting breeding stocks while increasing sustainable fish production.

Abstract: An equation is derived for yield per recruit of a fishery (or other exploited animal population) as a function of fishing intensity and age of first capture. The equation has the advantage that it does not require explicit estimates of natural mortality or individual growth rate parameters. Linear length growth is assumed until maximum size is reached, and mortality parameters are expressed relative to growth rate. Mortality parameters are estimated from average length samples of separate populations experiencing different fishing efforts in the same fishery. The equation may be used to compare existing fishing efforts and age of first capture with optimal values. Samples of the catfish Bagrus docmac from Lake Victoria (East Africa) are used to illustrate the method.

Abstract: The inshore fishery of Lake Victoria has symptoms of severe overfishing. Curvilinear multiple regression analysis was used to examine the large variation in fishing effort, gear composition, and catches around the lake and suggest fishing practices that would give highest yields. The overfishing problem is due to the kind of gear in use and not to excessive fishing per se. Heavy fishing is often associated with the use of small-mesh gill nets and seines to catch small fish such as Haplochromis, a practice that does not seem to justify the damage done to catches of the larger species by the same gear. The best strategy for maximizing the total tonnage yield is to fish optimally for the herbivorous genus Tilapia. This means using only the larger gillnets appropriate for Tilapia, as well as hooks, both at a very high fishing effort. The hooks capture large predators such as Bagrus, Clarias, and Protopterus, an abundant resource in themselves, and simultaneously appear to increase the Tilapia yields indirectly by reducing losses of Tilapia to predators.

Abstract: Lake Victoria’s artisanal fishery has an overfishing problem. A possible solution is suggested by records showing that fish catches are best where predator populations have been reduced by fishing. It may be possible to remedy overfishing by increasing fishing effort, provided the additional effort is directed toward predators.