Traditional analyses (e.g., Schnabel 1938 or Chapman 1954 of sequential mark-recapture experiments (Petersen and Schnabel type) yield population estimates with substantial negative bias and overly large confidence intervals if the combination of the number of animals marked and examined falls too low. To address these problems, sequential mark-recapture experiments are cast into a Bayesian framework using a "noninformative" discrete uniform improper prior (a priori theoretical) distribution. Some properties of the posterior distribution (probability of each population size given the data) are briefly and informally discussed (inference, convergence, mean, mode, median, and treatment of nuisance parameters). A sequential Bayes computational algorithm, suitable for microcomputers, is given. Several examples are presented as a practical guide to computing estimates.For relatively small sample sizes, the Bayesian approach yields larger mean abundance estimates than traditional methods. There is little difference in these estimates for larger sampling efforts. Advantages of the approach include the following: the probability of observing the data at all feasible population sizes is calculated exactly; the method works for all cases regardless of sample size or sampling procedure; a plot of successive posterior distributions can be used as a visual diagnostic of conformity with basic assumptions; and finally, inferences can be made directly, since the end product completely describes the uncertainty of the population size given the data.
We developed a Kemp's ridley (Lepidochelys kempii) stock assessment model to evaluate the relative contributions of conservation efforts and other factors toward this critically endangered species' recovery. The Kemp's ridley demographic model developed by the Turtle Expert Working Group (TEWG) in 1998 and 2000 and updated for the binational recovery plan in 2011 was modified for use as our base model. The TEWG model uses indices of the annual reproductive population (number of nests) and hatchling recruitment to predict future annual numbers of nests on the basis of a series of assumptions regarding age and maturity, remigration interval, sex ratios, nests per female, juvenile mortality, and a putative ''turtle excluder device effect'' multiplier starting in 1990. This multiplier was necessary to fit the number of nests observed in 1990 and later. We added the effects of shrimping effort directly, modified by habitat weightings, as a proxy for all sources of anthropogenic mortality. Additional data included in our model were incremental growth of Kemp's ridleys marked and recaptured in the Gulf of Mexico, and the length frequency of stranded Kemp's ridleys. We also added a 2010 mortality factor that was necessary to fit the number of nests for 2010 and later (2011 and 2012). Last, we used an empirical basis for estimating natural mortality, on the basis of a Lorenzen mortality curve and growth estimates. Although our model generated reasonable estimates of annual total turtle deaths attributable to shrimp trawling, as well as additional deaths due to undetermined anthropogenic causes in 2010, we were unable to provide a clear explanation for the observed increase in the number of stranded Kemp's ridleys in recent years, and subsequent disruption of the species' exponential growth since the 2009 nesting season. Our consensus is that expanded data collection at the nesting beaches is needed and of high priority, and that 2015 be targeted for the next stock assessment to evaluate the 2010 event using more recent nesting and in-water data.
Sturgeon (Acipenseridae) stocks worldwide are generally in decline, with many populations close to extirpation. One prominent species, the White Sturgeon (Acipenser transmontanus) with spawning populations distributed throughout three large, western North American watersheds (the Sacramento, Columbia, and Fraser rivers), has experienced population declines in the past decade. In 2003, the Committee on the Status of Endangered Wildlife in Canada designated all six populations of White Sturgeon in Canada “endangered.” To assist sturgeon recovery initiatives in the lower Fraser River (British Columbia), a stewardship‐based monitoring and assessment program was developed by the Fraser River Sturgeon Conservation Society. A descriptive population model was developed to provide reliable annual population estimates by size/age group and location, based on tag release and recapture data collected by trained volunteers. As of January 2011, the population estimate (from 40‐ to 279‐cm fork length) was 44,713 (95% confidence level 42, 634–46, 792). Group size analyses suggest that abundance decreases have been greatest for juvenile sturgeon under 100‐cm fork length. Recruitment decline may be the result of several factors, including destruction of important spawning and early life history rearing habitats; fewer successful adult spawners due to in‐river fisheries; and/or impacts of reduced food supply and ecological imbalances on both early life and adult stages.
