Neubauer P.,Dragonfly Science |
Jensen O.P.,Rutgers University
PeerJ | Year: 2015
Quantitative analysis of stable isotopes (SI) and, more recently, fatty acid profiles (FAP) are useful and complementary tools for estimating the relative contribution of different prey items in the diet of a predator. The combination of these two approaches, however, has thus far been limited and qualitative.We propose a mixing model for FAP that follows the Bayesian machinery employed in state-of-the-art mixing models for SI. This framework provides both point estimates and probability distributions for individual and population level diet proportions.Where fat content and conversion coefficients are available, they can be used to improve diet estimates. This model can be explicitly integrated with analogous models for SI to increase resolution and clarify predator-prey relationships.We apply our model to simulated data and an experimental dataset that allows us to illustrate modeling strategies and demonstrate model performance. Our methods are provided as an open source software package for the statistical computing environment R. © 2015 Neubauer and Jensen.
Richard Y.,Dragonfly Science |
Perriman L.,Moray Place |
Lalas C.,University of Otago |
Abraham E.R.,Dragonfly Science
PeerJ | Year: 2015
Demographic rates, such as annual survival rate, are generally difficult to estimate for long-lived seabirds, because of the length of time required for this kind of study and the remoteness of colonies. However, a small colony of northern royal albatross (Diomedea sanfordi) established itself on the mainland of New Zealand at Taiaroa Head, making possible regular banding and monitoring of its individuals since the first chick fledged, in 1938. Data on the presence/absence of birds, as well as on breeding outcomes, were available for the period from 1989-90 to 2011-12, and included 2128 annual resightings of 355 banded individuals of known age. The main goal of the present study was to estimate the annual survival rate of juveniles, pre-breeders, and adults at Taiaroa Head. These rates were estimated simultaneously in a single Bayesianmulti-state capture-recapture model. Several models were fitted to the data, with different levels of complexity. From the most parsimonious model, the overall annual adult survival rate was estimated as 0.950 (95% CI [0.941-0.959]). In this model, adult survival declined with age, from 0.976 (95% CI [0.963-0.988]) at 6 years, the minimum age at first breeding, to 0.915 (95% CI [0.879-0.946]) at 40 years. Mean annual survival of pre-breeders was 0.966 (95% CI [0.950-0.980]), and 0.933 (95% CI [0.908-0.966]) for juveniles. There was no discernible difference in survival between males and females, and there was no apparent trend in survival over time. Estimates of other demographic rates were also obtained during the estimation process. The mean age at first return of juveniles to the colony was estimated as 4.8 years (95% CI [4.6-5.1]), and the mean age at first breeding as 8.9 years (95% CI [8.5-9.3]). Because all the birds of the colony were banded, it was possible to estimate the total population size. The number of northern royal albatross present annually at the TaiaroaHead colony has doubled since 1989-90, and the current total population size was estimated to be over 200 individuals. The ratio of the total population size to the number of annual breeding pairs varied from5 to 12 among years, with an overall mean of 7.65 (95% CI [7.56-7.78]), and this high variability highlights the need for a sufficient number of surveys of seabird breeding populations before reliable conclusions on population trends can be made. Although long-termdata allowed estimates of demographic rates of northern royal albatross at Taiaroa Head, the location of the colony and the ongoing management by staff mean that the population dynamics may differ fromthose of the main population on the Chatham Islands. © 2015 Richard et al.
Thompson F.N.,Dragonfly Science |
Abraham E.R.,Dragonfly Science |
Berkenbusch K.,Dragonfly Science
PLoS ONE | Year: 2013
Marine mammals are regularly reported as bycatch in commercial and artisanal fisheries, but data are often insufficient to allow assessment of these incidental mortalities. Observer coverage of the mackerel trawl fishery in New Zealand waters between 1995 and 2011 allowed evaluation of common dolphin Delphinus delphis bycatch on the North Island west coast, where this species is the most frequently caught cetacean. Observer data were used to develop a statistical model to estimate total captures and explore covariates related to captures. A two-stage Bayesian hurdle model was used, with a logistic generalised linear model predicting whether any common dolphin captures occurred on a given tow of the net, and a zero-truncated Poisson distribution to estimate the number of dolphin captures, given that there was a capture event. Over the 16-year study period, there were 119 common dolphin captures reported on 4299 observed tows. Capture events frequently involved more than one individual, with a maximum of nine common dolphin observed caught in a single tow. There was a peak of 141 estimated common dolphin captures (95% c.i.: 56 to 276; 6.27 captures per 100 tows) in 2002-03, following the marked expansion in annual effort in this fishery to over 2000 tows. Subsequently, the number of captures fluctuated although fishing effort remained relatively high. Of the observed capture events, 60% were during trawls where the top of the net (headline) was <40 m below the surface, and the model determined that this covariate best explained common dolphin captures. Increasing headline depth by 21 m would halve the probability of a dolphin capture event on a tow. While lack of abundance data prevents assessment of the impact of these mortalities on the local common dolphin population, a clear recommendation from this study is the increasing of headline depth to reduce common dolphin captures. © 2013 Thompson et al.