The purpose of this study was to design and test a monitoring protocol for marine waterfowl in the central Alaskan Beaufort Sea. The study provides an important case-study of how a long-term monitoring program may be affected by unanticipated human disturbances. Because of its overwhelming and widespread abundance, relatively sedentary behavior, ease in counting, and the extensive historical database, the long-tailed duck (Clangula hyemalis) was selected as the focal species. Two null hypotheses were formulated concerning potential changes in the numbers and distribution of long-tailed ducks in relation to disturbance in an industrial study area, compared to a reference study area located about 50 km to the east. A 9-year historical database (1977-1984, 1989) of long-tailed duck densities and other important data recorded during systematic aerial surveys was analyzed retrospectively using multiple regression techniques. The retrospective analyses determined which of several predictor variables recorded were significantly related to long-tailed duck density. Separate analyses were conducted for two periods: (1) the overall period when long-tailed ducks were present in the lagoon study areas, and (2) the shorter adult male molt period. The results of the two analyses indicated that 57% and 68%, respectively, of the total variation in long-tailed duck density during the two periods could be explained by variables recorded during the surveys. Predictor variables representing habitat, day of the year, time of day, amount of ice, and wave height recorded on-transect during surveys were most closely associated with long-tailed duck density. Measurement error during the surveys, and influences outside the study area such as nesting success in tundra habitats and mortality during migration and in over-wintering areas likely also had strong influences on the results, but these factors were not measurable in our study. Based on results of the retrospective analyses, a long-term monitoring protocol consisting of a program of systematic aerial surveys and an analyses of variance and covariance (ANOVA and ANCOVA) statistical procedure was designed and initially tested in 1990 and 1991. This 2-year testing phase resulted in several revisions to the monitoring protocol. Refinements were made to the original sampling procedures, to the survey schedule, and to the recommended statistical analysis procedures. Results of the ANOVA and ANCOVA indicated that there was no evidence of a change in long-tailed duck densities that could be attributable to disturbance (from any source) in the industrial study area relative to a reference area with no industrial development. Other analyses indicated that the sampling and analysis procedures would be adequate to detect long-term trends in long-tailed duck density and localized disturbance effects, but that the monitoring program should be continued well beyond two years to detect statistically significant changes. As a result, additional aerial surveys of both study areas were conducted again...
We describe trends in growth and condition of Arctic cisco Coregonus autumnalis and broad whitefish C. nasus in the Prudhoe Bay region of northern Alaska for the period 1985–1989 to evaluate sublethal effects of changes in hydrography induced by building a causeway into the Beaufort Sea. Growth (increase in length) of age‐1 and age‐2 fish of both species was positively correlated with temperature, which explained 77–92% of the sample variance, Condition (whole wet body weight at a given length) was less correlated with temperature than was growth. However, small broad whitefish, which do not venture far from river mouths, showed a positive association between condition and salinity. Conversely, large broad whitefish, which range away from the rivers, exhibited a negative association with salinity. The foraging range of large broad whitefish may be restricted by high‐salinity water masses that, however, may deliver higher‐than‐normal levels of prey to the habitats near river deltas used by small fish. Arctic ciscoes appeared able to exploit a wide range of temperature or salinity conditions equally well, as indicated by their relatively constant condition over the 5‐year period. Growth of age‐1 and age‐2 Arctic ciscoes and broad whitefish was estimated to have been reduced by an average of 4–6% by causeway‐related temperature changes over the 5‐year study period (range of annual values was 0–10%). This magnitude of change is not believed to be important to the ecology of the two species.
Beginning in May 1998, the National Marine Fisheries Service has required that bycatch reduction devices be installed in penaeid shrimp trawl gear in the Gulf of Mexico. Changes in observer protocols were introduced, one of which was that all of the red snapper Lutjanus campechanus collected would be enumerated and measured when possible. This change has yielded catch and length information from the Gulf of Mexico shrimp fishery. To date, however, an integrated approach to determining the age composition, growth, and mortality of juvenile red snapper has not been attempted. To address this problem, we constructed a length‐based, age‐structured model to objectively estimate the growth and mortality parameters and age composition of the shrimp trawl bycatch of red snapper in the western Gulf of Mexico from 81 monthly length frequency data sets (a total of 239,521 fish were measured) from July 1999 to February 2007. Our modeling strategy was to make simple and straightforward representations of the deviations in growth from the mean trend, partial recruitment, density‐dependent mortality, and population dynamics. Bayesian parameter estimation was accomplished by calculating the mode of the posterior distribution. Important findings of this paper include (1) the shrimp trawl bycatch during the first and third trimesters is dominated (86–87%) by age‐0 fish, (2) the growth patterns of age‐0 and age‐1 red snapper suggest that the fish form an opaque annulus in winter months, (3) the natural mortality of age‐0 fish appears to be about double (a minimum 70% increase) the value used in the last stock assessment, and (4) the evidence for density‐dependent juvenile mortality is overwhelming. Inclusion of these findings in the red snapper stock assessment has the potential to substantially alter management practices.
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